On the performance of inelastically responding asymmetric structures designed by the modal response spectrum method of NZS 4203:1992

A M Chandler,X N Duan

doi:10.5459/bnzsee.27.2.96-106

Abstract

The primary objective of this paper is to evaluate the effectiveness of the 3-dimensional modal response spectrum method (MRSM), as specified in the seismic design regulations of NZS 4203:1992, in accounting for torsional effects arising in stiffness-asymmetric buildings. Such buildings are assumed to be excited well into the inelastic range of response, where ultimate limit state design criteria are applicable. Also included is a study of the horizontal regularity conditions of NZS 4203:1992. This preliminary study focuses on the influence that these regularity conditions have on the selection of an appropriate codified approach for the design of torsionally asymmetric buildings. In particular, restrictions on the use of the equivalent static method of design (ESM), leading to a requirement to employ the MRSM to account for dynamic torsional effects, are discussed in some detail. The dynamic response studies indicate that in systems designed by the MRSM, no significant increase in ductility demand arises in flexible-edge elements (compared with symmetric systems). Furthermore, the MRSM may be over-conservative for the design of flexible-edge elements, in cases of structures having intermediate or large eccentricity. However, significant additional ductility demand may arise in the stiff-edge element in highly asymmetric buildings subjected to strong ground motion. In such cases, therefore, structures designed by the MRSM may not achieve the design aim of consistent protection given to symmetric and asymmetric systems against excessive inelastic response and consequent structural damage.

Highlights

AND OBJECTIVESIt is widely accepted that the two main objectives of earthquake-resistant design are to ensure safety of life and to protect property
The main objectives are (a) to examine analytically the horizontal regularity conditions of NZS 4203: 1992, and to discuss their logic in the context of providing restrictions on the use of the equivalent static method of design (ESM) to design for torsional effects, (b) to assess whether or not designs based on the modal response spectrum method (MRSM) provide reasonably uniform protection to resisting elements situated on the flexible and stiff sides of stiffness-asymmetric structures, and (c) to determine whether the MRSM provides consistent control over inelastic structural response for both symmetric and asymmetric systems
These include the influence of lateral and torsional frequencies, the importance of adequate design of vertical resisting elements on both sides of the centre of stiffness, accidental eccentricity effects due to a variety of causes, and inelastic structural response. The last of these effects has been found to have a significant influence on the torsional structural response to severe earthquakes, fundamentally altering the dynamic response behaviour compared with systems responding in the elastic range

Summary

Introduction

AND OBJECTIVESIt is widely accepted that the two main objectives of earthquake-resistant design are to ensure safety of life and to protect property. To satisfy the above criteria, seismic building codes include various forms of torsional design provisions It is evident when reviewing such provisions that current design practice for torsional effects, whether using the MRSM or the ESM, is based largely on linear (or equivalent linear) elastic theory. The main objectives (in the order in which they are considered in the paper) are (a) to examine analytically the horizontal regularity conditions of NZS 4203: 1992, and to discuss their logic in the context of providing restrictions on the use of the ESM to design for torsional effects, (b) to assess whether or not designs based on the MRSM provide reasonably uniform protection to resisting elements situated on the flexible and stiff sides of stiffness-asymmetric structures, and (c) to determine whether the MRSM provides consistent control over inelastic structural response (and damage) for both symmetric and asymmetric systems

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