Proposed lateral force distribution for inelastic seismic design of steel moment frames considering soil-structure interaction effects

The lateral force distribution patterns proposed in the current seismic design codes are typically based on the results of elastic response of fixed-base structures studies without considering soil–structure interaction (SSI) effects. Based on proposed code approach, structural elements are designed primarily based on equivalent static forces in which the shape of the fundamental mode of the structure is dominant to determine the height-wise distribution of seismic design static forces and, hence, the role of SSI effect is not included in the proposed distribution pattern. In this paper, lateral force distributions are obtained from nonlinear dynamic analyses of several steel moment-resistant frame structures (SMRFs) designed based on the performance-based plastic design (PBPD) approach. The PBPD method is based on two key performance objectives, namely the pre-selected target drift and yield mechanisms. These two design parameters control the degree and the distribution of structural damages directly. For the proposed approach, determination of design base shear, lateral force distribution and plastic design with respect to the performance object are the main three components of design. Results are presented considering soil-structure interaction (SSI) effects and are compared with those recently proposed by researchers for fixed-base systems. The lateral force distribution taking into consideration of inelastic behavior is developed by using the distribution of relative maximum story shear strength of the fixed- and flexible-based SMRFs subjected to a family of 20 strong earthquake ground motions recorded on alluvium soil. Based on the results obtained from the present study, a significant discrepancy is concluded between the suggested lateral force distributions proposed recently based on fixed-base analysis and obtained lateral force distributions for SSI systems. The discrepancy is regarded as inaccurate predictions of force demands, which may lead to unpredictable deformation and, therefore, would result in an undesirable yield mechanism and performance objectives.
Generally, for low and high levels of structural ductility demands the story shear values increase as the aspect ratio increases, except for the cases in which the soil and structure flexibility is dominant enough. It is deduced that as the slenderness of soil-structure system increases, the relative story shear strength considerably decreases under severs SSI effects. It is revealed that the relative story shear distributions obtained from fixed-base nonlinear dynamic analyses of short-period systems are independent of the applied seismic design load patterns. The trend is verified for lower ductility demand and under both slight and severe SSI effects. For longer period structures, however, the relative story shear strength response is dependent on the seismic design load pattern, which is more considerable in lower floors and especially for higher levels of inelasticity demand. It is worthy to note that the dependency of response to the design load pattern alleviates as the SSI effect increases. The results show that, mostly, by increasing the ductility demand the relative story shear increases for all fixed- and flexible-base cases. Of course for the very flexible systems no specific difference is observed in relative story shear distribution of systems at higher levels of ductility demand (i.e., µ= 4, 6).
In conclusion, the nonlinear dynamic analysis results of this study show that the proposed equations of PBPD method cannot effectively predict the distribution of maximum earthquake-induced story shear strength along the height of the structures when the soil beneath the structure becomes softer. Therefore, an alternative equation should be adopted for the ultimate limit state to incorporate the SSI effects such as the soil-to-stiffness ratio. Based on nonlinear regression analyses, new practical equations are proposed for flexible-base systems under slight, moderate and severe SSI effects. Based on the proposed equation for various ductility demands, more realistic design lateral force distributions are obtained accounting for the inelastic behavior of structures with SSI effects.