In an age of ingenious designs, bridges have become the latest must-have commission for architects. Fortunately, engineers seem happy to share the plaudits with their new colleagues. Here, we look at the stunning results of architect Hakes Associates and engineer Buro Happold鈥檚 joint venture
Bridges are no longer the sole preserve of engineers: architects, landscape architects and even artists are all energetically jumping on the bandwagon. The latest bridge design to be unveiled, a collaboration between structural engineer Buro Happold and architect Hakes Associates that won an open international competition in Liverpool, exemplifies this latter-day interdisciplinary trend.
So what lies behind this recent approach? The slightly cynical answer might be that bridges, such as Tyneside鈥檚 Winking Bridge and the Millau Bridge in France, attract so much publicity that architects such as Wilkinson Eyre and Foster and Partners are keen to get a slice of the action. Engineers are sharing the limelight graciously. Davood Liaghat, who heads Buro Happold鈥檚 10-strong bridge engineering group, says that his colleagues 鈥渁ll want to design bridges that are significant icons. We all enjoy working with architects who can think outside the box.鈥�
Another interpretation of the interdisciplinary trend is that bridge commissions tend to be iconic by virtue of their being prominent landmarks in city centres and beautiful Countryside, and as such they call for attention to appearance as well as engineering principles. Liaghat adds: 鈥淐lients these days don鈥檛 want standard bridges: they want to spend money on something more imaginative or refined.鈥�
What鈥檚 more, many of today鈥檚 bridges are no longer large-span road and railway bridges, designed as heroic but straightforward structures, but pedestrian or cycle bridges that have to be intricately shaped to fit around existing buildings and landscape features. Julian Hakes, director of Hakes Associates, says this isn鈥檛 the only exacting challenge: 鈥淵ou have a captive audience along a bridge, especially a footbridge where users will be looking at every single detail and handling the handrail.鈥�
The competition-winning Liverpool bridge, which responds to a spectacular site leading up to the Anglican Cathedral with a delicate S-shape of steel tubing, presses all these buttons. It is one of eight sculptural bridge projects designed by Buro Happold and Hakes Associates. The others include a footbridge across Bristol鈥檚 floating harbour that resembles an elastic band made rigid, and a scheme in Boston, Massachusetts, where a pair of sinuous footbridges wind in and out of a riverside maze of railway and road bridges.
Two of the designs are on an even more imaginative plane, but as both are competition entries that failed to sway the juries, they are still in search of actual clients. Both are opening footbridges, with one resembling a pair of scissors (see previous page) and the other looking and operating like a Venus flytrap.
鈥淭hey look flamboyant but they are structurally efficient,鈥� says Hakes. 鈥淭heir forms are the literal translations of where the forces are going. If you added anything extra, you would be increasing the bridge鈥檚 weight and making it less efficient and more expensive.鈥�
So how does the architect鈥揺ngineer collaborative process work on this portfolio of bridge designs? 鈥淚t鈥檚 not a case of architects making it pretty and engineers making it stand up,鈥� says Hakes, who discovered the joys of structural engineering at the tender age of 13, when his model crane won a school competition.
鈥淭he first thing we do in a new project is to study all the constraints of site and purpose,鈥� says Liaghat. 鈥淭hen we come together around a table for a whirlpool of sketching. After that, we translate the sketches into 3D computer models, we look at all the alternative options, and finally we subject the selected design to structural analysis.鈥�
Often the design process also involves less sophisticated activities. 鈥淲e play around with models in paper or cardboard,鈥� says Hakes. 鈥淎nd for the Dublin Sailing Bridge, we brought in some raw clay, cut it up on the desk and poked sticks in it to see how the bearings might work. It鈥檚 all very simple stuff.鈥� Child鈥檚 play, perhaps, but hardly childish results.
LIVERPOOL鈥橲 BRIDGE OF HOPE
This competition-winning design for a new footbridge in Liverpool is a model of delicacy in form, inspiration and concept. In form, it is a slender S-shape of steel tubing that snakes around a deck and balustrading constructed in transparent glass. In inspiration, its architect Julian Hakes refers to 鈥渁n elevation that explores the path defined by the wing of a dove in flight鈥�. And its design concept is informed by the Fibonacci series, the same mathematical progression found in the spiral of a snail鈥檚 shell or sea shell in nature.
Such through-and-through delicacy is a response by Hakes Associates and Buro Happold to a particularly sensitive site next to Liverpool Anglican Cathedral. The bridge is planned to deliver pedestrians to the cathedral鈥檚 north porch across a deep chasm that was once a quarry and later became a cemetery. Its lightweight, soaring, bird-like form is intended as a foil to the massive stone bulk of Sir Giles Gilbert Scott鈥檚 huge building.
The ground conditions are as sensitive as the visual appearance is delicate. Historic graves occupy most of the chasm, while ancient catacombs had been built into the north bank. Any bridge supports should avoid disturbing these precious remains, as well as the cathedral foundations.
A hybrid structural design with a 75 m span was adopted as the solution to this combination of challenges. Starting at the cathedral end, a tied arch rises above the flat bridge deck, then flips down seamlessly into a 鈥渂ow-slung鈥� truss beneath it.
Meanwhile, the arch鈥檚 S-shaped outer rim of tubular steel snakes around the deck in one continuous bow. From the cathedral end it sweeps outwards and upwards. It then curves downwards, ducking beneath the deck and outwards on the other side and then back to the other end of the bridge.
Steel struts running from the arch support the load of the bridge deck, with those rising above the deck working in tension, and those below in compression.
A raking steel prop supports the span at the off-centre point where the overhead arch drops into the bow-slung truss. As this is the point of contraflexure between the two arches, where there are no bending moments, only vertical loads are transferred downwards through the prop. This positioning causes the least disturbance to the historic ground below.
Both the balustrading and the 3 m wide bridge deck are made of laminated glass panels.
The bridge has been named the Bridge of Hope, partly in reference to Hope Street at the north end, and partly to denote its spiritual symbolism.
The bridge is to be developed by the Liverpool chapter and diocese at a cost of 拢1.1m. Rob MacDonald of Liverpool John Moores University has been appointed project co-ordinator.
DUBLIN SAILING BRIDGE
A strikingly ingenious variant on the bascule bridge, the Dublin Sailing Bridge is a pedestrian bridge across the entrance to a riverside dock in Dublin. It was designed for an international competition that attracted 75 entries but was won by a local Irish firm.
The Hakes/Buro Happold entry is split into two folding arms, which are constructed as steel box girders and project from opposite banks. In the open position, the two arms stretch upwards to form a prominent gateway to the dock. To form the bridge, the arms fold down to the horizontal position, where they lock together to form a continuous pedestrian deck.
The bridge鈥檚 arms resemble a pair of scissors both in appearance and operation, with the key difference that the two blades do not share the same pivot.
VENUS FLYTRAP BRIDGE
The Venus Flytrap Bridge is an even more compellingly dynamic design for an opening pedestrian bridge than the Dublin Sailing Bridge.
It shares with Wilkinson Eyre鈥檚 Winking Bridge the concept of an arch that lies flat above the river in normal use but swings upwards to allow ships to pass below.
The mode of operation, however, is noticeably different. The two arches on either side swing up and down on pivots at either end, just as they do on the Winking Bridge. But the actual bridge deck passing along the centre line is not a solid arch but a series of discrete slabs that lie flat above the water surface. The slabs are connected by a series of cables that run through them lengthwise before taking a right-angled turn to connect with the two outlying arches, as shown in the picture and the sequence showing the opening process.
The cables control the bridge mechanism and create its arched open form. The two outlying arches are raised by reeling in the cables through the central deck sections. But as the lengths of cable shorten and the arches rise, they automatically raise the central deck sections with them. In the lifting process, the central deck sections are pulled apart to leave wide gaps between them.
The two outer arches close together about the raised central deck just like the jaws of a venus fly trap plant devouring an unsuspecting insect.
Like the Dublin Sailing Bridge, the Venus Flytrap Bridge was designed for the Dublin competition in 2003 but failed to win. Buro Happold and Hakes Design have since developed the design with the hope of applying it to other suitable sites.
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Cross-section of Bridge of Hope showing hybrid structure
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