Comparison of Structural Weight in Regular Moment Frame and Dual System Structures in low to Medium Rise Buildings, Designed Based on Iranian Standards

Selection of structural systems among the allowed ones of ISIR2800 is very important in designing phase and may affect the building costs. It is worth noting that designing of buildings has two phases; in the first one element sections are selected based on the working loads, including the seismic ones. In the second phase, the obtained structure should be controlled for the story drifts. In this phase the story drift should be less than 0.02 and 0.025 for the structures with natural period of vibration more and less than 0.7 second, respectively. The story drift is calculated by the reduction factor multiplied by 70 percent of the calculated drift in linear modeling of the structure for the seismic load (by considering pdelta effects). Therefore, despite the first phase, higher reduction factors intensify the second phase, in such a way that a lot of structural elements are practically determined by the drift controlling criteria. In low rise buildings, structural element sections are normally determined by the working loads and the drift criteria is passed automatically. However in medium to high rise buildings, the drift controlling criteria is dominated and most element sections are determined by these criteria. This research studies and compares costs of four structural systems, with different reduction factors, among ISIR2800’s accepted systems, including ordinary, intermediate and special moment frames as well as dual systems for regular steel buildings (with 4, 6 and 8 stories and 3 and 4 bays in the plan). In this study combination of intermediate moment resisting frame and concentric bracing is assumed for the dual system. All buildings are designed based on ISIR2800 and Iranian national building regulations (No. 10). The structural weight is calculated for each structural system as a representative of the structural cost, regarding that the cost is a linear function of the structural weight. The obtained results show that dual systems are more economical specially for higher buildings. In the assumed 4 story building the working loads determines the element's sections, therefore, the system with less reduction factor leads to smaller sections for beams. However the column dimensions are determined by compact section criteria. In the assumed 6 to 8 story building with moment frame systems, the sections are mostly determined by drift controlling criteria. In contrast, in similar building with dual system, the obtained sections for service loads could pass the drift criteria automatically and that's why it is more economical than the others.