Reliable fragility functions for seismic collapse assessment of reinforced concrete special moment resisting frame structures under near‐fault ground motions

A collapse fragility function shows how the probability of collapse of a structure increases with increasing ground motion intensity measure (IM). To have a more reliable fragility function, an IM should be applied that is efficient and sufficient with respect to ground motion parameters such as magnitude (M) and source‐to‐site distance (R). Typically, pulse‐like near‐fault ground motions are known by the presence of a velocity pulse, and the period of this pulse (T_p) affects the structural response. The present study investigates the application of different scalar and vector‐valued IMs to obtain reliable seismic collapse fragility functions for reinforced concrete special moment resisting frames (RC SMRFs) under near‐fault ground motions. The efficiency and sufficiency of the IMs as the desirable features of an optimal IM are investigated, and it is shown that seismic collapse assessments by using most of the IMs are biased with respect to T_p. The results show that (Sa(T₁), Sa(T₁)/DSI) has high efficiency and sufficiency with respect to M, R, T_p, and scale factor for collapse capacity prediction of RC SMRFs. Moreover, the multiobjective particle swarm optimization algorithm is applied to improve the efficiency and sufficiency of some advanced scalar IMs, and an optimal scalar IM is proposed.     

Fuzzy‐Valued Intensity Measures for Near‐Fault Pulse‐Like Ground Motions

Two fuzzy‐valued (FV) structure‐specific intensity measures (IMs), one based on squared spectral velocity and the other on inelastic spectral displacement, are presented to characterize near‐fault pulse‐like ground motions for performance‐based seismic design and assessment of concrete frame structures. The first IM is designed through fuzzying structural fundamental period to account for the period shift effect due to stiffness degradation, whereas the second IM is developed to take into account higher mode contribution in high‐rise buildings by employing a fuzzy combination of the first two or three modes for the lateral loading pattern in pushover analysis. A benchmark study of three example reinforced concrete frame structures shows that for moderate‐ to medium‐period structures, both of the proposed IMs improve prediction accuracy in comparison with the existing IMs. For short‐period structures, the FV inelastic spectral displacement is the best.     

基于增量动力分析的大型地下洞室群性能化地震动力稳定性评估

 基于现有研究成果,将增量动力分析引入大型地下洞室群地震动力稳定性评价领域,形成大型地下洞室群的性能化地震动力稳定性评价方法。首先讨论基于PEER-NGA数据库的强震记录选取方法,根据具体工程的地震地质特性针对性地搜索合适的强震记录,为开展增量动力分析提供最优的输入地震动。然后讨论适用于大型地下洞室群的结构损伤性能参数、地震强度因子的选取,为增量动力分析提供合适的地震强度指标和抗震性能表征。继而立足于现有规范,形成了适用于大型地下洞室群性能化地震动力稳定性评估的2级地震动水平和2级抗震性能水平。最后采用本文的方法及步骤对大岗山水电工程地下洞室群进行地震动力稳定性评估。结果表明,该方法较好地考虑了地震动的随机性,给出洞室群的抗震性能,并可进一步对地震动力稳定性进行概率分析,为大型地下洞室群地震动力灾变失稳提供准则。     

Performance‐based evaluation of tall buildings using advanced intensity measures (case study: 30‐story steel structure with framed‐tube system)

Performance assessment of high‐rise buildings has attracted peculiar attention among engineers. Care should be taken once higher‐mode effects are to be incorporated into analyses and designs. Recently, performance‐based evaluations have been widely used by designers to meet the required target capacities of engineering projects. A common tool to perform such studies is incremental dynamic analysis (IDA), which has been utilized for first‐mode‐dominant ordinary structures, whereas taller buildings demand other considerations to be made so that a thorough assessment of the structural response can be achieved. In this paper, performance‐based studies have been carried out for a sample 30‐story tall building, which takes advantage of tubular frame as lateral‐load‐resisting system. IDA is performed subsequently to quantify the structural response against a wide‐range of seismic loadings. Advanced intensity measures (IMs) are applied to optimize the capacity assessments resulting from multitude of non‐linear time‐history analyses. Finally, performance‐based evaluations have been carried out to provide a thorough assessment of target capacities that are normally advised by widely accepted codes. Results are also compared with regular short buildings where higher‐mode effects do not contribute significantly to structural response. Copyright © 2012 John Wiley & Sons, Ltd.     

Collapse behavior evaluation of asymmetric buildings subjected to bi‐directional ground motion

This paper discusses the collapse behavior of low‐rise plan‐asymmetric buildings under bi‐directional horizontal ground motions and utilizing strength and stiffness degrading nonlinear models. For this purpose, three‐dimensional three‐story and six‐story reinforced concrete frame buildings with uni‐directional mass eccentricities equal to 0% (symmetrical), 10%, 20% and 30% are subjected to nonlinear static (pushover) as well as incremental dynamic analyses using a set of far‐field two‐component ground motions and their performance are assessed on the basis of the safety margin against collapse and its probability of occurrence. Comparison of the collapse margin ratios as well as the fragility curves demonstrates significant reduction of the collapse‐level ground motion intensity with increasing eccentricity in plan. Results also indicate that current seismic design parameters including the response modification (R), overstrength (Ω) and ductility (μ) factors are not appropriate for buildings with high levels of plan eccentricity. Buildings with high values of plan eccentricity do not meet the design target life safety performance level on the basis of the calculated probability of collapse and safety margin against collapse. It appears that re‐evaluation of their design parameters is necessary. Copyright © 2015 John Wiley & Sons, Ltd.     

Seismic partitioned fragility analysis for high‐rise RC chimney considering multidimensional ground motion

In order to identify the vulnerable parts and areas of the high‐rise reinforced concrete chimney, this paper presents an effective method, which called partitioned fragility analysis. One 240‐m‐high reinforced concrete chimney was selected as the practical project, and its analytical model was created with ABAQUS software. The selected high‐rise chimney structure was divided into 17 parts, and then the damage probability of each part in different damage states was obtained with the fragility analysis considering multidimensional ground motions. Twenty ground motion records were taken from the Next Generation Attenuation database as the input motions, and the peak ground acceleration was selected as the intensity measure. The response of the chimney structure under multidimensional ground motions was obtained based on incremental dynamic analysis. The maximum strains of concrete and steel bars were defined as the damage limit states of the chimney structure. The fragility curves and surfaces obtained from this analysis showed that the vulnerable areas of the chimney structure appear at 0–20 m, 90–130 m, and 150–200 m along the height of the chimney respectively. Based analytical results, these vulnerable parts can be retrofitted to enhance the seismic resistance of existing chimney structures. And the partitioned fragility analysis method can also be used to improve the design of new chimney structures.     

Seismic fragility analysis of concrete encased frame‐reinforced concrete core tube hybrid structure based on quasi‐static cyclic test

Concrete‐encased frame‐core tube hybrid structural system has been widely employed in high‐rise buildings. This paper intends to analyze the seismic fragility of this structural system under ground motion excitation. The quasistatic cyclic test on a 1/5‐scaled, 10‐story three‐bay specimen is introduced. Fiber‐based finite element model is developed and integrated with numerical techniques that would be able to simulate the nonlinear response based on the OpenSees program. As the model is verified by the experimental data, a series of incremental dynamic analyses (IDAs) considering different frame‐tube stiffness ratios are carried out. IDA curves are drawn to describe each structural performance state. Fragility curves and probabilistic demand models are proposed for quantifying failure probability. The collapse margin ratio is employed to evaluate the collapse probability. The result shows that the collapse probability under rare earthquake still meets the requirement of Applied Technology Committee‐63 Report. The hybrid structure is proved to perform superior collapse resistance ability. The proper increase in the stiffness of core tube can reduce the collapse probability and enhance the collapse resistance capacity.     

Estimating the peak structural response of high‐rise structures using spectral value‐based intensity measures

With increasing trend towards performance‐based design in earthquake engineering, running nonlinear time history analysis is becoming the routing process to quantify the relationship between ground motions intensity measure (IM) and the structural responses. Because a high‐rise structure contains many higher modes, a newly proposed spectral value‐based IM is presented in this paper to quantify the structural response of high‐rise structures. The newly proposed IM uses the modal participation masses to combine higher modes. An actual high‐rise structure is taken as an example to demonstrate the efficiency of using the newly proposed IM to quantify the peak structural response of high‐rise structures. Five alternative IMs were compared in this study: (a) PGA ‐ peak ground acceleration; (b) S₁ ‐ spectra acceleration with only 1 mode; (c) S^* ‐ modified S₁ with the consideration of period elongation after structure yielded; (d) S₁₂‐ spectra acceleration with 2 modes; and (e) S₁₂₃ ‐ spectra acceleration with 3 modes. Linear regression is fitted between the peak structural response and the IM considered. The IM with the highest correlation coefficient to the engineering demand parameter is considered the most efficient IM. The results show that S₁ has better correlation to the structural response compared with PGA. S₁₂₃ has better correlation than S^* and S₁₂. It is found that the IM with higher modes can provide better correlation than IM with lower number of structural information. For engineering applications, IM with up to 3 modes (S₁₂₃) is sufficient to produce an accurate prediction to quantify the structural response of high‐rise structures.     

Seismic analysis of a coupled dam-reservoir-foundation system considering pressure effects at opened joints

In this paper, seismic response of a coupled dam-reservoir-foundation system is investigated under different types of artificially generated ground motions. Nonlinear behaviour of the arch dam is originated by modelling the vertical contraction and the curved peripheral joints based on discrete crack approach. Fluid–structure dynamic interaction is modelled based on Euleian–Lagrangian approach and the hydrodynamic pressure effect is considered inside the opened joints during the seismic action using heuristic approach. Foundation rock is modelled as massed medium and the viscous boundary model is applied on the exterior surface of the foundation finite element model. Artificial ground motions are applied to the coupled system using different durations and intensities. Various ground motion intensity measures (IMs) and engineering demand parameters (EDPs) are used for interpretation of the parametric study. It is shown that almost in all cases considering the pressure effect inside the joints, the demand parameter increases. Nonlinear behaviour of arch dams is sensitive not only to ground motion intensity but also its duration. This study shows that the specific energy density is the most appropriate IM almost for all the EDPs of concrete arch dams.     

Shaking table test on seismic resonant behavior of core‐outrigger structure

In order to investigate disastrous seismic resonant effect of resonant ground motions on tall building structures, a 1/40 scaled planar test model of a 56‐story core‐outrigger structure was tested on shaking table considering two types of ground motions, that is, non‐resonant and resonant ground motions. The non‐resonant ground motions were chosen from far‐field natural earthquake records, whereas the resonant ground motions were generated through scaling Fourier spectra of the non‐resonant ones in a target period region covering one of the first two natural periods of the test model. The test results showed that (a) with the same small peak ground acceleration of 0.07 g, the test model exhibited significant hysteretic behavior, residual displacement, and even overturning collapse under the resonant ground motions but behaved elastically under the non‐resonant ones; (b) the first‐period resonant ground motion caused extremely large displacements on the upper floors and large moments on the lower stories, whereas the second‐period resonant ground motion resulted in extremely large accelerations on the upper and middle floors; (c) the first‐period resonant ground motion was more probable to trigger overturning collapse of the core‐outrigger system after some exterior columns had reached their axial compression strength, even for those columns on the middle stories.     

Seismic response and fragility analysis of fiber-reinforced concrete frame-shear wall structure

为研究在关键部位采用聚乙烯醇纤维增强混凝土材料的结构的地震反应,利用Perform-3D软件对一栋10层的框架 剪力墙结构进行单向罕遇地震（50年超越概率2%）作用下的非线性动力时程反应分析。结果表明,随着结构地震损伤程度的增加,纤维增强混凝土的优良性能发挥更加充分,对结构的抗震性能改善作用也更加明显;结构基本周期对应的加速度反应谱强度S_a(T₁)能较好地反映结构的损伤程度,适合作为地震动强度衡量指标。依据FEMA P695建议的增量动力分析方法,对22对地震动记录进行标准化处理和调幅,并通过结构地震易损性函数,给出结构在不同强度地震作用下达到“防止倒塌”极限状态的失效概率。对于框架-剪力墙结构,建议可采用墙肢塑性铰转角作为其“防止倒塌”极限状态地震易损性分析的结构反应参数。     

Approximate method of estimating seismic performance of high‐rise buildings with oil‐dampers

When subjected to long‐period ground motions, many existing high‐rise buildings constructed on plains with soft, deep sediment layers experience severe lateral deflection, caused by the resonance between the long‐period natural frequency of the building and the long‐period ground motions, even if they are far from the epicenter. This was the case for a number of buildings in Tokyo, Nagoya, and Osaka affected by the ground motions produced by the 2011 off the Pacific coast of Tohoku earthquake in Japan. Oil‐dampers are commonly used to improve the seismic performance of existing high‐rise buildings subjected to long‐period ground motion. This paper proposes a simple but accurate analytical method of predicting the seismic performance of high‐rise buildings retrofitted with oil‐dampers installed inside and/or outside of the frames. The method extends the authors' previous one‐dimensional theory to a more general method that is applicable to buildings with internal and external oil‐dampers installed in an arbitrary story. The accuracy of the proposed method is demonstrated through numerical calculations using a model of a high‐rise building with and without internal and external oil‐dampers. The proposed method is effective in the preliminary stages of improving the seismic performance of high‐rise buildings.     

Analytically derived fragility relationships for the modern high‐rise buildings in the UAE

Investigating and planning for the expected damage that may hit the earthquake‐prone areas in the UAE should be undertaken in order to predict and mitigate earthquake losses. This paper discusses a framework for developing an essential driving engine in loss estimation systems, namely fragility relationships. Six reference structures, varying in height from 10 to 60 storeys, are selected due to the concentrated economic and human assets in this class of buildings. The reference structures are designed according to the building codes and construction practice adopted in this region. Inelastic fibre‐based simulation models are developed for the buildings using a verified analysis platform, which enables monitoring the spread of yielding and cracking during the multi‐step cyclic analysis. The ground motion uncertainty is accounted for using 20 input ground motions conforming to the latest understanding of the seismo‐tectonic characteristics of the UAE. A large number of inelastic pushover and incremental dynamic collapse analyses are deployed for the reference structures to derive the fragility relationships. The study illustrates the significance of assessing the vulnerability of a population of high‐rise buildings under the effect of various seismic scenarios and the need for expanding this study to cover other classes of structures in this region. Copyright © 2010 John Wiley & Sons, Ltd.     

Seismic performance of moment resisting steel frame subjected to earthquake excitations

This study presents static and dynamic assessments on the steel structures. Pushover analysis (POA) and incremental dynamic analysis (IDA) were run on moment resisting steel frames. The IDA study involves successive scaling and application of each accelerogram followed by assessment of the maximum response. Steel frames are subjected to nonlinear inelastic time history analysis for 14 different scaled ground motions, 7 near field and 7 far field. The results obtained from POA on the 3, 6 and 9 storey steel frames show consistent results for both uniform and triangular lateral loading. Uniform loading shows that the steel frames exhibits higher base shear than the triangular loading. The IDA results show that the far field ground motions has caused all steel frame design within the research to collapse while near field ground motion only caused some steel frames to collapse. The POA can be used to estimate the performance-based-seismic-design (PBSD) limit states of the steel frames with consistency while the IDA seems to be quite inconsistent. It is concluded that the POA can be consistently used to estimate the limit states of steel frames while limit state estimations from IDA requires carefully selected ground motions with considerations of important parameters.     

Seismic collapse risk assessment of super high‐rise buildings considering modeling uncertainty: A case study

Based on the multiple stripes analysis method and the first‐order second‐moment method, a seismic collapse risk assessment considering the modeling uncertainty is carried out for a 118‐story super high‐rise building with a typical mega‐frame/core‐tube/outrigger resisting system. The sensitivity of the median collapse capacity of the building to eight main parameters is analyzed, and then the modeling uncertainty is determined. Both the effects of the characterization methods of bidirectional ground motion intensities and the selection of the ground motion intensity measure (IM) on the aleatory randomness are investigated. The mean estimates approach and the confidence interval method are used to incorporate both the modeling uncertainty and the aleatory randomness, and then the annual collapse probability, the collapse probability at the maximum considered earthquake (MCE) intensity level and the acceptable values of the collapse margin ratios (CMRs) with different confidence levels for the building are calculated. The results show that the influence of the modeling uncertainty on the collapse capacity of the super high‐rise structure is negligible, the aleatory randomness caused by the record‐to‐record variability is significant, and an appropriate ground motion IM can significantly reduce the aleatory randomness.     

Seismic performance and cost‐effectiveness of high‐rise buildings with increasing concrete strength

The relationship between the seismic performance and economics of high‐rise buildings when designed to different material strengths is investigated in this paper. To represent the modern high‐rise construction, five 60‐story reinforced concrete buildings with varying concrete strengths, ranging from 45 MPa to 110 MPa, are designed and detailed to fine accuracy keeping almost equal periods of vibration. Detailed fiber‐based simulation models are developed to assess the relative seismic performance of the reference structures using incremental dynamic analyses and fragility functions. It is concluded that a considerable saving in construction cost and gain in useable area are attained with increasing concrete strength. The safety margins of high‐strength concrete in tall structures may exceed those of normal‐strength concrete buildings, particularly at high ground motion intensity levels. The recommendations of this systematic study may help designers to arrive at cost‐effective designs for high‐rise buildings in earthquake‐prone regions without jeopardizing safety at different performance levels. Copyright © 2014 John Wiley & Sons, Ltd.     

Determination of optimal probabilistic seismic demand models for pile-supported wharves

One of the basic components of the Performance-Based Earthquake Engineering (PBEE) framework of Pacific Earthquake Engineering Research (PEER) Center is the probabilistic seismic demand model (PSDM). PSDM is based upon a representative relation between ground motion intensity measures (IMs) and engineering demand parameters (EDPs). This research aims to develop an optimal PSDM for typical pile-supported wharf structures in western US ports by using probabilistic seismic demand analysis (PSDA). In this study, 4 bins with 20 non-near-field ground motions and 7 typical pile-supported wharf structures are used to determine an optimal PSDM by PSDA. The model geometry used in this study has a hybrid configuration incorporating many common field conditions. The optimal PSDM should be practical, sufficient, effective and efficient – all tested through several IM–EDP pairs derived by PSDA. The ground motion IMs used in this study include characteristics such as spectral quantities, duration, energy-related quantities and frequency content. Different EDPs are considered for local, intermediate and global response quantities. The considered optimal PSDM comprises a spectral IM, such as spectral acceleration and one of several EDPs. The EDPs of moment curvature ductility factor, displacement ductility factor and horizontal displacement of embankment and differential settlement between deck and behind land are considered for local, intermediate and global response quantities, respectively. Optimal PSDMs are used within PEER–PBEE framework, where they are coupled with both ground motion intensity and structural element fragility models to yield probabilities of exceeding structural performance levels under certain seismic hazard.     

近断层/远场地震动作用下隧道结构易损性研究

以川藏铁路线控制性工程——折多山隧道为研究对象,建立隧道动力时程分析模型。结合场地地震动设计反应谱,选取近断层脉冲型地震动及远场地震动记录,用于增量动力分析隧道工程结构的抗震性能水平。初步探讨适用于隧道结构的地震动强度指标IM,分析不同特征部位隧道结构易损性,对比分析近断层脉冲型地震动及远场地震动作用下隧道结构的地震易损性概率,并进一步给出在2种不同地震动作用下隧道结构在三级设防要求下的失效概率。结果表明:对于隧道结构,PGA为合适的IM指标;隧道左右边墙处衬砌为震害易损部位,可视作抗震设计的薄弱部位;在Ⅷ度多遇地震水平作用下,隧道结构仅发生轻微损伤甚至保持完好无损伤状态的概率较大,而在Ⅷ度罕遇与极罕遇地震水平作用下,隧道结构发生危及生命安全的严重损伤的概率较大;在相同强度的地震动作用下,近断层脉冲型地震动导致隧道结构发生更为严重破坏的可能性更大,具有更强的破坏性,在隧道抗震设计中,不可忽视近断层地震动的速度脉冲效应对隧道结构抗震性能的影响。     

Fragility assessment of high‐rise reinforced concrete buildings considering the effects of shear wall contributions

The effect of shear wall configurations on seismic responses of high‐rise RC buildings is investigated in this paper using fragility analysis method. Four lower high‐rise RC buildings that have the same plan dimensions and height but are different in configurations in lateral force resisting systems, were firstly designed following the standard code procedure. To consider uncertainties in earthquake motions, 16 real ground motion pairs were selected and scaled, then applied orthogonally to the four RC building models during the Incremental Dynamic Analysis (IDA). Fragility relationships were therefore derived based on the IDA results for the three limit states including slight damage, moderate damage and collapse to show the probabilistic comparison of seismic responses among the four buildings in both x and y‐directions. It was observed that generally adding shear walls will improve buildings' seismic performance at all limit states. However, shear wall configuration also plays a significant role in seismic behavior of the lower high‐rise regular RC buildings' and internal shear walls are generally more effective than external shear walls in improving building's seismic resistance. Copyright © 2016 John Wiley & Sons, Ltd.     

Seismic capacity assessment of postmainshock damaged containment structures using nonlinear incremental dynamic analysis

This paper investigates the structural performance and seismic capacity of the postmainshock damaged containment structure through the incremental dynamic analysis (IDA). To evaluate the seismic capacity with minimum uncertainty, the damage measure (DM) and intensity measure (IM) for IDA curves are selected in term of the coefficient of variation. The IDA using mainshock records is implemented to examine the seismic damage process of a containment structure and determine key damage states. The static cyclic loading analysis and aftershock IDA are also conducted on mainshock‐damaged containment structures to investigate the effect of initial damage states on the dynamic characteristics of structures. Finally, based on IDA results, limit states of a containment structure are defined using a generalized multidimensional limit‐state function that allows considering the dependence between the mainshock‐damaged level and residual seismic capacity. These proposed bidimensional limit‐state functions can be used in the fragility analysis and risk assessment of containment structures under mainshock–aftershock, which can improve the accuracy of seismic assessment.