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The present study is aimed at determining the effect of variability in soil type and structure height on soil-structure system responses. In order to explore this innovative idea, a wide range of soil types and structure heights are assumed, and the responses are analyzed with respect to changes in soil type and structure height. It is intended to address the three-fold problem of determining under what circumstances soil-structure interaction (SSI) exerts significant effects on the results, on what condition it could be ignored, and whether taking account of SSI leads to conservative results. To verify the numerical models, the results are compared with those derived from the NEHRP method. Five planar concrete frames are examined under the action of thirty earthquake records. Seven types of soil are considered. For modeling the soil-structure system, the direct method is employed. The plastic behavior of the soil is formulated based on the multi-surface (nested surfaces) concept; the yield surfaces of sand are of the Drucker-Prager type, and those of clay are of the Von Mises type. The exact Incremental Dynamic Analysis (IDA) is adopted for analyzing the soil-structure system. The results indicate that, for sandy soils, the SSI impact is greater for looser sub-soils. For clayey soils, the medium clay has the most powerful SSI effect. For low and medium rise buildings, the SSI effect is more profound as the structure becomes taller, but for high rise buildings, the SSI effect wears off with increasing height.

Viscoelastic (VE) dampers can be efficiently used for improving the seismic behavior of structures and also retrofitting of existing structures. Efficient nonlinear dynamic analysis methods like the Incremental Dynamic Analysis (IDA), especially the one which is based on Modal Pushover Analysis (MPA), which has proved especially popular among researchers, are required for analyzing the structures with dampers. As the IDA method is too complicated and time consuming, the MPA-based IDA method was introduced, a method which is fast and cost effective and the results are accurate enough to be utilized. Adoption of a static analysis in the procedure of MPA-based IDA method encourages the speculation that this method cannot be used for structures with dampers. This misconception has led to the fact that no one has ever used this method for structures with additional damping. In the present article, it is intended to present an approach to promote the applicability of the MPA-based IDA method for structures with VE dampers. The proposed approach is verified by the exact IDA method and validated by a numerical example. The results indicate the sufficient accuracy of the proposed technique, in which the MPA-based IDA is employed for structures with VE dampers.