Comparative study on the effect of outrigger on seismic response of tall buildings with braced and RC wall core. I: Optimum level and examining modal response spectrum analysis reliability

Abstract

SummaryDesigners usually use modal response spectrum analysis (MRSA) in design of tall buildings. This paper studies the reliability of this technique in predicting real seismic behavior of tall buildings with outrigger and belt truss. Four tall buildings utilizing either braced or reinforced concrete shear wall core with 28 and 56 stories were designed as the base models according to current design codes. Outrigger and belt truss were added at quarters of the structure height generating 44 structural models. Results obtained from MRSA of these models are discussed and compared to those obtained from nonlinear time history analysis (NLTHA). It was observed that MRSA unrealistically overestimates beneficial effects of outrigger on drift reduction of tall buildings with braced core. NLTHA showed that the maximum lateral displacements of the structures with braced core are less than those of buildings with RC shear wall, but they experience more residual displacements. Adding outrigger could reduce maximum lateral displacement of buildings with RC shear wall core (in excess of 30%) but had minor effect on this index in structures with braced core (less than 10%). Outrigger did not consistently reduce maximum drift under all records, except for the story where it is located. Although outrigger could not reliably decrease residual drift of structures with braced core, it significantly decreased this index when applied at any level of buildings with RC shear wall core, under all studied records. Applying outrigger increased story shears and NLTHA showed that it creates a sudden jump in shear at its placement story.