DETAIL engineering 4: SOM

SOM Structural Engineering

∂ engineering

4

Editor: Christian Schittich Editorial services: Cornelia Hellstern, Sandra Leitte Editorial assistants: Samay Claro, David Heilinger, Nina Müller, Jana Rackwitz, Hartmut Rändchen, Alina Reuschling SOM editorial team: Bill Baker, David Horos, Dmitri Jajich, James Crouch Copy editing: Susanne Hauger, Philadelphia (USA); Raymond D. Peat, Alford, Aberdeenshire (GB); David Wade, Lörrach (D); Stefan Widdess, Berlin (D) Translation into English: Terri White for keiki communication, Berlin (D); David Wade, Lörrach (D) Drawings: Ralph Donhauser, Kwami Tendar Production / DTP: Simone Soesters Reproduction: ludwig:media, Zell am See Printing and binding: Grafisches Centrum Cuno GmbH & Co. KG, Calbe

© 2015, first edition DETAIL – Institut für internationale Architektur-­ Dokumentation GmbH & Co. KG, Munich www.detail.de ISBN: 978-3-95553-223-9 (Print) ISBN: 978-3-95553-224-6 (E-Book) ISBN: 978-3-95553-225-3 (Bundle) This work is subject to copyright. All rights reserved. Reproduction of any part of this work in individual cases, too, is only permitted within the limits of the provisions of the valid ­edition of the copyright law. A charge will be levied. Infringements will be subject to the penalty clauses of the copyright law.

Bibliographical information published by the German National Library. The German National Library lists this publication in the Deutsche ­Nationalbibliografie; detailed bibliographical data are available on the Internet at: http://dnb.d-nb.de

The FSC-certified paper used fot this book is manufactured from fibres proved to originate from environmentally and socially compatibles sources.

Contents

Introduction 6

Efficiency + Economy 82

Preface 7 Past and future – reaching new heights 8

Structural art Constraints spur creativity Optimising design goals The economy of construction Sustainability Integrating discipline and play

Simplicity + Clarity 16 Architecture and engineering at SOM – in the genetic code Informing design

18 24

Scale + Form 34 Scale and proportion Clarity of design – giving things a name Sensory fields, self-reflection and the future Structural design of tall buildings Tall building case study – Burj Khalifa

36

Research + Future 104 Quo vadis – megatalls as the focus of the SOM Research Gang Structural optimisation – developing new design tools Research timeline

106 111 120

41 46 52 58

Hierarchy 62 The importance of hierarchy Structure as poetry Exchange House in detail

84 85 88 95 99 103

64 66 76

Projects + People 122 Catalogue of projects 124 People 140 Picture credits

144

SIMPLICITY + CLARITY

16

Architecture and engineering at SOM – in the genetic code Nicholas Adams

18

Informing design Nina Rappaport

24

SOM strives to design buildings that are simple and straight-forward, with clearly defined ­structural systems. Many of the firm’s greatest buildings express their function clearly through their structure and allow it to inform the aesthetic.

SOM’s approach to structural design embraces this philosophy. In many of their ­buildings, the “story of the structure” is plainly visible and ­intuitively obvious to anyone who cares to read it; in others, simplicity is expressed through bold forms or a clear focus on a single idea. SOM endeavours to strip away the visual clutter and obscuring detail that so often hides the structures of buildings in order to reveal the beauty and ­elegance of the bones beneath.

Good design is often a process of reduction and clarification, an “editing” job that refines and improves, leaving behind only what is essential.

In avoiding “complex structures and complicated solutions”, SOM adopts a “less is more” approach, striving to express the full essence of the building with the simplest possible ­solution without sacrificing function or beauty.

Whether the structure is quietly or boldly expressed, it helps define the architecture and is reflected by it in turn. It tells a story, and as in any great novel, the telling seems effortless.

17

SIMPLICITY + CLARITY

Informing design 2.1 Nina Rappaport is an architectural critic and curator as well as an ­educator and publications director at Yale School of Architecture. She has published a book about her research on the engineer’s role as a designer and written numerous arti­ cles on the topic for magazines and journals. Her current work focuses on the intersection of engineering and ­factory design for the future of urban manufacturing.

Structure expressed as form The Center for Character and Lead­ ership Development of the United States Air Force Academy in Colo­ rado Springs comprises an architec­ turally exposed diagrid of painted structural steel plates that forms a dramatic 32-m tall inclined skylight aligned with the North Star. Like for the Fishers Island Residence and the Roche Learning Center, the structural components and connec­ tions were developed in close col­ laboration with the architects and express the sharp corners and structural logic of structural steel.

In the final form, architecture and structure are indistinguishable.

The idea of communicating a clear and legible structure has many implications for an archi­ tecture /engineering firm such as Skidmore, Owings & Merrill. Historically, the company has approached both engineering and architecture with the respective teams collaborating from the outset of a project. The joint expertise of engineers and architects has been applied in developing innovative design solutions that have met numerous challenges with inspiring results.

The collaborative dialogue results in a clarity of structure. Projects in which engineers interact not only with architects but also with artists or other designers are evaluated from both the design and structural perspective with a shared language and design vocabulary.

The architects’ and engineers’ methods of work­ ing together on parallel but separate tracks, with numerous interwoven points along the design path, enable them to reveal elements of a building, bridge or art installation that inform their design. The collaborative process is evident even in the SOM office, where it is hard to tell which is the engineer’s and which is the architect’s workplace, as both display struc­ tural drawings and models, building details and design sketches on their walls. The office also has conference tables where the teams sit to work together on a project from the outset of a commission, rather than one taking the lead and calling in the other after the design has been sketched out. A fundamental struc­ tural conception is developed in which form relates directly to function, based on a deeper reading of a building. Yet, that does not mean that SOM sees the design as simplistic or ­obvious. Rather, the design unfolds as each layer is further explored, giving clear purpose to structure.

Typical questions arise in engineering struc­ tures. For example, what is the engineer’s role in design? Where do points of innovation exist that contribute to a project’s design from an engineering standpoint? And what parts of the problem-solving process are creative? Just as

2.2

24

architecture has multiple vocabularies, so does engineering have a grammar, the elements and syntax of which can be shifted and manipulated, stretching the norm and making invention pos­ sible. Structural typologies can be categorised in a new lexicon of forms, such as new types of truss and frame, cantilevers, tube extrusions, cable-net facades, column and mullion combin­ ations, and exoskeletons. When engineers com­ bine their calculations with intuition and experi­ ence, these give form to the structure [1]. These efforts, among others, to refine structure and integrate it into a holistic design are exemplified by the SOM projects featured here.

At certain times during the history of architec­ ture, a building’s structure was hidden by stone, metal or other type of cladding or element in line with the prevailing design aesthetic. In the modern era, the structure and its direct exposure have offered architects the means to achieve their design goals. Structure and design are stripped down and their intrinsic value is seen in buildings in the same way as it is in nature – through the bones supporting a shell or skin, or a self-supporting shell. Structure is inte­ grated, not as an afterthought, but seamlessly into the design.

In the recent engineering work of SOM, two aspects, which are explored here, continue to provide a valuable focus: that of simplicity and clarity, and that of structural expression.

Structural simplicity and clarity Carefully considered functional elements guide and express form with effects that are poetic and sometimes even sublime, as engineers rise to the challenge of simplifying complexity in an efficient way. This simplicity and clarity becomes a driving philosophy of the firm in terms of a theory of structure [2]. The conscious clarity of structural language produces an ­elegant form that allows articulation of the archi­ tecture in an efficient structural design reso­ lution. This is seen in the exacting details of many smaller projects, such as Fishers Island

Informing design

2.1

2.2

2.3

2.4 2.5

2.3

Residence, James Turrell’s Skyspace and Roche Labs, as well as those of larger projects, such as San Francisco International Airport and the US Force Academy’s Center for Char­ acter and Leadership Development (Figs. 2.1 and 2.2).

Lightness – Fishers Island Residence A client commissioned architect Thomas Phifer and Partners to design an open, transparent house connected to the landscape of Fishers Island in the Long Island Sound (Fig. 2.5).

SOM’s engineers often work with Phifer and, on this project, collaborated on the design of a minimalistic structural system, deceptively simple, through which an impression of lightness was created through close attention to detail.

Center for Character and Lead­ ership Development, United States Air Force Academy, Colorado Springs, Colorado (USA) 2015 Center for Character and Leadership Development, United States Air Force Academy Finite element model result showing 3D stress contours in casting and branches, Fishers Island Residence, New York (USA) 2007, architects: Thomas Phifer and Partners Detail of ductile iron branch arm, Fishers Island Residence Fishers Island Residence

2.4

Combining both prefabricated elements (trans­ ported to the island by ship) and custom­ detailed connectors (Fig. 2.4), the engineers employed two systems – one for the house and one for the surrounding canopy.

The 430­m2, open­plan glass house is sup­ ported by an exposed steel modular structure that adds a sense of weightlessness through a series of 56 ≈ 8.8­m steel roof beams. These span an uninterrupted interior space, clad in glass, with steel columns and mullions unified in one piece. To achieve this thinness, the engineers stripped everything down to the minimum supports, eliminating an entire system by making the small 7­cm square column and mullion one and the same.

Similarly, for the independent canopy structure

Thomas Phifer and Partners Architect Thomas Phifer founded his firm in 1997. Recent projects include the Corning Museum of Glass North Wing, Corning, New York, and the United States Court­ house, Salt Lake City, Utah. Thomas Phifer and Partners worked with the engineers from SOM on the Fishers Island Residence, the Turrell Skyspace and the Lee Hall III at Clemson University.

2.5

25

SIMPLICITY + CLARITY

2.20

2.21

which also support the glass panes.

Although, as engineer Aaron Mazeika notes, the large diagonal cable elements, forming a V-shape in elevation, fulfilled an important function in subdividing the facade, they actually introduced a significant design challenge, given the tendency of the diagonal cables to attract seismic forces during an earthquake.

While the engineers required highly preten­ sioned cables, as light and small as possible, to eliminate any sag, designing for seismic forces ran counter to that approach. The conceptual problem was how to disconnect the diagonal elements from the overall building system and decouple seismic behaviour from the facade system.

With the two diagonals sloping in different direc­ tions, any lateral movement of the building would mean that one would go into tension and the other into compression, so they realised there could be a way to offset these two forces.

An early solution was to replace the two diag­ onal cables by a single longer cable that starts at the bottom of the V-shape and travels diag­ onally to one of the top corners of the facade, across a pulley and horizontally to the other top corner, across another pulley and back down to the bottom of the V-shape. In this way, the cable force would be constant, with the two pulleys simply rotating as the building swayed back and forth, and the complicated pulley mechanisms could be hidden from view at the top of build­ ing. Yet, instead of this, the architectural team embraced the dramatic idea of exposing this critical structural element. The engineers thus placed the rocker mechanism at the base of the V-shape, in a prominent position on the access­ible roof of the “lantern” (Fig. 2.22). Two straight cables are then connected to the rocker arms, the rotation of which acts as a pulley to provide the necessary seismic isolation, while eliminating the complexity of the curved metal surfaces of a real pulley and the bending of the thick cables (Figs. 2.23 and 2.24). Engineers do not usually like things that move, so it is unusual for a building to contain a moving mechanism.

As such, the rocker is enthusiastically cele­ 2.22

32

Informing design

2.20 Mode shapes, first three fundamental modes of vibration, cable net facade, Poly Corporation Headquarters, ­Beijing (CN) 2007 2.21 Poly Corporation Headquarters 2.22 Rocker mechanism, Poly Corporation Headquarters 2.23 Pulley testing, Poly Corporation Headquarters 2.24 Detail of rocker, scale 1:50, Poly Corporation Head­ quarters 2.23

brated and has become the centrepiece in the atrium.

The other structural innovation is the glazed eight-storey glass “lantern”, suspended from the building structure within the atrium space, which houses a museum of bronze sculptures repatriated from the international auction market.

The cross-braced steel frame is suspended from the cable-net wall’s diagonal cables, while a backup steel frame allows it to cantilever from an adjacent building service core in the event of damage to the cable system. This backup frame provides a stressing support for pretension in the diagonal cable, the forces of which exceed the entire weight of the “lantern”. This combina­ tion allows for its unique, seemingly lightweight structure to float above the foyer floor. The effect is that of a transparent rectilinear space that seamlessly in­­terlocks with the building. Through their exposure, the structural elements inform the building design.

The synthesis of structure and form at SOM, combined with a clarity of innovation, has evolved historically from a company culture of technology and design to the makings of a structural theory. This structural theory reveals an understanding of the complexity of creating clear and direct forms, which could further expand investigations of new structural form.

The fact that SOM engineers realise the signifi­ cance of designing even structures that are hidden, covered by layers of building materials, ensures an honesty of expression deep below the surface. Exposed rational structures express a direct structural meaning and purpose which play a double role as form.

References: [1] Rappaport, Nina: A Deeper Structural Theory. In: Architec­ tural Design – Special Issue: The New Structuralism.

Design, Engineering and Architectural Technologies. July/ August 2010, pp. 122 –129 [2] Ibid.

[3] This is described in detail in: Rappaport, Nina: Deep ­Decoration. In: 306090 – Decoration, Vol. 10, 2006 [4] Structural Stainless Steel Case Study 10: Schubert Band Shell; available online at http://www.steel-stainless.org/ CaseStudies

1

2

3

4 5

6

7

8

1 151 ≈ 15.2 mm strand cable, bundled, Ø 226 mm 2 121 ≈ 15.2 mm strand cable, bundled, Ø 206 mm 3 Metal plate to receive strand anchors 4 Plate metal clevis attachment 5 Secondary steel pin 6 Upper primary steel pin 7 Lower primary steel pin 8 Web stiffener plate 2.24

33

SCALE + FORM

Floors 91 – 108

Floors 67 – 90

+

+

+

=

Floors 51 – 66

Floors 1 – 50

2.3

giving it a name derived from Western mythology in keeping with long-standing tradition.

Even the English eventually accepted the internationally more neutral and fitting name.

When modern biologists discover new species, they are constrained by the conventions of binomial nomenclature – even down to matters of Latin grammar and capitalisation – as to how they can choose and record a typically two-word scientific name. The first part of the name – the genus – is capitalised, but the second – the species – never is, not even if derived from a proper noun naming a person or place. The giant water lily, Victoria amazonica, for example, was originally improperly named “Victoria Regia”.

Moreover, while this was honorific to Queen ­Victoria, it was not descriptive of the plant’s origins. There is, of course, much more freedom in creating a common name for a new species of flora or fauna, which explains in part why such names are not always unique. The same holds for non-organic minerals, invented devices, and engineered structures and structural systems.

Non-living things tend to be named more descriptively than honorifically. For example, in the mid-16th century, an entirely new mineral that we now know as graphite was discovered in England’s Lake District. At first, it was referred to by names that described what it did and what it resembled. Whereas metallic lead had long been used to scribe faint guidelines on parchment and paper used for manuscripts, the new mineral made a heavy, dark black mark. Thus, imitatively, it began to be called by the Latin term “plumbago”, meaning “that which acts like lead”, in the sense of leaving behind a mark. In English, it was called, among other things, “black lead”, a simple descriptive name that helped understand the distinctive properties of the new mineral. This new substance was soon shaped to form the core of what we now know as a pencil. Prior to that, a pencil had been the name for a very fine-pointed brush. As a wooden shaft filled with graphite instead of animal hair could draw a fine line without having to be dipped into a wet medium, the implement was initially called

42

a “dry pencil”. In time, as the newer form of pencil became more common than the older one, its name was shortened and simplified to the now-familiar “pencil”. The marking medium was eventually given the unique name graphite, the Greek root of which means “to write”.

Naming an invented or designed thing is not always very easy or logical. The @ symbol, now ubiquitous in digital communication, had previously been widely used in commerce, which is why it typically appeared on typewriter keyboards. Its vestigial presence on the teletype machines used in early computer-to-computer communication made it an easy, if arbitrary, choice to designate early e-mail addresses.

However, at least in English, the symbol still has no agreed name and is often referred to reflexively as the “@ symbol”, with @ simply ­pronounced “at”. In other languages, it is often named after what it resembles: “pig’s tail” in Norwegian, “rollmops herring” in Czech, “spider monkey” in German.

Noun + adjective Individual finished bridges and buildings are typically named after the client or a politician, but the underlying structural genre is often associated with the engineer who invented and introduced it. Thus, the cable-stayed bridge is attributed to Fritz Leonhardt and the bundled tube building frame to SOM’s Fazlur Khan (Fig. 2.3).

However, the common names by which structural genres are known tend to be more descriptive, like the straightforwardly designated bridge types of beam, arch and suspension. The meanings of truss and cantilever may be less self-evident to the layperson, but once understood they should also become a comfortable part of the enthusiast’s vocabulary. Of course, as bridges evolved, their structural systems often became more varied and complicated. The arch form, which once implied that stone was the constituent material, had to be paired with the qualifiers stone, iron, concrete and steel. Trusses that were not simple were called continuous. Among the latest variations on a traditional bridge form is the self-anchored suspension type, most

Clarity of design – giving things a name

2.4

­ otably represented by the East Bay signature n span of the San Francisco-Oakland Bay Bridge.

Bill Baker observed that, for buildings, there are many structural systems – for example, “belt truss”, “bundled tube”, “buttressed core” – the names of which fall into the format of a noun modified by an adjective (noun + adjective), and this may be a preferred way to name any structural system. It could be seen as not unlike the binomial genus-species convention of biologists.

However, rather than the species following the genus in the name, in structural engineering the species precedes it, at least in English, as in the examples given. This could be seen as appropriate, since it is in the qualifier of the noun that the innovation is captured.

Yet not every structural system necessarily ­elicits the same noun + adjective pair from every structural engineer. As if there were a principle of relativity at work, the same tall building may be seen in different ways by different observers.

Fazlur Khan saw the Sears Tower as a bundle of nine tubes acting together as a single “bundled tube”, but another engineer might see the building’s “noughts-and-crosses” ground floor plan and its curtailed versions in the building’s upper storeys as the basis for a framed tube or a ­modular tube (Fig. 2.3). Baker sees the layout as the cross section of a giant beam with four webs. However the structure is viewed and no matter what it is called, an engineer’s understanding of its behaviour as a tall cantilever emerging from the streets of Chicago will be shaped by how it is imagined.

Given SOM’s emphasis on simplicity and ­structural clarity, the structural system of the Exchange House in London (see “Exchange House in detail”, p. 76 – 81) virtually names itself, as the hybrid building-bridge structure exposes the dominant tied arch (note the noun + adjective) that is central not only visually but also structurally. The appropriateness of this unique building-bridge structure over the wide expanse of railway tracks north of Liverpool Street Station might also be seen as a playful interpretation of the concept of a railway bridge. Similarly, the

long-span roof of McCormick Place Convention Center is a bold expression of a cable-stayed roof (taking its name from the bridge system of the same form; Fig. 2.6, p. 44). Both Exchange House and McCormick Place are long-span structures that share many of their traits with bridges, so it is not surprising that their structural systems evoke familiar bridge typologies.

But what of buildings that do not so directly suggest a bridge or any other familiar structure?

Naming the structure The frontispiece to Bill Baker’s Beyond Tall essay is an updated and augmented version of a graphic used by Khan in the 1960s to show how structural systems (and their names) evolved as the number of building storeys increased. As with bridges, a certain type of structural system is appropriate for a range of building heights. As the limit (whether structural, functional, economic or aesthetic) of an existing system was reached, a new system had to be created to build beyond the maximum height allowed by the old. As these systems evolved, they needed names to serve as a shorthand means of capturing the essential feature that distinguished them from one another.

For concrete office buildings, the names of the structural system as the buildings grew from 20 to 75 storeys are shown in the graph to have been, successively: frame, shear wall, frame-shear wall, framed tube, tube-in-tube and modular tube (Fig. 2.5, p. 44). The systems for steel structures, which are shown to have grown from 30 to a possible 140 storeys, have been given the names, successively: rigid frame, frame-shear truss, belt truss, framed truss, truss-tube with interior columns, bundled tube and truss-tube without interior columns.

(These names had evolved somewhat from those Khan had used, but that was to be expected as structural types were reinterpreted.) Certainly, all of these designations are clearly descriptive, but some are more ­mellifluous than others. How preferable it would be, from an aesthetic point of view, to stick to simple noun + adjective names.

2.3 Bundled tube structure of Willis (formerly Sears) Tower, Chicago, Illinois (USA) 1974 2.4 Shaped truss, International ­Terminal, San Francisco International Airport, California (USA) 2000

43

Storeys

16

30

40

40

50

50

60

70

100

100

110

140

Buttressed core

Trussed megaframe

Modular tube

Core-outrigger

Trussed tube

Tube-in-tube

Framed tube

Frame outrigger

Core-frame

Core

Frame truss

Rigid frame

Semi-rigid frame

SCALE + FORM

160+ 2.5

The observation that a tall building is, in essence, a vertical cantilever beam is a statement of clarity. Acknowledging this simple fact strips away the details of setbacks, facades and vortex-shedding strategies and enables us to see the building for what it is fundamentally.

Yet, since every building is a cantilever, that word contributes no informational content when striving to distinguish one structural building system from another. It is here that the noun + adjective scheme, which focuses not on the length of the cantilever but on the nature of its cross section, expresses organisation of thought. In fact, whatever SOM building systems are called, they express simplicity and structural clarity. The 7 South Dearborn structure, for example, was conceived to have an exceptionally simple gravity load path: all of the building’s upper floor loads are carried directly by the core (or “mast”). The stayed-mast structural system represents a new conceptualisation of a tall building as a single vertical “mast” elem­ ent that is laterally stayed. The John Hancock Tower in Chicago is another striking example.

Its gently tapered shape makes the building recognisable even in silhouette, and the simple pattern of steel, expressing the fact that it is clearly a braced tube, distinguishes it both structurally and architecturally. Even if the layperson does not see every nuance to the bold

X-bracing, he or she cannot help but understand that it is what makes the building work. The Hancock building structure remains an archetypal precedent that has been and continues to be reinterpreted. The morphing diagrid for SOM’s Lotte Tower (Fig. 2.9), for example, starts with the same basic structural system as Hancock, but projects the mega-bracing on a surface that gradually changes from a square (like Hancock) to a circle. Likewise, the structure that comprises the diagrid-tube Tower of Hope sculpture at the University of Nebraska (Fig. 2.7) is immediately recognisable as the functional framework that provides vertical and lateral rigidity. The diagrid-tube is one and the same as the sculptural expression.

New structural systems for higher buildings As buildings continue to grow in height, as they seem destined to do, there will be an ever-pres­ ent need to devise new structural systems and hence new names for them. The convention of using a noun + adjective pair to characterise these new systems will certainly add a rational approach, not only to naming the systems themselves, but also to categorising what we can expect will be an ever-growing number of types.

It was the conception and development of the buttressed core that made it possible to construct Burj Khalifa as tall as it is. The principle is

McCormick Place Convention Center, Chicago Completed in 1986, this significant expansion made McCormick Place the largest convention centre in the USA, a record it still holds. The proj­ ect combined advanced engineering and the utilisation of air rights to enable development on land once thought unbuildable. Constructed over active railway tracks, the add­i­ tion is divided into two distinct zones: a two-level exhibition hall and a storage area with loading docks. The roof is supported by steel cables hung from 12 distinctive pylons that rise up through the building.

2.6

44

Clarity of design – giving things a name

2.5 Tall building systems, their names and appropriate ­numbers of storeys 2.6 Cable-stayed roof, McCormick Place Convention Center, Chicago, Illinois (USA) 1986 2.7 Diagrid tube, Tower of Hope, University of Nebraska Medical Center, Omaha, Nebraska (USA) 2011, artist: James Carpenter 2.8 Finite element stress contours for connection nodes, Lotte Super Tower, Seoul (ROK), design 2008 2.9 Morphing diagrid, Lotte Super Tower 2.7

made even more understandable when we actually feel the forces involved through Baker’s anthropomorphic model of a man holding an open umbrella and bracing himself against the force of the wind in a horizontally blowing rainstorm by planting one foot behind him, as if in a fencing stance. The backwards extended leg functions, of course, as a buttress. D’Arcy Thompson saw such a structure in trees that grow in a setting of strong winds. He observed that “anchoring roots form powerful wind struts, and are most developed opposite to the direction of the prevailing winds.”

The buttressed core structural system will allow towers to be built about a kilometre high; to build taller than that is likely to require a new kind of structural concept. What that will be is not necessarily obvious. After all, in the evolutionary chain from skyscraper to supertall tower, the buttressed core did not spring to the mind of every structural engineer as the natural successor to the trussed tube. Taller structures will eventually be proposed, no doubt, but whether they are going to be built will depend upon a number of factors, not least of which will be the economic and political climate, which is intertwined with constructability and construction time. Of course, the basic structural concept will be the sine qua non.

Baker does not believe that Frank Lloyd Wright’s mile-high Illinois Tower could be built as proposed as it would take some 10 years to build and, if completed as designed, would lack torsional stiffness.

Yet, whatever present and future structural engineers see in their mind’s eye as a structural system that will make possible a tower approaching two or three kilometres, they will have to communicate it to other engineers, architects and clients. This task will be made easier if they can describe their structural concept not in a rambling sentence or disjointed paragraph, but in a few (preferably two) words.

If they can do that, they will demonstrate that they themselves do, indeed, clearly understand the structure and can proceed with confidence

to its completion. However, a name evolves with or follows the discovery of the thing, and so it will emerge only after sufficient structural engin­ eering and architectural thinking has been done to produce something on paper that is buildable in concrete and steel.

Engineers will start by envisaging their concept non-verbally in their mind’s eye. They will then communicate it to colleagues and consultants in gestures, sketches and drawings. In time, these will be accompanied by words in presentations and prospectuses. Initially, the words may outnumber the drawings because the design may not be fully fleshed out and may not be fully understood. Eventually, the words will become fewer because the concept will have been given a name – a name that was effectively discovered in the process of thinking and working through the design.

2.8

Bibliography: Baker, William F.: Beyond Tall.

Issues of Scale and the Evolution of Tall Buildings at SOM. In: SOM Journal 7, 2011, pp. 140 –146 Goldsmith, Myron: The Tall Building.

The Effects of Scale. M. S. Thesis, Illinois Institute of Technology, June 1953 Houston, Keith: Shady Characters.

The Secret Life of Punctuation, Symbols, & Other Typographical Marks. New York 2013 Khan, Yasmin Sabina: Engineering Architecture. The Vision of Fazlur R. Khan. New York 2004 Petroski, Henry: The Pencil. A History of Design and Circumstance.

New York 1990 2.9

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SCALE + FORM

Recycled inclusion

3.8 Parametric city model This model interfaces with programs such as Grasshopper, used to define geometry, combined with tools such as Galapagos and Karumba, which allow generic algorithms and structures to be defined.

The model accesses a database of hundreds of previously designed and built structures. The recorded data includes structural requirements relative to height, material type and site location (seismicity and wind conditions) along with space requirements for building systems, such as vertical transportation and mechanical systems.

of construction. In addition, the environment is becoming overburdened with waste materials that do not decompose and are not recyclable.

Materials such as lightweight waste plastics and polystyrene could beneficially reduce mass if strategically introduced into structures where concrete is not needed. The Sustainable FormInclusion System (SFIS; Fig. 3.8) – originally conceived for creating air voids in structures by placing capped, empty plastic beverage containers into structural systems – achieves these goals. More practically, the system can utilise bricks composed of ground and formed plastics or waste foamed polystyrene (such as styrofoam) cast into a lightweight mortar. Environmental responsibility could be taken further by using zero-cement concrete with the use of products such as Greencem, whereby cement is essentially eliminated and waste blast furnace slag used as a replacement.

Morphogenetic planning for the future Evaluating multivariable parametric building models on a district or city scale can be used to identify the best planning strategies for the

future. The parametric city model combines the weighted importance of form, structure, embedded carbon and efficiency of space use while considering orientation, including exposure to daylight and solar gain. The model is also capable of evaluating the embedded carbon impact of construction with regard to material type (steel, concrete, wood, masonry, etc.), fabrication and transportation of these materials, construction time and required equipment, and the number of construction workers and their transportation to and from the site. With the requirements for the structural and mechanical systems known, the commercial value of the net available space can be assessed based on its location within the building (such as floor level), access to daylight and views. The model is also capable of evaluating the environmental and financial benefit of incorporating advanced seismic systems into structures through the reduction of lifecycle carbon and anticipated damage over time, and the cost-benefit of addressing those risks at the time of construction. For slender structures or structures with complex geometries, parameters can be interactively evaluated with regard to the advantages of interlinkages or other geometric modifications.

These models can be translated into more sophisticated structural analyses to determine where structural material should be placed in order to ensure that the least amount of energy is expended when work is done to resist load.

In minimising the energy, forces and deformations should be distributed as evenly as possible throughout the structure by a synergetic placement of material. Forces will flow through the easiest, shortest and most natural load path of the structural form. Topographical optimisation techniques are used to map the structural response and define the most efficient placement of materials.

On a district or city scale, the environmental impact of planning can be interactively evaluated based on proposed or anticipated building material type, geometry and site conditions.

The type of use plays a significant role in the

3.9

50

Sensory fields, self-reflection and the future

3.8 Sustainable form-inclusion systems (SFIS) 3.9 Parametric city model, illustrating darker areas where occupied efficiency falls below targets 3.10 Construction, Poly International Plaza, Beijing (CN) 2015 3.11 Diagram of facade, Poly International Plaza 3.10

overall plan as the requirements for building systems and structure vary when comparing, for example, office, residential and mixed use occupancies (Fig. 3.9).

Rheological buildings for the future The envelope enclosure for structures represents the single greatest opportunity to consider flow and interaction between architectural, structural and building service systems (Figs. 3.10 and 3.11). Hundreds of millions of square metres of occupied area are enclosed each year with systems that essentially provide protection from the elements, safe occupancy and internal comfort. A closed-loop structural system integrated into an exterior wall and roof system that incorporates liquid-filled structural elements could provide a thermal store that heats up during the day and could be used for building service systems – such as a hot water supply or heat for occupied spaces – during the evening hours.

A solar collection system could be integrated into the network and incorporated into double wall systems, where it could be used to heat the internal cavity in cold climates. Transparent photovoltaic cells could be introduced into the glass and spandrel areas to capture more of the energy of the sun.

The concept of flow can be further developed into structures that are interactively monitored for movement. Through the measurement of imposed accelerations due to ground motions or wind, structures could respond by changing the state of the liquid within the system. For instance, the structure could use endothermic reactions to change liquids to solids within the closed network. Sensor devices could inform structural elements of imminent demand and ­initiate a state change in liquids that could be subjected to high compressive loads during which buckling could occur.

Magnetorheological or electrorheological (ER) fluids could be used to change the viscosity and therefore the stiffness of closed vessels and their damping characteristics. When subjected to a magnetic field, magnetorheological fluids greatly increase their apparent viscosity and can become viscoelastic solids. When subjected to an electrical field, ER fluids can reversibly change their apparent viscosity quickly, transitioning from a liquid to a gel and back again.

When storing liquids in very tall structural systems, pressures within the networked vessels become very large. With this level of pressure, for example, water could be supplied to the structure or to neighbouring structures of lesser height without requiring additional energy to move it. The energy required to store the water initially is minimised if water was collected at upper levels of the building, particularly roof and upper exterior wall areas. A continuous low velocity flow or a liquid with a low freezing point passing through these systems would keep it from freezing. Liquid in tuned-liquid dampers within the networked system would control motion, with fluid flow acting to dampen the structure when subjected to lateral loads from wind and earthquake events.

3.11

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SCALE + FORM

Normalised spectral energy of across-wind modal force

5.6 Wind tunnel testing 5.7 Reduction of wind forces 5.8 Wind tunnel test result: model frequency related to the recurrence interval for wind events.

The vertical axis is proportional to the resonant dynamic forces divided by the square of the wind velocity. a Original building configu­ ration b Configuration after several refinements of the architectural massing 5.9 Burj Khalifa 5.10 Tianjin CTF Financial Centre, Tianjin (CN), anticipated completion 2018 5.11 Wind tunnel workshop, Tianjin CTF Financial Centre, BMT Wind Tunnel 5.12 Wind tunnel tested schemes, Tianjin CTF Financial Centre

Initial scheme Base moment Acceleration 5.6

wind tunnel testing was undertaken, during which the structural and architectural teams refined the tower’s shape to increase its performance. Wind tunnel testing was performed in Rowan Williams Davies and Irwin Inc.’s (RWDI) boundary layer wind tunnels in Guelph, Ontario.

The wind tunnel programme included rigidmodel force balance tests, full multi-degree of freedom aeroelastic model studies, measurements of localised pressures, pedestrian wind environment studies and wind climatic studies (Fig. 5.6). Using the wind tunnel to understand and optimise wind performance was crucial to the tower’s design. Several rounds of force balance tests were undertaken as the tower’s geometry evolved and became refined.

1.6

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After each round of wind tunnel testing, the data were analysed, the building was reshaped to minimise wind effects and the building’s ­harmonics were refined (Fig. 5.7). In general, the number and spacing of the setbacks changed, as did the shape of the wings – ­originally, the setbacks were arranged in a ­spiralling counter-clockwise manner, which was reversed during testing to clockwise. Wind directionality was also studied, with respect to considering the direction of the frequent and strongest winds. As a result, the tower orientation was changed so as to better accommodate the most frequent strong wind directions for Dubai: northwest, south and east.

Through wind-tunnel testing, the tower’s struc-

1000 yr 4.21 Hz 100 yr 4.90 Hz 10 yr 5.69 Hz 1 yr 6.8 Hz

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Tall building case study – Burj Khalifa

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5.11

ture was “tuned” to minimise the effects of the wind. This was accomplished by using the results of the tests to perform parametric studies on the effects of varying the tower’s stiffness and mass distribution (Fig. 5.8). Along with this effort, the process described above of establishing and refining the shaping of the tower resulted in a substantial reduction in wind forces by “confusing” the wind and encouraging ­dis­organised vortex shedding over the height of the tower. These efforts also resulted in reduced wind forces and motions, such that the predicted building motions are within the ISO recommended values, without the need for auxiliary damping.

formwork system, allowing for quick floor cycle times with a minimal amount of crane usage.

Only the rebar cages needed to be hoisted by cranes. Concrete is distributed to each wing using concrete booms attached to the formwork system. Two of the largest concrete pumps in the world are utilised to deliver concrete to heights over 600 m in a single stage. The core and wing wall areas utilised an “up up” construction process where the walls and wind wall column could proceed several floors above the slab pours. This was much faster than the typ­ ical construction process.

Utilising concrete construction for Burj ­Khalifa was a natural choice. Concrete offers higher stiffness, mass and damping for controlling building motions and accelerations, which was critical in designing the world’s tallest building.

In fact, due to the stiffness of the system, SOM was able to design the tower to satisfy motion and acceleration criteria without the use of ­supplemental damping devices. Additionally, the tower’s flat-plate floor construction offers increased flexibility in shaping the building, as well as providing the minimum possible floor thickness in order to maximise the ceiling height.

SOM engineers and architects often work in partnership together using a wind tunnel to develop the design of a tall building. The development of the Tianjin CTF Financial Centre (Fig. 5.10) included intensive experimentation in the wind tunnel to test the effects of various building configurations, including the shape and porosity of the top, the shape of the corners, possible slots or vents and several other geometric details (Fig. 5.11). The wind tunnel testing revealed that the total wind overturning forces on the tower could be reduced by more than 50 % by adjusting the geometry of the tower. The resulting geometry resulted in great material savings and a more striking architectural form that directly expresses wind-engineering principles (Fig. 5.12; see sidebar “Confusing the wind”, p. 55).

Construction process Material technology and construction methods have a significant impact upon the design of supertall building systems. These elements must be incorporated early in the design process so as to provide a system that facilitates efficiency and constructability.

The construction sequence for Burj Khalifa has the central core walls being cast first, in three sections; the wing walls next; then the slabs for the core and wing wall areas; and the wing nose columns and slabs after these. Walls are formed using an automatic self-climbing

Base scheme

50 % crown porosity

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5.12

61

HIERARCHY

62

The importance of hierarchy Bill Baker Structure as poetry

Sigrid Adriaenssens

Exchange House in detail Dmitri Jajich, Bill Baker

64 66 76

Hierarchy is in concept that can help to identify, distinguish and organise the components of a structural system or building. Not all parts of a structure or complex building system are of equal importance. The relative significance ascribed to the different elements – their hierarchy – will affect the story that the building tells.

The concept of hierarchy can therefore serve as an organising principle that helps to focus the design process and identify the most critical aspects of the problem. Hierarchy is usually, but not always, related to the load path in a structure, but it is not necessarily the same. The hierarchy of systems and subsystems also often represents a conscious choice made by the designers and reflects their subjective judgements regarding the relative importance of the parts comprising a building. SOM believes that a hierarchical approach to finding a structural solution, especially as it pertains to constructability, is crucial in achieving a coherent result: a building that expresses its function clearly and candidly.

63

HIERARCHY

3 3 2 2

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1 1

1 Curtain wall, unitised aluminium Fritted glass, laminated Wood rib, glue-laminated Steel tension rod, galvanised Tapered compression strut wood Tapered curved wood rib, ­glue-laminated Wood louvre 2 Aluminium coping, powdercoated 3 20 in (50.8 cm) mullion exten­ sions with stainless-steel inserts laminated on south-facing side of each vertical mullion beyond top of window wall 4 Aluminium bar grating 5 4 ≈ 6 in (10.16 ≈ 15.24 cm) tube

2.13

72

galvanised steel for retractable ­window-washing davit 6 Clear insulated laminated low-E glass Aluminium skylight assembly 7 Continuous gutter and flashing stainless steel 8 Extruded aluminium drainable louvre with electric actuator 9 Standing zinc seam roofing Waterproofing membrane Mineral board sheathing 3 in (7.62 cm) semi-rigid insulation 1.5 in (3.81 cm) metal roof deck Steel framing Suspended runner subconstruction Gypsum board, painted

2.14

Structure as poetry

1 2

3-dimensional cable net End post truss 3 5

Roof structure 1 Stabilising cable, stainless steel 2 Horizontal cable stainless steel 3 Roof truss top chord, stainless steel pipe section 4 Roof truss bottom chord, stainless steel cable 5 Edge cable, stainless steel 6 Strut, stainless steel 7 End post, stainless steel pipe 8 Cables, stainless steel

4 6 7 8

2.15

found in the Christian vesica piscis symbol; and the second was the base isolation, crucial for the cathedral’s long-term survival in a high seis­ mic region. Geometry was used as a unifying rule to define forms in plan and section. The ­horizontal base isolation layer imposed horizontal stratification, which suited the architectural con­ cept of a more earthly layer (the mausoleum) and the more heavenly layers (the congrega­ tion area and the light enclosure above it). As a result, the structural systems are design features and architectural form-makers, successful in evoking devoutness, consolation and light.

Structural vocabulary To achieve the best expression in the physical realisation of a project, there must be quality at every level – from the most global sense to the most detailed individual elements and connec­ tions. The designer has to make choices at each hierarchal level. The manner in which forces are transferred from one subsystem to the next, and how an entire structural system performs as one unit, significantly depends on the elements and types of joints used. To achieve economically efficient structures, strategies for selecting and joining structural elements must also aim to avoid connections requiring extensive fabrica­ tion or time-consuming assembly. However, in SOM projects, the detailing can exceed this objective by also having a crucial role in estab­ lishing the overall design concept of a project.

SOM strives to produce structural clarity in its details for even the most physically complex and technologically advanced structures. Its strategy is to design for pure load paths with minimal interruptions caused by eccentricities between members.

reflect a high-tech, machine-like aesthetic that expresses engineering excellence. This design intent was captured by giving hierarchical importance to the lightweight, efficient tensile elements and their connections.

The Entrance Pavilion is a lenticular building with anticlastic, prestressed cable nets. The lenticu­ lar geometry was adopted to coordinate with the facade cable nets, consisting of horizontal, circular, segmented and vertical parabolic cables (Figs. 2.15 and 2.16). This anticlastic, pre­ stressed cable net resists all positive and nega­ tive wind pressure applied to the glass facade of the pavilion. The parabolic vertical cables are prestressed against the roof truss, which is sup­ ported at the vertical apex trusses. The cables, with their lightweight, high-tech appearance, were given hierarchical priority over all other structural members. To further express the pres­ ence of tension forces in the design of the pavil­ ion, the structural elements used were specific to the type of force they carry: tension in cables, and compression and bending in tubes. As a

2.13 Section, scale 1:200, Cathedral of Christ the Light, Oakland, California (USA) 2008 2.14 Interior view towards altar, Cathedral of Christ the Light 2.15 Isometric drawing, General Motors Entrance Pavilion, Detroit, Michigan (USA) 2004 2.16 General Motors Entrance ­Pavilion

In the design of the Entrance Pavilion for Gen­ eral Motors’ (GM) Global Headquarters in the Renaissance Center in Detroit, Michigan, for example, the detailing was key in realising the architectural concept and defining the hierarchy within the structural system. Given the worldclass corporation’s role as a vehicle manufac­ turer, the architectural design was meant to 2.16

73

HIERARCHY

2

3 4

1

1 Roof truss bottom chord: Ø 88 mm stainless steel cable 2 Roof truss top chord: Ø 2.75 in (324 mm) stainless steel pipe 3 Roof truss diagonal: Ø 22 mm stainless steel cable 4 Roof truss pipe strut: Ø 3.5 in (89 mm) stainless steel pipe 2.17

result, the cable net is form defining, and the entire pavilion appears lightweight and effi­ cient. Compression only appears in the com­ pression chords of the roof truss and the end post trusses, which are designed to be slender and not to distract attention from the tensile cable network. Their slenderness was made possible by partial bracing. Together, all elem­ ents form a self-stressed system with very clear articulation of tensile cable and slender compression elements, a minimalist structure emphasing function and efficiency.

This self-stressing global structure is organised based on a sequence of subsystems that resist gravity and wind loads. The lenticular geometry of the pavilion has a curvature that allows each of the single laminated glass panels to be planar and equally sized. The detailing of the glass panels allows the glass to pass the com­ pression chord of the end post truss and gov­ erns the spacing of the cable net. The glass facets follow the curve of the horizontal cables with radial geometry. The dead and wind loads acting on the glass panels are transferred through specially designed spider fittings to prestressed vertical cables by the extension of a cable net compression strut. These vertical cables are prestressed against the roof structure and foundations. The cables are designed not to go slack under any loading combination. As a result, they greatly inhibit deflection and rota­ tion of the roof and can be thought of as thin columns supporting the roof.

2.17 Schematic drawing of roof truss, General Motors Entrance Pavilion, Detroit, Michigan (USA) 2004 2.18 Cable net truss to roof truss connection, scale 1:20, Gen­ eral Motors Entrance Pavilion 2.19 End-post truss, scale 1:10, General Motors Entrance Pavilion 2.20 Interior view, General Motors Entrance Pavilion

74

While dead loads are simple to consider in design, wind forces are less predictable. They cause both positive pressure and suction, which may be uniform or vary in magnitude. Although cable and glass structures can handle these forces with significant deflections, the GM Entrance Pavilion was designed to be stiff and deflect minimally. Given its lenticular shape, the structure alone could handle uniform suction, but would not be able to withstand variable wind pressures. Therefore, the prestressed cable net provides stability, strength and stiffness for vari­

able wind forces through its anticlastic pre­ stressed form. As one set of cables in the net (e.g. the horizontal ones) takes larger tensile loads, due to wind suction for example, the forces in the set of opposite orientation (e.g. the vertical cables) are reduced. If non-uniform loads act on the structure, the cable net resists the forces with small allowable displacements.

These opposite sets of cables are separated by a compression strut made of stainless steel.

Additionally, a stabilising cable was added to the third points of the vertical cables to avoid out-of-plane translation of the strut and vertical cables under certain load combinations.

The vertical cable system needs to be pre­ stressed against the roof truss (which spans onto the end post trusses and further resists ­horizontal wind loads) in its horizontal plane, and gravity forces in its vertical plane. In the horizontal plane, the truss consists of one cen­ tral stainless steel compression member with stainless steel ribs radiating from it in a leaf-like pattern. These ribs transfer the prestressed ­vertical cable forces and the dead load of the roof glass panels back onto the truss.

In the vertical plane, the truss forms a double convex shape and spans the end post trusses.

The shape of the bowstring truss mimics the bending moment diagram due to gravity and prestress loads. The truss has a top and bottom chord and was triangulated for stability during the erection process.

It was important in this project to ensure all eccentricities between members were elimin­ ated to provide clear load paths and concur­ rent connection points. Resolving the compo­ nent forces into a single concurrent connection point is not a simple endeavour, especially in three dimensions. For example, the connec­ tion between the roof truss and the cable net involves seven intersecting elements. Therefore, a stainless steel casting is used to join the mul­ tiple elements at a single point to avoid creating eccentricities and bending stresses.

The desired high-tech efficiency was further expressed by articulation of the forces through

Structure as poetry

9 7

10

11

13 5 Stainless steel clevis, custommade for hydraulic prestressing 6 Stainless steel spider support system, custom made 7 Ø 4.5 in (11.43 cm) stainless steel pipe 8 Glass 9 Ø 2.5 in (6.35 cm) stainless steel pipe 10 Ø 2.5 –1.5 in (3.81– 6.35 cm) tapered rod 11 Ø 5 in (12.7 cm) stainless steel plate 12 Countersunk bolts 13 Adjustable stainless steel ­turnbuckle, custom-made

12

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8 5

2.18

2.19

the choice of structural members, e.g. tension in cables, compression and bending in tubes.

At the detail level, SOM stayed true to its over­ arching goal of structural simplicity. The strategy behind the connection design was to reduce the size of clamps and plates so that all tensile elements visually bisect the compression mem­ bers. For example, in the pre-stressed cable net, the vertical and horizontal cables intersect the compression strut and spider arm. When taking a closer look, it can be seen that the clamping bolts, which connect the cable net to the com­ pression strut and spider arm, are hidden for greater visual appeal. The detailing design phil­ osophy of the GM Entrance Pavilion clearly illus­ trates that the best realisation of a project relies on careful choices being made at the element and connection level. The form and nature of the components express the forces they carry.

Connections accentuate and do not distract from this force flow through the different sub­ systems. This design philosophy can be traced back to one of SOM’s original sources of inspira­ tion, Mies van der Rohe, who claimed: “God is in the detail”.

­ lements. The hierarchy in this project is defined e by the grammar rules of analytical geometry and horizontal stratification. At the micro scale, the design of the GM Entrance Pavilion prioritises tensile elements, their connections and their relationships to other elements with the objective of obtaining a clear visual load path and evoking engineering excellence.

Reference: [1] Coleridge, H. N.: Specimens of the table talk of the late Samuel Taylor Coleridge. London 1835

It is said that “an engineer is a (wo)man who can do for a dime what any fool can do for a dollar”. While it is customary for an engineer to be responsible for achieving a specific techno­ logical need at the lowest economic cost, this saying is crippling to both the engineer’s creativ­ ity and the design’s potential. Within the context of the large scale and programmatic complexity of the majority of SOM’s projects, the engineered structure is often expressed to accentuate the architectural design intent at the macro, meso and micro scales. The organising tool at these different levels is hierarchy. The San Francisco International Airport and Lee Hall III show how the expression of the convex trusses and the column trees are the global form-makers for the design concept, distinguishing themselves from other systems that are also crucial but not determinative. At the meso scale, the Cathedral of Christ the Light illustrates that hierarchy does not necessarily equate to load path or key 2.20

75

HIERARCHY

3.10

a

b

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3.12

system (Figs. 3.13 and 3.14). The diagonals are important to the overall system, but they are not the primary load path. Thus they are expressed hierarchically lower than the tied arch by the use of smaller round-shaped members.

components on the system to depict clear paths of load flow. For example, the columns and hangers were detailed to pass through the arch segments without interrupting the in-plane geo­ metric ­continuity (Figs. 3.15 and 3.16). This choice serves to emphasise the hierarchical relationship between the two elem­ents and clearly communicates that the building grid is suspended from the arch. The detailing of verti­ cal connections further ­elucidates the nature of the forces carried in ­different parts of the struc­ ture: hanger splices carry tension (Fig. 3.11) and are detailed as visible lap splices, whereas column splices carrying compression are expressed as butt splices (Fig. 3.10).

Hierarchy of components The principal components of the system are the arch segments, the arch nodes, the base node, the primary ties, the hangers /columns, the arch diagonal, the horizontal bar bracing and the end trusses. The detailing and expression of the structural steel components was intended to convey a consistent character and emphasise structural logic and hierarchy by the use of crisp, open forms. After discussions with the design team, a structural “bridge” detailing aes­ thetic rather than a “machine” aesthetic was chosen.

Equally important as the aesthetics was the need for simplicity, clarity and ease of fabrica­ tion and erection. The functional hierarchy of the arch system was expressed by layering the

The secondary nature of the arch diagonals is expressed by the use of smaller circular steel shapes which bypass the hangers with direct connections to the arch and primary tie.

Floor framing truss extensions protrude through the facade and are connected directly to the column /hangers, thus making clear that the

3.10 Typical exterior column splice detail, not to scale 3.11 Typical exterior hanger splice detail, not to scale 3.12 Exposed structural system components 3.13 Unsymmetrical load deforma­ tions a Without diagonals b With diagonals 3.14 Exterior arch elevation 3.15 Detail of arch node connection, not to scale 3.16 Construction of exterior arch 3.17 View of Exchange House from plaza

Bibliography: Iyengar, Hal; Baker, Bill; Sinn R. C.: Broadgate Exchange House – Structural Systems. In: The Struc­ tural Engineer, Vol. 71, No. 9, May 4, 1993 3.14

80

Exchange House in detail

1 Ø 195.7 mm pipe with clevis 2 Bolts, six per bearing plate 3 Web plate

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floor system loads flow from the floor to the column /hangers, then to the arch, and finally down to the bearings.

Realisation A high degree of dimensional control and craftsmanship during fabrication and erection was required to realise the structural details in an aesthetically consistent manner. To that end, SOM structural engineers provided highly detailed, fully-engineered structural connec­ tion details that were painstakingly coordinated with the architectural design. In contrast, the connections in a conventional steel building are typically designed and detailed by the steel fabricator, and the exact configuration of the details is often not fully understood during the architectural design process. SOM engineers worked collaboratively with both SOM archi­ tects and the steel fabricators early in the design process, prior to bid, to determine a comprehensive system of tolerances for ­fabrication and erection and to ensure that structural details allowed the necessary access and adjustment to correct for distor­ tions, movements and errors. Every detail was reviewed with the fabricator and erector, then modified as needed to optimise construction and maintenance.

The tied arch was erected on temporary shores, with adjustable jacks at each hanger acting as columns during erection. The shores were sup­ ported on the plaza structure, which was capa­ ble of supporting the steel and metal deck struc­ ture (excluding the floor slab concrete) up to the eighth floor – the level which would complete the arch (Fig. 3.17). Upon completion of the arch and tie system, the shores were removed by jacking up the entire structure at the eight sup­ ports by 50 mm. This operation ensured that load was removed from all shores simultan­ eously and that the arch was uniformly loaded.

The continuation of the hangers below the pri­ mary tie (to allow temporary support) was inten­ tionally expressed as a remnant of the construc­ tion process.

To a layperson, the simplicity of the structural concept and clarity of the detailing may belie the high degree of design and craftsmanship required to realise Exchange House. The para­ doxical – but now commonly accepted – convic­ tion of Mies van der Rohe that “less is more” could be amended in the case of Exchange House to “less is more work”. Indeed, the suc­ cess of Exchange House is a testament to the perseverance of the design team. The engineer­ ing goal of a minimum-material, well-propor­ tioned and hierarchical structure was comple­ mented by the sparsely elegant architectural expression that takes its form and beauty from the underlying hierarchical organisation of the building. Exchange House represents the suc­ cessful transformation of a difficult engineering challenge into architectural expression, and exemplifies the SOM tradition of building designs in which architects and structural ­engineers can, by working together, produce an elegant solution.

3.16 AIA Twenty-five Year Award The Broadgate Exchange House has been selected for the 2015 AIA Twenty-five Year Award. Recognis­ ing architectural design of enduring significance, the Twenty-five Year Award of the American Institute of Architects is conferred on a building project that has stood the test of time by embodying architectural excellence for 25 to 35 years. Proj­ ects must demonstrate excellence in function, in the distinguished exe­ cution of their original programme, and in the creative aspects of their statement by today’s standards.

Other SOM projects that won this award were Lever House, New York City (1980), Air Force Academy Cadet Chapel, Colorado Springs (1996), John Hancock Center, ­Chicago (1999), Weyerhaeuser Cor­ porate Headquarters, Federal Way (2001) and Hajj Terminal at King Abdulaziz Airport, Jeddah (2010).

3.17

81

EFFICIENCY + ECONOMY

82

Structural art Maria E. Moreyra Garlock, Annette Bögle

84

Constraints spur creativity

Maria E. Moreyra Garlock, Annette Bögle

85

Optimising design goals

Maria E. Moreyra Garlock, Annette Bögle, Nils Ratschke

88

The economy of construction

Maria E. Moreyra Garlock, Annette Bögle

95

Sustainability

Maria E. Moreyra Garlock, Annette Bögle

99

Integrating discipline and play

Maria E. Moreyra Garlock, Annette Bögle 103

SOM’s structural engineers strive to create effi­ cient and economical designs, where efficiency relates to minimisation of materials and economy to minimisation of cost. Since these two goals are not always aligned, engineers must strike a perfect balance to find solutions that are at once efficient, economical and practical.

Key to the search for efficient structural systems is the geometry, where architecture and structure intersect. The complex interplay between effi­ ciency and economy is addressed in the initial design phase using a variety of optimisation tools, where optimisation refers to maximising the most important design characteristics while min­ imising expensive or scarce resources. Often, optimising construction speed is crucial. Material expenditures can also be critical, but merely min­ imising the size of individual structural members is not sufficient, as their arrangement and the form of the overall system are often more impor­ tant. The efficiency of a system may be gauged through the study of load paths, where the load is defined as the product of force and member length. Most of the time, a form in which load path length is minimised is the form that couples the maximum stiffness and strength with a min­ imum of material. The demands placed on the optimisation of forms often lead to new architec­ tural modes of expression. But optimisation alone is not enough. SOM’s process of benchmarking and systematic bracketing and study of structural system options is as important as the optimisation results. Achieving efficiency and economy in a structure involves the parametric analysis of the effect of many different variables on the cost and efficiency of a design.

83

Efficiency + Economy

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100 Mount Street Building The development of the structural system for the 100 Mount Street Building in Sydney, which will be completed in 2019 (Fig. 2.13), provided an even more complex optimisation challenge.

Again the goals were maximum stiffness with minimal structural weight. But additional goals included maximum openings for prime views to the south, east and north. The core containing building services is located on the western end, where the building abuts existing construction.

The eccentric non-central placement of the core improves the architectural layout, but exposes the building to undesirable torsional or twisting forces. To address this issue, SOM engineers placed a mega-braced frame on the eastern facade. The frame balances the stiffness of the core walls to the west and pulls the tower’s centre of stiffness back near the geometric centre of the building.

In combination with the shear wall core, the mega-braced frame resists winds on the broad face and the torsion that they create. This frame needs to be as stiff as possible to balance the lateral stiffness of the shear wall core and to ­prevent torsional vibration from becoming a dominant mode. At the same time it should be as slender and elegant as possible to promote the least disrupted sight lines.

Different bracing layouts were studied and found to be suitable, but too much material was called for to achieve the required stiffness. Thus the engineers referred back to their experience with the Maxwell method and started with some hand sketches. This was ­followed by a computer-aided topology optimisation study to determine the best bracing layout, in which the SOM engineers made use of their previous research in topology optimisation. What emerged was an X-bracing system with an elevated central node at about threequarters of the height of the X (Fig. 2.12). At this node height, the stiffest brace geometry is achieved.

To realise the full potential of this frame geom­ etry, the form-determining optimisation process was followed by an intensive study of structural

=

Flange wall

System 2.11

behaviour resulting from a released node and the stability issues of the long diagonal compression members. Last but not least, there was an intensive discussion about structural detailing and the question of whether to choose concrete or steel for the frame. Because of the stiffness and strength requirements, concrete was recommended for both the core and the frame.

The Polestar Tower Requirements for optimisation design goals are complex and potentially antithetical (see “Structural Art”, p. 84). In some situations, benchmarking structural solutions may be a helpful tool to obtain reasonable arguments for one or the other solution. Detailed system-

2.13

Optimising design goals

2.10 Structural system components, 100 Mount Street ­Building, Sydney (AUS), design 2012 2.11 Typology optimisation process to find ideal truss ­geometry, 100 Mount Street Building 2.12 Topology optimisation: optimal bracing geometry a Problem statement b Free-body diagram c Topology optimisation results 2.13 100 Mount Street Building 2.14 Slenderness study for various heights, Polestar Tower, Gothenburg (S), anticipated completion 2019 2.15 Study of different outrigger configurations and normalised resulting material quantities, ­Polestar Tower 2.16 Comparison of deformed shapes due to lateral wind load for various outrigger configurations, Polestar Tower

atic parameter studies serve to determine the best system and evaluate its efficiency.

These studies should be carried out even before a schematic design is developed so as to facilitate a new approach to the structural solution.

When it is finished in 2019, the Polestar Tower in Gothenburg (Fig. 2.18, p. 94) will be 230 m tall, making it the tallest building in Scandinavia and scaling up the traditional development ­patterns in the country. The tower will be mainly for residential use, with a variety of flats pos­ itioned over several mixed-use podium blocks.

Other facilities may include a public restaurant, a gym, a residents’ lounge, a podium-level garden and a rooftop observation deck overlooking the city and the waterfront. The design is highly efficient in terms of both floor space and energy use. Its fluid geometry is accentuated by the balconies that provide each flat with flexible living space.

The aim of the benchmarking process is to determine whether the design of the structure is to be primarily strength /gravity-driven or ­serviceability /motion-driven. With increasing slenderness, the effect of the dynamic wind forces on and the resulting movements of the tower have more and more influence on the structural design process and thus on the evaluation of the structural material to be used. A first step involves an intensive study of local construction costs and the consent of the client, with the aim of arriving at unit costs of structural elements depending on the material and installation methods. This allows simple calculations and comparisons to be made. For the Polestar project, the process demonstrated that concrete was by far the most cost-effective material for the tower in this market.

In the next step of the benchmarking process, a series of different standard floor framing methods are considered and sketched. In addition to the quantity and cost of materials, particular aspects of the construction have an influence on the evaluation. For example, the total depth of the system has a feedback effect on the overall tower height and efficiency, the speed of

7.00 :1

7.33 :1

7.67:1

8.00 :1

8.33 :1

Direct outrigger A

B

8.67:1 9.00 :1 Hight : Building width 2.14

Indirect outrigger

No outrigger

C

D

E

1.059

1.096

1.101

0.156 0.309 0.531 0.047 0.017 1.059

0.148 0.358 0.531 0.042 0.017 1.096

0.168 0.346 0.531 0.040 0.017 1.101

F

1.217

Exterior Core Slabs Outrigger Other Total

1.000

1.015

0.170 0.237 0.531 0.049 0.012 1.000

0.148 0.274 0.531 0.047 0.015 1.015

0.188 0.447 0.531 0.035 0.017 1.217

2.15 A

B

C

D

E

F

2.16

93

Efficiency + Economy

4.1

environmental “friendliness” of a building design. It does not, however, attempt to measure carbon emissions.

Policies such as more stringent building codes are one approach to reducing carbon emissions; however, the initiative for a sustainable design goal must come from the designers, code or no code. The designers must have a green design objective and make a conscious decision to minimise the negative impact of materials, construction and form on carbon emissions. The most important decisions affecting the success of this design goal happen in the early stages of the project, so it is critical for both engineers and architects to work together to identify the proper form, building orientation, material, etc., since these will have enormous impacts on carbon emissions.

4.1 Learning and Development Center, Roche Diagnostics Corporation, Indianapolis, ­Indiana (USA) 2015 4.2 Comparison of embodied carbon for Timber Tower Research Project versus the original concrete design a Standard materials b Sustainable alternatives

100

Tall buildings and urbanisation “More than one half of the world population lives now in urban areas, and virtually all countries of the world are becoming increasingly urbanised”, according to the United Nations. As the population density increases in urban areas, the need to build tall grows.

This density in urban areas can have a positive impact on carbon emissions by promoting sustainable transport, since transport is currently the second-largest source of carbon emissions.

With proper urban planning, residents and commuters can rely on mass transit, bicycling or walking instead of cars. For example, when Sears was still in the Willis Tower, they had a large portion of the tower’s 405,000 m2 of floor space on one city block located close to the trains. Nearly everyone came to work by mass transit as there is very little parking. When Sears moved to the suburbs, they built low-rise buildings covering many more acres of land. Everyone needs to drive to work, which means more emissions. Furthermore, a large car park is required to accommodate all the cars, and each parking space is larger than the office space in a typical urban office. Sears’s move from downtown Chicago to the suburbs had a significantly negative impact on its carbon footprint.

In addition to reducing carbon emissions by encouraging the use of mass transportation, tall buildings can have a positive impact on oper­ ational energy. Modern technology leads to ­efficient central services, which consume less energy per square footage of floor space than single-family homes. In many cities, multiple buildings are linked so that they can share energy sources. Tall buildings also have a larger volume-to-surface ratio, so their indoor heating and cooling are less sensitive to the influence of the outdoor climate.

The challenge in tall building design is to minim­ ise the potentially negative impact of embodied energy. Relative to low-rise structures, more material is needed to support a tall building, for example larger columns and bigger lateral loadresisting elements. The first-floor columns of a 60-storey building will be much larger than the first-floor columns of a three-storey building with the same floor plan. The taller building’s columns need to support 59 storeys, whereas the shorter building only needs to support two. Lateral loads due to wind or earthquakes produce “bending moments” that are proportional to the height of a building squared. Therefore, taller buildings will have much larger bending moments and consequently larger structural members. The carbon cost of a building is very sensitive to its height. The earlier discussion of the “premium for height” (Fig. 2.2, p. 89) clearly applies not only to the cost in dollars, but also to the cost of the carbon footprint. Tall buildings must therefore be designed to minimise the premium for height.

In 2003, when SOM won the competition to ­engineer the Burj Khalifa, the sustainability movement was not as strong as it is today, but sustainable design was considered good practice by the SOM designers and thus incorporated from the very start. For example, the Y-shaped plan form provides for self-shading, so that only one sixth of the facade is in direct sunlight at any time, thus reducing the energy needed to cool the interior. In addition, a stainless-steel vertical rib, 20 cm deep, is placed at every ­mullion to reflect light, which serves to keep light

Sustainability

Steel and rebar

Concrete Embodied CO2 eq. [lb/sq ft]

Embodied CO2 eq. [lb/sq ft]

Construction 80 60

40

20

0

Total

60

40

20

0

-20

-20 a

Timber

80

Benchmark

Prototype

b

Benchmark

Prototype 4.2

from heating the building. The Burj Khalifa also harvests water. The building is located in a humid environment, and the condensed water from the air conditioning is captured and used in the building. The innovative cooling condensate recovery system can provide 14 Olympicsized swimming pools of fresh water annually.

In addition, the height was used to good advantage via “sky-source sustainability”. The tem­ perature and humidity decrease with elevation.

Engineers took advantage of this thermodynamic fact and pulled the cooler, fresher air from higher zones to lower zones.

Solutions for reducing the carbon footprint Sustainable engineering is integrated into the design process from the very beginning, because it is at this stage that one has the opportunity to make the largest impact on reducing the carbon footprint. One of the best solutions to reducing the operational energy is proper building form and orientation to take advantage of natural lighting. For example, in the Indianapolis campus of Roche Diagnostics, completed in 2014, daytime lighting is provided a combination of north-facing skylights and extensive exterior glazing (Fig. 4.1). Exterior mechanical shading devices provide protection from thermal gain. Other features that reduce operational energy include chilled beams, radiant panels and raised floors for mechanical and electrical distribution.

Three strategies are employed by SOM designers to reduce embodied energy: optimising the form to reduce the quantity of material required; using predictive tools for carbon emissions to inform design decisions; and selecting materials that use less carbon. The first strategy was discussed in “Optimising design goals” (pp. 88 – 94).

The second strategy should be employed in the conceptual design phase to evaluate design options for various forms and materials, and again in subsequent design phases as needed.

SOM has developed a new tool that estimates the equivalent carbon dioxide emissions em­­ bodied in structures of various building types: the environmental analysis (EA) tool. This tool

takes initial construction, service life, repair after hazardous events and deconstruction into consideration. Using inputs such as gross floor area, number of storeys, material type and quantity, location, seismicity and wind, the EA tool can be used to evaluate and assess the carbon implications of various design alternatives. The tool was developed with SOM’s advanced material quantity estimation algorithm generated from hundreds of previous SOM project designs. Project-specific inputs can override all parameters as projects progress through the final design and construction phases. The EA tool not only evaluates estimated carbon for building structures, it also performs a costbenefit analysis of enhanced structural systems (base isolation, for example) and estimates the damage expected over a building’s service life.

As mentioned previously, the production of materials used in the structure of a building is the largest source of embodied energy. ­Currently there are four principal construction materials: steel, concrete, masonry and wood.

Steel, concrete and mixed steel-concrete construction are the most common materials for commercial buildings, in particular tall ones.

The reason for this exclusive use of steel and concrete is that these materials are stronger than masonry or wood; also, non-combustible materials are usually required for buildings more than four storeys high.

Timber Tower Research Project SOM recently performed a research project on timber as an alternative building material for tall buildings. The results show that timber can reduce carbon emissions by 60 – 75 % as compared with that of the benchmark concrete structure.

Carbon sinks remove carbon dioxide from the atmosphere, and the largest natural sinks are oceans and forests. In fact, one half of the weight of dried timber is carbon. In addition, the overall amount of energy needed to produce wood (mass timber and glued elements) is significantly smaller than that for other construction materials, so SOM designers are examining the potential of wood for tall building construction.

101

RESEARCH + FUTURE

104

Quo vadis – megatalls as the focus of the SOM Research Gang

Annette Bögle, Nils Ratschke

106

Structural optimisation – developing new design tools Annette Bögle, Christian Hartz 111 Research timeline

SOM 120

Research into new technologies and typologies is central to SOM’s ethos and essential to the continued evolution of structural systems. Early SOM research into optimisation focussed on energy methods for maximising stiffness and minimising weight in structures controlled by drift. SOM was one of the first firms to use such techniques to proportion members in large structures. More recently, their research has been increasingly concerned with finding optimal forms and geometries that not only minimize weight and material, but also construction complexity, which includes making use of opportunities for repetition in the construction process.

Whenever research yields new forms, these must be rigorously tested. To date, large-scale testing has been used to validate deep beams and, more recently, pin-fuse joints and frames.

In the future, SOM will be using its own wind tunnel to make qualitative comparisons between new structures. Though a lot of research is specific to a particular project, “blue sky” studies are directed forward to identify and address the issues of tomorrow, and ideas that emerge from these studies inform the approach to new structural challenges. Research encourages engineers to look beyond their own discipline and their own project constraints to encompass the work of others, and thus facilitates creativity and the generation of new ideas.

105

Research + Future

Small filter (many smaller members)

Large filter (fewer bigger members)

Small filter (many smaller members)

Large filter (fewer bigger members)

solid = 15 % of total

solid = 30 % of total

solid = 50 % of total

solid = 70 % of total

solid = 85 % of total

used in combination with computer-assisted processes.

Form-finding and optimisation methods focus on different levels of a structure and different phases in its design process and so have a ­varying influence on the overall appearance of a building (Fig. 2.2). Where they concentrate on details of a design – that is, on the structure and its elements – in order to identify the opti­ mum cross-section of a beam or thickness of a slab, for example, the process is referred to as size optimisation. Here the requisite dimensions (values such as thickness, height, cross-sec­ tional area, moment of inertia, etc.) are deter­ mined according to the load on the structural elements for a given design; that is, the chosen statics system or the building’s shape remains unchanged. Size optimisation constitutes part of the traditional role of the engineer, and it there­ fore unfortunately tends to draw a disproportion­ ate amount of attention in everyday engineering situations. The art of architectural engineering, on the other hand, demands the virtuoso han­ dling of shapes and/or topology. Innovative solutions can be found only if the relationship between form and design is addressed during the design process, and there are various to­pology and/or shape optimisation methods avail­able to do this. In the design process, it is most likely the topology of the basic structural system – consisting of the nodal positions of the

Design domain

Topology

Shape

Size

2.2

112

structural elements and their connectivity – which is established first. An optional next step is to modify, adjust and optimise the shape, fol­ lowed by the fine-tuning of the result of the topology optimisation process using shape opti­ misation methods. The shape optimisation pro­ cess may necessitate further topology optimisa­ tion depending on the task at hand. Thus, struc­ tural optimisation methods need not adhere to a strict protocol or procedure. Rather, they are tools to be used individually and as needed, depending on the problem to be solved. Their efficacy and/or appropriateness is evaluated based on the end result.

Topology optimisation Benchmarking the footprint layout The topology of a skyscraper – its basic struc­ tural composition – is essentially determined by the cross-sectional layout described by its external form (circular, square, triangular, etc.) and by the arrangement of structural elements such as the core and lateral columns. These topological elements have as significant an impact on the footprint of a building as on the structural behaviour under wind or seismic loads. While wind load demands the stiffest structural solution possible for low deformations, an optimum structure for seismic load requires maximum ductility. Both affect footprint develop­ ment and thus the efficient use of space in the building. Obviously it is now possible to perform this function using computer-assisted topology optimisation, developing the basic structure outside the black box .

In addition to computer-assisted topology opti­ misation, however, the conscientious, committed engineer has another tool at his or her disposal: experience. Building on decades of such experi­ ence, SOM has benchmarked the optimal crosssectional shape for supertalls by performing a systematic analysis of a variety of building footprints and determining their associated ­efficiencies and their structural characteristics.

Fig. 2.4 illustrates the efficiency of material required under wind load in relation to the layout

Structural optimisation – developing new design tools

Small filter (many smaller members)

Large filter (fewer bigger members)

2.2 Idealised workflow for given design domain using ­topology, shape and size optimisation 2.3 Topology optimisation of a beam based on material intensity using an optimisation filter 2.4 Various footprint shapes ranked by a Moment of inertia b Moment of intertia divided by wind sail 2.3

of the footprint. If only the moment of inertia is taken into account, the triangular footprint with explicit columns at the vertices is the most ­efficient choice. But as soon as the wind sail is considered, it comes as no surprise that the squared footprint with mega-columns at the cor­ ners and a central core offers a more efficient solution.

Continuous topology optimisation The aim of computer-assisted topology optimi­ sation [2] is to establish the optimal basic design of a structure under load within a predetermined, continuous design space. A look at nature pro­ vides numerous examples of topology optimi­ sation. The structure of a bone, for example, varies according to its load, rather than being materially homogenous. In areas of high load, the bone is made up of tiny, lattice-shaped units, whereas areas subject to smaller loads are hollow or contain bone marrow.

Most of the computer-based calculation methods used in topology optimisation today were de­­ veloped in the automotive industry and aerospace engineering. At SOM, these methods are adapted for the whole of the building design process and applied specifically at the interface between architecture and engineering. This is done using commercial software programs and applications developed in academia as well as programs developed in-house exclusively for SOM (Fig. 2.1).

The basic procedure can be described as fol­ lows. First, a design space is defined based on the volume in which it is possible to create structural elements: the footprint and height (in the case of a tower) or span and width (in the case of a bridge) of the planned structure. Dis­ placement constraints and loads are established based on the available locations for supports and on loading conditions. The topology opti­ misation stage iteratively determines the optimal distribution of the structural material (the mate­ rial density) throughout the design space, where a negligibly small material density can be inter­ preted as a void within the structure. In addition, optimisation filters and post-processing tech­ niques can be applied to provide topological

a

b 2.4

113

Research + Future

2.5

variance, which can be used to influence the delicacy of the structure, for example (Fig. 2.3, pp. 112f.).

Since topology optimisation starts with a continu­ ous body, its results can be verified using what is known as continuum mechanics, in which a stress state is represented by principal stress trajectories that indicate the directions of ten­ sile and compression stresses. Principal stress trajectories are always perpendicular to one another and therefore intersect at an angle of 90 degrees. If as a result of topology optimisa­ tion, the structure follows these principal stress trajectories, a primary structure subject to either tensile or compression stress is created in line with the principles of efficiency.

During the process of topology optimisation, the nature of the design space changes, and the structural solution moves from an initially ­isotropic space to one that is concentrated along paths. The tension and compression load paths tend toward orthogonality. These paths are similar to, but not the same as, principle stress trajectories, as they are discrete and not a solution on a continuum.

The comparison of a fixed, vertical member under lateral wind load and the stages in the P/2

P

P

P

P

P

2.6

114

topology optimisation process show an opti­ mum match between the analytical and the opti­ misation results. These patterns were used, for instance, in the development of the superstruc­ ture of the office tower at 100 Mount Street in Sydney, Australia (Fig. 2.6).

The primary objective of topology optimisation in the design process is to develop the most material-efficient structural solution possible.

However, it is currently difficult for optimisation to be carried out for more than one stress state, though in reality a structure is exposed to many, sometimes conflicting, stresses. This results in conflicting optimisation objectives and evalua­ tion criteria, such as minimum deformation, maximum rigidity, ductility, natural frequencies and stability parameters. The topology optimi­ sation process generates a number of design alternatives, which provide vital data for the ­discussion about the relationship between form and structure that should ideally take place between architects and engineers.

Discrete topology optimisation In contrast to continuous topology optimisation, discrete topology optimisation is based on dis­ crete members. The Michell truss provides a clear example of this type of analytical optimisation.

The calculus of variations, a prerequisite for the mathematical solution of optimisation prob­ lems, was developed in the mid-18th century.

It was later applied to structural questions, in particular in the work of Scottish physicist James Clerk Maxwell (1831–1879; see “Maxwell’s ­theorem of optimal load path”, p. 89f.) and Aus­ tralian mechan­ical engineer Anthony George Maldon Michell (1870 –1958). In his 1904 publica­ tion “The limits of economy of material in framestructures”, Michell developed a particularly interesting frame structure of minimal weight for given ­support and load conditions (single load) and a defined design space. The individual frame members follow the principal stress lines, resulting in a “balloon-shaped” support [3].

Michell structures can be derived from continu­ ous topology optimisation by applying a single load to a continuum with a defined external geom­

Structural optimisation – developing new design tools

2.7

etry and a semi-circular support configuration (Fig. 2.7), for example. In the continuum, the principal stress trajectories represent the princi­ pal stress paths. The continuum is then reduced to these principal stress lines, which are trans­ ferred to a structure. If one transfers the con­ tinuum solution to a discrete one, the angles at the intersections deviate from orthogonality and vary based on the density of the pattern. When designing in accordance with this principle, it is possible to create a structure of extremely high rigidity.

Theoretical knowledge about simple model struc­ tures and their comparatively simple load sce­ narios can be applied on a project-by-project basis using a number of optimisation software

programs. Much of this software has grown out of collaborative work with scientists. The topol­ ogy optimisation software PolyTop, developed

by Professor Glaucio H. Paulino’s research group at the University of Illinois at Urbana-Champaign, was put to the test and used in the conceptual design of a SOM project while still in its develop­ mental phase. Later on, SOM engineers and interns from the same group working at SOM expanded this software to allow for pattern repe­ titions and other features, tailoring it to better address architectural problems [4]. It is avail­ able throughout the company to both engineers and architects. In addition, SOM developed an in-house implementation of the ground structure method, extending the application of topology

2.5 3D density-based topology optimisation of a bridge 2.6 PolyTop proof of concept for optimised brace configuration, 100 Mount Street Building, Sydney (AUS), design 2012 2.7 Development of a classic Michell structure using the Ground Structure software 2.8 Michell structure, CITIC Finan­ cial Centre, ­Shenzhen (CN), anticipated completion 2018 2.8

115

Projects + people

Architects Frank O. Gehry & Associates Client Consorcio Del Proyecto Guggenheim Bilbao Building type Museum Structural concept Modular lattice grid Completion 1997

Guggenheim Museum, Bilbao (E) Designed by Frank O. Gehry & Associates and one of the most visually unique buildings of the 20th century, the museum is a series of organically interconnected buildings featuring undulating walls and roofs. A large atrium provides a central focal point. Challenged to design a structural framework to accommodate the interplay of the compound curvilinear forms of the walls and roofs, SOM proposed an innovative, economical system in structural steel, allowing for prefabrication in a shop environment using computer-controlled techniques to achieve a high degree of accuracy for assembly in the field under tight tolerances. SOM structural engineers then created and developed a modular lattice steel grid system that could be adapted to the various curved surfaces of the undulating walls and roofs.

Client HKCEC, Hong Kong Trade Development Council, Wong & Ouyang (HK) Ltd. Building type Convention center Structural concept Long span steel roof trusses Completion 1997

Hong Kong Convention and Exhibition Centre, Hong Kong (CN) Surrounded by Victoria Harbour on three sides, the Hong Kong Convention and Exhibition Centre is in a highly visible location in the city. Built for ceremonies for the handover from British to Chinese rule on 30 June 1997, the centre was designed and constructed on a very tight schedule. The aluminium-clad roof structure is comprised of curved planes that overlap like wings and have been achieved by prefabricated, structural-steel trusses delivered by sea and lifted in place, creating a space 23 m high with an 80-m clear span. The primary trusses support a system of secondary trusses, stringers and galvanised steel roof decking. The interior reinforced concrete core walls that provide the lateral load-resisting system for the convention centre.

Client Korean Air Lines Building type Corporate headquarters and aircraft maintenance hanger Structural concept Trussed arches with balanced ­cantilever trusses Completion 1995

Korean Air Lines Operations Center, Seoul (ROK) The Operations Center for Korean Air Lines at Gimpo International Airport combines a dramatic clear-span hangar facility with perimeter office and operations facilities. The roof structure of the hangar bay spans 90 ≈ 180 m and can accommodate two wide-body aircraft or other combinations of smaller aircraft. In order to minimise overall building height while maintaining a clear span at the hangar doors, an innovative roof structural system was developed that essentially carries the weight of the entire roof on three columns. Tied arch trusses span from the corner columns at the hangar doors to a major column in the centre of the rear wall of the hangar bay. Intersecting rib trusses serve as balancing cantilevers on each side of the arch trusses. The graceful shape of the arch and rib trusses reflects an approximation of the moment diagrams under loading.

Client British Rail Property Board, Rosehaugh Stanhope Development PLC Building type Office tower on preconstructed raft structure over active railroad tracks Structural concept Fire-engineered exposed structural steel framing Completion 1993

1 Fleet Place, Ludgate Development, London (GB) The Ludgate Development is built over the Thameslink Railway in a densely populated area where the City of London adjoins the West End on a site dating to Roman times. 1 Fleet Place is framed in structural steel on a concrete raft above the active tracks, with the framing isolated for vibration control.

Architecturally, the exterior exposes the structural-steel framing, recalling the Roman grid of this part of the City of London and SOM’s historical background as a partnership founded on Miesian principles of structural rationality. The structural steelwork is fire-engineered to enable the majority to remain unprotected and exposed. The structural skeleton of the building is further emphasised by the changing planes of the exter­ ior walls.

Client The Travelstead Group Building type Mixed-use tower Structural concept Megaframe exoskeleton Completion 1992

Hotel Arts, Barcelona (E) The Vila Olimpica master plan, designed for the regatta and sailing programs of the 1992 Summer Olympics, uses a network of civic spaces and commercial facilities to connect the City of Barcelona with the Mediterranean Sea. The 45-storey tower – the centrepiece of the development that includes hotel and residential floors – is constructed in structural steel with a distinctive X-braced exoskeleton in white that is structural and establishes the strong architectural character of the building.

The window wall system is set back 1.50 m, thus satis­fying fire engineering criteria while allowing the exoskeleton to be fully expressed. The exoskeleton is an efficient exterior braced tube designed to resist all lateral loads.

136

Catalogue of projects

Broadgate Exchange House, London (GB) Exchange House, a 10-storey office block, sits at the heart of the multi-building Broadgate Development that has been constructed as an air rights project above the active railway lines of Liverpool Street Station. The building spans the 78-m railway tracks like a giant bridge. Four parallel, seven-storey tied arches – two expressed externally and two internally – extend to supporting foundation piers. The building is braced in the transverse direction by X-bracing. Fire-rated vision glass, set back from the structure, allows the exterior steel arch to remain exposed and untreated. The bridge structure provides for large, column-free office and trading floors as well as a large open plaza at ground level on the plaza structure above the tracks, also designed and engineered by SOM.

Client Rosehaugh Stanhope Development PLC Building type Office tower spanning over active railroad tracks Structural concept Steel tied faceted arch Completion 1990

McCormick Place North Building, Chicago, Illinois (USA) For the North Building expansion of Chicago’s McCormick Place convention centre, SOM used advanced engineering techniques to develop air rights over an active railway to enable the development of land previously thought unbuild­ able at an affordable cost. The roof of the main hall consists of a grid of 4.60 m-deep structural-steel trusses suspended by cables hung from 12 reinforced concrete pylons, spaced to provide the greatest flexibility for exhibition configurations and miss the active tracks. Each of the pylons supports 12 cables, which in turn support the 146 ≈ 238-m roof. The pylons also serve as air supply ducts keeping the upper hall free from ductwork. The main hall is clad in light grey aluminium and polished stainless steel with a band of vision glass just below the roof emphasising the structural nature of the cable support system above.

Client Metropolitan Pier & Exhibition Authority Building type Convention center Structural concept Long span, cable-stayed roof Completion 1986

National Commercial Bank, Jeddah (KSA) The desire to take advantage of the spectacular views of Jeddah and the Red Sea led to the design of this distinctive, triangular, travertine-clad, 27-storey office tower flanked by a helical parking garage. The verticality of the tower is interrupted by three large, seven-storey, triangular courtyards alternating at two of the facades with a linear service core at the third facade. The resulting V-shaped floor plates have glazing facing the shaded courtyards, producing an inward orientation typical of Islamic traditional design in response to the intense Saudi Arabian sunlight. The tower is framed in structural steel braced at the exterior for lateral resistance and trussed at the large atrium openings to accommodate columns for the floors above.

Client National Real Estate Company of Jeddah Building type Office tower Structural concept Steel braced frame Completion 1983

Hajj Terminal, King Abdul Aziz International Airport, Jeddah (KSA) The Hajj Terminal serves as a welcoming, culturally symbolic and structurally innovative portal for more than one million ­pilgrims during the annual pilgrimage to nearby Mecca.

Appropriately, SOM utilised the highly identifiable form of the Bedouin tent to create a marvel that was the world’s largest cable-stayed, fabric-roofed structure at the time of its construction. The roof consists of 10 modules of 21 semi-conical, Teflon-coated, fibreglass units, each measuring 45 m per side, stretched and formed by 32 radial cables. The modules are supported by 45-m high structural-steel pylons and paired at the perimeter of each module for lateral support. The inherent long-span characteristics of steel cable structures allow for the spacing of the columns to be far enough apart to create a very open feeling and great flexibility to the large support area covered by the roof.

Client Kingdom of Saudi Arabia – Ministry of Defence and Aviation Building type Airport terminal Structural concept Cable stayed stressed fabric roof and steel pylons Completion 1981

Baxter Travenol Laboratories Corporate Headquarters (now Baxter International), Deerfield, Illinois (USA) This large multi-building campus and corporate headquarters, consisting of four office pavilions, an executive pavilion, two garage structures and a central facilities building, was designed to provide a flexible framework in which a phar­ maceutical industry leader could continue to innovate and expand. The main central facilities building, containing an auditorium, training centre and cafeteria, features a stayedcable suspended roof supported by two steel pylons rising three storeys above the ground floor plane. The cable-hung suspended roof of the central facilities building provides a dramatic focus for the complex and permits a bold solution to the need for a large column-free interior space for the ­cafeteria.

Client Baxter Travenol Laboratories, Inc. Building type Campus of multiple buildings Structural concept Long span cable supported roof Completion 1975

137