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