System factor (R) and ductility of eccentrically braced steel frames

At present the force-based procedures are generally preferred for seismic design of structures. In force-based procedures, design base shear is obtained by dividing the elastic base shear into the seismic response modification factor (R). In seismic codes, the numerical values assigned to R factors differ between 1 to 8. When the seismic codes and relevant publications are examined, it can be found out that these values were generally selected through the experience and judgement of the members of the engineering committees and observed performance of buildings during past earthquakes. Systematic investigations are necessary for the verification of existing assigned values. Due to this necessity, R factors of eccentrically braced frames are investigated with an original approach in which 80 earthquake records those are utilised during the inelastic analyses of multi-storey structures. Eccentrically braced frames are chosen fort he numerical evaluations since they are new compared to conventional structural system types and are not investigated deeply yet. Consequently, though the main purpose of this dissertation is to investigate the R factors, too many research topics such as the design optimization of eccentrically braced frames and the effects of frame geometry on global ductility are also studied thoroughly. 105 frames with shear yielding links, 105 with flexural yielding links and 105 frames with intermediate links are designed by the coded program for this purpose. The basic characteristics of the designed frames and the effects of the frame geometry on the frame weights are given. A computer program is coded in order to produce the input files for DRAIN-2DX, which is utilised for the inelastic analyses of the model frames. During the investigation of R factors, an iterative procedure is used to determine the scale factor of the earthquake record that causes one of the links of the concerned frame to reach its plastic rotation capacity. The important findings of the push-over analyses of the designed frames about the global ductility of frames are given. The comparison of the static and dynamic inelastic analyses of the designed frame are based on the distribution of the maximum link rotation along the height of the structure. The results and original contributions are summarized and recommendations for design are also provided.