Numerical failure analysis of a continuous reinforced concrete bridge under strong earthquakes using multi-scale models

Previous failure analyses of bridges typically focus on substructure failure or superstructure failure separately. However, in an actual bridge, the seismic induced substructure failure and superstructure failure may influence each other. Moreover, previous studies typically use simplified models to analyze the bridge failure; however, there are inherent defects in the calculation accuracy compared with using a detailed three-dimensional (3D) finite element (FE) model. Conversely, a detailed 3D FE model requires more computational costs, and a proper erosion criterion of the 3D elements is necessary. In this paper, a multi-scale FE model, including a corresponding erosion criterion, is proposed and validated that can significantly reduce computational costs with high precision by modelling a pseudo-dynamic test of an reinforced concrete (RC) pier. Numerical simulations of the seismic failures of a continuous RC bridge based on the multi-scale FE modeling method using LS-DYNA are performed. The nonlinear properties of the bridge, various connection strengths and bidirectional excitations are considered. The numerical results demonstrate that the failure of the connections will induce large pounding responses of the girders. The nonlinear deformation of the piers will aggravate the pounding damages. Furthermore, bidirectional earthquakes will induce eccentric poundings to the girders and different failure modes to the adjacent piers.     

Aftershock collapse vulnerability assessment of reinforced concrete frame structures

In a seismically active region, structures may be subjected to multiple earthquakes, due to mainshock–aftershock phenomena or other sequences, leaving no time for repair or retrofit between the events. This study quantifies the aftershock vulnerability of four modern ductile reinforced concrete (RC) framed buildings in California by conducting incremental dynamic analysis of nonlinear MDOF analytical models. Based on the nonlinear dynamic analysis results, collapse and damage fragility curves are generated for intact and damaged buildings. If the building is not severely damaged in the mainshock, its collapse capacity is unaffected in the aftershock. However, if the building is extensively damaged in the mainshock, there is a significant reduction in its collapse capacity in the aftershock. For example, if an RC frame experiences 4% or more interstory drift in the mainshock, the median capacity to resist aftershock shaking is reduced by about 40%. The study also evaluates the effectiveness of different measures of physical damage observed in the mainshock‐damaged buildings for predicting the reduction in collapse capacity of the damaged building in subsequent aftershocks. These physical damage indicators for the building are chosen such that they quantify the qualitative red tagging (unsafe for occupation) criteria employed in post‐earthquake evaluation of RC frames. The results indicated that damage indicators related to the drift experienced by the damaged building best predicted the reduced aftershock collapse capacities for these ductile structures. Copyright © 2014 John Wiley & Sons, Ltd.     

Composed analytical models for seismic assessment of reinforced concrete bridge columns

This paper presents general composed analytical models to predict the behavior of reinforced concrete (RC) bridge columns. The analytical models were developed in OpenSees to represent the common hysteretic behavior of RC bridge columns. The proposed composed models can accommodate flexure failure, flexure‐shear failure, and pure shear failure, which are observed in existing RC bridge piers. The accuracy of the models was verified using data from the static cyclic‐loading experiments of 16 single columns and one multi‐column bent and dynamical experiment from two pseudo‐dynamic tests. The results showed that the analytical models could simulate the nonlinear behavior until the post‐failure behavior, including the strength degradation, the buckling of the reinforcement, and the pinching effect. Therefore, a global view of the behavior of reinforcement concrete is prescribed as simply as possible from the academic perspective, and these models are expected to provide sufficient accuracy when applied in engineering practice. Copyright © 2014 John Wiley & Sons, Ltd.     

Seismic fragility assessment of existing sub-standard low strength reinforced concrete structures

An analytical seismic fragility assessment framework is presented for the existing low strength reinforced concrete structures more common in the building stock of the developing countries.For realistic modelling of such substandard structures,low strength concrete stress-strain and bond-slip capacity models are included in calibrating material models.Key capacity parameters are generated stochastically to produce building population and cyclic pushover analysis is carried out to capture inelastic behaviour.Secant period values are evaluated corresponding to each displacement step on the capacity curves and used as seismic demand.A modified capacity demand diagram method is adopted for the degrading structures,which is further used to evaluate peak ground acceleration from back analysis considering each point on the capacity curve as performance point.For developing fragility curves,the mean values of peak ground acceleration are evaluated corresponding to each performance point on the series of capacity curves.A suitable probability distribution function is adopted for the secant period scatter at different mean peak ground acceleration values and probability of exceedance of limit states is evaluated.A suitable regression function is used for developing fragility curves and regression coefficients are proposed for different confidence levels.Fragility curves are presented for a low rise pre-seismic code reinforced concrete structure typical of developing countries.     

混凝土结构CFRP加固界面粘接质量DSSPI无损检测研究

本文将数字错位散斑技术引入CFRP加固混凝土界面粘接质量的检测。对预制缺陷模型检测的结果表明,该方法能够迅速准确地对CFRP加固粘接质量作出检测和评价,为混凝土结构加固界面粘接质量无损诊断提供一种新思路。     

钢筋混凝土框架结构在小震作用下失效相关性的拟动力模型试验研究

通过两榀二层三跨规范钢筋混凝土对称、非对称框架结构模型的拟动力试验,对试件在小震作用截面约束的失效相关性进行了分析,并与文献中的Monte-Carlo随机模拟结果进行了比较,进一步验证了规范钢筋混凝土框架结构在小震作用下失效独立的相关性结论。     

Seismic retrofitting of reinforced concrete frame structures using GFRP-tube-confined-concrete composite braces

This paper presents a new type of structural bracing intended for seismic retrofitting use in framed structures. This special composite brace,termed glass-fiber-reinforced-polymer(GFRP)-tube-confined-concrete composite brace,is comprised of concrete confined by a GFRP tube and an inner steel core for energy dissipation.Together with a contribution from the GFRP-tube confined concrete,the composite brace shows a substantially increased stiffness to control story drift, which is often a preferred feature in seismic retrofitting.An analysis model is established and implemented in a general finite element analysis program-OpenSees,for simulating the load-displacement behavior of the composite brace.Using this model,a parametric study of the hysteretic behavior(energy dissipation,stiffness,ductility and strength)of the composite brace was conducted under static cyclic loading and it was found that the area ratio of steel core to concrete has the greatest influence among all the parameters considered.To demonstrate the application of the composite brace in seismic retrofitting, a three-story nonductile reinforced concrete(RC)frame structure was retrofitted with the composite braces.Pushover analysis and nonlinear time-history analyses of the retrofitted RC frame structure was performed by employing a suite of 20 strong ground motion earthquake records.The analysis results show that the composite braces can effectively reduce the peak seismic responses of the RC frame structure without significantly increasing the base shear demand.     

Seismic resilience assessment of corroded reinforced concrete structures designed to the Chinese codes

The natural landscape in China exposes many existing RC buildings to aggressive environments.Such exposure can lead to deterioration in structural performance with regard to resisting events such as earthquakes.Corrosion of embedded reinforcement is one of the most common mechanisms by which such structural degradation occurs.There has been increasing attention in recent years toward seismic resilience in communities and their constituent construction;however,to date,studies have neglected the effect of natural aging.This study aims to examine the effect of reinforcement corrosion on the seismic resilience of RC frames that are designed according to Chinese seismic design codes.A total of twenty RC frames are used to represent design and construction that is typical of coastal China,with consideration given to various seismic fortification levels and elevation arrangements.Seismic fragility relationships are developed for case frames under varying levels of reinforcement corrosion,i.e.,corrosion rates are increased from 5%to 15%.Subsequently,the seismic resilience levels of uncorroded and corroded RC frames are compared using a normalized loss factor.It was found that the loss of resilience of the corroded frames is greater than that of their uncorroded counterparts.At the Rare Earthquake hazard level,the corrosioninduced increase in loss of resilience can be more than 200%,showing the significant effect of reinforcement corrosion on structural resilience under the influence of earthquakes.     

钢筋混凝土桥墩开裂刚度计算

以基本的力学理论为依据,对双筋矩形截面混凝土墩柱的受力模式进行简化,通过力学平衡方程,从理论上分析了墩柱受力过程中开裂区域的刚度。在此基础上,引入了三段式的概念,并通过三段式思想推算出截面刚度的简化公式。根据简化公式计算出力-位移曲线,继而推算出混凝土墩柱整体刚度。     

Damages on reinforced concrete buildings due to consecutive earthquakes in Van

Consecutive earthquakes occurred on October 23rd, 2011 in Ercis and on November 9th, 2011 in Edremit that are townships located 90 km and 18 km far from Van city in Turkey, respectively. A total of 28,000 buildings were damaged or collapsed in the city center and the surrounding villages after the Ercis earthquake. This number reached 35,000 after the Edremit earthquake. In the area where the earthquakes occurred, almost all the reinforced concrete buildings were affected.This study presents field observations of damages on reinforced concrete buildings due to the consecutive earthquakes that occurred in Van, Turkey. Damages appearing in the buildings may occur due to several reasons such as site effect, poor construction quality, poor concrete strength, poor detailing in beam-column joints, detailing of stronger beam than column, soft stories, weak stories, inadequate reinforcement, short lap splices, incorrect end hook angle, and short columns. Aftershocks also caused progressive damages on the buildings within 17 days after the earthquakes. According to the results of this study, most of the damaged buildings were not designed and constructed according to the Turkish earthquake code, the so-called Specification for Buildings to be built in Seismic Zones.     

Inelastic response analysis of reinforced concrete structures using modified force analogy method

A modified force analogy method (MFAM) is developed to simulate the nonlinear inelastic response of reinforced concrete (RC) structures. Beam–column elements with three different plastic mechanisms are utilized to simulate inelastic response caused by moment and shear force. A multi‐linear hysteretic model is implemented to simulate the nonlinear inelastic response of RC member. The P‐Δ effect of the structure is also addressed in MFAM. Static and dynamic inelastic response of structure, damage condition and failure type for structural element, structural limit state and collapse time can also be simulated using MFAM. Compared with the general algorithm, the MFAM provides less computational time especially in the case of large structural system. It is also easier to be written as computer program. Three test data groups, which include cyclic loading test data of a non‐ductile RC bridge column, a two‐storey RC frame, and dynamic collapse test data of a non‐ductile RC portal frame, are selected to confirm the effectiveness of applying MFAM to simulate the inelastic behaviour of structures. Copyright © 2007 John Wiley & Sons, Ltd.     

Seismic collapse analysis of reinforced concrete framed structures using the finite element method

A new finite element code using the Adaptively Shifted Integration (ASI) technique with a linear Timoshenko beam element is applied to the seismic collapse analysis of reinforced concrete (RC) framed structures. This technique can express member fracture as a plastic hinge located at either end of an element with simultaneous release of the resultant forces in the element. Contact between members is also considered in order to obtain results that agree more closely with actual behavior, such as intermediate‐layer failure. By using the proposed code, sufficiently reliable solutions have been obtained, and the results reveal that this code can be used in the numerical estimation of the seismic design of RC framed structures. Copyright © 2003 John Wiley & Sons, Ltd.     

既有钢筋混凝土框架结构的多指标云模型地震损伤评估

目前的既有钢混结构地震损伤研究没有同时考虑不同抗震设计规范差异和耐久性两个因素对结构抗震性能的影响,且损伤指标较简单,在动力损伤分析中也存在局限。基于云模型的特点,提出了包括弹塑性耗能差率、刚度损伤指数、层间位移角和顶点位移角的多元结构损伤状态综合评估方法,能够同时考虑结构各损伤指数的随机性和模糊性。考虑不同版本抗震设计规范造成的结构性能差异和耐久性下降对结构性能的影响,设计3个典型五层钢混框架结构,进行增量动力分析,验证损伤评估方法的准确性。结果表明：随着抗震规范版本的更新,结构的损伤程度有适当减轻;同一结构的损伤程度因混凝土碳化作用先减轻后加重;采用弹塑性耗能差率表征既有结构的地震损伤效果优于刚度损伤指数;基于多指标云模型损伤评估方法获得的云模型综合隶属度和综合损伤值能够更加细化和精确地描述结构损伤状态。     

Capacity-based inelastic displacement spectra for reinforced concrete bridge columns

Capacity-based inelastic displacement spectra that comprise an inelastic displacement ratio (C_R ) spectrum and the corresponding damage index (DI ) spectrum are proposed in this study to aid seismic design and evaluation of reinforced concrete (RC) bridges. Nonlinear time history analyses of single-degree-of-freedom (SDOF) systems are conducted using a versatile smooth hysteretic model when subjected to far-field and near-fault ground motions. It is demonstrated that the Park and Ang damage index can be a good indicator for assessing the actual visible damage condition of columns regardless of its loading history, providing a better insight into the seismic performance of bridges. The computed spectra for near-fault (NF) ground motions show that as the magnitude of pulse period ranges increases from NF1 (0.5-2.5 seconds) to NF2 (2.5-5.5 seconds), the spectral ordinates of the C_R and DI spectra increase moderately. In contrast, the computed spectra do not show much difference between NF2 and NF3 (5.5-10.5 seconds) when the period of vibration T_n≤  1.5 seconds, after which the spectral ordinates of NF3 tend to increase obviously, whereas those of NF2 decrease with increasing T_n . Moreover, when relative strength ratio R  = 5.0, nearly all of the practical design scenarios could not survive NF3. On the basis of the computed spectra, C_R and DI formulae are presented as a function of T_n , R , and various design parameters for far-field and near-fault ground motions. Finally, an application of the proposed spectra to the performance-based seismic design of RC bridges is presented using DI as the performance objective.     

Impulse-momentum and resistance analysis of reinforced concrete structures

The viability of most analytical approaches developed to predict the inelastic response of Reinforced Concrete (RC) structures requires the load-deformation relationship to have a positive slope. In many realistic cases the slope is not positive. When this happens, most analytical approaches consider the maximum point as the failure point or use unrealistic material laws, e.g., the assumption of a positive slope when it really is negative. Hence, there is a need to develop a new analytical approach which recognizes load-deflection relationships which have ascending and descending branches. A method based on the impulse momentum principle and the resistance force (IMR) of an RC structure is developed. The resistance force vector { R (t)} is used in place of the stiffness approach. The material non-linearity of RC systems can be elegantly accounted for in the { R (t)} vector. An examination of numerical properties of the IMR method shows that stability and convergence are dependent on a time step length ratio. Applications of the IMR method which predict the inelastic seismic response of RC structures are performed.     

Seismic safety of reinforced concrete members and structures

Based on cyclic load tests of large-scale reinforced concrete elements and assemblages, a probabilistic model of member failure is developed. The model gives the probability of survival at time t as a functional of damage ratio and dissipated energy up to t. After extension to multivariate survival of several members with correlated resistance, the model is used to calculate the safety of reinforced concrete frames subjected to given input motions. Results are in terms of the probability of local failure and of no failure anywhere in the system.     

钢筋混凝土加劲桁架悬索桥的模态分析

采用ANSYS软件对钢筋混凝土土加劲架悬索桥进行有限元分析，提出较为准确的建模方法和最优初始应变的取值条件，使计算结果能满足工程分析精度要求。在此基础上进行了模态分析，计算了钢筋混凝土加劲杵架悬索桥的动力特征及主要结构参数对动力学特性的影响，并对福建三明下洋悬索桥进行实桥测试与分析比较，研究结果有助于对该类悬索桥力学性能的认识。     

钢筋混凝土结构的抗震加固方法述评

本文讨论了钢筋混凝土结构抗震加固的原则,对目前常用的钢筋混凝土结构的加固方法进行了简要的论述,给出了各种方法的特点和适用范围以及一些需要注意的问题.文中还提出了加固方法将来可能的发展方向.     

钢筋混凝土结构倒塌分析的前沿研究

结构倒塌分析主要涉及三个难点，即不连续位移场的描述、接触碰撞分析以及结构倒塌过程中大位移、大转动的描述。本文就国内外研究现状针对结构倒竭的评定标准以及上述三个难点做出评述，同时介绍了作者近期工作，并对今后研究工作做了展望，以供参考。