Engineering Nature. Timber Structures

TIMBER STRUCTURES

Engineering

Ed dition

Contents

004

RESEARCH AND TECHNOLOGY 010 Mannheim Multihalle – The Power of the Temporary 016 Hardwood Load-Bearing Structures 024 Experimental Doubly Curved Gridshell Structures

030 Practical Timber Structure Design – Working with Specialist Firms 036 The Integral Stuttgart Timber Bridge 042 60 Metres: The Tallest Timber-Hybrid High-Rise in Switzerland

ROOFS 052 060 066 076 084

Archery Hall in Tokyo Chapel in Sayama Stadium in Nice Sports Hall in Rillieux-la-Pape Garage and Vehicle Workshop in Andelfingen 090 Sports and Leisure Pool in Surrey

100 108 116 124 134 142

St. Josef Parish Church in Holzkirchen Delicatessen Wholesale Store in Stuttgart Heuried Sports Centre in Zurich The Macallan Distillery in Aberlour Mactan Airport Terminal 2 Hall 10 at Messe Stuttgart

MULTI-STOREY BUILDINGS 152 162 172 180

Residential Tower in Heilbronn Conference Hall in Geneva Theatre near Boulogne-sur-Mer International House in Sydney

190 Timber Office High-Rise in Risch-Rotkreuz 198 Timber High-Rise in Brumunddal 208 Schönbuch Tower near Stuttgart

APPENDIX 218 Authors 220 Image Credits

005

222 Project Participants 224 Imprint

Foreword Jakob Schoof

Forward into the Wood Age

006

Back to the Future – this film title from the 1980s could best describe the renaissance that timber construction is currently experiencing around the globe. Ecological advantages in particular make timber construction full of promise for the future: wood is carbon neutral over its lifecycle. A timber structure becomes temporary storage for a material that can, in the ideal case, be reused after deconstruction or, where this is not possible, thermally recycled. Steel and reinforced concrete, on the other hand, are energy intensive to manufacture; the production of cement alone is currently responsible for around eight per cent of global CO2 emissions.

However, the timber renaissance also includes a good chunk of “Back”. Historian Joachim Radkau, who writes widely on the history of technology, describes the era shortly before industrialisation in Germany as the “Wood Age”. The renewable raw material was used, perhaps even overused, for almost everything: all kinds of commonplace objects, machines and in the construction of houses, ships and vehicles. This led to astounding craftsmanship in working with wood and a high level of knowledge about its properties.

Similar developments took place in other forested regions of the world. And even in the 20th century, as concrete and steel increasingly displaced wood as a primary building material, engineers were still producing structural masterpieces of timber construction.

In 1908, the nine-storey Butler Square warehouse in Minneapolis was completed with a load-bearing frame constructed using Douglas fir timbers up to 60 cm thick. In 1934, Deutsche Reichspost, Germany’s then provider of postal and broadcasting services, built the “Bavarian Eiffel Tower”, a radio transmission mast almost 160 m tall to the east of Munich. Just short of 40 years later, the tower had to be demolished using explosives because of severe deformations.

In comparison, today’s tallest wooden towers still have some way to go: the world’s current tallest timber building, Mjøstårnet in north Oslo, measures 85.4 m from its base to the top of its pergola. We discuss this building extensively in this book. However, in deciding the content, we were not led by such a collection of superlatives, but rather by the observation that timber structures are establishing themselves in many more areas of construction, and by the question of which structural and regulatory limits confront timber construction today.

In this book we also take a look at the design of timber structures and the technical developments that make contemporary timber construction possible. This includes the use of hardwoods, with their unsurpassed load-bearing capacity, and the increasing number of composite structures in which wood, steel and concrete interact optimally according to their material properties. Because that, too, is part of the truth in the new Wood Age: wood is not the only material making up a timber building. Connecting elements, tensioned cables, columns and girders made of steel and solid floor slabs made of concrete are almost always required to make today’s timber structures fit for purpose in terms of spans, sound insulation and fire protection.

007

Text Eike Schling, Rainer Barthel

Experimental Doubly

A

Curved Gridshell Structures A lattice structure based on asymptotic curves and built out of flat beech veneer strips exclusively orthogonal to one another

B Mannheim Multihalle Architect: Frei Otto

024

ESSAY

It is easy to design free-form building envelopes on the computer. Curved structures also offer the advantage of an efficient, spatial load transfer, like a shell. However, in building such structures, a higher degree of complexity must be taken into consideration during planning, design, fabrication, construction and in their logistics.

The authors have been studying the geometry and construction of curved surfaces as part of the research project “Repetitive Grid Structures” at the Technical University of Munich (TUM). This project does not aim to cover all digitally designed surfaces and their seemingly unlimited diversity. The focus rather lies on certain geometric structures that can be built from simple elements with a reduced number of parameters. The study shows that these repetitive structures follow fundamental principles of form that offer new possibilities for design and construction.

B

CONTINUOUSLY CURVED LOAD-BEARING STRUCTURES The starting point for the research described here was the timber gridshells of Frei Otto (Fig. B). These gridshells use the elasticity of their components to create a continuously curved lattice structure from straight wooden laths.

The question arises: what is the relationship between curvature and construction of the strained structure? The following article analyses the geometric properties of curves on doubly curved surfaces to derive new potentials for the fabrication and design of strained, load-bearing structures with continuous elements.

CURVATURE The simplest way to explain the term “curvature” is by considering a single curve in space: the curvature at any point is determined from the tangential circle of curvature at that point (Fig. C). The curvature equals the inverse of the radius (k = 1/r).

The curvature of a surface is determined at each point individually. This is done by creating the intersection curves through this point based on perpendicular planes. The two intersection curves with the maximum and minimum curvature are orthogonal to one another and determine the two principal curvatures k1 and k2.

The Gaussian curvature (K = k1 × k2) and the mean curvature H = (k1 + k2) / 2 can be calculated from these principal curvatures.

If the radii of curvature of the two principal curvatures lie on opposite sides of the surface, it gives rise to a negative Gaussian curvature. Such a surface is called anticlastic, e.g. like a horse’s saddle.

If the two radii lie on the same side, then the surface has a positive or synclastic curvature, e.g. like a football.

If one of the principal curvatures is zero, the surface is said to be singly curved. Surfaces with constant single curvature are developable.1 This means they can be unrolled in one plane without distortion or change of length, like a piece of paper.

Looking at a curve on a surface, every point can be referenced by a coordinate system consisting of a normal vector (z), tangent vector (x) and tangent normal vector (y). If this coordinate system (also known as the “Darboux Frame”) is moved along the curve, the rotations of the curve can be measured about all three coordinate axes. The three associated forms of curvature are called: geodesic curvature (about z), geodesic torsion (about x) and normal curvature (about y).

025

EXPERIMENTAL DOUBLY CURVED GRIDSHELL STRUCTURES

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axonometric illustration 1 50/12 mm longitudinal bar, cypress 2 36/36 mm bar, cypress

3 50/24 mm lateral bar, cypress 4 50/36/6 mm groove 5 wood screw M6

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ARCHERY HALL

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TOKYO (JP)

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TOKYO (JP)

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STADIUM

Appui néopréne Détail 8

PARAMETERISATION To set these geometrical constraints, we used Rhinoceros and Grasshopper parametrical software. At the same time, we set a number of parameters that allowed us to precisely control the shape of the roof in the subsequent stages of the project and adapt it during the six months of design development that followed. In the meantime, using information from the parametric model, we checked the resulting dimensions and roof slopes against the dimensioning criteria for the secondary structure and membrane cover.

At the macro level, the shape of the intrados sur-

face was controlled by the position of the circular arcs and the number of divisions per arc. The position of the circular arcs also gave the locations of the roof support points on the concrete stands. These parameters therefore controlled the number of repeated elements, the shape of the roof and the interfaces between roof and stands. At the micro level, parameters were set to control the aspect of the section. They also influenced the structural efficiency of the truss, the relationship between primary and secondary structures and the slope of the membrane cover.

MEMBRANE COVER The stadium is covered by 25,000 m2 of ETFE and is one of the first structures to use flat, single-layer ETFE over such a large area. Due to the material’s relatively recent appearance in France, ETFE structures are not yet covered by any regulations and the existing codes for fabric structures are not fully applicable. Rather,

E axonometric projection of a curved half-frame

F steel components of the end connection joint for “threaded-through” timber members with a thinner cross section

075

NICE (FR)

G steel components of the end connection joint for timber members with a thicker cross section.

they are considered as cladding systems and therefore have to be approved by obtaining an “avis technique experimental” (ATEx), which requires experimental tests to be performed.

We developed a method to justify the ETFE, which eventually led to biaxial tests to verify the allowable stress in the film.

H end connection joints for timber members with a thicker cross section after installation.

1

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1

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vertical section through roof and facade scale 1:20 1 roof waterproofing: PVC membrane 160 mm thermal insulation; vapour

barrier 22 mm veneered plywood board 170/70 mm rafters 25 mm wood wool acoustic panel 2 200/450 mm trussed girder top chord

082

SPORTS HALL

3 170/70 mm transverse timber beam 4 180 mm glulam truss diagonal 5 2× 90/720 mm trussed girder bottom chord

6 63 mm Douglas fir 19 mm 3-ply plywood 27/38 mm horizontal laths; 38/38 mm vertical laths membrane; 35 mm wood wool lightweight building board

360/140 mm cross beams, between them 360 mm straw bale thermal insulation in timber framing vapour barrier 18 mm OSB

120 mm vertical laths 19 mm 3-ply spruce plywood 7 160 mm PU thermal insulation waterproofing layer; 360 mm reinforced concrete

STIFFENING The large hall is stiffened horizontally by diagonal cross struts in solid wood that span between the rafters of every roof level. The facades and the inner frame construction are stiffened vertically by steel cross bracing with turnbuckles to transfer the horizontal

loads into the reinforced concrete base.

The trussed girders are stiffened transversely by timber frames connected to the double columns of the eastern facade. The column bases are cast into the concrete base to form a moment connection.

EXTERNAL WALLS The 2,000 m2 opaque facade consists of non-load bearing 36-cm thick prefabricated OSB box elements filled with straw bales. The

F

G

F connection principle for the two halves of a trussed girder scale 1:50

G detail principle for assembly of double columns on steel bracket scale 1:50

083

RILLIEUX-LA-PAPE (FR)

walls were boarded on both sides with threelayer plywood panels, the interior layer being spruce and the exterior Douglas fir.

Text

Paul Fast, Derek Ratzlaff

C

A

internal forces only in facade plane

support reactions

45 m

55 m

B

2 1

094

SPORTS AND LEISURE POOL

UNCONVENTIONAL STRUCTURAL ENGINEERING CONCEPT The long-span timber suspended roof of the GHAC demonstrates the potential of wood as an inexpensive, structurally efficient and aesthetically pleasing material for the construction of swimming pool buildings.

The architects proposed a longitudinally spanning roof. We suggested a slender, lightweight suspended roof with “cables” manufactured from glued laminated timber (GLT). After some initial scepticism, the architects decided in favour of our unconventional approach after we had demonstrated the advantages of timber in conditions of high air moisture content and the effects of chemicals used in swimming pools. The suspended construction has a structural depth of only 300 mm. Initial analyses quickly led to the introduction of V-shaped central supports between the two large pools to minimise complexity and cost. Pairs of 13 cm × 26.6 cm GLT profiles at 80 cm centres span

D

A structural system

E

B RC columns with foundations and prestressing elements scale 1:400 1 675 × 1000 mm concrete cross section

095

the 55 m and 45 m gaps between the central supports and seven reinforced concrete columns at each end of the building. A double layer of 16 + 12-mm thick plywood boards is attached to the tops of the GLT “suspension cables”. RC edge strips pick up the tensile forces from the suspension members and transfer them into the central and outer supports. Slab foundations acting with the backfill secure the columns against overturning.

Originally, the variably sloping roof geometry in the transverse direction led to 14 different radii for the GLT “cables”. We adjusted their lengths so that they all had the same radius of curvature. The steel tubular columns connecting the up to 20-m high facade structure to the roof are perforated and serve two functions: they carry the wind loads and act as integrated supply air distributors, largelydispensing with the need for internal ducts.

SURREY (CA)

with 6× Ø 46 mm threadbars 2 700 × 1200 mm concrete cross section with 6× Ø 46 mm threadbars

F

C deformation under uneven loads

D–F structural principle of the roof

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SPORTS AND LEISURE POOL

DEFORMATIONS Suspended structures change their shape in response to the load placed upon them and are particularly sensitive to varying, unevenly distributed loads. Initial calculations for an uneven snow load showed vertical deformations of up to 1200 mm. However, the aim was to limit this

to the manageable size of 200 mm, which was within the capacity of a proven sliding facade connection. The designers solved the problem by stiffening the supports and reducing the unevenness of the distributed loads from slipping snow by fitting snow retainers to the roof.

WIND UPLIFT The relatively light timber structure does not have enough self-weight to prevent wind uplift. Dismissing additional imposed load or steel cable stays inside the building, we designed the GLT members

as flat inverted compression arches to accommodate any possible wind uplift forces. In addition, a shear-transmitting connection of the boards with the GLT members provides composite action.

DYNAMIC BEHAVIOUR From the location, orientation and shape of the building, it was likely to be exposed to wind oscillations with a frequency of less than 1 Hz. In order to avoid vibration risks, the natural frequency of the roof structure needed to be

above 1.5 Hz. A 3D analysis estimated it to be 1.35 Hz. The calculations were checked by taking measurements on site with a metronome and accelerometers at a “jumping party” of test people and confirmed as 1.7 Hz.

ERECTION AND CONNECTING ELEMENTS Erection had to be completed quickly to protect the wood from rain. Transport restrictions limited the length of the GLT members to 25 m. The shorter span therefore required one on-site longitudinal joint, the longer span two. To save time, longitudinal joints consisting of 22-mm thick

G column head reinforcement scale 1:50

097

H GLT cross section I GLT connection

SURREY (CA)

J on-site connection scale 1:20

steel plates were each connected to two pairs of GLT members by a total of six bolts. A lifting frame was used for the short span while the long-span beams were lifted with two cranes.

The whole roof including the plywood layer was erected in 12 days.

1 266/130 mm GLT profile 2 140/52/180 mm GLT block every 5 m 3 12 + 16 mm glued plywood boards

4 800/220/6.4 mm galvanised bolted steel plate 5 Ø 25 mm bolts 6 200/22 mm steel plate 7 Ø 57 mm bolts

8 350/280/30 mm steel plate 9 225/200/16 mm steel plate

aa

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2

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9 7 8 6 4 3 section, floor plan scale 1:500 site plan scale 1:3,000

102

1 2 3 4 5

church chapel foyer vestry clock tower (existing) 6 rectory (2nd phase)

7 parish office (2nd phase) 8 parish community centre (2nd phase) 9 parish community hall (existing)

ST. JOSEF PARISH CHURCH

1 5

2

An extraordinary timber structure defines the atmosphere inside the new St. Josef Church in Holzkirchen.

Its predecessor, built in 1962, had become structurally unsound due to serious construction defects.

Because refurbishment had been shown to be uneconomical, the client issued an architectural design competition for the redesign of the whole parish centre complex and the construction of a new parish church (400 seats) with a weekday chapel (50 seats). The competition documentation expressed a wish for a timber structure. The successful architect’s design envisaged an open, inviting group of buildings that incorporated the existing church tower. Church and chapel stand opposite one another and take the form of two differently sized and inclined truncated cones with an elliptical footprint and rooflight. They are linked by a flat roofed vestibule, which is connected to the vestry and a covered path to the northern part of the parish centre complex.

The interior of the church expresses a contemporary interpretation of the liturgy with a centrally positioned altar. The conical building envelope clad on the outside with wooden shingles encloses the church’s interior and is both a roof and a wall. The rooflight and flat side window, as planar interface surfaces, differentiate themselves from the geometry of the building’s basic form and bring the 20 m high space to life with an exciting combination of light entering vertically and horizontally. Following dedication of the church, the community has access to a sacred space with a special aura.

Andreas Gabriel

103

HOLZKIRCHEN (DE)

Text

Peter Mestek

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ST. JOSEF PARISH CHURCH

157

157

157 710

157

40

TWO DIFFERENT STRUCTURES Although the church and the chapel are both inclined truncated cones, their load-bearing structures are remarkably different. The main structural element of the chapel comprises glued laminated timber (GLT) beams running in the principal direction of fall of the cone sides and restrained by a flexurally stiff ring at the crown of the roof. The system is stiffened by exterior wood composite board cladding. At

the large window opening, the beams are not supported on the ring foundation, but are connected to a parabolic, block-glued GLT arch.

The arch’s horizontal deflection was calculated using a 3D analysis to design the components and their connections. The result was that GLT rings, flexibly stiff to resist transport loads, were attached at the beam third points to stabilise the main structure of the chapel.

CHURCH MAIN LOAD-BEARING STRUCTURE The church has an elliptical floor plan with a diameter of approximately 34.5 m and a height of 21.6 m. The structure is terminated at its highest point with an inclined rooflight constructed from a slender steel grillage. The dissolved, exposed shell construction is composed of GLT struts, which form triangles and therefore fulfil stiffening and load-bearing roles. The corners of the triangles meet at nodes at the intersections of the fall lines of the cone with the differently inclined intersection planes. Thus the structure has approximately 350 of these nodes, which are normally a mirrored arrangement of two

E

A determination of the node positions

105

adjacent triangles. The elliptical rings within each intersection plane are one-piece, curved GLT beams with flexurally stiff joints. The diagonally running struts were initially idealised as members of a pin-jointed truss in the early 3D calculation model. However, they were predominantly loaded in compression and therefore later modelled as pure compression members that would fail under tension, in order to optimise the connections. The diagonals are kept in place by steel tie rods spanning between two adjacent rings at regular intervals and following the principal direction of fall lines.

F

B detailed section of a standard node scale 1:20

C vertical section through a special node at the side window scale 1:20

HOLZKIRCHEN (DE)

D node with steel tie rods

E standard steel in-built node component

F special beech LVL in-built node component

A

B

1

2

C

3

A At the tops and bottoms of the timber columns, steel plates form connections to the steel beams.

B The ends of the cross-laminated timber floor slabs are rebated to form bearings that fit on the steel beams.

C Non-load bearing external wall elements in timber-frame construction were pre-fabricated off site.

D The external wall panels are set between the load-bearing timber col-umns on steel beams.

E The steel beams are concealed in the outer walls at floor level and minimise deformation.

F Internally, the 300 mm deep HEM beams bear the columns and floor slabs with economical spans.

158

RESIDENTIAL TOWER

horizontal section: loggia scale 1:20

4

1 4 mm smooth alum. sheeting with grey wet paint coating; alum. backing to joints 82 mm rear ventilation with vertical supporting structure

D

E

F

6

9

4

10

7 1

9 8

5

6

3

2 base level: 400/400 mm reinforced concrete columns 3 upper storeys: 400/400 mm laminated spruce columns

4 timber-frame element: windbreak 18 mm gypsum plasterboard 80/280 mm timber studding with 280 mm insulation between

159

HEILBRONN (DE)

120 mm crosslaminated spruce boarding 5 20/60 mm flat steel balustrade 6 alum.-wood composite window with 119 mm integral low-E

glazing (Uw = d 1.0; g = 0.35) 7 rainwater pipe from roof 8 Ø 50 mm drainage and emergency inlet from loggia 9 drainage channel

10 timber-frame element to inner face of loggia: 8 mm fibre-cement sheeting 40 mm battens windbreak 18 mm gypsum plasterboard

80/140 mm timber studding with 140 mm insulation between

IMPRINT EDITOR Jakob Schoof

DESIGN strobo B M, Munich (strobo.eu)

EDITORIAL TEAM Roland Pawlitschko, Charlotte Petereit

TRANSLATIONS Raymond Peat PROOFREADING Meriel Clemett REPRODUCTION Repro Ludwig, AT– Zell am See

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APPENDIX