Multi-scale variability in the apparent stiffness of a nine-story reinforced concrete building

I am also interested in engineering and urban seismology. In particular, environmental phenomena like rainfall can have a non-negligible impact on both site and structural response, which has generally not been quantified. I recently examined a 20-year continuous record of structural vibrations at CI.MIK, a permanent strong-motion station installed on the 9th floor of a reinforced concrete building in Pasadena, CA, in 2001. On short time-scales the building exhibits complex nonlinear elasticity: (1) during strong shaking the apparent stiffness of the soil-structure system decreases in proportion to the peak acceleration, and (2) following shaking the apparent stiffness recovers over the scale of minutes with a log-linear time dependence. Over longer time-scales, the building appears to be gradually healing, with a >10% increase in apparent structural stiffness over the past 20 years. The building also exhibits >10% variability in stiffness on diurnal to seasonal scales in response to temperature changes, wind, rainfall, and soil moisture. While the short-time behavior is similar to trends observed in laboratory rock physics experiments and the environmental sensitivities can largely be explained in terms of poroelastic and thermoelastic effects, the long-term healing trend is a mystery. If such dramatic time-dependent behavior is common in reinforced concrete structures, then current standards for, e.g., post-earthquake damage assessment and design-stage fragility analysis are grossly inadequate and significantly underestimate uncertainty.

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