Failure analysis of a collapsed flat gymnasium roof

Abstract A 1300 m2 flat roof of a three year-old gymnasium collapsed in the early morning of a winter day, fortunately before the opening hours. The snow load was rather high, but still considerably below the design limit. Therefore, the snow could only be the trigger of the collapse. The visual findings, material tests and norm conformity checks pointed towards the buckling of the girder’s stiffener-less web at its supported ends to be the root failure cause. However, relevant contributions of other structural deficiencies still could not be excluded. In order to prove the “buckling-hypothesis”, the optimistic–realistic ultimate resistance of the girders had to be determined and compared with the reconstructed loads acting at the moment of the failure. The present paper focuses on the calculation of the ultimate resistance of the girder’s ends including the non-linear post-buckling process. The procedure consists of a concerted two-stage combination of linear and nonlinear finite element analyses. A sensitivity analysis regarding the girder’s initial geometrical imperfection and the consideration of the real elastic–plastic material behavior were essential to obtain the adequate range of the girder’s ultimate resistance. Based on the quantitative results, the girder’s web buckling under the regular service load due to lack of stiffeners could be proven as the root cause of the accident. The conclusion is supported additionally by the consistency between the retracted collapse and the characteristic visual findings. The presented case study demonstrates the importance of finding the realistic–optimistic load carrying capacity when proving a corresponding failure hypothesis. This required a different approach compared to the stage of design in terms of appropriate assumptions and accuracy.