Designing bridges

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The Milne lecture

Designing bridges Angus M. Low, MA CEng, MICE, Arup Keywords: Bridges, Design, Case Studies, Arstaviken Railway Viaduct, Stockholm, Sweden, Drachten Ring Cycle Bridge, Netherlands , Designers, Role, Architects

This paper is based on the Milne lecture which was held at The Institution of Structural Engineers, 11 Upper Belgrave Street, London SW1X 8BH on 13 March 2008 Bridges are wonderful things, and they can take many forms. As a designer it is important that you are always seeing the full breadth of possibilities. Designing is totally absorbing, it is a challenging, intellectual process that has so many aspects to it. I have been lucky enough to have spent the last 35 years designing bridges, within the Bridge Group at Arup. This paper reflects on the nature of bridge design, how it is practiced currently, and how it could be improved.

The entry requirements for the ‘Milne Medal for excellence in structural design’ require details of three projects in the last 7 years in which the entrant has contributed significantly to the design. I was lucky in that I had three projects which reached completion within a year, so it seemed a good time to enter.

Arstaviken Railway Viaduct, Stockholm This bridge (Fig 1) provides two much-needed additional tracks into Stockholm from the south across the Arstaviken inlet. It lies alongside a traditional viaduct which has a small version of the Sydney Harbour Bridge over the navigation channel. The island it steps across, the wooded slopes and the inlet together make a delightful scene, and the locals objected strongly to the idea of an additional viaduct. They pressed for a tunnel instead, which initial estimates indicated could cost four times as much. Banverket (Swedish Railways) realised they needed a design which would woo the objectors and in 1994 they organised an international design competition. Arup won this with architect Foster and Partners, and after many delays it was opened in 2005.

From the outset we knew we wanted a design that was fluent and seductive and we explored forms which were quite strongly curved in section, and gently curved in elevation. I realised that we were breaking some sort of taboo by ignoring the cusped form of the moment diagram over the supports but we studied the flow of stresses in a Finite Element model using shell elements, and it seemed to work quite well.

The prestressed concrete structure is raised up and around the railway tracks (Fig 2) with flanges on each side at 1100mm above track level. This is the design step that solves most of the issues. It

resolves the tight dimension between the track level and the shipping clearance, it allows good views of the existing viaduct – a request of the brief, the flanges provided the space needed for a cycleway on one side with the bonus of vehicular maintenance access on the other side, and it contains much of the noise that radiates from the railway. It also provides secure derailment containment. Finally, because the section follows a continuous curve, it makes it easy to provide transverse prestress – which is relevant because the Swedish prestressed concrete code imposes strict limits on principal tensile stresses.

The architects’ particular contribution was their desire to span the shipping channel without any overt structural expression.

Initially the dimensions made this look unachievable, and the scheme submitted for the competition had a stretched version of the standard span over the channel, which was working very hard.

There is something serene about a viaduct which effortlessly crosses everything in its path without changing step. The jury saw this and they encouraged us to negotiate with the navigation authority. We were able to find a solution which met all the needs, with repeating spans of 68m. Part of its special character comes from the architects’ decision to make it red, picking up a colour used for traditional Swedish timber houses.

We designed the structure for cantilever construction, and we thought out how this would allow the standard shutters to follow the plan curvature at the northern end. The project suffered extended delays and our commission ended after we had completed a detailed scheme design. When the project was revived it passed through different hands. It was decided to build it using heavy falsework which supported complete spans. There were changes in detail, but the basic scheme was unchanged.

The Drachten Ring, Holland The design and construction of The Drachten Ring (Fig. 3) has been for me both extremely satisfying and extremely frustrating.

The satisfaction happened by mistake. We heard that the local authority of Smallingerland was seeking designers for a landmark cycle bridge across the A7 motorway to connect Drachten to a new business park. Many people use the A7 but they pass Drachten without noticing it because it lies behind an extensive noise bund to one side of the motorway. They wanted the new bridge to put Drachten on the map. We were on a short-list of four teams and I travelled to Drachten in 2001 to present my approach to bridge design. I was surprised to learn that the other three teams were all architects, and the engineering commission had already been let to Grontmij. Arup won the design commission and Grontmij agreed that we should undertake the concept for the

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Opposite: Arstaviken Viaduct Section over a support The Drachten Ring The crescent beam

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bridge and its approaches and the engineering design for the main steel structure. They would do the concrete substructures and the approaches. In this way I found myself as the sole concept designer for a landmark bridge, its alignment, the structure and the landscape.

I reached the final form quite quickly, but my route was not an obvious one. Because of future road widening there was no possibility for a tower between the road and the canal so, as I saw it, I had to have a tower in the middle of the road. This generated a central formality which I had not originally expected. I like structures where there is a clear separation of parts, and I wanted to see where this led. Often the deck of a cable-stayed bridge appears to be cramped by the legs of its tower. What happens if you open out the legs of the tower so there is clear space all round the deck? The cable planes become something spatial and interesting and the tower – what does it become? It became a tall ellipse and, quite quickly, because of the plan curvature of the deck, it became a tilted ellipse. I remembered old photographs of racing cars where a tilted ellipse represents a wheel at speed, and it was a small step to see the tower as a speeding bicycle wheel whose spokes have burst outwards to become stays for the bridge.

Because of the spatial arrangement of the stays, they meet the deck at a variety of angles when viewed along the deck.

Conceptually it seemed appropriate that the edge member should be a circular tube so it could have an equal relationship to the different stays. Of course this has the very practical effect that a common, simple cable anchor detail can be used. Designers always have half-formed ideas that they want to use somewhere and I realised that the edge tubes would allow me to use a crescent beam support detail (Fig 4) at the two piers and the two abutments. I see this detail as another manifestation of the

separation of parts. The crescent is formed by pulling the top of the pier down, generating some space between the pier the deck – and the crescent with the deck plate as its tie acts as an effective crossbeam.

In the Netherlands the design impact force on the bridge deck from an over-high vehicle is 2MN, which is very high. Fortunately my tubular edge beam had one more trick up its sleeve – over the highway it could be filled with concrete. This solved the problem at the point of impact, but there were still problems elsewhere. I was worried that the deck would jump out of conventional bearings.

Luckily the crescent beam detail introduced sufficient flexibility at the supports to absorb the temperature expansion, so it was possible to use robust pin bearings instead.

One problem continued to intrigue me. The radius of the wheel ellipse is tight at its ends. How do the longitudinal forces in the flange plates of the box section find their way around the bend? I knew that, under slight curvature, the effect of the transverse bending stresses in the plates due to the radial equilibrium forces had both beneficial and adverse effects on the mean effective yield stress in the flange. I wrote a program, Curp (curved plates), in Quickbasic which split the flange into strips and, through a structured trial-and-error procedure, found the maximum longitudinal mean stress in the flange which was consistent with both equilibrium and Von Mises’ yield criterion. Initially I had shown a longitudinal stiffener splitting the outer flange into two panels.

Curp showed this was not needed.

But this is where the frustration enters the story. Our relations with Grontmij had deteriorated. We had completed the main part of the detailed design, and we were expecting to be mobilised for the final phase which related to the detailing of connections. We were never mobilised. We heard that the project had gone to tender, with the contractor responsible for the detailed design and The Structural Engineer 87 (5) 3 March 2009 27

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5 Nesciobrug 6 A14 Viaduct 7 Crossbeam within the concrete deck

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the connections. This is quite a normal form of contract in many countries. We had believed that our agreement gave us a role throughout design and construction. We later learnt that our role would have been protected under Dutch law if we had described ourselves as architects, instead of designers. As a firm we employ many architects and we would have been perfectly within our rights to describe ourselves as architects. A lesson learnt! The Curp study was never incorporated into the design, but I have published its findings1,2.

Bicycles are important in Drachten, they account for 56% of all journeys. The bridge was opened on 12 July 2006 by a cycle mounted brass band – they steer specially adapted bicycles with their elbows! About 200 locals followed on their bicycles, many decked out with garlands, and I knew they were going to appreciate their new bridge.

Nesciobrug, Amsterdam Nesciobrug (Fig 5) is another Dutch cycle bridge. It connects the new suburb of IJburg to Amsterdam. It has won several awards and has already been written up in two papers3,4. The second paper describes its details.

The nature of bridge design Bridges offer a particularly pure form of design, in the sense that high over a river they are not interacting with much apart from the elements and, despite their size they have relatively few components to design. There are usually many more design criteria to satisfy than there are components to shape. It follows that each component is serving several functions and so there is a complex interaction between all the design requirements, and the different ways they can be satisfied. Because of this the design process occurs in a single fusion, when all the pieces fall into place. This is proceeded by many trials and studies, finding what works well together and quantifying the relationships. When working with 3D form there are always more different solutions than are immediately apparent. And when you have finished shuffling the parts about, and honing the form, if all goes well, it will 28 The Structural Engineer 87 (5) 3 March 2009

emerge looking serene and calm and simple, with no hint of the twists and turns that have been negotiated on the way.

And yet, when you start a project, you will usually find that the requirements do not fit together. It is often easy to jot down a logical chain which shows that your desired goal is not possible.

As a designer you have to have to believe that there is a good solution, waiting to be found. You have to question everything. You have to extend the borders of the problem, or redefine your aims, or add another problem into the melting pot. It takes time and perseverance. The old cliché about creativity being more perspiration than inspiration is certainly true – and the inspiration part is often an echo from some previous experience. A designer needs a large reservoir of past details and half-formed ideas on which to draw.

Here is an example of design interaction (Fig 6). We won the design competition for the A14 viaduct at Nanterre on the outskirts of Paris, including architect Odile Decq et Benoit Cornette and engineer RFR. The new motorway crosses a future park, and the design explores ways in which the perception of severance can be minimised. Light penetrates between the split carriageways. The continuous curved soffit – there are no re-entrant corners – softens the presence of the roadway above. The slim and playful steel supports carry the decks lightly. Technically the connections between the steel and concrete parts are interesting. Fig 7 shows how the concrete crossbeams – within the deck silhouette – deliver their loads to the steel arches. Because of the overall concept the concrete section is quite shallow here. The transverse prestress is anchored directly into the steelwork. This is an unusual detail which, I realised, would give a significant benefit to the design as a whole. It greatly reduces the shear carried on the concrete section and so allows the width allocated to the crossbeam to be minimised. In turn this minimises the longitudinal prestress because the French have a 1:3 rule. The effective section in the plane of the support is derived from the section at the face of the crossbeam with the void dimensions on this face reduced by a third of the half-width of the crossbeam, so the width of the crossbeam reduces the effectiveness of the deck section. As an

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8 Bridge in Korea

aside, the 1:3 rule is an approximation for the laws of physics so it applies equally in other countries as well.

Design is a journey of discovery, and it is a largely unrecorded journey. We don’t produce design reports explaining the whys and hows and the what ifs. Back in the 1970s we used to. A carefully crafted design report was the natural outcome of all our deliberations during the concept phase.

Design criteria Design can be seen as the solving of a self-contained problem, but what are the criteria you are solving for? Some relating to loads and strengths are quite hard edged and covered by codes, although under closer inspection there are more gaps than you might imagine. In a recent talk I gave on Eurocodes (NCE 24.10.07) I included a slide which showed several: – wind load on a rectangular section inclined at an angle, – the strength of a curved steel plate, as in the Drachten Ring, – the curved concrete of a shell arch tunnel. This last one is surprising because there are papers5 which seem to address the issue, but they only report tests on flat slabs.

It is not only the less common situations that are omitted.

Possibly the commonest situation which arises in bridge design is a wheel being carried on a concrete deck slab near a support.

Surprisingly we have no way to check for shear failure in the slab.

This omission does not cause a problem because designers in the UK do not design deck slabs less than 200mm thick. Why not?

They do in the US, but in the US the deck slab has traditionally been treated as a replaceable item, like the surfacing.

More interesting are the soft design criteria which require levels of judgment and personal opinion. Issues of comfort come into this category, and there are societal issues which change over time.

Not long ago if a ship knocked down a part of a bridge it was reported in the press as the ship’s fault. Now we see it as the bridge‘s fault for being vulnerable. Within my career span, designing for hazards has risen from almost nowhere to top of the list. And if the worst happens how much of the bridge collapses?

The concept of ‘disproportionate collapse’ has been around for many years. It is a matter of judgment what is disproportionate.

The words in regulations may stay the same, but the expectations of the society we live in change the way the words are interpreted – so precedent cannot be relied upon.

The designer The discussion of the soft issues brings us to a discussion of the designer who is continually making judgments and who has to be aware of the changing expectations. Their judgments are tempered by the physical possibilities – what can reasonably be achieved in the circumstances.

I have not yet mentioned the need for some overall design

quality, too often referred to as ‘aesthetics’ as if it is some separate topic. A common belief among engineering designers is that there is a natural form which will emerge from their deliberations which, with a minimum amount of tidying, meets their aspirations. This design represents a minimal, pure, culture-free starting point from which others – architects – may depart in ways to meet their own aspirations. Ove Arup said it in a different way in a paper6 on bridge design: ‘There will always remain a number of more or less arbitrary decisions about proportions or detail design which do not greatly affect economy or functional efficiency, and which have to made on purely aesthetic or sculptural grounds. I suggest however that the best result is obtained if there are very few of such arbitrary decisions to be made.’ Part of the reason for this belief is that engineers are often working as part of a consensual team, and to maintain the comfortable sense of equality within the team it is necessary to believe the resulting design is emergent, rather than imposed by particular interests. In recent years I have come to question this belief. There is an analogy with the way we use language to convey meaning. We talk of the ‘voice’ within a piece of writing. The voice is not some added effect. It is always there.

When working recently in Korea, Mr Suh, an engineer we were working with, showed us a picture of a recent bridge of his (Fig 8).

I was immediately struck by how Korean it looked and I mentioned this. He didn’t understand my comment. If I understood him correctly he saw it as a design that had emerged naturally from responses to the functional requirements. His natural form was not my natural form, and it follows that my natural form is, to some extent, an expression of my own unconscious culture.

So what is my culture? It is not a question an engineer usually addresses but I think my conclusion on ‘voice’ requires me to do so. I have written7 about an aspiration for timelessness that I call ‘long wave design’, and in past talks I have admitted to being inspired by the curves of an axe handle – functional, tactile and beautiful. I see myself as a North European and at heart I am a child of the Modern Movement. From within Arup I have worked with many leading modernist architects. In 1969 when I was a fresh graduate there was a flu epidemic in London. As the only ones standing from our respective offices Max Fry and I sat down at a large drawing board and designed together a complete building in one sitting. In his early years he had been the partner of Walter Gropius, the first director of the Bauhaus. He had also been Le Corbusier’s assistant on Chandigarh. The boiler house at the Open University is not great architecture, but for me it is a special place! Later, I joined the Arup Bridge Group under Bill Smyth, and found a strong tradition for simple, well-mannered designs which were truthful to their purpose. Hopefully, over the years, we have not strayed too far from these intentions but, in a different age, our designs do come out differently. I like using gentle curves. I argue that a gentle curve is just a straight line with some of its stiffness taken out.

The designer is part of a project team that has been assembled by the client. Many people will be interacting with the design and the designer in many ways. The dreaming process of design meets the bustling world of deadlines. I visualise a time line (Fig 9). The designer is dreaming of the next 200 years, with backward glances over the past 2000. The project exists within a hub of about 6 years or so for design and construction, and this is where everyone else is focussed.

In this world the designer needs well-honed interpersonal skills.

Taken all together the range of skills and knowledge needed by a bridge designer is as wide as you are likely to find in any profession. It crosses the all divides between the arts and sciences, theory and practice, academia and commerce.

How do you become a bridge designer? This is a surprisingly difficult question. Coming back to the ‘relatively few parts’ and ‘single fusion’ it becomes clear why it is difficult to split the work between the senior engineer and the junior engineer other than in the traditional divide between designer and analyst. The two tasks are of an entirely different nature. So how do you know that the junior engineer has what it takes to become a designer when, eventually, they get their opportunity? There are several answers to this question, but they all depend on the question being The Structural Engineer 87 (5) 3 March 2009 29

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9 Time line

recognised. I was lucky. My first 21/2 years in Arup I was in a group designing building structures where there are many secondary elements which can be passed to the junior to design.

So what drives a designer? It is a complex topic made much easier for me because I have discovered a recent book, The Craftsman8 by Richard Sennett. It is a somewhat philosophical work which has many of the answers. He uses the term craftsman to cover a range of individuals from ancient times, up to software writers today. He draws a picture of someone who derives much of their personal fulfilment through mastery of their own field.

Project management So many meetings, and they get larger and larger. Is the world going mad? Management is principally about managing interfaces.

The dialogue between the separate teams is carefully monitored.

The babble tends to emphasise the periphery of each activity, and draws attention away from the centre, where most of the design happens. This is why the Design Report has fallen out of use. The number of interfaces depends on the number of compartments that the overall activity is split into – to a first approximation it increases with the square of the number of compartments. The size of each compartment is determined by the breadth of skill and experience of each team leader. I suspect that a rational review of the whole process would find that it would be much more efficient if more could be contained within each compartment, and the number of interfaces could be greatly reduced. This would require a conscious determination to use ongoing professional development to broaden skills significantly.

Not only are there too many compartments, there are far too many steps. Design management tends to bring with it the staged design approach which is appropriate to buildings. Bridge design is essentially a single step process. The fusion I have spoken about which is preceded by a process of study and insight. It is right that the design process should be monitored, but it is the degree of insight that should be monitored not, as often happens, intermediate design decisions which might constrain later findings. I foresee a role for a Design Log which records all the logical chains from the studies, and their implications for the resulting design. By making this document permanently accessible it would demonstrate an openness between the parties, and remove the need for much intermediate reporting. It would require considerable discipline to ensure that the log was always intelligible, but that would be a good thing anyway. And it would focus attention on a wider range of solutions.

Increasingly the design role is passed around like a parcel. The contractors for Drachten and Nanterre were given the responsibility for detailed design. For various reasons Arstaviken was passed through three sets of hands. After Nanterre was complete Arup received a claim. The contractor’s designer had not managed to match the economy of our scheme design. When I went through the details I discovered that they had missed the point that I made earlier – even though we had been required to submit calculations – in French. It is difficult handing over a design from one team to another without losing some of the subtleties, and the resulting economies.

The activities of project managers should be scaled down. I 30 The Structural Engineer 87 (5) 3 March 2009

would like to see them renamed ‘project servants’ and, to be fair, the best of them do have the qualities of Jeeves, inconspicuous but always present with the next piece of vital information when it is needed.

The role of the architect Who designs a bridge? Traditionally engineers designed bridges. In recent years architects have been brought into the process. What is the role of the architect? In the early years of the Arup Bridge Group we often brought architects in to help us in the process of evolving our forms, and to produce presentational drawings.

Sometimes the client did not recognise the need for an architect and their quite small role was paid for out of the engineering fees.

Now the architect usually has a direct commission from the client.

The expectations are different. I pose the question knowing that in practice it is not as significant as it might seem. Around the design table it serves no purpose to define roles. Instead it is the personal chemistry of the individuals that determines which way the design ideas develop. The roles overlap. When designing Nanterre I had been working with Hugh Dutton for several months before I discovered he was an architect! I told him I had thought he was an engineer and he graciously took it as a compliment.

Architects have been introduced into the process because some engineering designers were designing bad bridges. But I think there is another story. Traditionally our public sector clients were senior professionals leading large in-house teams, who were interested in talking to their engineering consultants about the broader issues of bridge design. They were traditional patrons. In modern slimmed down government the client is more likely to see themselves as a budget holder, and the patron role is contracted out to a suitable person who can talk sensitively and intelligently.

An architect is ideal for this role.

But working with architects does make a difference. Architects are limited in their range of design forms by their own sense of what is appropriate or acceptable – which is why they are employed. But different architects will have different limits. And often their limits and sensibilities have been developed in the world of building design. I feel strongly that infrastructure requires a different aesthetic. Infrastructure provides the permanent stage on which transient actors, buildings, display their playful acts. I recognise that some bridges, or parts of bridges, can be players also, but there is an underlying intention to provide cool forms, and the challenge is often the challenge of minimalism – how to take away so much so that what is left shines. It is often left to the engineer to represent this essence of infrastructure. The engineer must have a strong enough design sense to match any architect in the arguments which are part of the design process.

Going back to the question ‘who designs a bridge’, the simple answer is ‘a bridge designer’. The important thing is that they have a familiarity with the issues, and a relevant track record. I accept that the term can be applied to both engineers and architects. On relatively simple bridges the architect/engineer role can be similar to that on buildings. Once the structural behaviour of the bridge becomes a significant issue the engineer needs to lead the design.

There are many ways that the design process can be assisted or inspired or monitored by other professionals – usually architects

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Conclusion My main point is that by opening up the design process – the Design Log is a key component – management structures will be better able to support it, and get better results. And we need to encourage the critical tradition. This way the role of the best designers will be appreciated, and others can learn. Above all we need to inspire the next generation.

I have happy memories of working with Bob Milne, and the medal itself is an inspiration. It bears the iconic image of the first Forth Bridge, and I am currently part of a team sketching schemes for the third Forth Bridge.

Angus Low studied engineering at Cambridge University and joined Arup in London in 1968. Initially he worked on the design of building structures. After a spell with Christaini & Nielsen on site, building roads and sewers, he rejoined Arup in its Bridge Group in London where he is still designing bridges today.

References

10 Pero’s Bridge, Bristol

but, for Pero’s Bridge (Fig 10) in Bristol I worked with the sculptor Eilis O’Connell. In contradiction of all good management practice I find it essential not to define roles. I always learn a lot from working with other designers.

I haven’t addressed the question that I am often asked – why do architects get so much of the attention? They are recognised for the integrity of their design sense. This comes from a background centred in design, which is founded on the practice of design criticism at their schools. We need to expose the fuller design story which will come from our design reports and logs, and we need peer written design reviews in our journals.

1 Low, A.: ‘The strength of curved steel plates’. First Inter. Conf. Advances in Bridge Engineering, Brunel University, June 2006 2 Low, A.: ‘The strength of curved steel plates under in-plane forces’, IABSE Symp., Budapest 2006 3 Low, A and Ichimaru, Y.: ‘Nesciobrug, Amsterdam’. The Structural Engineer, 85/5, London, 6 March 2007 4 Low, A and Ichimaru, Y.: ‘Nesciobrug, Amsterdam. A symbol for a new suburb, inspirational and functional’. IABSE Symp., Weimar 2007 5 Polak, M.A. and Vecchio, F. ‘Reinforced concrete shell elements subjected to bending and membrane loads’, ACI Structural Journal, May-June 1994 6 Arup, O.N. ‘Trois projets de ponts’. (Designs for three bridges). L’Architecture d’Aujourd’hui, 1963, 110, p46-51.

7 Low, A. M. ‘The role of culture in civil engineering design’, ICE Proc. Municipal Engineer, 158, September 2005 8 Sennett, R.: The Craftsman. London, Allen Lane, 2008

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