The large number of bridge collapses that have occurred in recent earthquakes has exposed the vulnerabilities in existing bridges. One of the emerging tools for protecting bridges from the damaging effects of earthquakes is the use of isolation systems. Seismic isolation is achieved via inserting flexible isolator elements into the bridge that shift the vibration period and increase energy dissipation. To date, the structural performance of bridges incorporating sliding seismic isolation is not well-understood, in part due to the lack of adequate models that can account for the complex behavior of the isolators. This study investigates and makes recommendations on the structural performance of bridges utilizing sliding type seismic isolators, based on the development of state-of-the-art analytical models. Unlike previous models, these models can account simultaneously for the variation in the normal force and friction coefficient, large deformation effects, and the coupling of the vertical and horizontal response during motion. The intention is to provide support for seismic risk mitigation and insight for the analysis and design of seismically isolated bridges by quantifying response characteristics. The level of accuracy required for isolator analytical models used in typical highway bridges are assessed. The comparative viability of the two main isolator types (i.e. sliding and elastomeric) for bridges is investigated. The influence of bridge and sliding isolator design parameters on the system s seismic response is illustrated.

<p>Seismic isolation has been widely adopted for structural protection. This technique, which introduces a flexible layer between the structure and the support, isolates the structure from earthquake ground motions by lengthening the structural period. The lead rubber bearing (LRB) is one of the most commonly used seismic isolators. The sizes of the rubber bearing and the lead core determine its stiffness and damping characteristics. The parameters, which characterize the seismic performance of a LRB, are the elastic stiffness (k₁), post-elastic stiffness (k₂), yield strength (F_y) and the total weight (w) of the isolated structure. In this study, an assessment of the nonlinear performance of LRB isolators is carried out using a series of spectra, which are referred to as bilinear spectra, as they are based on the bilinear behaviour of LRBs. The LRB parameters are non-dimensionalized using post-to-pre elastic stiffness ratio (n=k₂/k₁) and yield strength to weight ratio (r=F_y/w) to construct the bilinear spectra. Feasible ranges of n and r have been considered according to design code recommendations. The spectra are constructed from statistical analyses of LRB responses due to sets of real earthquake ground motions. These spectra plot the displacement and the shear force response of isolated structures for various combinations of n and r, vs. the elastic period.</p> <p>The results of the study show that displacement decreases as the lead content increases, as expected. However, the corresponding shear forces fluctuate over different isolated periods. An increase in the rubber bearing size increases only the shear response, but has negligible influence on the displacement. It is also found that earthquakes with a lower ratio of PGA/PGV tend to result in higher displacement and shear force responses of the LRB compared to ground motions with higher PGA/PGV ratios.</p> <p>A new chart-based method (referred to as the Chart Method) is developed by using a regression-based bilinear spectrum for estimating the LRB isolator displacement and shear force responses. The design capability of the Chart Method is compared to a more conventional method for designing LRBs, by solving several examples. The study concludes that the Chart Method has improved accuracy and versatility and can be used to evaluate the design suitability of commonly available LRB sizes.</p> / Master of Applied Science (MASc)

In this study influences of vertical ground motion on seismically isolated bridges were investigated for seven different earthquake data. One assessment of bearing effect involves the calculation of vertical earthquake load on the seismically isolated bridges. This paper investigates the influence of vertical earthquake excitation on the response of briefly steel girder composite bridges (SCB) with and without seismic isolation through specifically selected earthquakes. In detail, the bridge is composed of 30m long three spans, concrete double piers at each axis supported by mat foundations with pile systems. At both end of the spans there exists concrete abutments to support superstructure of the bridge. SCBs which were seismically isolated with nine commonly preferred different lead&amp / #8208 / rubber bearings (LRB) under each steel girder were analyzed. Then, the comparisons were made with a SCB without seismic isolation. Initially, a preliminary design was made and reasonable sections for the bridge have been obtained. As a result of this, the steel girder bridge sections were checked with AASHTO provisions and analytical model was updated accordingly. Earthquake records were thought as the main loading sources. Hence both cases were exposed to tri&amp / #8208 / axial earthquake loads in order to understand the effects under such circumstances. Seven near fault earthquake data were selected by considering possession of directivity. Several runs using the chosen earthquakes were performed in order to be able to derive satisfactory comparisons between different types of isolators. Analytical calculations were conducted using well known structural analysis software (SAS) SAP2000. Nonlinear time history analysis was performed using the analytical model of the bridge with and without seismic isolation. Response data collected from SAS was used to determine the vertical load on the piers and middle span midspan moment on the steel girders due to the vertical and horizontal component of excitation. Comparisons dealing with the effects of horizontal only and horizontal plus vertical earthquake loads were introduced.

Simplified methods of analysis described in codes and specifications for seismically isolated structures are always used either directly in special cases or for checking the results of nonlinear response history analysis (RHA). Important predictions for seismically isolated structures by simplified methods are the maximum displacements and base shears of the isolation system. In this study, the maximum isolator displacements and base shears determined by nonlinear RHA are compared with those determined by the equivalent lateral force (ELF) procedure in order to assess the accuracy of the simplified method in the case of bi-directional excitations with near-field characteristics. However, although there are currently many methods for ground motion selection and scaling, little guidance is available to classify which method is more appropriate than the others in any applications. Features of this study are that the ground motions used in analysis are selected and scaled using contemporary concepts and that the ground excitation is considered biv directional. The variations in response of isolated structures due to application of ground motions uni-directionally and bi-directionally are also studied by employing a scaling procedure that is appropriate for the bi-directional analysis. The proposed new scaling methodology is an amplitude scaling method that is capable of preserving the horizontal orthogonal components and it is developed especially for dynamic analysis of isolated structures. Analyses are conducted for two different symmetric reinforced concrete isolated structure for two different soil conditions in structural analysis program SAP2000. Effect of asymmetry in superstructure on isolator displacement is also investigated with further analyses considering 5% mass eccentricity at each floor level. Furthermore, once the significance of the orthogonal horizontal component on the response of isolation system is shown, the biaxial interaction of hysteretic behavior of lead rubber bearings is implemented in OpenSees by developing a subroutine which was not readily available.

Isolator displacements, floor accelerations, roof displacements, base shear and torsional moments are basic parameters that are considered in the design of seismically isolated structures. The aim of this study is to evaluate the effects of bidirectional earthquake excitations of near fault records on the response of base isolated structures in terms of basic parameters mentioned above. These parameters computed from nonlinear response history analysis (RHA) and they are compared with the parameters computed from equivalent lateral force procedure (ELF). Effect of asymmetry in superstructure is also examined considering mass eccentricity at each floor level. Torsional amplifications in isolator displacements, floor accelerations, roof displacements and base shear are compared for different level of eccentricities. Two buildings with different story heights are used in the analyses.The building systems are modeled in structural analysis program SAP2000. The scaling of ground motion data are taken from the study of &ldquo / Response of Isolated Structures Under Bi-directional Excitations of Near-fault ground Motions&rdquo / (Ozdemir, 2010). Each ground motion set (fault normal and fault parallel) are applied simultaneously for different range of effective damping of lead rubber bearing (LRB) and for different isolation periods.