Soil-structure interaction effects on demand and probabilistic confidence level of geometric vertically irregular steel buildings

Authors
H. sh 1
F. Homaei 2
1 1
2 Tarbiat Modares University
Abstract
In this paper, the effect of soil-structure interaction is investigated on the maximum drift demand and probabilistic seismic confidence level of geometric vertically irregular steel buildings. A series of vertically irregular steel building with different setback ratios were designed based on the regulations in the earliest version of Iranian seismic design code. Foundation design of the structures was accomplished with the assumption of sandy soil with shear wave velocity of 200m/s under the strip footings. Three dimensional model of nonlinear soil-structure system was built in OpenSees. Concentrated plastic hinges were used at the end of frame members to model the nonlinear behavior of these elements. Soil-foundation system of the structures was modeled with Beam on Nonlinear Winkler Foundation (BNWF) approach. In this method, a series of nonlinear springs are used to model the soil behavior under the dynamic excitation. The seismic analysis of the structures was performed under the simultaneous action of orthogonal components of real ground motions. An ensemble of twenty ground motions that have a reasonable response spectrum matching with the design spectrum was chosen for the time history analysis. Incremental dynamic analysis (IDA) was accomplished to estimate the structural performance of the regular and vertically irregular setback buildings from the earlier linear behavior to the nonlinear phase and up to the global instability of the structures. Based on the results, the median IDA curve of fixed and flexible base buildings was evaluated. Four common performance objectives namely Immediate Occupancy (IO), Life Safety (LS), Collapse prevention (CP) and Global Instability (GI) were specified on the median IDA curve of each fixed and flexible base structure. Following the performance-based earthquake engineering framework, the confidence level of meeting a specific performance level was evaluated at each limit state. Based on the given results, curves were generated to specify the confidence level of meeting a specific performance level for a range of earthquake intensities and corresponding maximum inter story drift ratio. The performance based confidence level of flexible base setback buildings was compared to that of the fixed base structures at five seismic hazard levels. The seismic hazard of earthquakes was chosen to cover a wide range of return period from 25 years to 4975 years. It is observed that all the fixed and flexible base buildings have the ability to continue their immediate occupancy with the confidence level of 100% under the excitation of earthquakes with low to medium hazard levels (i.e. with the return period of 25 to 43 years that know as the service level). However, as the level of seismic hazard increases the difference between the confidence level of flexible base structure and the fixed base ones increases. Depend on the position and ratio of the setback, 40 to 60% of reduction is observed in the performance based confidence level of flexible base structures. Meanwhile, demand evaluation of structures at each hazard level shows that soil-structure interaction increases the maximum drift demand in structures. Based on the given results, it is observed that up to 35% increase of maximum drift happens in vertically irregular structures with flexible foundation.

Keywords

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Modares Civil Engineering journal
Volume 16, Issue 5
November and December 2016
Pages 215-229