Recovery of the resonance frequency of buildings following strong seismic deformation as a proxy for structural health

Earthq Engng Struct Dyn

2020

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The elastic properties of buildings change during earthquakes. In particular, fundamental frequencies are observed to shift rapidly during the co-seismic phase and to recover slowly once the strong motion has finished. Although the frequency shift is usually correlated with loading amplitude, the co-seismic frequency variations observed in real structures are not only determined by the absolute amplitude of the strain value. In order to interpret the uncertainties of the prediction of engineering demand parameters for a given intensity measure, we analyze the influence of loading rates (i.e., strain rates) on resonance frequency variations in buildings with different structural states during the loading and unloading phases caused by seismic events. Our observations suggest the existence of a strain rate threshold that activates the nonlinear response of the structure, characterizing the activation of cracking and indicating a strong nexus between the elastic structural response, structural state, and the loading process. K E Y W O R D S building response, earthquakes, frequency variations, strain rates 1 | INTRODUCTION Recent studies have shown that the elastic properties of different systems are significantly modified during strong seismic motions. Fundamental frequency shifts are observed in buildings 1-4 and seismic velocity changes in the Earth's crust. 5 These are interpreted by the authors as a co-seismic softening of the medium, linked to the activation of cracks. When loading ceases, the elastic properties begin to recover over time according to a slow dynamic process related to the closing of cracks; this has also been reported by Ostrovsky and Johnson 6 in rock samples. This recovery is dependent on the level of residual damage. 2 During the loading phase, the amplitude of shift is generally well correlated with the excitation amplitude, with stronger loadings causing larger shifts. For example, Astorga et al. 1,2 observed the maximum co-seismic shift of resonance frequencies in buildings to be proportional to maximum structural deformation and peak acceleration. In observations of the Earth, seismic velocity reductions due to earthquake-related processes are also proportional to the excitation amplitude. 7,8 Similarly, laboratory experiments in rocks and granular media have shown that modulus softening and successive recovery appear to be dependent on both effective loading and duration. 9-11 Johnson and Jia 10 observed the resonance frequency variation in rock samples as a function of strain, detecting a strain threshold (max. 10 −6) below which the material behaves in a linear-elastic manner. The resonance frequency shift is therefore dependent on strain for strain values above this threshold. Moreover, Guéguen et al. 12 showed significant nonlinear-elastic behavior in the relationship between frequency variation and strain in buildings under weak and

Section: Case 1: the Anx Buildingmentioning

confidence: 94%

Section: Case 1: the Anx Buildingmentioning

confidence: 99%

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Influence of seismic strain rates on the co‐ and post‐seismic response of civil engineering buildings

Earthq Engng Struct Dyn

2020

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“…Secondly, the ratio τ max / τ min computed from the relaxation function in Figure 3B 23 increased from ∼6 to ∼23 in the ANX building, and from ∼9 to ∼18 for THU. This ratio denotes the different time‐scale mechanisms that act in the time‐logarithmic segment of the recovery and characterizes the diversity of the crack sizes 14 (ie, preexistent and new ones). From this we can infer that after the 2011 event the variety of the cracks in the ANX and THU structures was quadrupled and doubled, respectively.…”

Section: Observations In Japanese Buildingsmentioning

confidence: 99%

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

2020

Engineering Reports

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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.

“…Over the past three decades, scientific progress has mainly come from numerical and analytical models, although some pioneering instrumental observations have confirmed the role and origins of these rotations in seismic damage patterns. As stated by Trifunac et al [ 12 ] and recently confirmed by Astorga et al [ 13 , 14 , 15 ], significant progress in the understanding of the dynamics of buildings and the physics of the processes activated during earthquakes can only be achieved by generalising experimental recordings in more instrumented structures, in spite of the development of increasingly sophisticated numerical models. With regard to rotation, Lee and Trifunac [ 16 ] noted that despite recurrent engineering studies that have shown the importance of rotations in the response of buildings, the development and deployment of specific strong motion instruments to record rotations have remained somewhat rare.…”

Section: Introductionmentioning

confidence: 99%

The Torsional Response of Civil Engineering Structures during Earthquake from an Observational Point of View

2021

Sensors

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This paper discusses the origins of torsion and its effect on the response of structures with a focus on the contribution of experimental data. The fact that torsion increases the stresses in structures, augmenting strain and damage during earthquakes, was confirmed in the 1960s. Over the years, the torsional response of structures has mainly been analysed through numerical studies, because few buildings are equipped with translational sensors, and even fewer are equipped with rotational sensors. This is likely to change as building instrumentation becomes more widespread and new generations of rotational sensors are developed. Therefore, this paper focusses on a number of scientific questions concerning the rotational response of structures during earthquakes and the contribution of experimental data to the understanding of this phenomenon.