رفتار غیرخطی و سه بُعدی پویای ساختار تیرچه های فولادی

As the trend towards developing performance-based design specifications for the seismic design of structures gains momentum, it is clear that very little is known about the performance of light industrial structures under large lateral loads. Among the main outstanding issues related to the seismic design of these structures are (1) the determination of appropriate response modification factors .R; Cd and 0/, (2) the establishment of drift limits to avoid damage of structural and nonstructural components, and (3) clarification of the role that the roof diaphragm plays on the seismic behavior of light-weight roof structures. This study attempts to elucidate some of those issues for a particular class of light-weight industrial structures, those composed of one-story, weak columnstrong beam joist girder frames. Two types of analysis models were developed for the nonlinear dynamic analyses of these structures. The first is a simplified 2-D analysis model, using SAP2000 and the second is a complex 3-D analysis model, using ABAQUS. Nonlinear time history analyses were performed for sites in Los Angeles (CA), Boston (MA), and Memphis (TN). The accuracy of the simplified 2-D model was verified by comparison with the results from the 3-D model. The results indicate that the behavior of these structures is almost always in the elastic range, and that substantial roof bracing should be installed for this type of structure, to prevent excessive drifts in the weak direction. When two horizontal components of excitations were applied concurrently to check the effect of torsion of the frame, it was found that torsional effects were negligible for structures regular in plan, and that a 2-D model can provide reasonable analysis results. Column base fixity effects on the dynamic behavior were also investigated and it was determined that column base fixity should be considered, to obtain more accurate dynamic behavior of the steel joist girder structures.

Joist girder frame structures consist of repetitive, open, tall, one-story frames with or without additional bracing along the perimeter (Fig. 1). These structures are inherently very flexible, and in the past their design has controlled by drift criteria for wind. Their performance during past earthquakes has been satisfactory, with damage limited to brittle façade elements and poorly detailed column bases [1]. This is in spite of the fact that no specific seismic design guidelines exist for these structures, and that these structures are generally weak column strong beam systems. With the advent of seismic performance-based design (PBD), there is a need to evaluate the performance of these structures under a wide range of seismic loads, in order to provide rational design guidelines

Currently, there are no established seismic design guidelines for the steel joist girder structures, because the dynamic characteristics of these structures have not been sufficiently investigated. Much experimental and analytical work is needed to determine reasonable R values for these structures. For the particular case of industrial structures with moment frames incorporating joist trusses, this study concludes that: The results of the analyses argue for an elastic design, as from the results of the nonlinear analyses there is little or no inelastic behavior under low or moderate seismic excitations.