Walsall's bus station was meant to fuel the town's regeneration but four months after its planned opening date, contractors are still working to correct its sagging roof. What went wrong?
It was to be catalyst for the midland town's renaissance. A stunning bus station bang in the centre of Walsall, topped by a spectacular concrete roof carpeted with plants. For the young architect that produced the daring design, the bus station was to be the scheme that launched it into the big league, and for the structural engineer, it would be proof that such designs could be built.

But the project has run into difficulties. Beset by construction problems, the subject of a safety report and the focus of local political debate, the scheme is still incomplete four months after its planned opening date. Now, as the contractor struggles to finish the project, an air of resignation hangs over the design team as it awaits the legal wrangling and accusations of blame that are set to begin once the building is complete.

The venture began optimistically in 1995. The design, by architect Allford Hall Monaghan Morris, was the winning entry in a competition to raise passengers' expectations of bus travel. The architect's produced an exciting scheme that forsook the conventional rows of canopied islands in favour of a single roofed building, the idea being to create the feeling of a terminus, says director Peter Morris.

The design featured a double-curvature elliptical roof measuring 80 × 45 m, carried by 12 slender steel "trees", each with three supporting branches. A series of 10 ventilation cowls, resembling extinct volcanoes and open to the outside, dot the roof's surface, their layout tracing the route of the bus lanes below. Between these, six smaller, glazed cowls light the pedestrian waiting areas.

Tucked into the south-west corner of the station is a concrete box that houses the station's staff offices, toilets and shops. This is surrounded by full-height, glazed walls, rounded to echo the elliptical shape of the canopy above. The scheme also includes a small public square to the south, which is a pedestrian link to the main shopping street, while to the west is a satellite canopy with a sculpted aerofoil roof served by three additional bus stops.

After the euphoria of the competition win, the architect had to wait three years while the client, Centro, raised funding for the £6.5m scheme. The canopy was the first thing to be built. The design, by structural engineer Atelier One, called for a 325 mm thick elliptical lightweight concrete slab supported by a 5 m wide, 500 mm deep, dense concrete edge beam. At tender stage, the structural engineer had intended to use a precast concrete system for the edge beam and the rooflights, but after discussions with the main contractor, Shepherd Construction, an insitu cast concrete system was used.

At the beginning of September 1999, when the roof was nearing completion, things started to go wrong. As the propping used to construct the roof was removed, the edge beam gave cause for concern. Councillor Richard Worrell, chair of West Midlands Passenger Transport Authority, the body responsible for setting the region's transport policy and advising Centro, says it sagged by more than 50 mm.

The edge beam sagged by more than 50 mm

Richard Worrell, chair, West Midlands Passenger Transport Authority

The deflection occurred in the south-west corner of the roof, at the point where the edge beam is at its maximum span – 27 m between a supporting tree and the elliptical office. The roof had been precambered to allow for some deflection between supports and to assist in rainwater drainage, but the deflection in the beam was "more than anticipated," says Atelier One director Scott Nelson.

Immediately after the deflection was discovered, two reports were commissioned – one by Centro using structural engineer Anthony Hunt Associates, and one by Shepherd – to assess the safety of the structure. The reports disagreed about the main bone of contention, which was the sagging. However, Nelson says there was some contradiction between the reports. "The report by Shepherd's engineers calculated the same figure for deflection that we calculated, but the client's report predicted deflection twice as much as we'd calculated," he explains.

Nelson puts the discrepancy down to the different briefs the teams were working to. He says the brief given to Anthony Hunt "was not a full one, and it was working on limited information to produce the report. Whereas the report for Shepherd was done regarding the temporary works". Since then, Atelier One has had discussions with the client's engineer and, Nelson claims, "it does not disagree with our calculations".

The deflection did not affect the whole roof; it "only affected one particular bay in the first area to be de-propped," says Nelson. He says this sagging did cause some concern "to the contractor and client and had a knock-on effect on the programme". However, the remainder of the roof has deflected "as expected", he said.

Anthony Hunt's report confirmed that it was safe to proceed with the removal of the propping, but highlighted another problem: that of impact safety if a bus were to crash into one of the supporting columns. A separate report was subsequently commissioned by Centro into the impact resistance of the columns.

Large lozenge-shaped lumps of concrete have now been positioned at the foot of each column, at a cost of £50 000. "Centro asked for the column protection to be put in place," says Worrell. "You'd have to ask the architect if column protection was thought necessary, but it had not put in any," he adds. Morris disagrees: "Impact protection was provided in the form of steel deflector rails to protect against a sensible degree of risk. The client has simply asked for enhanced protection to meet a greater extent of risk."

It’s not the roof’s strength and safety that is at issue, but the long-term serviceability

Scott Nelson, director, Atelier One

From the street, the canopy looks magnificent, a huge concrete mass levitating surreally over the unfinished concourse. From above, however, the roof looks in a sorry state. Puddles of water dot the waterproof membrane as work is under way to correct the area where deflection occurred. "We bought a green roof, but we are not proposing to plant water weeds," says Worrell tersely.

The problem of ponding is exacerbated by the rainwater downpipes concealed in the supporting trees, which naturally form the high points on the roof. Between these high points, the screed is being built up. "It was impossible to precamber all the areas of the roof," says Morris "We had always assumed we'd be doing some screeding between these points to build up a fall." Work is still to be completed on the roof and, says Morris, the client has decided to put the green roof on hold for 12 months, "to monitor the roof effectively".

The debate set to erupt once the project is complete will be over who is to blame for the deflection. "We are of the definite opinion that it is more to do with the actual procedure for de-propping," says Atelier One's Nelson. But councillor Worrell, whose ward the bus station is in, says the architect is at fault for failing to ask main contractor Shepherd to "produce a method statement for the prop removal". Nelson is keen to play down the problem. "It's not the roof's strength and safety that is at issue, but the long-term serviceability," he says.

Further delays have hit the construction of a linear satellite canopy. The canopy will be constructed from a lightweight ferrous cement roof installed over a prefabricated frame covered in galvanised steel mesh and supported on two branched steel supporting trees. "It's the first time since the 1950s that such a construction has been used," says Morris. He puts the delay down to "various procurement issues", but will not expand further. The canopy is being shrouded in scaffolding ready for the application of the concrete.

The concern now is for the future. Centro has taken the precautionary step of issuing an instruction for an additional column footing to be installed beneath the area where the edge beam has deflected on the main canopy, "on the basis of what might be needed", says Worrell.

"It's not a concern we share," says Morris. "If the client wants to put a lump of concrete in the ground, that's up to him." In any case, he adds, if the deflection gets worse, "an additional column might not be the answer".

Worrell says Centro is about to commission another report "on the issue surrounding the roof's long-term durability", to assess if it will last 30 years and if it will cost more to maintain. Morris does not see this as a problem. "The roof can accept standing water, it does not affect guarantees and it is not a structural issue," he says.