Many buried structures incorporate a layer of granular material supported by the roof of the structure. The weight of the gravel, combined with the self weight of the roof, may lead to a fundamental frequency of the roof structure in the frequency range of peak spectral acceleration in the input seismic ground motion resulting in large values of In-Structure Response Spectra (ISRS) which must be accommodated in the design of roof supported Systems, Structures, and Components (SSCs). Current code guidance and standard engineering practice for reinforced concrete structures is to use 4% damping when developing ISRS. For a concrete roof structure, this results in a SDOF Fourier amplification factors on the order of 1/(2x0.04)= 12.5, further exacerbating the challenge of designing roof supported SSCs. For the case of roof structures that support a significant thickness of gravel material, additional damping in the system may be mobilized through non-linear behaviour within the supported gravels and possible sliding behaviour at the gravel/concrete interface. These additional contributions to damping of the structural response may lead to significant reductions in seismic demand on SSC supported by the roof. This paper quantifies the structural damping appropriate for the concrete roof system supporting overlying gravel materials and examines the effect of the combined gravel-concrete roof damping on the response.
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