A consideration of P-delta effects in ductile reinforced concrete frames

Abstract

The likely effects of secondary moments due to the gravity load, which is being laterally displaced during inelastic seismic storey drift, upon ductile reinforced concrete frames is examined. Existing building code recommendations and design procedures relevant to the phenomenon are briefly reviewed. The probable effect of P-delta moments on inelastic dynamic
frame response is discussed. With the aid of illustrations various design considerations are presented. It is suggested that if strength demand
due to P-delta effects exceeds 15% of the ideal lateral load carrying capacity of a subframe, this strength demand should be met. From the comparison of the elastic and inelastic deformations of a frame due to earthquake loading, recommendations are made for the estimation of critical inelastic drifts in the lower half of the frame. This is subsequently used to quantify the problem with the aid of the "stability index". The quantitative evaluation of P-delta effects for an 18 storey frame is illustrated in the appendix.