Javier Llarena explains a project to extend the roof of the San Mames football stadium in Bilbao, Spain. The project won The Award for Long Span Structures at The Structural Awards 2017.
The cable roof extension project for San Mames stadium was mainly about improving the spectator experience through better rain cover. It was quite an engineering challenge, as we were tasked with increasing the roof span by 13 to 23m, while minimising the required reinforcements on the original roof - without any disruption on football games. I’m proud to say we achieved all this in record time.
The main structural challenge was the fact that our new structure had to be supported by an original structure which had not been designed to withstand an additional roof. We used advanced engineering tools, from non-linear calculation models (which analyse the structural integrity of the structure through the whole erection process), to physical mock-ups and wind tunnel modelling to ensure we delivered the best possible solution.
Starting from an existing structure considerably limits potential solutions: it is not only necessary to design and calculate the new extension, but to check the entire original structure (from the original roof to the foundations) and, in the case that something need to be reinforced, find a feasible solution for that too.
Moreover, from a construction point of view, the erection process for a cable roof extension is more complex than for a whole cable roof, as the starting geometrical configuration of the cable system (radial cables start almost in vertical position, unlike in other cable roofs for stadiums) makes the initial lifting stage more problematic.
This roof extension minimised loads on the existing structure as it was composed of lightweight cable and ETFE cladding, while the cable system preload “tenses” the original cantilevered roof, compensating the additional compression loads on their bottom chords (and, thus, avoiding the buckling failure) caused by the extra vertical loads. At the same time, this tensioned state leads to a minimum reduction of the eigenfrequencies of the whole roof structure, so that the dynamic amplification of wind loads is minimised.
This kind of roof extension method could easily be used for other projects. For instance, it could be very useful for those stadiums converted from athletics arenas to football stadiums, which require smaller pitch areas and allow extension of the lower stands – which would require roof extension cover.
The client was very satisfied with the upgrade solution as it not only improved shelter performance without disruption to games, but also improved the atmosphere by creating a more enclosed space.
We were delighted to win The Award for Long Span Structures category at The Structural Awards 2017. The Award is recognition for a lot of hard work and innovation and we were proud to take home the prize against such good competition.
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