Accounting for P-Delta Effects in Structures When Using Direct Displacement-Based Design

Abstract eng:
As part of the development of Direct Displacement-Based Design, a method for explicitly including P-∆ in the design process is presented. The differences in sensitivity to P-∆ of both elasto-plastic (approximating steel response) and stiffness degrading (reinforced concrete) hysteresis are discussed, from which a proposed multiplicative factor is derived to account for the enhanced performance of reinforced concrete structures. The derivation and an example the parametric behaviour of the proposed factor is presented here, following which a four-storey frame designed for both reinforced concrete and steel response is tested using non-linear response history analysis with a suite of seven spectrum-compatible ‘massaged’ real records. It is found that the proposed account for P-∆ satisfactorily reduces the storey drift amplifications, such that the design performance targets are maintained at the original level when second-order effects are not included in the analyses. Design recommendations are also presented based on simplifications to the derived equation through the assumption of reasonable values for typical new structures.

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