Deflection Limits in Tall Buildings

Serviceability criteria in the form of lateral deflection and acceleration limits under wind loading are often the governing structural issues for tall buildings. Whilst the basis for acceleration criteria has been the subject of research, rational refinement and consensus over recent years, deflection limits are still rather arbitrary. Current guidance on deflection limits in international design codes is very limited and is based primarily on experience with typical low and medium-rise buildings. The issues with lateral deflection in very tall buildings are different to those of low-rise buildings, and depend on structural form. Rational choice of deflection criteria for tall buildings therefore requires further consideration of the nature of the deformations and the effects they have on the functional aspects of the building. This paper identifies the functional issues, the relevant deflection measures, and reviews current international practice. New design approaches are challenging previous design norms, and the paper describes a performance-based approach, which allows greater freedom in choosing appropriate constraints for structural and non-structural components. Whilst the subject matter is not new, the use of new design tools and processes leads us to re-examine rules of thumb previously used. The paper focuses primarily on wind events, but comparisons with seismic events are drawn.