Structural health building response induced by earthquakes: Material softening and recovery

Abstract

Under proper loading conditions, micro-to-nanoscale heterogeneities (ie, the bond system) that are commonly found within the materials of a system can coalesce until causing macroscopic alterations of the system properties. The bond system is responsible for atypical and invariant-scale nonlinear elastic processes in granular media, from laboratory-tested materials (mm) to the Earth's crust (km). The unusual observed behavior involves slow recovery, or relaxation, of the elastic properties after dynamic loading. Several models have been designed to explain non-linear elasticity, although their physics is still partially unknown. Here, we show that recovery processes are also observed at intermediary scales (m) in civil engineering structures, and that they might be related to structural health due to the healing of cracks. For Japanese buildings subjected to earthquakes, we observe rapid co-seismic reductions of their resonance frequency, followed by fascinating recoveries over different time scales. For the first time, slow recovery is presented in buildings over short times (ie, seconds) for a single earthquake; over intermediate times (ie, months) for a sequence of aftershocks; and over long times (ie, years) for a series of earthquakes. This study focuses on the analysis of long-term recovery and recovery during aftershocks, in order to detect permanent damage. By comparing two buildings with different damage levels after the 2011 Tohoku earthquake, we show how relaxation models can characterize the level of cracking caused by damaging events. Our results demonstrate that nonlinear elasticity, combined with existing monitoring techniques, can be a powerful approach for damage detection and damage characterization. K E Y W O R D S bond system, building damage, earthquakes, recovery process, resonance frequencies, seismic structural health 1 INTRODUCTION Damage is understood as any change in the material of a system that negatively affects its current or future performance, meaning a loss of its optimal and original design. 1 All damage begins at the scale of the material, usually as a small This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

Variability in the Natural Frequencies of a Nine-Story Concrete Building from Seconds to Decades

“…Although a small permanent reduction in the natural frequencies of Caltech Hall has been observed for nondamaging historical earthquakes, the frequencies recover rapidly to their pre-event levels for the majority of events (Clinton et al, 2006). Such rapid dynamic softening (often termed anomalous nonlinear fast dynamics or ANFD) followed by slow recovery (often termed slow dynamics) has been widely documented in laboratory rock mechanics (tenCate et al, 2000;Johnson and Sutin, 2005) and recently reported for several structures (Kohler et al, 2005;Gueguen et al, 2016;Astorga et al, 2018;Astorga and Gueguen, 2020). Example spectrograms with a resolution of 12.8 s are plotted in Figure 4 for two earthquakes, showing that the apparent system frequencies of Caltech Hall drop suddenly at the onset of strong motion and then recover slowly following an approximately log-linear curve over the scale of minutes.…”