Effects of Structure Height on Seismic Demands of Under-designed RC Frames Considering Soil-Structure Interaction

Abstract

Existing under-designed structures, which have been built due to the past insufficient constructional knowledge, are an important issue and anticipation of their dynamic responses to seismic events may be a cumbersome task. It is crucial to account for seismic demands of such structures for later retrofitting plans. In this research, three under-designed RC frames with different heights are considered to represent low-, mid- and high-rise structures. By performing non-linear response analyses, maximum seismic demands are calculated subject to five earthquake motions considering soil-structure interaction. The structures are designed for gravity loads and, especially for the high-rise, they lack about 30 percent rebar with respect to requirement for equivalent Special Moment-Resisting Frames. The major factors controlling the results are the input motion and soil conditions. The maximum inter-story drifts differ and critical stories shift upward or downward and may violate code-provided limits when the underlying soil state changes. Judgment can be made about the effects of loose and medium-dense underlying soils on structural responses. The critical sub-soils for low- and high-rise structures are medium-dense and loose sands, respectively. Subject to one single record, when the structure is high-rise, the maximum base shear is bigger with the base of the structure being flexible. For weak low- and mid-rise flexible-base structures, compared to the fixed-base state, the base shear is always smaller. The peak roof acceleration is generally greater than peak ground and bedrock accelerations, with exceptions in low-rise flexible-base structures.