عنوان مقاله [English]
Structural design of building frames depends on the exerted loading. The simplified lateral loading in seismic design codes is, in fact, height-wise distribution of base-shear and depends on the designed section profiles for structural members. The present study seeks for the suitable, meanwhile, smooth and simplified distribution of base-shear for practical seismic design and compares it with the current code practice. The problem is, thus, two fold. First, a lateral load pattern is sampled using linear combination of a few simplified patterns and their corresponding scale factors, second, the optimal sizing design under such a generated loading is determined. An integrated optimization platform is presented, including Harmony Search, for the first part, dealing with both continuous and discrete variables and Ant Colony Optimization, with further local search for the second part to do the discrete sizing design of the building frame. Consequently, a penalized objective function is developed to consider not only code-based steel design and drift constraints but also remunerate pioneering plastic-hinge formation to occur at upper levels of the frame. Using the proposed algorithm, examples of steel moment frames with different width and height are treated. New smooth patterns of base-shear distribution are obtained, including a uniform part at mid-height with a linear increase near the roof and a linear decrease to zero at the base level. Optimal sizing designs under these patterns and the code-based pattern were then provided to be further compared.It was found that the new designs are closer to the structural constraints, including stress and displacements limits. In another words the code-based pattern resulted in non-economic over-designed structures. The achieved designs are verified employing several static, spectral and time-history analyses. Considering the story displacement and drift response, the corresponding spectral and static results under new-optimal patterns are found to be compatible. Standard deviations of the inter-story drift response for new-pattern designs were generally less than the code-based designed models. Such drift uniformity among the frame stories is desired in seismic design as it leads the entire structure to uniformly participate in resisting earthquake effects. The most critical stress points as the plastic-hinge starting locations, occurred in upper stories of the designed models under optimal patterns with respect to the code-based practice. The new smoothed patterns of base-shear distribution stand superior to the current codified pattern, regarding the abovementioned points especially to prevent progressive collapse, Thus, they may be considered as safer seismic designs than the current practice.