Effect of abutment modeling on the seismic response of bridge structures

Abutment behavior significantly influences the seismic response of certain bridge structures. Specifically in the case of short bridges with relatively stiff superstructures typical of highway overpasses, embankment mobilization and inelastic behavior of the soil material under high shear deformation levels dominate the response of the bridge and its column bents. This paper investigates the sensitivity of bridge seismic response with respect to three different abutment modeling approaches. The abutment modeling approaches are based on three increasing levels of complexity that attempt to capture the critical components and modes of abutment response without the need to generate continuum models of the embankment, approach, and abutment foundations. Six existing reinforced concrete bridge structures, typical of Ordinary Bridges in California, are selected for the analysis. Nonlinear models of the bridges are developed in OpenSees. Three abutment model types of increasing complexity are developed for each bridge, denoted as roller, simplified, and spring abutments. The roller model contains only single-point constraints. The spring model contains discrete representations of backfill, bearing pad, shear key, and back wall behavior. The simplified model is a compromise between the efficient roller model and the comprehensive spring model. Modal, pushover, and nonlinear dynamic time history analyses are conducted for the six bridges using the three abutment models for each bridge. Comparisons of the analysis results show major differences in mode shapes and periods, ultimate base shear strength, as well as peak displacements of the column top obtained due to dynamic excitation. The adequacy of the three abutment models used in the study to realistically represent all major resistance mechanisms and components of the abutments, including an accurate estimation of their mass, stiffness, and nonlinear hysteretic behavior, is evaluated. Recommendations for abutment modeling are made.     

强震作用下桥台对斜交连续梁桥抗震性能的影响研究

为研究强地震作用下,桥台及台后土体对斜交连续梁桥抗震作用的影响。以一座三跨连续斜交箱梁桥为依托,应用sap2000建立不同斜度的模型,针对有、无桥台两种工况,采用非线性时程分析方法,研究了纵向不同地震动强度输入下,桥台及台后土体作用对不同斜度的连续梁桥主梁和桥墩位移的影响规律,并对桥墩的延性性能进行分析。研究结果表明:桥台及台后土体的存在会抑制主梁的纵向位移,大大增加主梁梁端的横向位移,地震动幅值越大,这种作用越明显;桥台及台后土体作用会减小墩顶纵向位移和墩底纵向弯矩,降低桥墩纵向位移延性需求,提高桥墩纵向安全性,斜交角越大,该影响效果越小;桥台作用对桥墩的横向反应几乎无影响。建议在桥梁抗震设计时应考虑桥台以及台后土体的作用,并针对不同斜度的连续梁桥采取相应的抗震措施,以提高其抗震性能。     

Higher order model for the seismic response of bridge embankments

The paper presents a new higher order model for the dynamic analysis of embankments. By considering a Legendre polynomial expansion to describe the motion at a generic point of the embankment, the application of the Lagrange-D’Alembert principle in conjunction with a through-the-width closed-form integration allows reducing the 3D physical domain into a 2D analytical domain. 4-node isoparametric elements with linear interpolating functions are used to numerically solve the problem. The model is suitable for bridge embankments by introducing a kinematic rigid constraint to account for the presence of the abutment. The embankment frequency dependent impedances and the displacements to be imposed to the abutment in bridge seismic analyses are obtained by condensation. The model has been validated comparing results with those furnished by high-fidelity 3D finite element models. The application to the approach embankment of an instrumented bridge subjected to a severe earthquake has demonstrated the model capability to capture both occurrence and intensity of main response peaks, as well as the frequency content of the response.     

无缝桥抗震性能的对比研究

本文以一座三跨总长60 m的整体桥为案例桥,分别试设计了同跨径的半整体桥、延伸桥面板桥和常规连续梁桥。通过Midas/Civil软件建立四种桥型的有限元模型,并对其进行了E1和E2反应谱分析和时程分析,对比了四种桥型的结构反应峰值（墩顶位移、桥墩及桩基剪力与弯矩、台底位移、桥台桩基剪力与弯矩）。计算结果表明:当桥梁存在15°的斜交角,整体桥、半整体桥在地震动沿平行于桥台长边方向及其垂直方向输入时更不利,而延伸桥面板桥和常规连续梁桥在地震动沿顺桥向和横桥向输入时更不利。四种桥型在地震作用下:整体桥抗震性能最优异,但其台底位移、桥台桩基的剪力和弯矩最大;半整体桥台底位移、桥台桩基的剪力和弯矩最小,其墩顶位移、桥墩及桩基的剪力和弯矩仅比整体桥大;延伸桥面板桥和常规连续梁桥的墩-梁相对位移远大于整体桥和半整体桥,不适用于地震基本烈度高的区域。     

Forces acting on bridge abutments over liquefied ground

Large earthquake-induced displacements of a bridge abutment can occur, when the bridge is built on a floodplain or reclaimed area, i.e., liquefiable ground, and crosses a water channel. Seismic responses of a bridge abutment on liquefiable ground are the consequence of complex interactions between the abutment and surrounding soils. Therefore identification of the factors dominating the abutment response is important for the development of simplified seismic design methods. This paper presents the results of dynamic three-dimensional finite element analyses of bridge abutments adjacent to a river dike, including the effect of liquefaction of the underlying ground using earthquake motions widely used in Japan. The analysis shows that conventional design methods may underestimate the permanent abutment displacements unless the following two items are considered: (1) softening of the soil beneath the liquefiable layer, due to cyclic shearing of the soil surrounding the piles, and (2) the forces acting on the side faces of the abutment.     

Extending the application of integral frame abutment bridges in earthquake‐prone areas by using novel isolators of recycled materials

Integral abutment bridges (IABs) are jointless structures without bearings or expansion joints which require minimum or zero maintenance. The barrier to the application of long‐span integral abutment bridges is the interaction of the abutment with the backfill soil during the thermal expansion and contraction of the bridge deck, that is, serviceability, or when the bridge is subjected to dynamic loads, such as earthquakes. The interaction of the bridge with the backfill leads to settlements and ratcheting of the soil behind the abutment and, as a result, the soil pressures acting on the abutment build up in the long term. This paper provides a solution for the aforementioned challenges by introducing a novel isolator that is a compressible inclusion of reused tyre‐derived aggregates placed between the bridge abutment and the backfill. The compressibility of typical tyre‐derived aggregates was measured by laboratory tests, and the compressible inclusion was designed accordingly. The compressible inclusion was then applied to a typical integral frame abutment model, which was subjected to static and dynamic loads representing in‐service and seismic loads correspondingly. The response of both the conventional and the isolated abutment was assessed based on the settlements of the backfill, the soil pressures and the actions of the abutment. The study of the isolated abutment showed that the achieved decoupling of the abutment from the backfill soil results in significant reductions of the settlements of the backfill and of the pressures acting on the abutment. Hence, the proposed research enables extending the length limits of integral frame bridges subjected to earthquake excitations. Copyright © 2016 John Wiley & Sons, Ltd.     

Seismic response of skewed RC box-girder bridges

It is critical to ensure the functionality of highway bridges after earthquakes to provide access to important facilities. Since the 1971 San Fernando earthquake, there has been a better understanding of the seismic performance of bridges. Nonetheless, there are no detailed guidelines addressing the performance of skewed highway bridges. Several parameters affect the response of skewed highway bridges under both service and seismic loads which makes their behavior complex. Therefore, there is a need for more research to study the effect of skew angle and other related factors on the performance of highway bridges. This paper examines the seismic performance of a three-span continuous concrete box girder bridge with skew angles from 0 to 60 degrees, analytically. Finite element （FE） and simplified beam-stick （BS） models of the bridge were developed using SAP2000. Different types of analysis were considered on both models such as： nonlinear static pushover, and linear and nonlinear time history analyses. A comparison was conducted between FE and BS, different skew angles, abutment support conditions, and time history and pushover analysis. It is shown that the BS model has the capability to capture the coupling due to skew and the significant modes for moderate skew angles. Boundary conditions and pushover load profile are determined to have a major effect on pushover analysis. Pushover analysis may be used to predict the maximum deformation and hinge formation adequately.     

Analytical investigation of bidirectional ductile diaphragms in multi-span bridges

In the AASHTO Guide Specifications for Seismic Bridge Design Provisions,ductile diaphragms are identified as Permissible Earthquake-Resisting Elements(EREs),designed to help resist seismic loads applied in the transverse direction of bridges.When adding longitudinal ductile diaphragms,a bidirectional ductile diaphragm system is created that can address seismic excitations acting along both the bridge’s longitudinal and transverse axes.This paper investigates bidirectional ductile diaphragms with Buckling Restrained Braces(BRBs)in straight multi-span bridge with simply supported floating spans.The flexibility of the substructures in the transverse and longitudinal direction of the bridge is considered.Design procedures for the bidirectional ductile diaphragms are first proposed.An analytical model of the example bridge with bidirectional ductile diaphragms,designed based on the proposed methodology,is then built in SAP2000.Pushover and nonlinear time history analyses are performed on the bridge model,and corresponding results are presented.The effect of changing the longitudinal stiffness of the bidirectional ductile diaphragms in the end spans connecting to the abutment is also investigated,in order to better understand the impact on the bridge’s dynamic performance.     

Bridge–embankment interaction under transverse ground excitation

Seismic performance and dynamic response of bridge–embankments during strong or moderate ground excitations are investigated through finite element (FE) modelling and detailed dynamic analysis. Previous research studies have established that bridge–embankments exhibit increasingly flexible performance under high‐shear deformation levels and that soil displacements at bridge abutment supports may be significant particularly in the transverse direction. The 2D equation of motion is solved for the embankment, in order to evaluate the dynamic characteristics and to describe explicitly the seismic performance and dynamic response under transverse excitations accounting for soil nonlinearities, soil–structure interaction and imposed boundary conditions (BCs). Using the proposed model, equivalent elastic analysis was performed so as to evaluate the dynamic response of approach embankments while accounting for soil–structure interaction. The analytical procedures were applied in the case of a well‐documented bridge with monolithic supports (Painter Street Overcrossing, PSO) which had been instrumented and embankment participation was identified from its response records after the 1971 San Fernando earthquake. The dynamic characteristics and dynamic response of the PSO embankments were evaluated for alternative BCs accounting for soil–structure interaction. Explicit expressions for the evaluation of the critical embankment length L_c are provided in order to quantify soil contribution to the overall bridge system under strong intensity ground excitations. The dynamic response of the entire bridge system (deck–abutments–embankments) was also evaluated through simplified models that considered soil–structure interaction. Results obtained from this analysis are correlated with those of detailed 3D FE models and field data with good agreement. Copyright © 2007 John Wiley & Sons, Ltd.     

Isolation parameters for improving the seismic performance of irregular bridges

Bridge structures are usually built on irregular topographical surfaces which create substructures with different pier heights and non uniform stiffness distribution. Three irregularity types of typical reinforced concrete (RC) medium length bridges located in a high seismicity zone of Mexico, were analyzed aimed at determining the best strength and stiffness parameters of an isolation system. The isolation system is composed by lead rubber bearings (LRB) located on each pile and abutment. The variation of the bridge characteristics and the isolation parameters produced 169 models that were subjected to ten seismic records representative of the subduction zone in the Pacific Coast of Mexico. A total of 1690 nonlinear time history analyses (NLTHA) were carried out in longitudinal and transverse directions of the structures. The maximum pier drifts, bending moments and shear forces demands were analyzed to identify the best isolation properties for reducing the concentration of damage in one or two elements and for improving the structural behavior of irregular bridges. Additionally, the analysis of the seismic response of the bridges supported on traditional neoprene bearings was carried out.     

Effect of torsional stiffness of prestressed concrete box girders and uplift of abutment bearings on seismic performance of bridges

Uplift of certain abutment bearings during earthquake may be utilized as a safety mechanism for the protection of the bridge against excessive stress. For this reason design codes such as Eurocode 8 permit the uplift of such bearings under certain conditions. Uplift of an abutment bearing occurs when the torque at the deck end exceeds a critical value. Therefore the torsional stiffness of the deck, before or after concrete cracking, is an important factor. In this work realistic values for the cracked torsional stiffness of the bridge deck are estimated from a parametric study of typical thin-walled prestressed box girders, based on non-linear analysis of softened space truss models proposed in the literature. Moreover the interaction between bearing uplift and pier flexural response is investigated and recommendations for the seismic design of bridges are proposed, taking into account the possibility of uplift of abutment bearings before or after yielding of the piers.     

Effects of spatially variable ground motions on the seismic response of a skewed, multi-span, RC highway bridge

This paper presents a study of the influence of spatially variable ground motions on the longitudinal seismic response of a short, three-span, 30-degree skewed, reinforced concrete highway bridge. Linear and nonlinear finite element models are created for the bridge and linear elastic and nonlinear inelastic time history analyses conducted. Three different types of illustrative excitations are considered: The first utilizes spatially variable ground motions incorporating the effects of variable soil conditions, loss of coherency and wave passage as input motions at the structures' supports. The time history with the smallest peak displacement and the one with the largest peak displacement from the spatially variable ones are then used as uniform input motions at all bridge supports. The comparative analysis of the bridge model shows that the uniform ground motion input with the largest peak displacement cannot provide conservative seismic demands for all structural components—in a number of cases it results in lower response than that predicted by spatially variable motions. The present results indicate that there is difficulty in establishing uniform input motions that would have the same effect on the response of bridge models as spatially variable ones. Consequently, spatially variable input motions need to be applied as excitations at the bridge supports.     

Probabilistic seismic performance and loss assessment of a bridge–foundation–soil system

This paper presents the probabilistic seismic performance and loss assessment of an actual bridge–foundation–soil system, the Fitzgerald Avenue twin bridges in Christchurch, New Zealand. A two-dimensional finite element model of the longitudinal direction of the system is modelled using advanced soil and structural constitutive models. Ground motions at multiple levels of intensity are selected based on the seismic hazard deaggregation at the site. Based on rigorous examination of several deterministic analyses, engineering demand parameters (EDP's), which capture the global and local demand, and consequent damage to the bridge and foundation are determined. A probabilistic seismic loss assessment of the structure considering both direct repair and loss of functionality consequences was performed to holistically assess the seismic risk of the system.It was found that the non-horizontal stratification of the soils, liquefaction, and soil–structure interaction had pronounced effects on the seismic demand distribution of the bridge components, of which the north abutment piles and central pier were critical in the systems seismic performance. The consequences due to loss of functionality of the bridge during repair were significantly larger than the direct repair costs, with over a 2% in 50 year probability of the total loss exceeding twice the book-value of the structure.     

高原大桥桥台地震破坏机理与抗震措施分析

通过现场震害调查和有限元分析,研究了汶川大地震高原大桥桥台的地震破坏机理和提高桥台抗主梁撞击能力的工程措施。研究表明,基于pushover静力分析技术可较好地模拟桥台在主梁撞击下的破坏形态。对不同的有限元模型假定,高原大桥虹口侧桥台可表现出"上弱下强"、"上下等强"和"下弱上强"等3种不同的破坏模式,且桥台胸墙、前墙、前墙与翼墙交界面是抗震薄弱点,在主梁撞击下易发生脆性开裂;而高原侧桥台的破坏则基本为自胸墙开始,沿45°夹角向斜下方扩展,最终导致桥台胸墙撞碎、后侧翼墙脱落倒塌。素混凝土桥台在主梁撞击下表现出明显的脆性,提高桥台混凝土强度可有效增大桥台抗撞击的强度,而在桥台混凝土中配置一定量的分布钢筋则可有效延缓桥台破坏过程。     

Effects of structural characterizations on fragility functions of bridges subject to seismic shaking and lateral spreading

This paper evaluates the seismic vulnerability of different classes of typical bridges in California when subjected to seismic shaking or liquefaction-induced lateral spreading. The detailed structural configurations in terms of superstructure type, connection, continuity at support and foundation type, etc. render different damage resistant capability. Six classes of bridges are established based on their anticipated failure mechanisms under earthquake shaking. The numerical models that are capable of simulating the complex soil-structure interaction effects, nonlinear behavior of columns and connections are developed for each bridge class. The dynamic responses are obtained using nonlinear time history analyses for a suite of 250 earthquake motions with increasing intensity. An equivalent static analysis procedure is also implemented to evaluate the vulnerability of the bridges when subjected to liquefaction-induced lateral spreading. Fragility functions for each bridge class are derived and compared for both seismic shaking （based on nonlinear dynamic analyses） and lateral spreading （based on equivalent static analyses） for different performance states. The study finds that the fragility functions due to either ground shaking or lateral spreading show significant correlation with the structural characterizations, but differences emerge for ground shaking and lateral spreading conditions. Structural properties that will mostly affect the bridges＇ damage resistant capacity are also identified.     

Use of rubberised backfills for improving the seismic response of integral abutment bridges

Reuse of the 1.5 billion waste tyres that are produced annually is a one of the major worldwide challenges, as waste tyres are toxic and cause pollution to the environment. In recognition of this problem, this paper introduces the reuse of tyres, in the form of derived aggregates in mixtures with granulated soil materials, as previous studies indicated the potential benefits of these materials in the seismic performance of structures. The objective of the present research study is to investigate whether use of rubberised backfills benefits the seismic response of Integral Abutment Bridges (IABs) by enhancing soil-structure interaction (SSI) effects. Numerical models including typical integral abutments on surface foundation with nonlinear conventional backfill material and its alternative form as soil-rubber mixtures are analysed and their response parameters are compared. The research is conducted on the basis of parametric analysis, which aims to evaluate the influence of different rubber-soil mixtures on the dynamic response of the abutment-backfill system under various seismic excitations, accounting for dynamic soil-abutment interaction. The results provide evidence that the use of rubberised backfill leads to reductions in the backfill settlements, the horizontal displacements of the bridge deck, the residual horizontal displacements of the top of the abutment and the pressures acting on the abutment, up to 55, 18, 43 and 47 % respectively, with respect to a conventional backfill comprising of clean sand. Small change in bending moments and shear forces on the abutment wall is also observed. Therefore, rubberised backfills offer promising solution to mitigate the earthquake risk, towards economic design with minimal damage objectives for the resilience of transportation networks.     

抗震销的模拟及其对异型板桥抗震性能影响分析

抗震销在上海莘庄立交工程的设计中,由于构造的特殊性,其力学特性难以用常规方法表达。基于位移和多级设防的抗震思想,以莘庄立交工程中的一异型板桥为例,首次以两个包含极端边界条件的结构分析模型考虑抗震销这一特殊构造措施对结构的影响,并按照实际结构构造充分考虑材料非线性及连接构件的非线性等因素,分别以弹塑性纤维梁柱单元、空间滑动支座单元和接触单元模拟桥墩屈服、支座滑动、相邻结构之间碰撞,建立了空间非线性时程反应分析模型,并进行了设计和罕遇地震水平两个概率水准下的地震反应分析和抗震性能评估,并根据分析结果的对比,提出强化抗震销刚度的建议。     

地震作用下曲线梁桥双向碰撞作用影响分析

碰撞作用直接影响到桥梁不同构件的地震响应,是桥梁抗震研究中一直关注的问题。针对地震作用下曲线梁桥因主梁面内转动而发生主梁与切向桥台和径向挡块碰撞的现象,以1座3跨预应力混凝土连续梁桥为例,采用非线性时程分析方法,对曲线连续梁桥的双向碰撞作用影响进行研究,并分析了不同减撞措施的效果。结果表明:考虑双向碰撞作用后,下部结构响应有明显增加,主梁转动现象变得复杂,曲线梁桥地震响应分析中应通过建立精细化模型来考虑主梁双向碰撞作用的影响;在切向桥台处设置限位拉锁装置能起到较好的减轻双向碰撞作用的影响,以及采用减隔震设计后,减撞效果更明显,桥梁抗震性能明显改善,但合理减撞措施设计参数应结合曲线梁桥约束体系及结构设计参数进行体系分析确定。     

Clear-water scour around semi-elliptical abutments with armored beds

Experimental study has been carried out under a clear-water scour condition to explore the local scour around semi-elliptical model bridge abutments with armor-layer bed, compared with the local scour process around semi-circular abutment. Two types of model bridge abutments, namely semi-elliptical and semi-circular abutments have been used in this experimental study. The model abutments had a ratio of streamwise length of abutment to the length of abutment transverse to the flow of 2 for semi-circular abutments and 3 for semi-elliptical abutments. In total, 50 Experiments have been designed and conducted under different flow conditions such as bed shear velocities, flow depth, and dimensions of bridge abutment model, as well as grain size of the bed material. Based on these experiments, the scour process around bridge abutments has been assessed. The dependence of the equilibrium scour depth of the scour hole on hydraulic variables has been studied. Empirical equation describing the equilibrium scour depth of the scour hole around bridge abutments has been developed.     

结构参数对高低墩刚构连续梁桥体系地震响应的影响

地震作用下,相邻主梁间的碰撞会改变桥台-引桥-刚构连续梁桥结构体系的动力响应。为了探究主桥结构形式、墩高、引桥跨数和伸缩缝间距等结构参数对伸缩缝处碰撞效应和桥梁结构地震响应的影响,以某实际桥梁为背景,考虑碰撞能量耗散、桩土相互作用、桥台与台后填土相互作用以及支座和桥墩的非线性行为,采用CSIBridge建立桥台-引桥-刚构连续梁桥结构体系的有限元模型进行碰撞弹塑性动力分析。研究结果表明：不同主桥结构形式的主桥墩受力区别较大,相邻主桥墩高差较大时,选择连续梁桥结构体系更加合理。墩高增加使主引桥间动力差异增大,碰撞效应更加显著,仅对刚构墩受力影响较大。引桥跨数增多和伸缩缝间距增大分别使伸缩缝处碰撞效应增大和减小,碰撞抑制作用的增强和减弱也使得刚构墩内力和变形分别减小和增大,但对于其他桥墩基本无影响。