The effects of large displacements on the earthquake response of tall concrete frame structures

Peter J Moss,Athol J Carr

doi:10.5459/bnzsee.13.4.317-328

Abstract

Many analytical techniques have been developed for the inelastic analysis of multi-storey framed structures subjected to earthquake excitations. Most of these analyses have ignored the influence of the geometric effects due to the large lateral displacements on the response of the structure. However, there has been an increasing interest recently in these second order effects and, in particular, their consequences for the design of the column members in the frame. This paper describes the modification of an inelastic frame analysis to include the effects of large displacements and then the application to three typical New Zealand concrete structures subjected to a variety of earthquake excitations. Comparisons are then made with results obtained from analyses ignoring these effects. The results are then reviewed in order to determine the nature of the problems, to determine when these second order effects should be considered and discusses methods of limiting these displacements.

Highlights

Most analyses of multi-storeyed reinforced concrete frames subjected to earthquake excitation have ignored the secondary effect due to the combination of gravitational forces and large displacements
For the second analysis, where the strength was increased by 25% while maintaining the original stiffness, the increase in maximum interstorey drift was reduced to 21% of the non P-Delta value for the original frame
The original program has been extended and modified so that the ultimate strengths of the members are given in terms of maximum positive and negative moments for beams, while for column members sufficient information can be given to define the maximum extent of the load-moment interaction curve in tension and compression

Summary

Introduction

Most analyses of multi-storeyed reinforced concrete frames subjected to earthquake excitation have ignored the secondary effect due to the combination of gravitational forces and large displacements. The top storey deflections and interstorey drifts under Parkfield and Pacoima are greater than the code allowable values though the maximum plastic hinge rotations are capable of being achieved in well detailed structures.

Results

Conclusion