SEISMIC BEHAVIOUR OF A R/C WALL: NUMERICAL SIMULATION AND EXPERIMENTAL VALIDATION

This paper describes the numerical simulation of the seismic behaviour of a mock-up of a six-floor building, constituted by two parallel R/C walls and experimentally tested on a shaking table. Within the scope of an international benchmark the mock-up was submitted to three earthquakes with intensities up to 0.71 g, which induced nonlinear behaviour in the concrete and reinforcement. For the numerical simulations concrete is discretised with 2D finite elements, and its behaviour reproduced via a constitutive model with two scalar damage variables. Steel rebars are discretised with 2-noded truss elements, and their constitutive behaviour under cyclic conditions reproduced by the Menegottb-Pinto model. Specific attention is devoted to Rayleigh damping, focusing on two different strategies: (i) disregarding the damping contribution, or (ii) adopting a damping matrix that takes into account the stiffness changes during the nonlinear analyses. Main results and strategies for simulating the benchmark axe presented, with emphasis on the comparison between the numerical and the experimental results, which show good agreement when the damping contribution is neglected.     

CONCEPT AND DEVELOPMENT OF HYBRID SOLUTIONS FOR SEISMIC RESISTANT BRIDGE SYSTEMS

The development of alternative solutions for precast concrete buildings based on jointed ductile connections has introduced innovative concepts in the design of lateral-load resisting frame and wall systems. Particularly efficient is the hybrid system, where precast elements are connected via post-tensioning techniques and self-centring and energy dissipating properties are adequately combined to achieve the target maximum displacement with negligible residual displacements. In this contribution, the concept of hybrid system is extended to bridges as a viable and efficient solution for an improved seismic performance when compared with monolithic counterparts. Critical discussion on the cyclic behaviour of hybrid systems, highlighting the most significant parameters governing the response, is carried out.

The concept of a flexible seismic design (displacement-based) of hybrid bridge piers and systems is proposed and its reliability confirmed by quasi-static cyclic (push-pull) and nonlinear time-history analyses based on lumped plasticity numerical models.     

Numerical Analysis of RC Bridge Piers with Rectangular Hollow Cross-section Retrofitted with FRP Jackets

The research presented in this article deals with the seismic retrofit of bridge piers with rectangular hollow cross-section using fiber-reinforced polymer (FRP) jackets. A two-level numerical approach that combines finite element method (FEM) analyses and fiber modeling is proposed. The FEM is used to study the effect of FRP jackets on the properties of concrete. The analyses show that the existing empirical laws for FRP-confined concrete are not suitable for piers with hollow cross-section, as the effect of confinement is not uniform within the cross-section and the stress–strain curves show softening after peak strength. Fiber modeling is used to study the global behavior of reinforced concrete piers with rectangular hollow cross-section wrapped with FRP jackets. To account for confinement, the properties of the concrete fibers are modified according to the results of the FEM analyses. The proposed method is validated against experimental results and used for an extensive parametric study. It is found that the effectiveness of jacketing is conditioned by the axial load, longitudinal reinforcement, and jacket dimensions. An empirical design equation is formulated on the basis of the numerical analyses.     

Nonlinear Modeling and Mitigation of Seismic Pounding between R/C Frame Buildings

A multi-link finite element contact model reproducing Jankowski’s nonlinear viscoelastic relationships is devised and implemented for a time-history analysis of seismic pounding. The mechanical and algorithmic parameters are calibrated with a numerical test case study, represented by two single-story R/C frames. A real case study, concerning two adjacent buildings representative of a stock of R/C multi-story frame structures with insufficient separation joints at rest, is then examined. A damped interconnection-based mitigation solution consisting in linking the two structures with fluid-viscous dissipaters is proposed. The benefits of this retrofit strategy are discussed by comparison with the response in original conditions.     

A Checking Method for Multiple Seismic Performance Objectives of Bridge Piers Designed According to Code Provisions

Seismic bridge design codes require that bridge piers designed according to prescribed design rules should attain specified multiple seismic performance objectives. However, design codes do not explicitly require checking the attainment of specified performance objectives for designed bridge piers. In this article, seismic performance levels have been correlated with engineering damage parameters. A checking method for multiple seismic performance objectives of bridge piers has been outlined and validated with experimental results. The application of the method has been demonstrated by checking the performance of a bridge pier designed according to a code provision for a wide range earthquake ground motions.     

Analytical Seismic Assessment of a Tall Long-Span Curved Reinforced-Concrete Bridge. Part I: Numerical Modeling and Input Excitation

The seismic response of bridges is affected by a number of modeling considerations, such as pier embedment, buried pile caps, seat-type abutments, pounding, bond slip and architecturally flared part of piers, and loading considerations, such as non-uniform ground excitations and orientation of ground motion components, which are not readily addressed by design codes. This article addresses a methodology for the nonlinear static and dynamic analysis of a tall, long-span, curved, reinforced-concrete bridge, the Mogollon Rim Viaduct. Various modeling scenarios are considered for the bridge components, soil-structure interaction system, and materials, i.e., concrete and reinforcing steel, covering all its geotechnical and structural aspects based on recent advances in bridge engineering. Various analysis methodologies (nonlinear static pushover, time history response to uniform and spatially variable seismic excitations, and incremental dynamic analyses) are performed. For the dynamic analyses, a suite of nine earthquake accelerograms are selected and their characteristics are investigated using seismic intensity parameters. A recently developed approach for the generation of non-uniform seismic excitations, i.e., spatially variable simulations conditioned on the recorded time series, is used. Methods for the evaluation of structural performance are discussed and their limitations addressed. The numerical results of the seismic assessment of the Mogollon Rim Viaduct are presented in the companion article (Part II). The sensitivity of the bridge response to the adopted modeling, loading and analyzing strategies, as well as the correlation between structural damage and seismic intensity parameters are examined in detail.     

Seismic Protection of the Piers of Integral Bridges using Sliding Bearings

Seismic resilience and continued operation of bridges after earthquakes are important seismic design criteria. A new seismic protection concept for integral bridge piers is explored that uses sliding bearings to separate the superstructure from the piers. The influence of sliding bearings on the seismic response of a representative 3-span integral highway bridge is investigated. With sliding bearings, the pier column shear force was limited to the bearing design friction force. Furthermore, the abutment ductility demands were found to be insensitive to the friction forces in the sliding bearings because the bridge displacement demands were controlled by the equal displacement rule.     

Analysis of Carbon Fiber Sheet-Retrofitted RC Bridge Columns Under Lateral Cyclic Loading

In recent years, the use of carbon fiber sheet (CFS) to provide lateral confinement for enhanced ductility and strength of reinforced concrete bridge columns has been increasing. While the monotonic behavior of CFS-confined concrete has been studied extensively, its cyclic response has not been fully understood. Most of the available studies are experimental investigations, hence there is a need to develop an analytical model to simulate the experimental results. Analysis of the hysteretic behavior of CFS-retrofitted circular columns is presented in this article using the fiber element that is based on cyclic constitutive models of longitudinal reinforcement and concrete confined by both CFS and tie reinforcement. The analysis was verified based on available cyclic test data and the analysis provides good agreement with the experimental results. Results show that flexural strength and ductility of columns wrapped with CFS increases as CFS ratio increases. However, as tie reinforcement ratio increases, there is no much difference on the hysteretic response for low tie reinforcement ratios. Using the fiber element analysis, the effect of CFS retrofit on the seismic response of a 7.5 m tall prototype pier built in the 1970s to 1980s is also clarified.     

Predicting Ductile Crack Initiation of Steel Bridge Structures Due to Extremely Low-Cycle Fatigue Using Local and Non-Local Models

The strain-based prediction model combining the Miner's rule and Manson-Coffin's relationship provides a local parameter for evaluating the ductile crack initiation of steel structures, and some modified models based on it were proposed to evaluate extremely low-cycle fatigue (ELCF) behaviors of steel structures. Previous research has confirmed these local models to be an accurate index for ductile crack initiation in steel bridge piers, however it is found to quite depend on the mesh size of the numerical model used. In this study, a non local damage parameter is presented and successfully applied to ductile crack initiation life assessment of steel bridge piers subjected to earthquake-type cyclic loading. The non local damage parameter is based on averaging the strains over the effective plane using a weight function in the exponential form, and introduces the non local damage parameter to replace the local state variable. Finite element analysis with three different mesh sizes is employed. Comparisons of the local and non local solutions with those of experiments indicate that the non local prediction model can predict the ductile crack initiation of steel bridge piers with good accuracy regardless of the specimen geometries and loading histories, meanwhile the mesh independent nature of the non local model is also demonstrated.     

INFLUENCE OF CAPACITY DESIGN METHOD ON THE SEISMIC RESPONSE OF R/C COLUMNS

Abstract

This paper aims at assessing the influence of the design procedure followed in designing the columns of a reinforced concrete (R/C) building on the performance of the columns, aa well as of the structure as a whole, when subjected to seismic loading; to identify potential weaknesses in currently adopted procedures; and to present a new procedure which is based on currently-available, powerful analytical tools, and results in increased reliability with regard to seismic loading. Two case studies are presented, involving multistorey reinforced concrete buildings with frame and dual structural systems subjected to various appropriately-scaled input accelerograms. The results obtained indicate that capacity design of columns results in adequate safety margins against failure, even when the adopted overstrength factors are quite low, but hinging in columns is not avoided unless very high overstrength factors are used. The suggested novel technique of capacity design led to very satisfactory seismic performance, and offers the possibility of cost reduction by achieving an appropriate balance between provided flexural strength and corresponding confinement.     

Effect of Nonlinear Seismic Torsion on the Performance of Skewed Bridge Piers

This article presents an analytical investigation on the effect of seismic torsion on the performance of a skewed bridge. A nonlinear torsional hysteretic model developed by the authors is applied to idealize the torsional behavior of bridge piers. Deterioration of the torsional strength of piers due to combined flexure is considered and deterioration of flexural strength due to torsion is not taken into account. The effects of pounding between deck and abutments, cable restrainers, and damage of bearing supports are also included in analysis. It is found that the eccentric impact force due to lock of bearing movement results in extensive torsion in piers.     

Performance-Based Seismic Assessment of Superelastic Shape Memory Alloy-Reinforced Bridge Piers Considering Residual Deformations

The application of superelastic Shape Memory Alloy (SMA) reinforcement in plastic hinge regions of bridge piers has been proven to reduce the residual displacement after a strong shaking owing to its unique shape recovery characteristics; however, the maximum deformation of the piers could increase due to the relatively lower modulus of elasticity of SMA bars and lower hysteretic energy dissipation capacity. In this context, this article applies a recently formulated probabilistic performance-based seismic assessment methodology that considers both the maximum and the residual deformation simultaneously to evaluate the performance of SMA reinforced bridge piers.     

SEISMIC RESPONSE CONTROL OF CABLE-STAYED BRIDGE USING DIFFERENT CONTROL STRATEGIES

The goal of this research is to study the effect of cable vibration through a number of control cases of a cable-stayed bridge. In order to consider the complicated dynamic behaviour of the full-scale bridge, a three-dimensional numerical model of the MATLAB-based analysis tool has been developed by the complete simulation of the Gi-Lu bridge. The dynamic characteristics of cables in the cable-stayed bridge are verified between the field experiment and the result from numerical simulation using geometrically nonlinear beam elements in MATLAB program. Three types of control devices are selected to reduce the response of the bridge deck which includes: actuators, viscous-elastic dampers with large capacity, and base isolations. Moreover, two types of control devices, MR dampers and viscous dampers, are installed either between the deck and cables and/or between two neighbouring cables for controlling the cable vibration. A modified bi-viscous model combined with convergent rules is used to describe the behaviour of MR dampers. Finally, through evaluation criteria the control effectiveness on the cable-stayed bridge using different control strategies is examined.     

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.     

NONLINEAR ANALYSIS OF REINFORCED CONCRETE SHEAR WALL UNDER EARTHQUAKE LOADING

A constitutive model for predicting the cyclic response of reinforced concrete structures is proposed. The model adopts the concept of a smeared crack approach with orthogonal fixed cracks and assumes a plane stress condition. Predictions of the model are compared firstly with existing experimental data on shear walls which were tested under monotonic and cyclic loading. The same model is then used in the finite element analysis of a complete shear wall structure which was tested under a large number of cyclic load reversals due to earthquake loading at NUPEC's Tadotsu Engineering Laboratory. Two different finite element approaches were used, namely a two-dimensional and a three-dimensional representation of the test specimen. The ability of the concrete model to -reproduce the most important characteristics of the dynamic behaviour of this type of structural element was evaluated by comparison with available experimental data. The numerical results showed good correlation between the predicted and the actual response, global as well as local response being reasonable close to the experimental one.     

SEISMIC SOIL-STRUCTURE INTERACTION: BENEFICIAL OR DETRIMENTAL?

The role of soil-structure interaction (SSI) in the seismic response of structures is reex-plored using recorded motions and theoretical considerations. Firstly, the way current seismic provisions treat SSI effects is briefly discussed. The idealised design spectra of the codes along with the increased fundamental period and effective damping due to SSI lead invariably to reduced forces in the structure. Reality, however, often differs from this view. It is shown that, in certain seismic and soil environments, an increase in the fundamental natural period of a moderately flexible structure due to SSI may have a detrimental effect on the imposed seismic demand. Secondly, a widely used structural model for assessing SSI effects on inelastic bridge piers is examined. Using theoretical arguments and rigorous numerical analyses it is shown that indiscriminate use of ductility concepts and geometric relations may lead to erroneous conclusions in the assessment of seismic performance. Numerical examples are presented which highlight critical issues of the problem.     

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.     

SIMPLIFIED MODELLING STRATEGIES TO SIMULATE THE DYNAMIC BEHAVIOUR OF R/C WALLS

A continuous damage model and different simplified numerical strategies are proposed to simulate the behaviour of reinforced concrete (R/C) walls subjected to earthquake ground motions. For 2D modelling of R/C walls controlled primarily by bending, an Euler multilayered beam element is adopted. For 3D problems, a multifibre Timoshenko beam element having higher order interpolation functions has been developed. Finally, for walls with a small slenderness ratio we use the Equivalent Reinforced Concrete model. For each case, comparison with experimental results of R/C walls tested on shaking table or reaction wall shows the advantages but also the limitations of the approach.     

Probabilistic Calibration and Experimental Validation of the Seismic Design Criteria for One-Story Concrete Frames

This article investigates the seismic performance of one-story reinforced concrete structures for industrial buildings. To this aim, the seismic response of two structural prototypes, a cast-in-situ monolithic frame and a precast hinged frame, is compared for four different levels of translatory stiffness and seismic capacity. For these structures an incremental nonlinear dynamic analysis is performed within a Monte Carlo probabilistic simulation. The results obtained from the probabilistic analysis prove that precast structures have the same seismic capacity of the corresponding cast-in-situ structures and confirm the overall goodness of the design criteria proposed by Eurocode 8, even if a noteworthy dependency of the actual structural behavior from the prescribed response spectrum is pointed out.

The experimental verification of these theoretical results is searched for by means of pseudodynamic tests on full-scale structures. The results of these tests confirm the overall equivalence of the seismic behavior of precast and cast-in-situ structures. Moreover, two additional prototypes have been designed to investigate the seismic behavior of precast structures with roof elements placed side by side. The results of these further tests show that an effective horizontal diaphragm action can be activated even if the roof elements are not connected among them, and confirm the expected good seismic performance of these precast systems. Finally, the results of the experimental tests are compared with those obtained from nonlinear structural analyses. The good agreement between numerical and experimental results confirms the accuracy of the theoretical model and, with it, the results of the probabilistic investigation.     

EFFECT OF SOIL-STRUCTURE INTERACTION ON SEISMICALLY ISOLATED BRIDGES

The objective of this study is two-fold: first, to assess the effects of soil-structure interaction (SSI) on the response of seismically isolated bridge piers and, second, to develop a method that considers SSI and can be easily applied to the preliminary design of bridges. Emphasis is given on pier behaviour, because piers together with the abutments are the most critical components of a bridge with a high potential for concentration of ductility demands during earthquakes. The relative importance that several parameters of the bridge-isolators-soil system play on design is examined. Conclusions and suggestions that can lead to safer and more economical isolated pier design are also presented. Cases in which SSI needs to be incorporated in seismically isolated bridge design are identified and ways to take advantage of SSI in order to enhance safety level and reduce design costs are recommended.