Almost all of the existing seismic design codes suggest a so-called "response modification factor" for taking into account the plastic behavior of the structure. This factor appears in the calculation of the building total seismic shear force rather than the lateral load distribution calculations, which is believed to be more dependent on the plastic behavior of the system in its various story levels. To find out how far the actual distribution of lateral loads is from the assumed patterns of the design codes some sets of concentrically braced steel buildings as the most common type of moderately high rise steel buildings, having up to 18 stories and up to 5 bays, have been analyzed by a nonlinear time history analysis (NLTHA) program. The maximum values of lateral loads experienced by the building in each case have been compared with the lateral load pattern of the code, used for the design of the building. Great differences are observed between these two patterns in all cases. Then, buildings have been redesigned by the average load pattern obtained in the previous stage, and the NLTHA has been repeated. This time a good agreement is observed between the actual load distribution and the average pattern used for the redesigning of buildings. By calculating the ratio of the values given by this average load pattern to those values given by the suggested linear pattern of the code a somehow new concept of "story-dependent response modification factor" can be defined. Results show that this ratio is more than unity in lower and higher stories of the building and less than unity in the intermediate stories.
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