Numerical analysis of a clad portal frame structure tested to destruction

A full-scale steel portal frame structure, explicitly designed to utilize stressed skin action, was tested to destruction in the early 1970′s. At this time, it was not possible to perform any detailed analysis of such structures; however, using modern computational techniques, detailed Finite Element Models (FEMs) have now been developed. These FEMs show an excellent agreement with the reported test results, including following the nonlinear behaviour to complete collapse. The validated model was then extrapolated into a multi-bay portal structure of more realistic length, and the effect of cladding on the strength and flexibility of the frames was established under both vertical and horizonal loading. Under a distributed vertical load, the cladding initially had little influence although, once the internal frames reached a complete collapse mechanism, the cladding prevented structural collapse until the cladding also failed. However, under horizontal load, the coupling between frames was significant, particularly in the multi-bay case, leading to high forces in the cladding and the structure/cladding interface. The tested frame was stocky and low rise, typical of its time. Today, with taller, more flexible modern frames, stiffer and potentially less robust cladding systems will resist an even more significant proportion of the horizontal load; a warning to the construction industry that parasitic diaphragm action should not be ignored.