Although the importance and the positive effects of structural redundancy havebeen long recognized, structural redundancy became the focus of research only after the1994 Northridge and 1995 Kobe earthquakes. Several researchers have investigated thebenefit of redundancy to structural system. However, the definition and interpretation ofstructural redundancy vary significantly and it remains a controversial subject.A reliability/redundancy factor, p, was introduced in NEHRP 97, UBC 1997, andIBC 2000. It is used as a multiplier of the lateral design earthquake load and takes intoaccount only the floor area and maximum element-story shear ratio. It lacks an adequaterationale and can lead to poor structural designs (e.g. Searer G. R. and Freeman S. A.,2002, Wen and Song, 2003). A new reliability/redundancy factor, primary a function ofplan configuration of the structures such as the number of moment frames in the directionof earthquake excitations, has been adopted in NEHRP 2003 and also proposed in ASCE-7. This new factor attempts a more reasonable and mechanism-based approach, and it islikely to be implemented in other codes in the near future. However, the uniformmultiplied factor (1.3) of lateral design force for non-redundancy structures fails toaccount for different structural configurations and could lead to serious damage in apoorly designed structure. In view of the complicated nonlinear structural behaviors andthe effects of uncertainty in demand and capacity, redundancies of structures underseismic loads can be measured meaningfully only in terms of reliability of a given system.Therefore, a systematic and probabilistic study of redundancy in structural system isneeded and a uniform-risk redundancy factor is used for reliability assessment ofstructural redundancy.To accurately describe the inelastic connection behaviors, the Bouc-W en model isused and incorporated into the ABAQUS computer program. A 3-D finite element modelis developed, which allows one to examine the effects of 3-D motions including torsionoscillation and biaxial bending interaction. The capacity uncertainties of connections thatwere documented in the FEMAISAC projects are included in the Bouc-Wen model andused in the reliability analysis.Finally, a framework is proposed for evaluation of structural redundancy against·incipient collapse limit state. In this framework: (1) the maximum column drift ratio(MCDR) or biaxial spectral acceleration (BSA) is used to measure both demand andcapacity of a given building; (2) the demand and capacity analyses of a building areperformed, from which the probabilistic demand curves and the distribution of capacityare constructed. The demand of a building is determined by conducting a series of timehistory analyses under a given probability level. The capacity of a building againstincipient collapse is determined by performing the Incremental Dynamic Analyses (IDA);(3) both aleatory and epistemic uncertainties in demand and capacity are taken intoaccount; (4) based on the results of (2), a uniform-risk redundancy factor, R, for designto achieve a uniform reliability level for buildings of different redundancies is obtained.This method is also used to evaluate the redundancy of a given structural system. The pfactors in NEHRP 97 and in NEHRP 2003 and the proposed RR factor are compared andthe inadequacies of p factors are pointed out.
展开▼
