Assessment of the Effects of Torsion in Steel Structures with Non-ParallelLateral Resisting System with an Asymmetric Plan

Since Iran is located in a seismic region and due to the existence of trapezoidal or triangular terrains, because cities were not constructed based on a grid format in the past, in addition to the interest of contemporary architecture in designing irregular structures and structures with non-parallel lateral resisting system as defined in the 4thedition of the Standard 2800, thus this paper studies the seismic behavior of such structures. In this paper, three-dimensional modelling of steel structures with five different plans, each representing a percentage of irregularities of the lateral resisting system and three-dimensional modelling of a completely regular plan is presented for comparison. Besides, the torsional behavior of structures with lateral resisting systems is studied and compared with regular structures. The letter alpha is used with indexes indicating the level of anomaly of the carrier systems. The value of the alpha index is obtained by dividing the length of the removed openings by the length of the entire structure. In order to investigate the effect of height, the structures were modelled and studied using two, four and twelve floor models. The modelled structures are made of steel and possess a specific steel converging bracing system in line with Y and a modular steel folding frame along X. Moreover, the soil on the land is of type 2 in accordance with the 4thedition of the Standard 2800, and the structures are in accordance with articles six and ten of National Building Regulations. To analyze the structures, pushover analysis and dynamic linear analysis were implemented. In the pushover analysis, a load was analyzed in line with X and another load was analyzed in line with Y. Furthermore, for dynamic linear analysis, the peak acceleration recorded for Tabas, Iran and El Centro, USA earthquakes were used. The results obtained from this investigation and comparisons with regular structures suggests that in structures with non-parallel lateral resisting system, the highest torsion is related to the floor that has a distance of 60% to 70% from the base level. Besides, it was evident that during loading for the non-parallel lateral resisting system in line with Y, the torsion for longer order structures is up to 56 times, and for shorter order structures up to 8 times more than that of more parallel structures (structure model α0.2). Moreover, this ratio for loading on the lateral resisting system (X) was 6 and 8 times, respectively. Thus, it is noteworthy that for these types of structures, loading in (Y) direction has more disastrous results compared to other types of loading. The results obtained from torsion in elastic mode for pushover analysis in Y direction were different compared to other types. For pushover analysis in X direction, the elastic and non-elastic behavior of structures was not significantly different and could be neglected. The results obtained from dynamic linear chronological analysis were such that for models α0.8 and α1, loading for both directions experienced sudden aperture rotation compared to other structures. For structures of different floors and identical plan, the aperture of maximum twists can be reduced by increasing structure height. In short order structures, the torsion of irregular structures is up to 18 times more than the torsion of structure model α0.2. This torsion for high order structures is up to five times more than that of structure model α0.2. Increasing the height reduces the torsion. Besides, increasing height reduces the torsion ratio of irregular structures to more regular structures. The final point is that loading in a state of non-parallel lateral resisting system (Y) will entail more unpredictable and disastrous results.