Variation of Dynamic Properties with Displacement in a 3-Story Reinforced Concrete Flat Plate Structure

Abstract eng:
A full-scale three-story reinforced concrete flat-plate building was subjected to a series of quasi-static load cycles. At various stages of the quasi-static test dynamic tests in the forms of free and forced vibrations, ambient vibrations and modal impact-hammer tests, were performed to collect low-amplitude dynamic response data from the building. This paper presents results from the structure after three different drift levels during the quasi-static test. These drift levels are: as-built/zero drift, 1.5% overall drift and 3.0% overall drift (damaged condition). Data on natural frequencies and damping ratios are provided and discussed. It is shown that at low displacement levels the structure exhibits non-linear elastic behavior regardless of its past displacement level. Consequently, natural frequency is a displacement-amplitude-dependent parameter in the range of low displacement amplitudes. It is recommended that comparison of natural frequency before and after an event be made using results from tests with same displacement amplitude. The equivalent viscous damping ratio was observed to remain nearly constant in the range of low displacements while being sensitive to past maximum displacement level. However, once the structure reached damaged condition, damping ratio became sensitive to the displacement level of the dynamic tests as well. More data are needed to conclude whether a robust health monitoring technique based on damping measured at small amplitudes could be suggested for reinforced concrete structures.

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