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Abstract
Braced steel frames have been proved to be cost-efficient lateral load resistance systems for multi-story steel buildings subjected to wind or earthquakes. They are capable of providing sufficient strength and stiffuess in order to minimize the deformations of the multi-story steel buildings.
The present study deals with the seismic performance of the braced steel frames designed according to the Egyptian code. The evaluation of the seismic performance of newly designed buildings in Egypt is required in order to determine the seismic level of protection afforded to these buildings by the new design provisions which have been introduced to the code as an implication of the 1992 Cairo earthquake.
The ductility characteristics can be considered as one of the main factors that affect the seismic performance of the building structures. To evaluate the ductility characteristics, a six story braced steel frame has been designed according to the Egyptian code using both concentric and eccentric bracing. Three different types of concentric brace configurations as well as one eccentric brace configuration were considered in this study. The concentric brace cases include regular X, split-X and inverted-V configurations. An inverted-V configuration was used as an eccentric brace.
nonlinear dynamic analysis of building structures. The brace elements were modeled using a buckling element which is capable of representing the post-buckling behavior of brace elements. The shear links were modeled using link elements that are capable of representing the shear and flexural behavior of shear links. The ductility characteristics of the frame have been evaluated in terms of the global deformations (represented by the roof drifts, story drifts), and the local deformations of frame elements (represented by axial displacements in the brace elements and the deformation angles of shear link elements). The results indicated that the eccentric bracing exhibits the best deformability among 811 the bracing cases. Also, it was found that some ductility requirements have to be considered in the design to insure satisfactory performance under the effect of lateral loading.
Earthquake analysis of six and eighteen story braced steel frames (representing low-rise and high-rise buildings) was performed to evaluate the seismic performance of such building.
The frames have been designed according to the Egyptian code using both concentric and eccentric bracing taking into account the ductility requirements. The DRAIN-2DX computer program for inelastic dynamic response analysis was utilized. The seismic excitation was represented by scaled versions of ten ground motion records selected to cover a wide range of frequency content and duration. The seismic performance parameters include the global and local deformations.
The relationships between the (mean + standard deviation) levels of the frame performance parameters and the peak ground acceleration level (PGA) of the earthquake were obtained. The results of the dynamic analysis indicated the high efficiency of the eccentric bracing. Also, it was found that the mean and the (mean + standard deviation) of the performance parameters differ significantly which indicated that these parameters are dependent on the characteristics of the ground motions, such as, the frequency contents and duration of earthquakes. Moreover, the (mean + standard deviation) levels of the frame performance parameters at the design PGA level were determined. It was found that the braced steel frames designed according to the current Egyptian code requirements are expected to achieve satisfactory performance if some ductility requirements have been taken into consideration.