The seismic performance of a single-storey steel building with vertical X-bracing and a flexible roof diaphragm is examined through nonlinear dynamic analysis. The influence of the flexibility of the roof diaphragm, in-plane stiffness eccentricity and accidental variation in the yield strength among the vertical bracing is studied. Diaphragm flexibility was found to increase the ductility demand on the vertical bracing beyond current design assumptions. The torsional response of the structure with stiffness eccentricity was also underestimated by a static analysis procedure. In particular, deformations and ductility demand on the bracing located on the flexible side were found to exceed the anticipated values. Slightly reducing the yield strength of this bracing, to simulate inherent variation in member capacity, further amplified the torsional behaviour.
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