Exciting architecture and a positive environment are well understood to improve performance in the workplace. However, one struggling school in South London secured Government funding to help transform its uninviting structure, identified as a key factor in explaining its poor results.

The school now boasts an extraordinary covered quadrangle, flooding the reclaimed space with light and creating a more open and inviting space for pupils and staff alike. The ambitious design of the quadrangle required the expertise of structural design consultants, Michael Hadi Associates, using structural analysis software from ISS.

The chosen design involved redeveloping the central courtyard and covering it so it could be used throughout the year. Walkways would be created through the courtyard for easy access across the school, with the rest of the space remaining open for flexible use such as group work or mealtimes.

Roof of concept

The design of the roof was obviously the main challenge. John Roycroft is Project Engineer at Michael Hadi: "Our challenge was to find a roof design that would satisfy several needs," he explains. "Firstly, we had to ensure that the weight of our proposed roof was as low as possible, since the 1960s steel frame of the existing school would be put under lateral pressure that it was not originally designed to bear."

Michael Hadi explored several possibilities, including a folded plate roof and a ply roof, but the final design was a cable-supported arch structure using transparent ETFE cladding. The panels would contain three variable skins, controlled by sensors. The skins move nearer or further apart to control the light entering the quadrangle. "The big advantage of ETFE is that it is really light," says Roycroft. "The quadrangle is 40m by 80m, which is quite a big space, but the roof design only weighed in at 40kg per metre squared. It's only the steel arches that have any significant weight. These created the lateral loads on the existing building that we needed to analyse carefully."

Underneath the arches

The chosen design of the roof created a significant structural engineering challenge for Roycroft and his team. The roof comprises ten arches of different shapes and sizes, which needed to be pre-stressed to compensate for the wind speeds they would encounter once in position. "We had to define what the curvature of each arch would need to be prior to installation, and then what the final position of each arch would be once attached to the school," explains Roycroft. To complicate matters further, the slenderness limits of the arches were designed outside the limits set by the normal BS5950 standards, in order to meet the need for weight reduction and aesthetic appeal. In addition, since the entire structure was to be supported by cables, detailed nonlinear analysis was required to ensure the correct tension was used.

At Michael Hadi Associates, the structural engineers prefer to use analysis and simulation software to confirm their hand calculations, rather than immediately turning to their computers for a quick answer. But on this occasion, Roycroft knew he needed some specialist assistance to ensure the arches were pre-stressed in the right way.

"We had to be confident that second order effects were being modelled accurately, particularly as we were using a compression member beyond slenderness limits," he explains. "We hadn't got a program here that modelled cables in a nonlinear fashion - crucial for the accurate positioning and tension of the arches. We tried several structural analysis packages, but we quickly determined that Robot Millennium was the best tool for the job."

Structural support

Robot Millennium from ISS is a complete structural analysis solution, specialising in cable analysis, featuring model generation, calculations, analysis results, codes and design checks and a calculation report. The straightforward system offers the ability to model complex structures quickly and accurately and, crucial for Michael Hadi, Robot is a true' nonlinear solution. Using Robot, Roycroft could confidently model both the initial pre-stress and in situ pre-stresses.

"Robot is very user-friendly and as it's Windows based the interface is intuitive and quick to learn," says Roycroft. "In addition, if you do come up against difficulties, the Robot support team is excellent. You can email them an analysis problem and they are quick to help and sort it out. It's also important that they understand the engineering challenges we face, not just the software issues. We can explain situations from a technical point of view and they have a good understanding of how best to find the right solution."

Full marks

The pre-stressing of the arches needed to be absolutely accurate to avoid the adverse affects of temperature, loading and wind. "Having done a lot of calculations ourselves beforehand, we were confident that even though our solution went beyond slenderness limits it would still work. Even so, the positive and accurate results we got from Robot gave us the confidence we needed," says Roycroft. The solution has also saved the team a considerable amount of time. Roycroft estimates that to manually calculate the pre-stress requirements of the ten different arches would have taken about three weeks. "It was a weekend's work using Robot," he says.

On the day of installation, each arch arrived on site in three pieces before being bolted together. Special lifting and pre-stressing cradles had been designed to ease each section into position. When the roof was finally ready to install, Roycroft and his team checked the pre-stress levels on site and discovered that Robot's results had indeed been very accurate. "It's fair to say this was a very successful project," says Roycroft. "From our point of view this was a real team effort, and the people at Robot were a part of that team."

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