Ambient and Forced Vibration Testing of a Reinforced Concrete Building before and after Its Seismic Retrofitting

Summary A full‐scale five‐story reinforced concrete building was built and tested on the NEES‐UCSD shake table during the period from May 2011 to May 2012. The purpose of this test program was to study the response of the structure and nonstructural components and systems (NCSs) and their dynamic interaction during seismic base excitation of different intensities. The building specimen was tested first under a base‐isolated condition and then under a fixed‐based condition. As the building was being erected, an accelerometer array was deployed on the specimen to study the evolution of its modal parameters during the construction process and placement of major NCSs. A sequence of dynamic tests, including daily ambient vibration, shock (free vibration) and forced vibration tests (low‐amplitude white noise and seismic base excitations), were performed on the building at different stages of construction. Different state‐of‐the‐art system identification methods, including three output‐only and two input‐output methods, were used to estimate the modal properties of the building. The obtained results allow to investigate in detail the effects of the construction process and NCSs on the dynamic parameters of this building system and to compare the modal properties obtained from different methods, as well as the performance of these methods. Copyright © 2015 John Wiley & Sons, Ltd.

Section: Introductionmentioning

confidence: 99%

Influence of the construction process and nonstructural components on the modal properties of a five‐story building

Astroza

Ebrahimian

Conte

et al. 2016

“…Identification of dynamic characteristics of civil structures from response recorded during strong ground shaking has been a subject of research for more than three decades [1][2][3][4]. System identification techniques are not only useful for condition assessment, but also for improving future designs, validating predictive models, and verifying retrofit procedures [5]. Active and semi-active control systems that are now being frequently employed in important structures throughout the world also make use of system identification techniques, which provide intelligent feedback from the structure so that the active and passive (damping) forces supplied to the system can be properly controlled [6].…”

Section: Introductionmentioning

confidence: 99%

Response‐only modal identification of structures using strong motion data

Ghahari

Abazarsa

Ghannad

et al. 2012

SUMMARY Dynamic characteristics of structures — viz. natural frequencies, damping ratios, and mode shapes — are central to earthquake‐resistant design. These values identified from field measurements are useful for model validation and health‐monitoring. Most system identification methods require input excitations motions to be measured and the structural response; however, the true input motions are seldom recordable. For example, when soil–structure interaction effects are non‐negligible, neither the free‐field motions nor the recorded responses of the foundations may be assumed as ‘input’. Even in the absence of soil–structure interaction, in many instances, the foundation responses are not recorded (or are recorded with a low signal‐to‐noise ratio). Unfortunately, existing output‐only methods are limited to free vibration data, or weak stationary ambient excitations. However, it is well‐known that the dynamic characteristics of most civil structures are amplitude‐dependent; thus, parameters identified from low‐amplitude responses do not match well with those from strong excitations, which arguably are more pertinent to seismic design. In this study, we present a new identification method through which a structure's dynamic characteristics can be extracted using only seismic response (output) signals. In this method, first, the response signals’ spatial time‐frequency distributions are used for blindly identifying the classical mode shapes and the modal coordinate signals. Second, cross‐relations among the modal coordinates are employed to determine the system's natural frequencies and damping ratios on the premise of linear behavior for the system. We use simulated (but realistic) data to verify the method, and also apply it to a real‐life data set to demonstrate its utility. Copyright © 2012 John Wiley & Sons, Ltd.

“…For instance, consider the simple example of a single-story, single bay with length of two times the story height, concrete moment frame with fixed supports, and rigid connections (beams are considered axially rigid with no shear deformation). After careful review of such buildings in the literature, consisting of 18 buildings listed in [12,17,[19][20][21][22][23][24] and another 21 buildings subjected to the 1994 Northridge earthquake and its aftershocks [25,26], the following observations are made: (i) the strong-motion modal frequencies are decreased by a maximum of 30% and 40% of the corresponding values extracted from ambient vibration records for steel and concrete buildings, respectively. Changes in modal characteristics and wandering of natural frequencies were also observed in undamaged structures (with slight or not visible damage) subjected to strong motion [12].…”

Section: Modified Three-dimensional Seismic Assessment Methodsmentioning

confidence: 99%

Modified three‐dimensional seismic assessment method for buildings based on ambient vibration tests: extrapolation to higher shaking levels and measuring the dynamic amplification portion of natural torsion

Mirshafiei

McClure

2016

Summary This paper presents applications of the modified 3D‐SAM approach, a three‐dimensional seismic assessment methodology for buildings directly based on in situ experimental modal tests to calculate global seismic demands and the dynamic amplification portion of natural torsion. Considering that the building modal properties change from weak to strong motion levels, appropriate modification factors are proposed to extend the application of the method to stronger earthquakes. The proposed approach is consistent with the performance‐based seismic assessment approach, which entails the prediction of seismic displacements and drift ratios that are related to the damage condition and therefore the functionality of the building. The modified 3D‐SAM is especially practical for structures that are expected to experience slight to moderate damage levels and in particular for post‐disaster buildings that are expected to remain functional after an earthquake. In the last section of this paper, 16 low to mid‐rise irregular buildings located in Montreal, Canada, and that have been tested under ambient vibrations are analyzed with the method, and the dynamic amplification portion of natural torsion of the dataset is reported and discussed. The proposed methodology is appropriate for large‐scale assessments of existing buildings and is applicable to any seismic region of the world. Copyright © 2016 John Wiley & Sons, Ltd.