Soil-Structure Interaction (SSI) as a complex phenomenon resulting in positive or negative effects on seismic behavior of structures has long been a focus for research. The general belief of its decreasing effect on structural forces has resulted in it being usually neglected in the design of ordinary buildings. This is while the soil-structure interaction effects change the dynamic responses and this alteration can have a decisive effect. Today, with the spread of the performance-based design concepts, accounting for soil-structure interaction has become increasingly more important. Soil-Structure Interaction (SSI) can alter the performance of structure totally including its dynamic characteristics, response maxima and more important, distribution of nonlinear response through structure the accurate calculation of which is vital for performance evaluation. Collapse prediction of special RC moment frames has been subject of many research works in the past. For instance, FEMA P695 proposes a methodology for implementing the same task, although without regard for soil structure-interaction. This study describes an application of FEMA P695 methodology for assessing collapse performance of reinforced concrete (RC) structures including the effect of soil-structure interaction. The collapse capacity of structure is determined using an incremental dynamic analysis with a suite of appropriate earthquake records and the fragility curves are derived for structural collapse. In the current study, the method of beam on nonlinear Winkler foundation is utilized. The soil model consists of spread nonlinear vertical springs along the foundation being able to include the rocking, subsidence, and uplift degrees of freedom and the radiation damping. In addition, Winkler foundation is made of horizontal springs to model lateral slip of the foundation as well as the passive soil pressure. Also in this study the collapse of structure envelopes dynamic instabilities including lateral deformations, P-? effects, and strength degradation of structural elements. In this research, special RC moment frames in 4,8,12 and 16 stories are designed for B,C and D type soils of ASCE 7-10 and then their fragility curves are produced using an incremental dynamic analysi procedure. Effect of a flexible base is included in the model with the help of a Winkler foundation. Results point out the increasing probability of failure for a structure on softening bases for the same spectral acceleration. Keywords: Collapse, soil-structure interaction, RC special frame, Winkler spring, incremental dynamic analysis
