The validity of AS4100 clause 5.13.4 was put in doubt following an enquiry into the failure of a beam inadvertently raised the issue of its conservativeness. After a theoretical investigation, it was revealed that the theory behind the clause in question had been abbreviated for simplicity and under specific conditions it may be possible for the web to buckle at as little as one-sixth of the capacity given by AS4100.
In most engineering applications that use this clause, the top flange is adequately restrained and no problems would arise. However it is possible to design a beam that does not have the top flange restrained. This is difficult to achieve as was found while trying to achieve these conditions during the physical test but nonetheless is possible and should be properly addressed.
By using the buckling theory behind the clause, approximate buckling loads were evaluated and the initial predictions made for a test case of a 2m long 310UB32. Finite element modelling and physical testing were then employed to verify the initial findings. The finite element results confirmed that the buckling loads were significantly lower than clause 5.13.4 allows for but the physical test buckled at loads exceeding the capacity evaluated by the Australian Standards. The results are inconclusive but show that the matter is a real issue and should be treated as such.
During the testing, it was also found that the restraint conditions at the support had very little effect on the buckling loads and that the bearing buckling problem only occurred in relatively short beams. Coupled with the complications in loading a beam without restraining the top flange, and this problem appears unlikely to occur much and could be easily solved by the addition of web stiffeners.
However the fact is that the problem could occur and the Australian Standards does not account for it, therefore as an interim measure while a comprehensive study is carried out, a recommendation to the Standards Australia committee will be made to alter clause 5.13.4 to include an effective web length factor into the assumed slenderness ratio of the clause.