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Nonlinear Response Analysis of SDOF Systems Subjected to Doublet

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Nonlinear Response Analysis of SDOF Systems Subjected to Doublet Engineering Structures 110 (2016) 281–292 Contents lists available at ScienceDirect Engineering Structures journal homepage: www.elsevier.com/locate/engstruct Nonlinear response analysis of SDOF systems subjected to doublet earthquake ground motions: A case study on 2012 Varzaghan–Ahar events ⇑ Saman Yaghmaei-Sabegh a, , Jorge Ruiz-García b a Department of Civil Engineering, University of Tabriz, Tabriz, Iran b Facultad de Ingeniería Civil, Universidad Michoacana de San Nicolás de Hidalgo, Edificio C, Planta Baja, Ciudad Universitaria, 58040 Morelia, Mexico article info abstract Article history: A moderate earthquake ground motion with moment magnitude (Mw) 6.4 took place in the North- Received 28 February 2015 Western area of Iran on August 11, 2012, which was followed by an aftershock (Mw = 4.9) just 6 min Revised 14 November 2015 later. Eleven minutes after the first event, the affected area was struck by another moderate seismic event Accepted 19 November 2015 (Mw = 6.3) coming from a different seismogenic source. Earthquake ground motions of this special type Available online 29 December 2015 of seismic sequence were recorded in the cities of Varzaghan and Ahar. Therefore, the first part of this study investigated the energy distribution and frequency content characteristics of the Varzaghan– Keywords: Ahar earthquake ground motions records through wavelet transform analysis. The second part of this Doublet earthquake ground motion study presents the results of the nonlinear response of SDOF systems subjected to as-recorded Strength reduction factor Ductility demand Varzaghan–Ahar seismic sequences. The nonlinear response was measured in terms of constant- SDOF ductility strength reduction factors, R, and inelastic displacement ratios, IDR, which were compared with Wavelet analysis predictive capabilities of previously proposed predictive expressions that take into account seismic sequences. Results indicate that the predictive expressions for R and IDR underestimate values obtained in this study for the intermediate- and long-period spectral region. Ó 2015 Elsevier Ltd. All rights reserved. 1. Introduction Large aftershocks after the 2011 Tohoku earthquake (Mw = 9.0) caused significant structural damages in suffering region of Japan 1.1. Motivation [2]. Field investigations after the recent 2011 earthquakes of the Pacific coast of Tohoku, Japan, and Christchurch, New Zealand, Seismic design of structures requires controlling the resulting reported loss of stiffness and strength in structural components inelastic lateral deformation to acceptable level of ductility or the of reinforced concrete moment frames and accordingly failure of allowable drift. Such controlling procedure is commonly utilized structural systems due to multiple events [3]. Damage to monu- under a single design-loading scenario. A survey on past earth- mental masonry constructions caused by the 2009 L’Aquila, Italy, quakes occurred in different earthquake-prone regions worldwide earthquake sequences has been reported by Parisi and Augenti has shown that the seismic events with strong or moderate inten- [4]. Penna et al. [5] studied the damage to old existing clay brick sity levels may be followed by aftershocks with considerable level masonry buildings in the area affected by the 2012 Emilia earth- of intensity. For instance, about 21 aftershocks with moment mag- quake ground motion sequences. The numerical analyses of Acito nitude (Mw) greater than 6.0 occurred in a period of two months et al. [6] provided a valuable picture on collapse of the clock tower after the February 2010 Chile earthquake (Mw = 8.8). The main- in Finale Emilia after the 2012 Emilia Romagna earthquake shock of the 2010 Haiti earthquake (Mw = 7.0) was followed by sequence. Damaged structures due to the mainshock generally 14 aftershocks of earthquake magnitude among 5.0–6.1 [1]. The cannot be repaired due to the short intervals of time and conse- China Earthquake Network Center reported five strong aftershocks quently strong aftershocks may increase the damage degree of with earthquake magnitudes greater than 6.0 after the occurrence the structures, which should be considered in post-earthquake per- of the 2008 Sichuan, China mainshock earthquake (Mw = 8.0). formance evaluation procedure. It should be noted that besides aftershocks, structures could be ⇑ Corresponding author. Fax: +98 (411) 334 4287. subjected to rare sequence events such as doublet mainshock E-mail address: [email protected] (S. Yaghmaei-Sabegh). earthquake ground motions (i.e., in this investigation, ‘‘doublet http://dx.doi.org/10.1016/j.engstruct.2015.11.044 0141-0296/Ó 2015 Elsevier Ltd. All rights reserved. 282 S. Yaghmaei-Sabegh, J. Ruiz-García / Engineering Structures 110 (2016) 281–292 earthquakes” refers to a pair of seismic events which take place subjected to multiple near-field motions was suggested by Hatzi- closely spaced in time and location). For instance, a moderate georgiou [18] based on a comprehensive nonlinear regression anal- earthquake ground motion with Mw = 6.4 took place in the North ysis of 26,124,000 dynamic inelastic analyses. Hatzigeorgiou [18] Western area of Iran on August 11, 2012, which was followed by proposed ductility demand spectra for SDOF models under multi- an aftershock (Mw = 4.9) just 6 min later. Eleven minutes after ple near- and far-field earthquake ground motions in terms of the first event, the affected area was struck by another moderate the period of vibration, the damping ratio, the post yield stiffness seismic event (Mw = 6.3) coming from a different seismogenic and the force reduction factor. In this study, artificial earthquake source [7]. ground motion sequences were constructed by a combination method based on real single events. However, previous studies 1.2. Literature review showed that secondary sequences often occur due to repeated rup- ture of the fault (within the same zone along a fault) that ruptured As a first attempt to study the effects of aftershocks, Mahin [8] in the mainshock [19,20]. investigated analytically the response of elastoplastic single- degree-of-freedom (SDOF) systems under the mainshock–after- 1.3. Objectives shock ground motions recorded at 1972 Managua earthquake. Years later, Aschheim and Black [9] studied the seismic response Therefore, the main objectives of this study were threefold: (1) of degrading SDOF systems having prior damage (e.g. damage trig- to investigate the frequency content characteristics of the recorded gered by prior earthquake ground motions to the design-level Varzaghan–Ahar seismic sequence; (2) to evaluate the inelastic earthquake) under a set of 18 earthquake ground motions with dif- displacement and strength demands due to the Varzaghan–Ahar ferent ground motion features. Prior damage was modeled as a seismic sequence; and (3) to review the accuracy of recently pro- reduction in the initial stiffness assuming that residual displace- posed empirical equations to estimate constant-ductility inelastic ments were negligible. Aschheim and Black [9] found that prior displacement ratios and strength-reduction factors taking into damage does not have significant impact in increasing peak dis- account seismic sequences for the Varzaghan–Ahar seismic sce- placement demands in degrading SDOF systems with positive nario. For this purpose, a wavelet-based analysis is performed in post-yield stiffness. The effect of repeated earthquake ground the first part of this study to explain the characteristics of the seis- motions on the seismic response of nonlinear SDOF systems having mic sequences recorded at Varzaghan and Ahar stations, which different hysteretic behavior (i.e., bilinear, and stiffness-degrading experienced the highest level of ground shaking. The second part with positive or negative strain hardening) were examined by of this paper focuses on evaluating the inelastic response of SDOF Amadio et al. [10]. They discussed how the response of simple systems due to the Varzaghan–Ahar doublet events. Particularly, structures under repeated earthquakes depends on the period of the efficiency of recently proposed empirical relationships for the vibration, type of sequence and system’s available ductility. Analy- prediction of force reduction factor and inelastic displacement sis of Fragiacomo et al. [11] based on real seismic sequences ratio taking into account the effect of seismic sequences is also recorded at the 1940 El Centro, 1977 Romania, 1985 Michoacan examined in this part of the study. (Mexico) and 1976 Friuli (Italy) earthquakes along with two gener- ated earthquake sequences revealed a strong vulnerability of SDOF 2. Ground-motion records systems when subjected to repeated seismic events. In the study of Ruiz-García and Negrete-Manriquez [12], the response of the exist- On 11th of August 2012, two earthquakes took place in the ing steel frames was measured in terms of the peak and residual North-Western part of Iran. The doublet earthquakes measured drift demands at the end of the mainshock–aftershock sequence’s as 6.4 and 6.3 on the moment magnitude scale, respectively. The excitation. They also demonstrated that the frequency content of two events affected the cities of Varzaghan and Ahar, and, as a con- the mainshock and the aftershocks is weakly correlated. Goda sequence, seriously damaged about 20 villages, killed about 327 [13] looked at nonlinear response potential of mainshock–after- people and injured more than 3000 people [21]. Fig. 1 shows the shock sequences from the K-NET and KiK-net
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