Quantification of fundamental frequency drop for unreinforced masonry buildings from dynamic tests

Abstract

The knowledge of fundamental frequency and damping ratio of structures is of uppermost importance in earthquake engineering, especially to estimate the seismic demand. However, elastic and plastic frequency drops and damping variations make their estimation complex. This study quantifies and models the relative frequency drop affecting low-rise modern masonry buildings and discusses the damping variations based on two experimental data sets: Pseudo-dynamic tests at ELSA laboratory in the frame of the ESECMaSE project and in situ forced vibration tests by EMPA and EPFL. The relative structural frequency drop is shown to depend mainly on shaking amplitude, whereas the damping ratio variations could not be explained by the shaking amplitude only. Therefore, the absolute frequency value depends mostly on the frequency at low amplitude level, the amplitude of shaking and the construction material. The decrease in shape does not vary significantly with increasing damage. Hence, this study makes a link between structural dynamic properties, either under ambient vibrations or under strong motions, for low-rise modern masonry buildings. A value of 2/3 of the ambient vibration frequency is found to be relevant for the earthquake engineering assessment for this building type. However, the effect of soil-structure interaction that is shown to also affect these parameters has to be taken into account. Therefore, an analytical methodology is proposed to derive first the fixed-base frequency before using these results.

Numerical and experimental evaluation of site effects at ridges characterized by complex geological setting

“…The noise measurements showed that first vibration modes are in the range 4-6 Hz for both buildings. Even considering a small reduction (about 20 %) of the vibration frequencies during earthquakes because of the nonlinear behavior of masonry structures (Michel et al 2011), resonance phenomena must have occurred during 2009 earthquake, thus justifying the anomalous damage suffered by the two villages.…”

Data-Interpretation Methodologies for Non-Linear Earthquake Response Predictions of Damaged Structures

“…However, a well-known drawback of ambient vibration measurements is the very low amplitude of excitation, resulting in mostly linear responses. In addition, despite controversial results, modal properties derived from ambient vibrations have been shown in the past to be potential indicators of structural damage (Mucciarelli et al, 2004;Clinton et al, 2006;Michel et al, 2011). As a consequence, many proposals for structural identification based on vibrations measurements for damage detection have emerged (Chellini et al, 2010;Moaveni et al, 2010;Behmanesh et al, 2017).…”

Evaluating seismic retrofitting efficiency through ambient vibration tests and analytical models

“…Thus, frequency is representative of the linear elastic structural response only. Although the real frequency related to higher excitation amplitudes will be lower than the frequency characterizing the ambient vibration response, approximate relationships between the two frequencies are available [11]. Also, it is assumed that the retrofit will remain at least equally sound as the rest of the structure during an earthquake.…”