This CD-ROM consists of seven papers that were presented at a session sponsored by ACI Committee 341 at the ACI Fall Convention in St. Louis, MO, in November 2008. The papers focus on the most recent advancements in performance-based seismic design of reinforced concrete bridges, including analytical and experimental studies, and design and construction practices with relevant information related to reinforced concrete bridges based on performance-based design. Note: The individual papers are also available. Please click on the following link to view the papers available, or call 248.848.3800 to order. SP-271

This paper investigates the performance-based evaluation of reinforced concrete (RC) bridge circular columns under combined bending, shear, axial, and torsion using decoupled damage index models. The main feature of the proposed damage index model is the feasibility of decoupling these combined actions according to various damage limit states. Research has shown that under combined bending, shear, axial, and torsion loads, the main parameters in the structural performance of RC bridge columns that are affected the most are their strength, deformation capacity, and failure mode. Response of RC columns under these combined actions is very complex and requires the implementation of numerical tools that can quantify the progressive nature of damage under the influence of various parameters. A proper damage index should thus include the main parameters that describe the hysteretic behavior under these combined loadings. Existing damage index models are modified to account for these combined actions in a decoupled scenario which are then used to evaluate the progression of damage under the combined loads. Under combined loads damage limit states that can be identified are flexural and/or shear/torsion cracking, yielding of transverse and/or longitudinal reinforcement, spalling of concrete cover, and fracture of transverse and longitudinal reinforcement. The main variables that are considered in the study to characterize the damage index are (i) the ratio of torsion-to-bending moment (T/M) for circular columns and twist-to-displacement (q/D) for square columns, (ii) the level of detailing for high and moderate seismicity (low or high transverse reinforcement ratio) and (iii) level of shear (low or moderate). Progression of damage in RC columns due to the interaction between bending and torsion is also evaluated as a function of the transverse reinforcement ratio. Results show that the columns’ lateral displacement ductility as well as its torsion rotation ductility are decreased under combined loads. The progression of damage is found to be amplified due to the effects of torsion. An important observation from this study that can have a significant impact in the seismic design of RC columns under combined loads is that an increase in the transverse reinforcement ratio helps delay the progression of damage, thereby changing the response of the columns from a torsional response to a predominately flexural response.

This paper presents a collection of practical and readily implementable recommendations for the modeling of highway bridges and overpasses subjected to earthquake ground motions. The specifications were developed particularly for Ordinary Standard Bridges in California as defined according to the Caltrans Seismic Design Criteria. Bridge components that require special modeling considerations and nonlinear characterization are identified in this paper, establishing specific criteria for the level of sophistication required. To reduce possible errors that arise during modeling and analysis of bridge structures using a particular structural analysis program, a comparison between bridge models using SAP2000 and OpenSees analysis packages was carried out to assess sensitivities and characterize important modeling parameters. Comparisons were made between the two software packages using modal, pushover and nonlinear time history analyses. A total of six typical reinforced concrete bridges in California with box-girder superstructure and different geometries and cross sections were considered. Inconsistencies between the two analysis packages were found for peak displacements obtained through nonlinear time history analysis. Two methods of obtaining response estimate bias factors between the two programs are illustrated for the six bridges analyzed under three seismic hazard levels (50%-, 10%-, and 2%-in-50-year probabilities of exceedance).

Post-earthquake repair costs and repair times are important for evaluating the performance of new bridge designs and existing bridges in regions where bridges are subject to seismic hazards. Hazard and structural demand models describe the probabilistic structural response during earthquakes. Damage and decision models link the structural response to decisions on bridge repair actions and repair costs. A step-by-step probabilistic repair cost and repair time methodology is proposed in this paper to probabilistically evaluate repair metrics for different bridge components and the bridge as a system, corresponding to varying degrees of damage. Repair actions, quantities, times, and costs are input into spreadsheet templates, and a numerical tool evaluates the expected value and variance of both repair costs and repair times for a range of earthquake intensities. This methodology uses the concept of performance groups—groups defined to account for bridge components that are repaired together. Spreadsheets are used to track all the necessary data: bridge information, structural response, component damage states, repair methods and repair quantities, and unit costs or production rates. Data can be customized for repair methods and bridge types particular to different regions. A multi-span, reinforced concrete highway overpass bridge in California is used to illustrate the methodology.

In the last two decades, seismic design of reinforced concrete bridges has shifted from a purely “life-prevention” design approach to a broader approach that also addresses “damage control” and “loss reduction” issues. This shift in philosophy requires the use of numerical tools that more accurately simulate the response of various bridge components. As importantly, a greater emphasis is placed on understanding the effects that these bridge components have on the overall seismic response of bridges. This paper presents the results of a series of nonlinear time history analyses of a RC bridge that was simulated under various modeling conditions using a finite element program called Opensees. These time history analyses were performed according to the following 16 modeling conditions: (a) two options for the nonlinear modeling of the columns, (b) two modeling conditions at the bridge columns’ foundation, and (c) four types of modeling conditions at the bridge abutments. Additionally, these modeling conditions were evaluated under two design earthquake levels that characterize the maximum considered earthquake and the frequent earthquake. To enforce the analysis, two more earthquake records were used: Inca-Peru and Northridge. Analytical results confirm that the various modeling options have significant influence on the seismic response of bridge systems, especially when nonlinear response of the abutment shear keys are included in the analyses. The different modeling runs were numerically evaluated and compared to each other in terms of the displacement ductility imposed on the columns. Within a performance-based design methodology, detailed results from these analyses are presented and discussed in further detail in the paper.