Seismic structural health from drop and recovery of resonance
frequencies in buildings
Abstract
Under proper loading conditions, micro-to-nanoscale heterogeneities
(i.e., 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 non-linear 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: over short times (i.e. seconds)
for a single earthquake; over intermediate times (i.e. months) for a
sequence of aftershocks; and over long times (i.e. years) for a series
of earthquakes. 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 bridge the gap between the laboratory and seismological
observation scales, verifying in this way the universality of recovery
processes, and demonstrating their value for the detection and
characterization of damage.