SEISMIC PERFORMANCE OF DUAL STEEL STRUCTURES CONSISTING OF NON-GEOMETRICAL IRREGULARITY ALONG THE HEIGHT

The effects of irregularities along the height of structure on the basis of mass, stiffness, strength, and the combination of stiffness-strength are investigated in this study by means of dual steel structures consisting of special steel moment frame and special concentric braced frames. The Performance-based earthquake engineering has been implemented in the form of probabilistic confidence level by means of incremental dynamic analysis in order to obtain seismic demand and capacity. A set of ten-storeyed steel moment frames is taken into consideration with different irregularity cases along the height of structure. All considered structures are first carefully designed based on 4th edition of the Iranian practice for seismic-resistance design of buildings. Several modeling issues are taken into consideration including stiffness and strength drop due to cyclic nonlinear behaviour in beams and columns, post-buckling behaviour, and low-cycle fatigue in braces and gusset-plate behaviour in the brace connections. Generally, the irregularity decreases the seismic capacity when structure behaves in the nonlinear range. Irregularity in the form of weak story decreases ductility capacity when compared to regular structures: however, other irregularity increases the seismic demand in the irregular section of structure, especially in the simultaneous strength and stiffness irregular cases; consequently, increasing the possibility of limit state exceedance and decreasing the confidence level associated to a given limit state. It is worth noting that these results are strongly dependent on the irregular conditions. The combined stiffness and strength irregularity as well as mass irregularity, respectively, are associated with the maximum and minimum effects on the structural behaviour. The results show that the chance of meeting the immediate occupancy confidence level decreases by having irregularity, specially in the case of stiffness irregularity. However, the collapse prevention limit state confidence level is not affected much by the irregularity in all cases.