© 2021 Elsevier LtdDetermination of shear modulus along with damping ratio values in the field when shear strains exceeds those maximum strains obtained from laboratory tests has a crucial importance in the seismic design of structures whose interaction with soil is more pronounced. In this study, shear modulus of undisturbed samples, obtained from Adapazarı city center during drilling works conducted within the scope of the geotechnical investigation following the 1999 Kocaeli earthquake, were determined. Elasticity of undisturbed Adapazarı samples was measured using a cyclic triaxial test rig. The aim is to obtain relationships between the elasticity moduli from the stress–strain relations of specimens in dynamic and static loading setups, and the axial strains following termination of dynamic tests. Minimum and maximum shear moduli were determined for non-plastic soils and for soils with the plasticity index of PI = 40%. The elasticity modulus ratio (EN=20/EN=1) for samples with plasticity index of PI = 40% is higher than those with PI = 20% at the same level of deformation. As the plasticity increases at the same deformation level, both of the Eds/EN=1 and Eds/EN=1 ratios increase. Moreover, seismic performance of a reinforced concrete frame is evaluated for both fixed and soil-structure system. Seismic demand in soil-structure model with the frame and soil profile considered in this study is observed to be higher as compared to fixed-base frames.