Comparative study on the effect of outrigger on seismic response of tall buildings with braced and Wall Core. II: Determining seismic design parameters

In this paper, the effect of outrigger on key seismic design parameters of tall buildings with braced or shear wall core is studied. Based on the results of modal response spectrum (MRS) and nonlinear time history (NLTH) analyses of 44 models with braced or wall core, discussed in the companion paper and pushover analysis and presented in this part, the effect of outrigger and its placement level on parameters of seismic design including response modification factor R, overstrength factor omega(0), deflection amplification factor C-d, and damping ratio of structures are studied. New local measures for C-d and omega(0) are proposed based on the NLTH results. It is observed that outrigger results in greater increase in strength and stiffness of structures with wall core than buildings with braced core, but decreases the ductility of both systems. The obtained R is less than the values given by the code in most cases. For the main structures without outrigger, using the code prescribed value for C-d underestimates the real displacement of the structures with wall core and overestimates it for those with braced core. Outrigger results in increasing C-d of structures with braced core and reduction of C-d in those with wall core. The considerable variation of two proposed measures for C-d shows the inappropriateness of application of a single value of C-d for all stories. Outrigger increased the damping ratio of buildings with shear wall core considerably.