Ascent 112

Slowly, tentatively, gingerly, a stunning landmark has risen from a muddy north-west London construction site to take its place on the capital’s skyline. Three weeks ago, on 22 June, the construction team for the Wembley National Stadium succeeded in raising its 133 m steel latticework arch to its final, inclined position.

The imposing structure now dwarfs not only the temporary welding sheds and plant that clutter the busy site but the tower cranes and assorted crawler cranes that circle the arena. Even the raking steelwork of the part-completed stand appears tiny beneath the arch.

Lifting such a structure into place was always going to be a bravura piece of engineering – all the more so as its erection early in the project was fundamental to the whole programme. “The arch affects the programming of the whole job,” says Riccardo Petaccia, steelwork package manager for main contractor Multiplex. Only now that the 450 m long, 7.4 m diameter arch is raised can Multiplex finish construction of the stands and roof. “The arch is a key structural element: it supports two thirds of the weight of the finished roof,” says Petaccia.

The project has not been without problems: sections of the tubular steel latticework frame were found to be out of position and had to be replaced before the lift could commence, while problems with the concrete foundations delayed the lift further. The unusual method of raising the 1600-tonne arch, by pivoting it around the base of its legs, meant that 1800 tonnes of temporary steelwork had to be assembled and put into place before the lift could begin.

Before the team could tackle the challenge of lifting the arch, however, they had to build it. Mulitplex decided to assemble the arch flat on the ground. The structural engineer on the job was Connell Mott MacDonald. David Satchell, its design package leader for the stadium roof, says other assembly methods were considered, such as building in situ, but ruled out on safety grounds. “We could have constructed the arch using temporary supports and by lifting up preassembled sections,” he says. “We chose to construct the arch on the ground and then roll it up to minimise the amount of work that had to be done at height.”

The arch’s steel lattice was built by steelwork contractor Cleveland Bridge – its final triumph on the project before it was replaced by Dutch outfit Hollandia. It used preassembled sections about 30 m long. Each comprised three circular members linked by criss-crossing tubular steelwork to form the circular truss. The preassembled sections were then joined to form the arch using the “hit-and-miss” method of assembly. This involved positioning the preassembled circular arch sections adjacent to one other and an equal distance apart. The “missing” criss-cross tubular steelwork trusses, or “straws” as Petaccia refers to them, were then welded into position to “stitch” all the preassembled components together.

The arch was assembled at ground level, wrapping around the south side of the pitch, to allow construction of the more complex north stand to continue uninterrupted.

Before Christmas the talk on site was that the task of raising the arch would begin in February. However, a survey of the completed arch steelwork revealed that 16 of the tubular “straws”, had been welded so far out of position that the engineers feared that the structure would not perform as intended. These straws had to be replaced before the lift could begin.

Problems with the concrete used in the arch’s foundations also contributed to the delay. Petaccia refers to “suspect concrete” used in the construction of the massive concrete foundations of the arch bases. Samples of the concrete used in the foundations’ construction failed a strength test, which meant it had to be broken out and replaced.

While this remedial work was being addressed, Multiplex set about installing the arch lighting system and the temporary work platforms on the prone arch so that they were in place once the lift had taken place.

Preparations for raising the arch started three months before the actual lift with a weekly workshop for all those involved. “We tried to cover every possible scenario for things to go wrong,” says Satchell. Petaccia puts it more succinctly: “There was a lot of planning and a bit of anxiety,” he says.

To lift the arch five huge strand jacks were installed to the north of the stadium to pull it up using massive steel cables. Five jacks were also needed on the south side to lower the arch once it had passed through the vertical. The whole operation was computer controlled to ensure the arch was lifted evenly. Temporary lifting struts were attached to pivots to enable the jacks to pull the arch upwards (see photos).

Saturday 22 May was the day chosen for the big lift. Multiplex emptied the site of all but 80 key personnel. “We quarantined the site,” says Petaccia.

The first step in lifting the arch was a modest one: the arch was raised just 100 mm clear of its supports. “It was a big step for us,” Petaccia says. In this position, many of the forces on the arch and lifting mechanism are at their greatest. “It was a big comfort to us that, if anything did go wrong with the lift, the arch would only fall 100 mm,” adds Satchwell.

The arch was held in this position for four hours before anybody was allowed to go near it. “We knew that if the arch passed the four-hour test there was a very good chance we were not going to have to bring it down again,” Petaccia explains. Once that was passed the arch was surveyed to ensure the structure was behaving as planned. Thankfully it was, so over the next four days, Cleveland Bridge set about removing the temporary supporting feet and painting the underside of the arch in preparation for the lift proper.

The lift was originally scheduled to take place over four weekends. However, Petaccia says this regime was too restrictive because a couple of days of bad weather would disrupt an entire weekend window. Instead, the lift was scheduled to take place on a daily basis between 4pm and 10 pm, by closing the site early, and all day Saturday and Sunday.

After Saturday’s cautious beginning, the first big 5° lift took place on the following Thursday. This succeeded in raising the centre of the arch 10 m clear of the ground. Again, the arch was surveyed to ensure that it was behaving as planned. Each day, the arch was raised a little higher so that by the end of the weekend it had reached the 30° milestone, when the cable net that would support the arch in its final position was attached.

As the lift continued, the engineers kept a check on the surveys. “We were looking for trends in the survey results that would signify a problem,” says Satchwell. As it happened, the actual measurements were almost exactly the same as the theoretical values: “They were spot on.”

The next milestone in the arch’s upward journey was the 65° mark. As it was getting closer to vertical and its point of balance, the five restraining cables were attached to its south side so it could be gently lowered through 90° to 102°. Once beyond this point, the weight of the five turning struts started to become a significant downward force on the arch. These temporary cables will hold the arch in place until it is connected to the roof structure.

At 100°, the three central lifting trusses were removed to reduce the downward load on the arch. The two smaller, outer-turning struts, however, remained in place to keep a limited downward force on the arch until it was lowered to its final resting place at an angle of 112° from its starting point. Finally, the last two lifting struts were removed so that its canted weight was supported on just five cables strung from the five strand jacks. “We’ve parked it in position,” declares Petaccia.

With the arch now in position, the next critical stage is to complete construction of the stands. Only when the stands and the perimeter roof truss are complete can the permanent backstay cables be connected. After that, construction of the stadium’s roof can start.

But any thoughts that the project will be plain sailing from here are quickly dismissed.

“If anything, the roof structure will be even more difficult to erect,” says Petaccia, somewhat ominously.