Seismic Vulnerability Analysis of RC Bridges Based on Kriging Model

This paper presents a Kriging model-based method for seismic vulnerability analysis of reinforced concrete (RC) bridges. It aims at reducing the computational effect when the Monte Carlo technique is used for establishing the structural vulnerability curves. The general procedure of the proposed method is put forward firstly. In the procedure, the uncertainties existing in the structures and ground motions are both taken into account, and the uniform design (UD) technique is adopted for generating the random samples. The reliability of the proposed method is demonstrated by the vulnerability analysis of an single degree of freedom (SDOF) system using the Latin hypercube simulation (LHS) method. Vulnerability analysis of an RC bridge system is then carried out using the proposed method. The vulnerability curves of the bridge obtained by the Kriging model-based method are compared with those obtained by the LHS method. Additionally, three simulation schemes adopting different UD tables are employed to investigate the convergence and stability of the proposed method. The results show that the proposed method used for the seismic vulnerability analysis of RC bridges can reduce the computational effort and time to a large extent without much compromise on the accuracy.     

IDENTIFICATION AND VERIFICATION OF SEISMIC DEMAND FROM DIFFERENT HYSTERETIC MODELS

The goal of this paper is to develop a modified Bouc-Wen hysteretic model from cyclic loading test data for reinforced columns, including the behavior of stiffness degradation, strength deterioration, pinching and softening effects of RC members. Seismic demands on this inelastic single degree of freedom system when subjected to both near-fault ground motion and far-field ground motion excitations were examined.

The cyclic loading test of reinforced concrete columns was experimentally observed and a system identification computer program was developed to solve each control parameter of the hysteretic model. A least-squared method for identifying parameters of the model is proposed in this paper. The hysteretic constitutive law produces a smoothly varying hysteresis such as the control-parameters for strength deterioration, stiffness degradation, pinching and softening effects. Two implementations of (1) flexure damage and (2) shear damage were conducted to provide better understanding of hysteretic behavior of RC structural members. A pseudo-dynamic experiment was also developed to verify the model parameters.

Based on the developed hysteretic model, the seismic demand of this inelastic model was investigated by using both near-fault ground motion data and far-field ground motion data as input motion. An R-μT inelastic response spectrum from different hysteretic models was generated.     

Towards Damage-Based Design Approach for RC Bridge Columns Under Combined Loadings Using Damage Index Models

This article investigates a damage-based design approach for circular reinforced concrete (RC) columns under combined bending, shear, and torsion using decoupled damage index models. The combination of bending moment, shear, axial, and torsional loading affects the structural performance of bridge columns with respect to strength, deformation capacity and progression of damage. The damage index model proposed here permits decoupling these combined actions according to various damage limit states. This work evaluates the interaction between bending and torsional damage indices in terms of progression of damage. It also investigates the effects of the transverse reinforcement ratios and shear span. Based on experimental and analytical results increase of torsion amplified the progression of damage. The increase in transverse reinforcement ratio was found to have delayed the progression of damage and to have changed the torsional dominated behavior to flexural dominated behavior under combined bending and torsion.     

STRUT-AND-TIE COMPUTER MODELLING OF REINFORCED CONCRETE BRIDGE JOINT SYSTEMS

Nonlinear inelastic force-displacement response envelopes of large scale bridge tee-joints and multi-column bridge bents were established using strut-and-tie models (STMs) representing the entire structural system. The computer based STMs employed in the current study were formulated based on the force transfer mechanisms in the joint panel region reported in the literature, in conjunction with the formulation procedure for beam and column members that has been previously reported by the authors. Analysis was performed using the nonlinear analytical program Drain-2DX. Obtained analytical results, including response envelopes and sequences of structural failure, were found to correlate satisfactorily with those obtained from the experimental data.     

Natural Frequency of Single Pier Bridges Considering Soil-Structure Interaction

Because of the crucial role of free vibration frequency of a structure (e.g., a bridge) in design procedure, more realistic estimation of the frequency ends up in safer and more optimized design. As obtaining the free vibration frequencies of a bridge, considering soil-pile group-structure interaction, provide more realistic values, development of an analytical model to obtain such free vibration frequencies is studied in this research work. Most researchers have studied models with a single pile foundation. The purpose of this study is to assess soil-structure interaction (SSI) effects on dynamic performance of pile group supported bridges. A new analytical model is proposed to predict seismic analysis of these bridges. Applying the dynamic equations of motion for the system, SSI effects have been estimated. Based on the suggested analytical model, a new approximate equation is proposed for calculating natural frequency of pile group supported bridges. Equation accuracy has been investigated by comparing the results with those achieved by previous studies. Most periods calculated by the approximate equation are similar to those given for other case studies, indicating that the model could be applicable to other projects. Since the proposed model is very similar to real soil-pile-pier systems, this approximate equation can be used in preliminary seismic design of bridges.     

Shake Table Test and Numerical Analysis of a Bridge Model Supported on Elastomeric Pad Bearings

Elastomeric pad bearings are widely applied in short- to medium-span girder bridges in China, with the superstructure restrained by reinforced concrete (RC) shear keys in the transverse direction. Field investigations after the 2008 Wenchuan earthquake reveal that bearing systems had suffered the most serious damage, such as span falling, bearing displaced, and shear key failure, while the piers and foundations underwent minor damage. As part of a major study on damage mechanism and displacement control method for short- to medium-span bridges suffered in Wenchuan earthquake, a 1:4 scale, two-span bridge model supported on elastomeric pad bearings were recently tested on shake tables at Tongji University, Shanghai. The bridge model was subjected to increasing levels of four seismic excitations possessing different spectral characteristics. Two restraint systems with and without the restraint of RC shear keys were tested. A comprehensive analytical modeling of the test systems was also performed using OpenSees. The experimental results confirmed that for the typical bridges on elastomeric pad bearings without RC shear keys, the sliding effect of the elastomeric pad bearings plays an important role in isolation of ground motions and, however, lead to lager bearing displacement that consequently increases the seismic risk of fall of span, especially under earthquakes that contain significant mid-period contents or velocity pulse components. It is suggested from the test results that RC shear keys should be elaborately designed in order to achieve a balance between isolation efficiency and bearing displacement. Good correlation between the analytical and the experimental data indicates that the analytical models for the bearing and RC shear key as well as other modeling assumptions were appropriate.     

Transverse Seismic Behavior Studies of a Medium Span Cable-Stayed Bridge Model with Two Concrete Towers

Due to lack of investigation on nonlinear seismic behavior of cable-stayed bridges under strong earthquake excitation, the concrete towers, as the main gravity-carrying component, are usually required to remain nearly elastic. However, in order to achieve this high seismic performance objective, the reinforcement ratio of the tower legs and the tower struts need to be greatly increased in addition to its static loading requirement. To study the potential plastic region and possible failure mode of the cable-stayed bridge, a 1/20-scale full bridge model from a typical medium span concrete cable-stayed bridge was designed, constructed and tested on 4 linear shake tables using a site specific artificial wave in the transverse direction. Test results showed that the damage characteristics of the bridge model were as follows: (1) the severe damage was observed at the upper strut, with several steel bars fractured at both ends; (2) the repairable damage was observed at tower legs at the bottom and the middle part, with concrete cover spalling and exposure of steel bars; (3) the minimal damage was observed at the lower strut and the both sides of the side bents, with only slightly concrete spalling; and (4) no damage was observed at the auxiliary bents, the superstructure and the cables. Numerical results and test results were further compared and showed good agreement in low amplitudes of excitations. The test also proved that the bridge system was stable in flexural failure of upper struts, and had the negligible residual displacement subjected to high amplitudes of excitations.     

Fiber-Based Modeling of Circular Reinforced Concrete Bridge Columns

This article presents the application of fiber-based analysis to predict the nonlinear response of reinforced concrete bridge columns. Specifically considered are predictions of overall force-deformation hysteretic response and strain gradients in plastic hinge regions. This article discusses the relative merits of force-based and displacement-based fiber elements, and proposes a technique for prediction of nonlinear strain distribution based on the modified compression field theory. The models are compared with static and dynamic test data and recommendations are made for fiber-based modeling of RC bridge columns.     

Macro-Modeling of Reinforced Concrete Structural Walls: State-of-the-Art

During the past decades, various analytical macroscopic models of structural walls have been developed for simulating the seismic behavior of reinforced concrete (RC) walls. Due to the inherently complicated characteristics of RC walls, macroscopic models that can capture all the important response characteristics with good accuracy and applicability are very challenging to establish. A thorough review of the four main types of mathematical macro models of RC walls, i.e., the vertical-line-element-model, the 2-D shear panel element model, the equivalent truss model and the fiber-based model, is presented to discuss the methodology behind each model and examine the corresponding merits and disadvantages. Suggestions are also made for the further research of the macro modeling of structural walls.     

Developing Representative Dual Loading Protocols for the Columns of Steel Special Moment Frames based on the Seismic Demands on These Members

The column members of steel moment frames undergo high axial forces as well as inelastic rotations during a severe seismic event. The boundaries of these simultaneous structural demands on the columns of special moment frames have been investigated in this research. Based on the results of this investigation, dual cyclic loading protocols have been developed that represent both axial force and lateral deformation demands. Contrary to other loading scenarios that have been implemented in previous studies on steel columns, the loading protocols developed in this study include simultaneous axial and lateral loading cycles with varying amplitudes. The level of axial forces and story drifts tolerated by the columns of some typical Special Moment Frames (SMFs) has been investigated through performing nonlinear dynamic analyses. These frames have been selected with several configurations and different number of stories. The results of the nonlinear dynamic analyses have been processed to assess cumulative and instantaneous seismic demands on the columns of the chosen typical frames. Subsequently, dual cyclic loading protocols have been developed such that exerting these loading protocols on individual steel columns can result in structural effects close to the general seismic demands assessed in this study. Two separate dual loading protocols have been introduced for Design Earthquake (DE) and Maximum Considered Earthquake (MCE) seismic intensity levels.     

MODELLING OF RC BEAM-COLUMN JOINTS AND STRUCTURAL WALLS

The deformation of beam-column joints may contribute significantly to drift of reinforced concrete (RC) frames. In addition, failure may occur in the joints due to cumulative concrete crushing from applied beam and column moments, bond slip of embedded bars or shear failure as in the case of existing frames with nonductile detailing. When subjected to earthquake loading, failure in RC structural wall is similar to failure of frame joints as it may occur due to cumulative crushing from high flexural stresses, bond slip failure of lap splice, shear failure or a combination of various mechanisms of failure. It is important to include these behavioural characteristics in a simple model that can be used in the analysis of RC frames and RC walls to predict their response under earthquake loading and determine their failure modes.

Global macro models for the beam-column joint and for RC structural walls are developed. The proposed models represent shear and bond slip deformations as well as flexural deformations in the plastic hinge regions. The models are capable of idealising the potential failure mechanism due to crushing of concrete, bond slip or shear with allowance for the simultaneous progress in each mode. The model predictions are compared with available experimental data and good correlation is observed between analytical results and the test measurements.     

Inelastic Displacement Response of RC Systems with Cyclic Deterioration—An Energy Approach

Inelastic displacements of reinforced concrete systems are investigated by employing an energy-based approach. A hysteresis model is developed that accounts for stiffness degradation, strength deterioration and pinching. The model is calibrated by using experimental data from literature. Inelastic displacement ratios are calculated under a specific set of ground motion records with long effective durations. The results reveal the importance of deteriorating behavior under long duration excitations, especially for short and medium period structures. The last part of the study is devoted to the introduction of a simple empirical relationship for estimating the inelastic displacement demands of degrading RC structural systems.     

Seismic Retrofitting of Columns with Lap Spliced Smooth Bars Through FRP or Concrete Jackets

The effectiveness of RC jacketing or FRP wrapping for seismic retrofitting of rectangular columns having smooth (plain) bars with 180° hooks lap-spliced at floor level is experimentally investigated. The relatively low deformation capacity and energy dissipation of five unretrofitted columns is found not to depend on lap length, if lapping is not less than 15 bar-diameters. Six columns cyclically tested up to ultimate deformation after RC concrete jacketing demonstrate force and deformation capacity and energy dissipation sufficient for earthquake resistance, regardless of the presence or length of lap splicing in the original column. Another ten columns cyclically tested to ultimate deformation after wrapping of the plastic hinge region with CFRP show that FRP wrapping of the splice region is more effective than concrete jackets for enhancement of the deformation and energy dissipation capacity of old-type columns with smooth bars lap-spliced at floor level, provided that wrapping extends over the member length sufficiently to preclude plastic hinging and early member failure outside the FRP-wrapped length of the column.     

Hysteretic Model for Flexure-shear Critical Reinforced Concrete Columns

A model for predicting the cyclic lateral load-deformation response of flexure-shear critical reinforced concrete (RC) columns subjected to combined axial load and cyclic shear is proposed. Strength deterioration in the primary curve due to the effect of shear after yielding is considered by a modification coefficient. Rules for unloading and reloading branches of the hysteretic curve are obtained from regression analysis of test results. Unloading stiffness is fitted as a function of displacement ductility and secant stiffness of the point with maximum displacement in the primary curve. Pinching is simulated by changing the slope of reloading branch. Path-based cyclic strength deterioration is incorporated in the proposed model. In the expression of cyclic strength deterioration, the effects of aspect ratio and axial-load ratio are considered. Comparison between the predicted cyclic response and experimental results indicates that the proposed model can predict the observed hysteretic response of flexure-shear critical RC columns well.     

Impact of Vertical Ground Excitation on a Bridge with Footing Uplift

Ground motions recorded in the epicentral region of an earthquake often have a strong vertical component with dominant high frequencies. Damage to bridges in near-source regions due to strong vertical ground motion has been reported. The beneficial effects of footing uplift on structural performance in form of reduction of seismic response of structural members have been confirmed in previous research. The uplift of bridge piers has been utilised in a very limited number of bridge structures, e.g., the South Rangitikei railway bridge in New Zealand. However, the near-fault seismic behaviour of bridges with footing uplift has been even less addressed. In this study shake table investigations were carried out on the response of a single-span bridge model with footing uplift subjected to simultaneous vertical and horizontal excitations. Near-fault ground motions recorded in the Canterbury earthquake sequences of 2010 and 2011 were used. The experimental results show that inclusion of vertical ground motions produce stronger axial force in the pier and larger bending moment in the deck. Concurrent horizontal and vertical excitations may also cause more frequent footing uplift than the solely horizontal excitations.     

Experimental Studies and Vulnerability Assessment of Timber-Arched Lounge Bridges

ABSTRACT

There are many preserved timber-arched lounge bridges in China and some of these bridges are hundreds of years old. The woven arch is the main bearing structure of a timber-arched lounge bridge. Few studies still exist about woven arch mechanical performance, a fact that contributes to the improper repair. This article concerns the structural performance of the woven arch. The results of experimental studies on deformation, stress distribution, force-transferring mechanism, and failure modes were presented. Two woven arch models with and without П-shape braces under symmetrical load and asymmetrical load were investigated. The results indicated that the woven arch had high bearing capacity, especially under symmetrical load. The comparison of experimental results obtained for models with and without brace showed that braces modified the failure mechanism and increased the load carrying capacity and integrity. Based on the failure conditions observed experimentally, limit criteria were defined in order to facilitate vulnerability assessment of timber lounge bridges.     

Seismic Behavior of L-Shaped RC Squat Walls under Various Lateral Loading Directions

Considerable progress has been made on the research of non-rectangular reinforced concrete (RC) squat walls over the past decades. However, the experimental data of L-shaped RC squat walls remain limited, especially for their seismic behaviors under non-principal bending actions. This paper presents an experimental and numerical investigation on L-shaped RC squat structural walls with an emphasis on how varying the directions of lateral cyclic loading influences the seismic responses of these walls. Four L-shaped specimens are tested under lateral cyclic displacements and low levels of axial compression The variables are axial loads and lateral loading directions. The performance of specimens is discussed in terms of cracking patterns, failure mechanisms, hysteretic responses, deformation components and strain profiles. Furthermore, three-dimensional finite element models are developed to supplement the experimental results. The direction of lateral loading is found to have a significant effect on the peak shear strength of L-shaped RC squat walls.     

Experimental Evaluation of Seismic Performance of Squat RC Structural Walls with Limited Ductility Reinforcing Details

This article describes an experimental study carried out on of reinforced concrete (RC) walls of less confining reinforcement than that recommended by ACI 318. A total of eight RC walls with boundary elements comprising of five walls with aspect ratio of 1.125 and three walls with aspect ratio of 1.625 were tested by subjecting them to low levels of axial compression loading and simulated seismic loading, to examine the structural performance of the walls with limited transverse reinforcement. Conclusions are reached concerning the failure mode, drift capacity, strength capacity, components of top deformation, and energy dissipation characteristics of walls on the seismic behavior with limited transverse reinforcement. The influences of axial loading, transverse reinforcement in the wall boundary elements, and the presence of construction joints at the wall base on the seismic behavior of walls are also studied in this paper. Lastly, reasonable strut-and-tie models are developed to help in understanding the force transfer mechanism in the walls tested.     

Critical Assessment of Intensity Measures for Seismic Response of Italian RC Bridge Portfolios

The seismic assessment of a road network depends largely on the characterization of the fragility of its bridge components. The accuracy of bridge seismic demand estimates and the use of proper intensity measures (IM) will significantly influence such task. The available literature has mainly focused on buildings or a limited number of bridge configurations and IMs, which may not be representative for bridge portfolio assessment studies. In this paper, the correlation quality between a larger pool of traditional and innovative IMs and the nonlinear dynamic response of typical Italian RC bridges is investigated to identify the best-performing IMs.     

Pseudo-Dynamic Testing and Analytical Modeling of AAC Infilled RC Frames

Two concrete frames were tested by the PsD procedure. One frame was bare and the other was infilled with AAC blocks in the middle bay. The objective was to determine the effect of AAC infills on the seismic performance of reinforced concrete frames and developing an AAC strut model. Based on the test results, it was found that AAC infill panels did not modify the deformation response of the RC test frame significantly; however, shear in diagonal strut must be considered in boundary column design. A shear design procedure is proposed for the boundary columns in infilled frames. The drift limits of AAC infill panels measured during the tests were 0.005, 0.008, and 0.014 during diagonal cracking, corner crushing, and severe damage states, respectively.