Modeling RC column flexural failure modes under intensive seismic loading

The aim of this work is to model beam‐column behavior in a computationally effective manner, revealing reliably the overall response of reinforced concrete members subjected to intensive seismic loading. In this respect, plasticity and damage are considered in the predominant longitudinal direction, allowing for fiber finite element modeling, while in addition the effect of inelastic buckling of longitudinal rebars, which becomes essential at later stages of intensive cyclic loading, is incorporated. Α smooth plasticity‐damage model is developed for concrete, accounting for unilateral compressive and tensile behavior, nonlinear unloading and crack closure phenomena. This is used to address concrete core crushing and spalling, which triggers the inelastic buckling of longitudinal rebars. For this reason, a uniaxial local stress‐strain constitutive relation for steel rebars is developed, which is based on a combined nonlinear kinematic and isotropic hardening law. The proposed constitutive model is validated on the basis of existing experimental data and the formulation of the buckling model for a single rebar is developed. The cross section of rebar is discretized into fibers, each one following the derived stress‐strain uniaxial law. The buckling curve is determined analytically, while equilibrium is imposed at the deformed configuration. The proposed models for concrete and rebars are embedded into a properly adjusted fiber beam‐column element of reinforced concrete members and the proposed formulation is verified with existing experimental data under intensive cyclic loading.     

Centrifuge modeling of rocking‐isolated inelastic RC bridge piers

Experimental proof is provided of an unconventional seismic design concept, which is based on deliberately underdesigning shallow foundations to promote intense rocking oscillations and thereby to dramatically improve the seismic resilience of structures. Termed rocking isolation, this new seismic design philosophy is investigated through a series of dynamic centrifuge experiments on properly scaled models of a modern reinforced concrete (RC) bridge pier. The experimental method reproduces the nonlinear and inelastic response of both the soil‐footing interface and the structure. To this end, a novel scale model RC (1:50 scale) that simulates reasonably well the elastic response and the failure of prototype RC elements is utilized, along with realistic representation of the soil behavior in a geotechnical centrifuge. A variety of seismic ground motions are considered as excitations. They result in consistent demonstrably beneficial performance of the rocking‐isolated pier in comparison with the one designed conventionally. Seismic demand is reduced in terms of both inertial load and deck drift. Furthermore, foundation uplifting has a self‐centering potential, whereas soil yielding is shown to provide a particularly effective energy dissipation mechanism, exhibiting significant resistance to cumulative damage. Thanks to such mechanisms, the rocking pier survived, with no signs of structural distress, a deleterious sequence of seismic motions that caused collapse of the conventionally designed pier. © 2014 The Authors Earthquake Engineering & Structural Dynamics Published by John Wiley & Sons Ltd.     

Experimental study of the seismic behavior of partially concrete‐filled steel bridge piers under bidirectional dynamic loading

The seismic behavior of steel bridge piers partially filled with concrete under actual earthquake conditions was investigated by using 20 square section specimens subjected to static cyclic loading tests and single‐directional and bidirectional hybrid loading tests. Acceleration records of two horizontal NS and EW directional components for hard (GT1), medium (GT2), and soft grounds (GT3), obtained during the 1995 Kobe earthquake, were adopted in dynamic tests. Experimental results clearly showed that maximum and residual displacements under actual earthquake conditions cannot be accurately estimated by conventional single‐directional loading tests, especially for GT2 and GT3. A modified admissible displacement was proposed on the basis of bidirectional loading test results. The concrete fill can effectively improve the seismic resistance performance if the concrete inside the steel bridge piers is sufficiently high in quantity. Copyright © 2013 John Wiley & Sons, Ltd.     

Modelling exterior unreinforced beam‐column joints in seismic analysis of non‐ductile RC frames

The seismic response of non‐ductile reinforced concrete (RC) buildings can be affected by the behaviour of beam‐column joints involved in the failure mechanism, especially in typical existing buildings. Conventional modelling approaches consider only beam and column flexibility, although joints can provide a significant contribution also to the overall frame deformability. In this study, the attention is focused on exterior joints without transverse reinforcement, and a possible approach to their modelling in nonlinear seismic analysis of RC frames is proposed. First, experimental tests performed by the authors are briefly presented, and their results are discussed. Second, these tests, together with other tests with similar features from literature, are employed to calibrate the joint panel deformability contribution in order to reproduce numerically the experimental joint shear stress–strain behaviour under cyclic loading. After a validation phase of this proposal, a numerical investigation of the influence of joints on the seismic behaviour of a case study RC frame – designed for gravity loads only – is performed. The preliminary failure mode classification of the joints within the analysed frame is carried out. Structural models that (i) explicitly include nonlinear behaviour of beam‐column joints exhibiting shear or anchorage failure or (ii) model joints as elements with infinite strength and stiffness are built and their seismic performance are assessed and compared. A probabilistic assessment based on nonlinear dynamic simulations is performed by means of a scaling approach to evaluate the seismic response at different damage states accounting for uncertainties in ground‐motion records. Copyright © 2016 John Wiley & Sons, Ltd.     

A comparison of intensity‐based demand distributions and the seismic demand hazard for seismic performance assessment

This paper compares the seismic demands obtained from an intensity‐based assessment, as conventionally considered in seismic design guidelines, with the seismic demand hazard. Intensity‐based assessments utilize the distribution of seismic demand from ground motions that have a specific value of some conditioning intensity measure, and the mean of this distribution is conventionally used in design verification. The seismic demand hazard provides the rate of exceedance of various seismic demand values and is obtained by integrating the distribution of seismic demand at multiple intensity levels with the seismic hazard curve. The seismic demand hazard is a more robust metric for quantifying seismic performance, because seismic demands from an intensity‐based assessment: (i) are not unique, with different values obtained using different conditioning intensity measures; and (ii) do not consider the possibility that demand values could be exceeded from different intensity ground motions. Empirical results, for a bridge‐foundation‐soil system, illustrate that the mean seismic demand from an intensity‐based assessment almost always underestimates the demand hazard value for the exceedance rate considered, on average by 17% and with a large variability. Furthermore, modification factors based on approximate theory are found to be unreliable. Adopting the maximum of the mean values from multiple intensity‐based assessments, with different conditional intensity measures, provides a less biased prediction of the seismic demand hazard value, but with still a large variability, and a proportional increase the required number of analyses. For an equivalent number of analyses, direct computation of the seismic demand hazard is a more logical choice and provides additional performance insight. Copyright © 2013 John Wiley & Sons, Ltd.     

Near‐collapse response of existing RC building under severe pulse‐type ground motion using hybrid simulation

Column shear‐axial failure is a complex response, which lends itself to physical experimentation. Reinforced concrete structures built prior to the mid‐1970s are particularly susceptible to such failure. Shear‐axial column failure has been examined and studied at the element level, but current rehabilitation practice equates such a column failure with structural collapse, neglecting the collapse resistance of the full structural system following column failure. This system‐level response can prevent a column failure from leading to progressive collapse of the entire structure. In this study, a hybrid simulation was conducted on a representative pre‐1970s reinforced concrete frame structure under severe seismic ground motion, in which three full‐scale reinforced concrete columns were tested at the University of Illinois at Urbana Champaign. The analytical portion of the model was represented in the computer program OpenSees. Failure occurred in multiple physical specimens as a result of the ground motion, and the hybrid nature of the test allowed for observation of the system‐level response of the tested columns and the remaining structural system. The behavior of the system accounting for multiple column shear‐axial failure is discussed and characterized. Copyright © 2016 John Wiley & Sons, Ltd.     

Long‐term seismic performance of reinforced concrete bridges under steel reinforcement corrosion due to chloride attack

This work presents a new seismic evaluation methodology for corroded reinforced concrete bridges on the basis of nonlinear static pushover analysis. Corrosion of steel reinforcement by chloride attack is considered. At the material level, the effects of corrosion are considered by modeling the degradation of the mechanical properties of steel reinforcement, softening of cover concrete under compression, degradation of core concrete due to confinement steel corrosion, and reduction of bond strength between concrete and steel reinforcement. At the structural level, the effects of corrosion on both flexural behavior and shear behavior, and their interaction are considered. Eleven bridges of various structural types in Taiwan that are located within 6.5 km of their nearest coastline are analyzed to identify their long‐term seismic performance. Relationships between the yield and collapse peak ground accelerations (PGAs), and service time and corrosion level are established for each bridge. Analysis results show that chloride corrosion starts in 2–32 years. The transverse steel reinforcement typically starts corroding before the longitudinal steel reinforcement, as the former has a thicker cover. Research results show that collapse PGA reduces by 0.94% or 1.23% per 10 years when the mean value plus 1 or 2 standard deviation of the collapse PGA values are considered, respectively. Therefore, we suggest increasing the design PGA from 4.70% to 6.15% for a bridge adjacent to a coastline to ensure adequate long‐term seismic performance for 50 years, the typical design life span of a regular bridge. Copyright © 2013 John Wiley & Sons, Ltd.     

中欧规范RC桥墩抗震设计及验算方法对比研究

地震作用下钢筋混凝土（RC）桥墩容易损坏。为完善RC桥墩的抗震设计及验算方法,对比最新中国和欧洲规范中关于RC桥墩的延性抗震设计及验算方法的不同之处。基于Midas/Civil软件所建立的常规连续梁桥有限元模型,对比分析采用中欧规范开展的RC桥墩延性抗震设计及验算结果。结果表明:中欧规范中关于RC桥墩的延性抗震设计理念、抗剪和变形验算方法及延性构造细节均有区别。基于中欧规范设计的RC桥墩配筋情况存在差异。与中国规范相比,欧洲规范关于RC桥墩的横向钢筋配筋率和纵筋最小配筋率要求较高,有利于保证结构的抗剪强度和延性;箍筋最大间距要求较低,不利于防止纵筋压曲。     

Hybrid experimental performance of a full‐scale two‐story buckling‐restrained braced RC frame

This paper proposes a novel implementation of buckling‐restrained braces (BRB) in new reinforced concrete (RC) frame construction. Seismic design and analysis methods for using a proposed steel cast‐in anchor bracket (CAB) to transfer normal and shear forces between the BRB and RC members are investigated. A full‐scale two‐story RC frame with BRBs (BRB‐RCF) is tested using hybrid and cyclic loading test procedures. The BRBs were arranged in a zigzag configuration and designed to resist 70% of the story shear. The gusset design incorporates the BRB axial and RCF actions, while the beam and column members comply with ACI 318‐14 seismic design provisions. Test results confirm that the BRBs enhanced the RCF stiffness, strength, and ductility. The hysteresis energy dissipation ratios in the four hybrid tests range from 60% to 94% in the two stories, indicating that BRBs can effectively dissipate seismic input energy. When the inter‐story drift ratio for both stories reached 3.5% in the cyclic loading test, the overall lateral force versus deformation response was still very stable. No failure of the proposed steel CABs and RC discontinuity regions was observed. This study demonstrates that the proposed design and construction methods for the CABs are effective and practical for real applications. Copyright © 2016 John Wiley & Sons, Ltd.     

A preliminary prediction of seismic damage‐based degradation in RC structures

Estimation of structural damage from a known increase in the fundamental period of a structure after an earthquake or prediction of degradation of stiffness and strength for a known damage requires reliable correlations between these response functionals. This study proposes a modified Clough–Johnston single‐degree‐of‐freedom oscillator to establish these correlations in the case of a simple elasto‐plastic oscillator. It is assumed that the proposed oscillator closely models the response of a given multi‐degree‐of‐freedom system in its fundamental mode throughout the duration of the excitation. The proposed model considers the yield displacement level and ductility supply ratio‐related parameter as two input parameters which must be estimated over a narrow range of ductility supply ratio from a frequency degradation curve. This curve is to be identified from a set of recorded excitation and response time‐histories. Useful correlations of strength and stiffness degradation with damage have been obtained wherein a simple damage index based on maximum and yield displacements and ductility supply ratio has been considered. As an application, the proposed model has been used to demonstrate that ignoring the effects of aftershocks in the case of impulsive ground motions may lead to unsafe designs. Copyright © 2001 John Wiley & Sons, Ltd.     

Yield frequency spectra and seismic design of code‐compatible RC structures: an illustrative example

The seismic design of an eight‐story reinforced concrete space frame building is undertaken using a yield frequency spectra (YFS) performance‐based approach. YFS offer a visual representation of the entire range of a system's performance in terms of the mean annual frequency (MAF) of exceeding arbitrary global ductility or displacement levels versus the base shear strength. As such, the YFS framework can establish the required base shear and corresponding first‐mode period to satisfy arbitrary performance objectives for any structure that may be approximated by a single‐degree‐of‐freedom system with given yield displacement and capacity curve shape. For the eight‐story case study building, deformation checking is the governing limit state. A conventional code‐based design was performed using seismic intensities tied to the desired MAF for safety checking. Then, the YFS‐based approach was employed to redesign the resulting structure working backwards from the desired MAF of response (rather than intensity) to estimate an appropriate value of seismic intensity for use within a typical engineering design process. For this high‐seismicity and high‐importance midrise building, a stiffer system with higher base shear strength was thus derived. Moreover, performance assessment via incremental dynamic analysis showed that while the code‐design did not meet the required performance objective, the YFS‐based redesign needed only pushover analysis results to offer a near‐optimal design outcome. The rapid convergence of the method in a single design/analysis iteration emphasized its efficiency and practicability as a design aid for practical application. Copyright © 2017 John Wiley & Sons, Ltd.     

Rehabilitation of earthquake damaged external RC beam‐column joints by joint enlargement using prestressed steel angles

The effectiveness of a rehabilitation method based on joint enlargement using prestressed steel angles to enhance the seismic behavior of damaged external reinforced concrete beam‐column joints was experimentally investigated. Three half‐scale joints having either non‐seismic or seismic reinforcement details were tested both before and after rehabilitation by applying lateral cyclic loading of increasing amplitudes. Two defects were considered for the two non‐seismic units, being the absence of transverse steel hoops and insufficient bond capacity of beam bottom steel reinforcing bars in the joint panel zone. The damaged specimens were rehabilitated by injecting epoxy grout into existing cracks and installing stiffened steel angles at the re‐entrant corners of the beam‐column joint, both above and below the beam, that were mounted and held in place using prestressed high‐tensile strength bars. The test results indicated that the seismic performance of the rehabilitated specimens in terms of strength, stiffness, and ductility was fully recovered and comparable with the performance of the seismically detailed specimen. Copyright © 2016 John Wiley & Sons, Ltd.     

Methodology for preliminary seismic design of extended pile‐shafts for bridge structures

Seismic design of extended pile‐shafts requires a careful consideration of the influence of the surrounding soil on the overall response of the soil–pile system. In this paper, a procedure that incorporates soil properties into the process is developed for preliminary seismic design of extended pile‐shafts. The method follows the well‐accepted approach of using a force reduction factor to determine the lateral strength of the structure. The procedure involves an iterative process to arrive at the required amount of longitudinal reinforcement. Other outcomes of the procedure include the appropriate lateral stiffness and strength, as well as an estimation of the local curvature demand and ultimate drift ratio that can be used to ensure a satisfactory lateral response. The design procedure is capable of providing reliable results for a practical range of structural and soil properties. The versatility of the procedure is illustrated using two numerical examples of extended pile‐shafts constructed in different soil sites. Copyright © 2006 John Wiley & Sons, Ltd.     

基于位移设计的钢筋混凝土桥墩抗震性能试验研究(I):拟静力试验

通过拟静力试验研究了基于位移设计钢筋混凝土桥墩的抗震性能。利用基于位移抗震设计方法和桥梁抗震规范方法设计了各2根和1根1:2.5比例钢筋混凝土桥墩试件,对低周反复荷载作用下试件试验破坏形态、承载力、位移延性、滞回耗能、刚度退化等方面进行了比较分析,可以认为基于位移设计的钢筋混凝土桥墩能够达到预期的延性抗震要求,并且在相对耗能能力(与理想弹塑性模型相比)、刚度退化性能方面与现行规范抗震设计方法设计的桥墩相当。试验表明建议的钢筋混凝土桥墩基于位移的抗震设计方法是实际可行的。     

Ground motion selection for simulation‐based seismic hazard and structural reliability assessment

This paper examines four methods by which ground motions can be selected for dynamic seismic response analyses of engineered systems when the underlying seismic hazard is quantified via ground motion simulation rather than empirical ground motion prediction equations. Even with simulation‐based seismic hazard, a ground motion selection process is still required in order to extract a small number of time series from the much larger set developed as part of the hazard calculation. Four specific methods are presented for ground motion selection from simulation‐based seismic hazard analyses, and pros and cons of each are discussed via a simple and reproducible illustrative example. One of the four methods (method 1 ‘direct analysis’) provides a ‘benchmark’ result (i.e., using all simulated ground motions), enabling the consistency of the other three more efficient selection methods to be addressed. Method 2 (‘stratified sampling’) is a relatively simple way to achieve a significant reduction in the number of ground motions required through selecting subsets of ground motions binned based on an intensity measure, IM. Method 3 (‘simple multiple stripes’) has the benefit of being consistent with conventional seismic assessment practice using as‐recorded ground motions, but both methods 2 and 3 are strongly dependent on the efficiency of the conditioning IM to predict the seismic responses of interest. Method 4 (‘generalized conditional intensity measure‐based selection’) is consistent with ‘advanced’ selection methods used for as‐recorded ground motions and selects subsets of ground motions based on multiple IMs, thus overcoming this limitation in methods 2 and 3. Copyright © 2015 John Wiley & Sons, Ltd.     

基于纤维梁柱单元的桥梁墩柱地震反应模拟方法研究

为讨论利用纤维梁柱单元进行钢筋混凝土桥墩地震反应分析的建模方法,分别以4个悬臂式单柱墩和1个双柱墩拟静力加载试验,以及1个悬臂式单柱墩的振动台试验结果为依据,基于OpenSees数值分析平台建立了桥墩的地震反应分析模型。通过改变单元数量,分析了基于力的纤维梁柱单元和基于位移的纤维梁柱单元对桥墩地震反应的模拟精度。结果表明:对悬臂式单柱墩的拟静力和振动台试验,可沿墩高仅建立1个基于力的纤维梁柱单元,并在墩底串联1个考虑纵筋塑性渗透和粘结滑移的转动弹簧单元,即可获得很好的模拟结果。当采用基于位移的纤维梁柱单元时,应沿墩高至少建立2个单元,且塑性铰区至少有1个,才能保证获得较高的模拟精度。对双柱墩拟静力试验,采用基于力的纤维梁柱单元建模,沿每个墩高建立2个单元即可;以基于位移的纤维梁柱单元建模,建议沿每个墩高建立3个单元,且其中2个单元布置在塑性铰区。当数值模型可对静力滞回曲线取得很好的模拟结果后,该模型一般可对动力作用下墩顶最大位移和墩底最大剪力进行较为准确的模拟,但对墩顶残余位移的模拟精度无法保证。     

偏心水平荷载下内藏钢桁架混凝土核心筒抗震试验研究

对两个1／6缩尺的核心筒结构模型进行了偏心水平荷载作用下的低周反复荷载试验研究,其中包括一个普通混凝土核心筒和一个内藏钢桁架混凝土组合核心筒。在试验的基础上,分析了两个试件的承载力、刚度、延性、滞回特性、耗能能力、破坏特征以及抗震机理。试验研究表明：偏心水平荷载作用下,内藏钢桁架混凝土组合核心筒比普通混凝土核心筒抗震能力显著提高。     

按中、欧规范设计的钢筋混凝土框架结构抗震性态的对比分析

本文在确认我国GB 50011-2001规范及欧洲EC-EN1998-1规范在设计加速度反应谱取值、对R-μ基本规律的贯彻程度以及框架柱抗弯能力增强措施等方面存在差异的前提下,严格按照中国规范和欧洲规范两个延性等级分别设计0.4g高地震风险区的3个3跨6层钢筋混凝土框架结构算例,进行多条地面运动输入下的非线性动力反应分析并详细对比其在强震下的抗震性能,得出了中国9度0.4g区按一级抗震等级设计的框架结构的抗震性态总体上与同一地震风险区欧洲规范高延性和中等延性两个等级的框架结构相近的初步结论;同时对这两部规范设计规定的差异给抗震性态带来的有利或不利影响分别作了分析评价。     

基于位移设计的钢筋混凝土桥墩抗震性能试验研究(Ⅰ):拟静力试验

通过拟静力试验研究了基于位移设计钢筋混凝土桥墩的抗震性能。利用基于位移抗震设计方法和桥梁抗震规范方法设计了各2根和1根1：2．5比例钢筋混凝土桥墩试件,对低周反复荷载作用下试件试验破坏形态、承载力、位移延性、滞回耗能、刚度退化等方面进行了比较分析,可以认为基于位移设计的钢筋混凝土桥墩能够达到预期的延性抗震要求,并且在相对耗能能力（与理想弹塑性模型相比）、刚度退化性能方面与现行规范抗震设计方法设计的桥墩相当。试验表明建议的钢筋混凝土桥墩基于位移的抗震设计方法是实际可行的。