J.M. Jara

Procedure for determining the seismic vulnerability of an irregular isolated bridge

A seismic vulnerability procedure, based on the capacity/demand ratio approach, is applied to an irregular isolated bridge. Special features are incorporated in both, demand estimation and capacity evaluation. The seismic demand is represented by an average pseudo-acceleration spectrum derived from 159 earthquake accelerograms recorded in the region where the bridge is located. The capacity spectrum method is adopted for estimating the structural expected performance for several limit states. The capacity curve derived from a static non-linear procedure is obtained by means of a lateral load pattern that follows the displacement configuration, previously assessed by the use of time history analyses of the bridge supported on non-linear isolator bearings. Based on a moment–curvature analysis of the piers sections, the maximum curvature ductility was established for each of the four defined performance limit states. Finally, probability density functions of the bridge capacity and demand were assessed and fragility curves were proposed aimed at determining the expected behaviour of the bridge as function of peak ground acceleration (PGA) of the typical strong motions recorded in the area.

Comparative structural response of two steel bridges constructed 100 years apart

This paper presents a comparative numerical analysis of the structural behaviour and seismic performance of two existing steel bridges, the Infiernillo II Bridge and the Pinhão Bridge, one located in Mexico and the other in Portugal. The two bridges have similar general geometrical characteristics, but were constructed 100 years apart. Three-dimensional structural models of both bridges are developed and analysed for various load cases and several seismic conditions. The results of the comparative analysis between the two bridges are presented in terms of natural frequencies and corresponding vibration modes, maximum stresses in the structural elements and maximum displacements. The study is aimed at determining the influence of a 1 century period in material properties, transverse sections and expected behaviour of two quite similar bridges. In addition, the influence of the bearing conditions in the global response of the Pinhão Bridge was evaluated.

Seismic energy dissipation and local concentration of damage in bridge bents

Seismic damage on bridge systems is basically concentrated on the piers. Hence, it is necessary to develop comprehensive studies oriented to improve our understanding of the bridge bents seismic response. Accordingly, the seismic behaviour and local concentration of damage within a typical bridge bent were analysed, considering different rotational restrictions of the foundation-soil system and different steel ratio arrangements. The contributions to structural damage of both, maximum deformation and cumulative damage, were assessed for records with different characteristics. The results showed that the amount of damage dissipated by the bent and the distribution of damage were strongly affected by the flexural strength variation along the columns. In models with non-uniform strength distribution, the inelastic demands were concentrated on few plastic hinges and bent degradation and failure were reached before the energy dissipation capacity of all potential hinges occurred. The analyses results also showed that the rotational restriction imposed by the foundation had a notable influence on the energy dissipation capacity and location of damage. Finally, the determination of damage indices in plastic hinge regions showed that most of the damage is produced by the cumulative effect of plastic energy dissipation, independently of the ground motion characteristics.

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.

Stochastic collocation-based nonlinear analysis of concrete bridges with uncertain parameters

Abstract This paper focuses on the stochastic response of concrete bridges considering uncertainty in bearing and abutment stiffness. A multi-span simply supported bridge with concrete girders is selected. A 3D-dimensional model is prepared, and nonlinear response history analyses are performed. For the numerical dynamic simulation, the non-sampling stochastic method based on generalized polynomial chaos (gPC) expansion is utilised. The uncertain parameters include the vertical and shear stiffness of bearings and the lateral stiffness of abutments are presented by the truncated gPC expansions. Furthermore, the system response such as base shear, acceleration, velocity and displacement in different columns is presented by gPC expansion with unknown deterministic coefficients. The stochastic Galerkin projection is employed to calculate a set of deterministic equations. A non-intrusive solution, as a set of collocation points, determines the unknown gPC coefficients of the system response and the results are compared with Monte Carlo simulations. The key advantage of spectral discretization is the combination of the mentioned method with the spatial discretization, e.g. finite element model. This study also emphasises the accuracy in results and time efficiency of the proposed non-sampling method for uncertainty quantification of stochastic systems comparing to sampling procedure (e.g. Monte Carlo simulation).

Expected damages of retrofitted bridges with RC jacketing

The bridge infrastructure in many countries of the world consists of medium span length structures built several decades ago and designed for very low seismic forces. Many of them are reinforced concrete structures that according to the current code regulations have to be rehabilitated to increase their seismic capacity. One way to reduce the vulnerability of the bridges is by using retrofitting techniques that increase the strength of the structure or by incorporating devices to reduce the seismic demand. One of the most common retrofit techniques of the bridges substructures is the use of RC jacketing; this research assesses the expected damages of seismically deficient medium length highway bridges retrofitted with reinforced concrete jacketing, by conducting a parametric study. We select a suite of twenty accelerograms of subduction earthquakes recorded close to the Pacific Coast in Mexico. The original structures consist of five 30 m span simple supported bridges with five pier heights of 5 m, 10 m, 15 m 20 and 25 m and the analyses include three different jacket thickness and three steel ratios. The bridges were subjected to the seismic records and non-linear time history analyses were carried out by using the OpenSEEs Plataform. Results allow selecting the reinforced concrete jacketing that better improves the expected seismic behavior of the bridge models.

Engineering demand functions for RC medium length span bridges

The evolution of the seismic design philosophies and the strong correlation of some structural demands with damage, motivate the research community to improve the knowledge about the pier demands in bridges. The seismic demands can be determined by conducting non-linear time history analyses, however, the computational time required can be excessive and the outcome are usually limited to the type of bridge studied and to the ensemble of records applied. Recently, several studies have proposed analytical expressions to assess Engineering Demand Parameters (EDP’s) as a function of intensity measures (IM’s). The objective of this research is twofold: to determine IM-EDP functions for typical highway RC bridges, and to quantify the influence on the bridge EDP’s by considering the seismic data of two important seismic sources jointly or separately. For this purpose, the response of a family of bridge models located in four different seismic zones was analyzed. The bridges were subjected to 124 seismic records from earthquakes with origin on the two most important seismic sources in Mexico, namely: subduction fault and deeper inslab fault. The IM selected to establish the relationships, was the spectral acceleration for the fundamental period of the bridge and the EDP’s were the pier Park and Ang damage index and the pier drift ratio. The proposed expressions are based on the 7,936 nonlinear time history analyses conducted in both directions of the bridge models.

Strengthening and Retrofitting of Steel Bridges

This chapter begins with a brief description of the most common typologies of steel bridges. Typical structural deficiencies and damages of steel bridges are outlined to identify the basic needs of rehabilitation actions. It includes the damage produced by corrosion, fatigue, increasing of live loads, seismic actions, poor detailing and vehicle collision. Initially, a description of the characteristics and performance of traditional rehabilitation techniques is outlined. Later, the superstructure rehabilitation techniques based on the use of composite materials as carbon, aramid or glass fibre plates, as bonded external reinforcement, and aramid or glass fibre rods for prestressing are considered. The experience about the behaviour of strengthened beams under overloading and fatigue conditions are also outlined and the heat strengthening of steel girders after a collision is commented as well. The use of steel jacketing of columns is also described and the benefit of using link beams to improve the transverse seismic response of multicolumn bents. Base isolation as an appealing strategy for reducing the seismic demand in piers and foundations and the advantages of its application is discussed and the use of cable restrainers added for limiting the longitudinal displacement is also analysed. In the end, the methodologies more employed to assess the seismic vulnerability of bridges to select the best retrofit technique, and the parameters to be considered for a better selection of the bridge intervention are described.

Probabilistic Seismic Performance Analysis of RC Bridges

ABSTRACT This paper focuses on the evaluation of the seismic vulnerability of pre-1990 highway concrete bridges, based on the development of analytical fragility curves. The parameters considered are the span length, column height, lap splices, reinforcement yield strength, and concrete compressive strength. Nonlinear response history analyses using 3D models were conducted for each set of bridge samples subjected to earthquake ground motions with different intensities. Moreover, this study analyzes the influence of the earthquake fault type on the seismic vulnerability of the bridges, using as a performance parameter the displacement ductility demand of the piers. Based on this parameter, to support future seismic-risk mitigation efforts fragility curves are utilized to assesses the seismic vulnerability of typical concrete bridge classes in Iran. The selected suite of seismic records originates in strike-slip and reverse fault seismic sources. The bridges subjected to reverse fault records present pier demands and seismic vulnerability larger than the bridges subjected to strike-slip accelerograms. The study shows that the span length, lap splice, column height, and seismic fault type have significant effects on the seismic vulnerability of bridge piers. Additionally, the results are useful to identify and prioritize retrofit actions of the most seismically vulnerable bridges.

Strength and stiffness parameters of energy dissipation devices for the seismic protection of building on soft soils

Energy dissipation devices are continuously implemented in several buildings and bridges around the world. The dynamic characteristics of the strong motions in soft soil sites make attractive the use of this type of passive control system; however, its applicability remains limited. In spite of the important number of papers studying the seismic behavior of structures with passive control devices, only few of them consider low frequency excitations and even less studies combine low frequency excitations and yield type devices. One of the main impediments regarding to the use of energy dissipation devices is the lack of specific studies, showing their applicability as a function of the dynamic characteristics of structural systems with dissipation devices located on soft soil sites. This study conducts a parametric study of a single degree of freedom system with metallic yielding devices, with the aim of proposing graphs for the preliminary design of buildings with energy dissipation devices, by means of drift, frame ductility and dissipator ductility response spectra. The excitation of the models is a collection of seismic strong motions recorded in soft soil sites. 21,600 nonlinear time history analyses were processed for the entire combinations of the displacement and stiffness parameters of the structures and devices selected. The results allow an easy way to select initially the mechanical and geometrical properties of the energy dissipation devices with elastoplastic behavior, as a function of the fundamental period of the building. Finally a case study of a six-story RC building with metallic energy dissipation devices demonstrates the applicability of the proposed response spectra.