Effect of soft ground story on the collapse possibility of the moment resistance frames under near and far-fault earthquakes

Soft story in the ground floor is established due to the open space for establishing shops, parking lots or even schools. The 2008 regulation suffices to only define the soft story. That is, when the lateral rigidity of each story is less than 70% of the lateral rigidity of the higher story or is less than 80% of the average lateral rigidity of the three stories above them, this story is called the soft story. The previous earthquakes such as those of Chichi in 1921, Koobeh in 1995, Bam in 2003 and Keshmir in 2005 had destructive effects on structures with soft stories and even caused them to collapse. Through conducting further studies on the destructive effects of the earthquakes near to and far from the fault on the regular and irregular structures, conducting precise analyses such as the IDA increasing dynamic analysis, and drawing fragility curves obtained from the statistical relationships, we can study the performance of the structures with soft stories which are of great significance such as schools, etc. so that we can better know the performance of these kinds of structures under the earthquakes near to and far from a fault to be able to strengthen these structures to be less vulnerable against the future earthquakes. In This paper aims to study the effect of near and far fault earthquakes on two series of nine-story steel bending frames, one with 3.2 meters and another with disordered in ground story with the height of 4.5 and 5.5 meters constituting soft story. The IDA increasing dynamic analysis is utilized for the estimation of the different performance level of buildings. It means that the structural models are modeled under one or more earthquake records with different intensities and one or several reply curve (s) is (are) produced due to the quake’s severity level. The severity of the records applied on the structure, which is increased during the analyses, and the analyses’ output, which is the structure’s reply, are presented by IM and DM parameters, respectively. The IDA curves are in fact the outline of the relationship between the structure’s reply (DM) and the severity change (IM) of the records. The selected parameters for IM and DM should well indicate the impact of the earthquake and the behavior of the structure. In this paper the parameter of IM corresponding to the maximum of inter-story drift and the parameter of DM corresponding to the spectral acceleration of the first mode (T1, 5%) Sa were considered. In addition, the collapse-preventing performance level of CP was analyzed. The results obtained from the fragility curves in the mode of CP indicate that with the height increase of soft story and consequently becoming softer, the collapse possibility of 10% of structure was greater in near fault records than the far fault ones. Also, the destruction probability of 10% of the regular and irregular structures under the records near to the fault is almost the same, whereas the destruction probability of the regular structure under the records far from the fault is averagely 15.35% more than other structures under the records near to and far from the fault, which indicates that the regular structure under the records far from the fault is more subject to destruction. Also through increasing the height of the soft story from 4.5 m to 5.5 m, the destruction probability of 10% under the records far from the fault compared to the regular structure, underwent a more reduction.