IDARC-BRIDGE

Introduction

The computer program IDARC-BRIDGE was conceived as a platform for nonlinear analysis of bridges, in which various aspects of bridge behavior can be modeled. The program was officially released in September 1997, however, it was used in research prior to that date. The current version includes the following element types:

  1. Column Elements
  2. Beam Elements
  3. Shear Wall Elements
  4. Edge Column Elements
  5. Transverse Beam Elements

Column elements were modeled considering macromodels with inelastic flexural deformations, and elastic shear and axial deformations. Beam elements are modeled using a nonlinear flexural stiffness model with linear elastic shear deformations considered. Shear wall include inelastic shear and bending deformations, with an uncoupled elastic axial component. Edge column elements were introduced considering only inelastic axial deformations. Transverse beam elements, that have an effect on the rotational deformation of the shear walls or beams to which they are connected, are modeled using elastic linear and rotational springs.

One of the significant features incorporated in the program, to implement inelastic behavior in the macromodels, is the distributed flexibility model that replaced the commonly used hinge model developed for steel frames. The hinge model is not suitable for reinforced concrete elements since the inelastic deformation is distributed along the member rather than being concentrated at critical sections (Park et al., 1987). To trace the hysteretic response of a section a three parameter model was developed. Through the combination of three basic parameters and a trilinear skeleton curve stiffness degradation, strength deterioration and pinching response can be modeled.

The original version of the program included the damage model developed by Park and Ang (1984) to provide a measure of the accumulated damage sustained by the components of the structure, by each story level, and the entire building. This damage index included the ratio of the maximum to ultimate deformations, as well as the ratio of the maximum hysteretic energy dissipated to the maximum monotonic energy, therefore capturing both components of damage.

Program Enhancements

For the new release of the program, Version 4.0, a number of enhancements were made to the previous releases:

The major highlights of each improvement are briefly described below.

  1. The spread plasticity model in the original release of the program was reformulated to enhance numerical precision and computation efficiency. The spread plasticity formulation includes the effect of shear distortions in the elements. The revised formulation can now handle flexural or shear failures with the possibility of numerical overflow eliminated. This effort is part of a larger project to model element collapse (loss) during analysis.
In addition to the reformulation of the spread plasticity model, yield penetration rules were introduced to allow for varying plastic length zones. The formulation can capture the change in the plastified length under single or double curvature conditions. The penetration length is updated at each step in the analysis as a function of the instantaneous moment diagram in the element, but the penetration length is never allowed to become smaller than the previous maximum.
  1. Response snapshots during analysis.
    One of the new features of the program is that the user can request a series of response snapshots during the analysis. The response snapshots provides the user with displacement profile, element stress ratios, collapse states, damage index states, and dynamic characteristics (eigenvalues and eigenvectors) of the building at an instant during the analysis.
     
  2. New pushover options.
    Pushover analysis are used to determine the force-deformation response characteristics of a structure. Using the results from this analyses, the actual nonlinear dynamic response of the structure can be estimated (Valles et al., 1996). Furthermore, new set of dynamic evaluation procedures, as suggested in the ATC-33 50% Draft (1995), utilize the results obtained with pushover analyses.
     
    A number of different options for the pushover analysis were added to the program: displacement control, user defined force control distribution, a generalized power distribution, and a modal adaptive lateral force distribution. These options allow a more realistically force distribution to be used in the pushover analysis. The generalized power distribution is also suggested in the ATC-33 50% Draft (1995) to determine the load distribution as a function of the fundamental period of the structure. The modal adaptive force distribution is able to capture the changes in the lateral load distribution as the building responds in the inelastic range.
  1. Reprogrammed for improved efficiency.
    Most of the solution routines, including the eigenvalue routine, the shear calculation, the spread plasticity and yield penetration routines, and the matrix condensation routines were revised and reprogrammed to improve computational efficiency in the analysis. With these modifications the program can readily be executed in a personal computer.
     
  2. New case studies for program validation.
    Verification examples have been included to highlight the program capabilities and features, as well as to validate whenever possible numerical models with experimental results. The case studies will also help new users of the program to get familiar with IDARC capabilities and input formats.
     
  3. Mail user group and Internet site.
    A mail user group for the program is available for questions, suggestions or comments related to the program:
    Email: reinhorn@eng.buffalo.edu
     
    A web site in the Internet has been created where news, updates, comments and current developments will be posted:
    http://civil.eng.buffalo.edu/idarc-bridge

References

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13 July 2004