The effect of earthquake aftershocks on the steel buildings with irregular plan

Earthquake sequences occur at many regions around the world where complex fault systems exist. These fault systems usually do not relieve all accumulated strains at once when the first rupture takes place. Therefore, high stresses formed at different locations causing sequential ruptures until the fault system is completely stabilized. The sequential ruptures along the fault segment(s) lead to multiple earthquakes which are often hard to distinguish them as fore, main and after-shocks, or a sequence of earthquakes from proximate fault segments. The most recent one sequences occurred are two foreshocks of magnitude 6.2 and 6 followed by a magnitude 7 main shock between 14 and 15 April 2016 caused severe damage and injuries. The event of two earthquakes by magnitude 2.6 and 6, which rocked the cities of Ahar, Varzaghan and Harris in East Azerbaijan in Iran in August 2012, is also a seismic sequence. Field investigations reported failure of structural systems under earthquake sequences, especially where structural retrofitting was not provided due to the short time between the earthquake sequences. In most failure cases the reported damage is mainly due to loss of stiffness and strength of structural elements as a result of material deterioration under earthquake sequences loadings. Buildings may have different configurations depending on the construction location and have different plan dimensions that would lead to irregularity in their planning. Limited research has addressed the seismic behavior of structures subjected to earthquake sequences especially irregular structures. This study investigates the effect of earthquake aftershocks on the steel buildings with irregularity in plan. For this purpose, we studied on structures of 8, 12 and 20 number of stories with special steel frame system under the earthquake sequences. Each of these structures consists of three cases: regular, irregular 1, and irregular 2 models that were designed in accordance to Iranian codes by SAP 2000 software. Geometric irregularities in the plan of the structure created in accordance to Iran's seismic code, a recess should be created in proportion to more than 2% of the total length of the building. In this paper, first irregularity has a recess by 5 meters (one span) and the second irregularity with a recess by 10 meters (two spans), which is 25% and 50% of the length of the building, respectively. The spectral dynamical analysis method has been used to design the structures. For nonlinear analysis, we use Perform-3D software, In Perform-3D, a frame element is used to model beams and columns. Then these structures were evaluated by nonlinear dynamic analysis under actual earthquake sequences. Six sequential earthquake categories that have a major earthquake spacing with corresponding aftershock occurred in less than a week in the area were selected from the PEER (Earthquake Strength Database). Finally, after discussion on parameters such as roof displacement, residual inter story drift and maximum inter story drift, it is observed that aftershock leads to increasing the response of the structures and as a result failure under aftershock. By increasing the irregularity in the plan, the instability of the structure increases under aftershock, which may cause structural failure.