Column strength requirements in multi-storey seismic frames

E L Blaikie

doi:10.5459/bnzsee.22.3.135-144

Abstract

This paper examines factors affecting the strength requirements of columns in multi-storey frames responding to seismic ground motions. The examination is carried out using an inelastic static analysis approach and the concept of an "equivalent condensed frame". In particular, the influence of higher modes and the effect of varying the pattern of beam flexural strength over the frame height are evaluated. It is suggested that the current capacity design approach of the NZ Concrete Design Code overstates the importance of higher mode effects while neglecting the potentially more important influence of the beam flexural strength pattern that is provided for a frame. Some tentative modifications to the current column design procedure are suggested for future evaluation under inelastic dynamic response conditions.

Highlights

These observations have led to the suggestion, given in the commentary to NZS 3101, the NZ Reinforced Concrete Design Code [l], that columns should be designed to resist seismic loads using a capacity design approach that takes into account the effects of concurrency, higher modes and beam flexural overstrength on the column strength requirements
This paper examines the possible affects of an additional factor that is not currently considered and suggests modifications to the current capacity design procedure for columns
By reducing the frame to an 'equivalent condensed frame' with only one column and examining the behaviour under static loads that model some of the characteristics of dynamic loads, factors influencing column strength requirements are more examined

Summary

Introduction

Inelastic dynamic analysis of multi-storey frames has established that relatively high column strengths are required if the ductility demand generated by seismic ground motions is to be spread throughout a frame [2 , 4 JHigh ductility demand concentrated in a few structural members may exhaust their ductility capacity and is undesirable in elements such as columns carrying high axial loads.These observations have led to the suggestion, given in the commentary to NZS 3101, the NZ Reinforced Concrete Design Code [l], that columns should be designed to resist seismic loads using a capacity design approach that takes into account the effects of concurrency, higher modes and beam flexural overstrength on the column strength requirements.This paper examines the possible affects of an additional factor that is not currently considered and suggests modifications to the current capacity design procedure for columns.To enable the factors affecting column strength requirements in multi-storey ductile frames to be evaluated, the concept of an 'equivalent condensed frame' with its 'Load' and 'Resistance' lines is developed.as only a static inelastic analysis approach is used, the conclusions and suggestions arrived at can only be considered as tentative until they are evaluated under inelastic dynamic loading conditions.The objective of the paper is to raise questions and, hopefully, to generate some discussion. Inelastic dynamic analysis of multi-storey frames has established that relatively high column strengths are required if the ductility demand generated by seismic ground motions is to be spread throughout a frame [2 , 4 J. High ductility demand concentrated in a few structural members may exhaust their ductility capacity and is undesirable in elements such as columns carrying high axial loads. These observations have led to the suggestion, given in the commentary to NZS 3101, the NZ Reinforced Concrete Design Code [l], that columns should be designed to resist seismic loads using a capacity design approach that takes into account the effects of concurrency, higher modes and beam flexural overstrength on the column strength requirements.

Objectives

Conclusion