P-delta effects on short-period systems subjected to earthquake excitation

Abstract

Short period structures are commonly associated with reduced P-delta (PΔ) effects, and some codes prescribe fundamental periods below which these effects can be neglected. However, relevant material nonlinearities and different hysteretic responses can highly influence both the amplification of the displacement demand and the limit values of the fundamental period for negligible PΔ effects, at a specific effective height. Considering a large earthquake database, a large variation of the ductility, and several hysteretic models, this paper examines the amplification of the displacement demand due to PΔ effects of short period single-degree-of-freedom (SDOF) systems. It was found that limiting the maximum fundamental period to T0=0.4 s for bilinear, to T0=0.5 s for Takeda & Sina, and to T0=0.6 s for flag-shaped hysteretic systems, the variation of the PΔ displacement amplification with the stability coefficient obtained is, with good approximation, linear. Under these period limitations, simple and accurate analytical estimates of the amplification of the displacement demand have been obtained. For effective heights equal to or larger than he=4 m, and ductility levels up to μ=8, median amplifications of the displacement demand lower than 5% for bilinear, Takeda & Sina, and flag-shaped hysteretic systems required the fundamental period to be limited to T0=0.4 s, T0=0.8 s, and T0=1.0 s, respectively.