Consideration of seismic demand in the design of braced frames

This paper addresses several salient issues associated with the design of typical configurations of concentrically braced (CB) frames, with particular emphasis on European seismic provisions. A brief assessment of the underlying design philosophy and strength demand considerations for capacity design checks incorporated within Eurocode 8 is presented first. It is shown that brace overstrength in conventional CB frames is largely dependent on the idealizations related to the treatment of brace buckling in compression, and that the level of such overstrength decreases as brace slenderness increases when tension-only design approaches are adopted, as is the case with Eurocode 8. Results from extensive non-linear response-history analyses on a set of CB frames are presented, and are used to illustrate the main behavioural aspects related to ductility demands on CB buildings for moderately stiff, stiff and soft soil conditions. These results are also used to examine the applicability of the 'equal displacement' rule typically adopted in Eurocode 8 for steel structures. The findings indicate that this rule offers reasonable upper-bound estimates of deformations in CB structures, but only for those with initial periods >0.5 s built on moderately stiff to stiff soils and for moderate to relatively low slenderness values. The influence of column stiffness continuity and brace capacity-to-demand ratio on the distribution of plasticity in CB buildings is also demonstrated, and its implications on the interpretation of code provisions are highlighted. Finally, based on a parametric investigation of simplified frame models, the study shows that properly designed secondary systems such as gravity frames can play a significant role in mitigating the probabilities of dynamic instability in CB buildings.