The current study examined the effectiveness of considering the nonlinear behavior of lead rubber bearings (LRBs) on the response of seismically isolated bridges. Also, the effect of the vertical component of the strong ground motion on the critical buckling capacity of LRBs was studied. 3D modeling of a seismically isolated bridge with nonlinear time history analysis under near-fault ground motion was applied. The results showed lack of consideration of the cavitation, post-cavitation and strength degradation behavior in tension and force softening under compression produces a large axial force and small axial displacement in LRBs and axial force-deformation appears to be perfectly linear. The results revealed that the average maximum base shear force in the bridge piers increased by 116% and 29.8% in the longitudinal and transverse direction respectively in comparison with the simple modeling. It also increased the average maximum displacement of bridge pier about 113% in the longitudinal direction and 31% in the transverse direction. Simplifying the LRBs modeling led to produces smaller stress and base shear forces of the bridge columns relative to actual conditions. Since the AASHTO regulation requires a seismically isolated bridge substructure to remain in the linear range, so in the bridges with a simplified model of LRBs, the nonlinear behavior of some columns of isolated bridges will be ignored under strong near-fault ground motion.

https://ajce.aut.ac.ir/article_3682_7d29e0b49a12f77a42ac6237fe92d34d.pdf

Effect of behavioral parameters of base isolators on the seismic response of the bridge to near-fault ground motions

The objective of this paper is to evaluate the structural condition of an existing reinforced cement concrete T-beam and slab bridge and to check its survivability under two imposed ground motions. A three dimensional finite element model of the bridge was developed using Structural Analysis and Program Software SAP2000. The finite element model of the bridge was subjected to ground motions along the transverse and longitudinal directions. A nonlinear static pushover analysis was carried out and the capacity curves were obtained. The seismic demands and the capacities were established. The performance of the bridge was assessed and the results are reported.

/pdf/performance-assessment-of-an-existing-reinforced-cement-51hagsv2rv.pdf

Performance Assessment of an Existing Reinforced Cement Concrete T-Beam and Slab Bridge Using Pushover Analysis

An existing reinforced cement concrete T-beam bridge was evaluated using inelastic analysis procedures namely capacity spectrum method (CSM) and modal pushover procedure (MPA). MPA was performed in both the transverse and longitudinal directions of the bridge structure independently. The response of the bridge structure to El Centro and Kobe earthquake ground motions was evaluated by the CSM. The capacity curves that represent the response of the bridge in transverse and longitudinal directions for the particular modes of the vibration are generated using MPA. The capacity-demand spectra for mode#1, mode#8 (transverse direction) and mode#2 (longitudinal direction) were obtained using SAP2000 analysis software. When the bridge was subjected to an earthquake similar to the El Centro Earthquake in transverse and longitudinal directions, the bridge capacity spectrum curve extended through the envelope of the demand curves, indicating that the bridge would survive in both the directions. Whereas for an earthquake similar to the Kobe one, the demand was much greater than the capacity and the bridge failed to survive. In the transverse modes, the structure indicates large energy absorption capacities in the inelastic range, without a significant loss of strength and stiffness. The bridge has more displacement ductility in the transverse direction than in the longitudinal direction. Hence, retrofitting applications to the multi-column bents are suggested, to enhance the global stability.

Analysis of T Beam bridge for seismic characterization

An existing reinforced cement concrete T-beam bridge is evaluated using inelastic analysis procedures namely capacity spectrum method (CSM) and modal pushover procedure (MPA). In this paper, MPA is performed in both the transverse and the longitudinal directions of the bridge structure independently. The capacity curves that represent the response of the bridge in transverse direction for the particular modes of the vibration are generated using MPA. Modal Criticality Index (MCI), a value which is the ratio of the Spectral acceleration (Sa) for demand and the Spectral acceleration (Sa) for capacity is determined. Modal criticality index value was calculated, considering the initial stiffness and the secant stiffness of the bridge structure, for identifying the critical mode. From the calculated modal criticality index value, the critical mode of the structure was inferred to be the higher mode irrespective of the stiffness (initial/secant) adopted.

Modal criticality index of an existing reinforced cement concrete T-beam bridge

/pdf/analysis-of-t-beam-bridge-for-seismic-characterisation-3j5g4vmagn.pdf

Analysis of T–beam bridge for seismic characterisation

The need to maintain the functionality of critical transportation lifelines after a large seismic event motivates the strategy to design certain bridges for performance standards beyond the minimum required by bridge design codes. To design a bridge to remain operational, one may stiffen and strengthen the load carrying members to increase the capacity, or alternatively use response modification devices such as seismic isolators to shift the dynamic characteristics of the bridge, henceforth reducing the seismic demands. Seismic isolation systems are attractive because they are directly conducive to accelerated bridge construction techniques. The two strategies are compared for a typical Utah highway bridge, using a three-span, pre-stressed concrete girder bridge that crosses Legacy Highway as a case study example. The existing Legacy Bridge, which was designed as a Standard bridge for a 2500-year return period earthquake, is evaluated as an Essential bridge for a 1000-year return period earthquake. Subsequently, this bridge is redesigned and evaluated as a seismically isolated bridge. Configuration changes needed to accommodate a seismic isolation system are discussed, and reductions to column and foundation elements are proposed. Example seismic isolator designs are provided for several different types of isolation systems commonly used in the United States. Inspection and maintenance practices for seismically isolated bridges are discussed.

Keri L Ryan

Papers

Design, Analysis, and Seismic Performance of a Hypothetical Seismically Isolated Bridge on Legacy Highway