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Damping Evaluation Studies for Structural Systems


Structural systems (Reinforced Concrete Frame, Steel Frame) under dynamic forces such as wind, earthquake, blast, impact, etc. suffer varying degrees of damage leading to economic and human loss. Protection of such structural systems is of prime importance and is an active research area. Structural systems represented by their mass, stiffness and damping properties are crucial parameters to determine their dynamic response. Damping parameter related to energy dissipation plays a vital role in reducing the dynamic response of the system under dynamic forces but its estimation is a non-trivial task due to the co-existence of many dissipation mechanisms in the system. Therefore, for better estimation of the dynamic response of structural systems, the damping parameter requires a non-linear model against the widely adopted linear viscous model. Various non-linear models for damping estimation are developed but were tested under the harmonic periodic loadings only, and their assessment under transient loading is a research gap.

This research gap was one of the objectives for the research done by Mr Tushar Bhoraniya, PhD student at Nirma University, under the mentorship of Dr Sharadkumar Purohit, Professor, Civil Engineering Department, Nirma University. He studied the dynamic response of the structural system with linear and non-linear damping models under harmonic as well as transient loading. A First Cycle Damping Coefficient (FCDC) approach is proposed to determine damping co-efficient for the structural system under the transient loading and its efficacy is established through experimental validation.

Most of the existing structural systems have red clay brick as infill material, but newly constructed systems utilise Fly Ash Brick (FAB) or Autoclaved Aerated Concrete (AAC) blocks nowadays. These newly practised infill materials are likely to alter the dynamic response of the structural system in comparison to red clay brick infill material, and thus the behaviour of such infill materials with structural system subjected to lateral load becomes important. Apart experimental investigations on structural system infilled with FAB and/or AAC block under lateral loading are scantily available in the literature.

Thus, the second objective of the research is to study the dynamic behaviour of the structural system through quasi-static testing. Single storey single-bay Moment Resisting RC Frame (MRF) test specimens cast monolithically and built non-integrally with FAB and AAC infills were tested for quasi-static testing under lateral loading by Mr Tushar Bhoraniya at Heavy Structures Laboratory (HSL) of Civil Engineering Department. Normalised response parameters quantifying post-yield behaviour of masonry infilled RC frame test specimens were evaluated. A free vibration test on each RC frame test specimen rarely performed constitutes the third objective of the research. The test was conducted after each cycle of lateral loading to estimate the damping ratio. Dr Purohit says that masonry infills (FAB and AAC) increases the lateral load-carrying capacity of the MR frame when compared to the bare MR frame. It improves its energy dissipation capacity and is evident from the experimental investigations.

A case study of three G+5 storey RC buildings with and without masonry infills designated as; BMRF, MRFFAB and MRFAAC is considered to study their dynamic behaviour under strong motion and pulse-type seismic excitations. RC buildings are modelled as shear buildings with discrete degrees of freedom. Post-yield damage modelling cases for both bare and masonry infilled RC building are derived by modifying stiffness and damping properties based on post-yield damage observed at different stages of lateral loading and lateral displacement during quasi-static testing. Non-linear time history analysis was performed to evaluate the seismic response of RC buildings with post-yield damage modelling cases, the fourth objective of the research work. Dr Purohit observed that the post-yield damage modelling cases yield higher seismic response for masonry infilled RC buildings vis-à-vis bare RC building. Out of the various post-yield seismic parameters considered for the masonry infilled RC buildings, it’s found that the stiffness degradation of the building heavily affects its seismic response for the present case study.

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