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Structural Research Series 636PDF


Title:Redundancy in Steel Moment Frame Systems Under Seismic Excitations
Author(s):Liao, K-W.
Contributor(s):Wen, Y-K.
Subject(s):Reliablity/redundancy factor
Earthquake motions
Earthquake engineering
Structural redundancy
Abstract:Although the importance and the positive effects of structural redundancy have been long recognized, structural redundancy became the focus of research only after the 1994 Northridge and 1995 Kobe earthquakes. Several researchers have investigated the benefit of redundancy to structural system. However, the definition and interpretation of structural redundancy vary significantly and it remains a controversial subject. A reliability/redundancy factor, p, was introduced in NEHRP 97, UBC 1997, and IBC 2000. It is used as a multiplier of the lateral design earthquake load and takes into account only the floor area and maximum element-story shear ratio. It lacks an adequate rationale 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 of plan configuration of the structures such as the number of moment frames in the direction of 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 is likely to be implemented in other codes in the near future. However, the uniform multiplied factor (1.3) of lateral design force for non-redundancy structures fails to account for different structural configurations and could lead to serious damage in a poorly designed structure. In view of the complicated nonlinear structural behaviors and the effects of uncertainty in demand and capacity, redundancies of structures under seismic 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 is needed and a uniform-risk redundancy factor is used for reliability assessment of structural redundancy.To accurately describe the inelastic connection behaviors, the Bouc-W en model is used and incorporated into the ABAQUS computer program. A 3-D finite element model is developed, which allows one to examine the effects of 3-D motions including torsion oscillation and biaxial bending interaction. The capacity uncertainties of connections that were documented in the FEMAISAC projects are included in the Bouc-Wen model and used 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 and capacity of a given building; (2) the demand and capacity analyses of a building are performed, from which the probabilistic demand curves and the distribution of capacity are constructed. The demand of a building is determined by conducting a series of time history analyses under a given probability level. The capacity of a building against incipient collapse is determined by performing the Incremental Dynamic Analyses (IDA); (3) both aleatory and epistemic uncertainties in demand and capacity are taken into account; (4) based on the results of (2), a uniform-risk redundancy factor, R, for design to 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 p factors in NEHRP 97 and in NEHRP 2003 and the proposed RR factor are compared and the inadequacies of p factors are pointed out.
Issue Date:2004-08
Publisher:University of Illinois Engineering Experiment Station. College of Engineering. University of Illinois at Urbana-Champaign.
Series/Report:Civil Engineering Studies SRS-636
Genre:Technical Report
Date Available in IDEALS:2009-11-17
Identifier in Online Catalog:5037543

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