Analytical Investigations and Design Implications of Seismic Response of a Two-Span ABC Bridge System

Latest Progress Report:

Final Report

ABC-UTC Guidlines for Analytical Investigations and Design Implications of Seismic Response of a Two-Span ABC Bridge System

Background:
Various connections and prefabricated components have been developed and investigated under seismic loading in the past decade.  Component studies have been essential in understanding the local behavior of connections and have provided invaluable information.  However, to determine the adequacy of the connections in a realistic bridge, the connections along with prefabricated elements should be integrated and studied in a bridge system. Through a study at UNR funded by the ABC-UTC Cycle  2 funding allocation, a two-span bridge system with steel superstructure, full-depth precast deck panels, SDCL (simple for dead, continuous for live) connections, and a two-column bent that integrates precast columns with pocket and grouted duct connections is under construction for testing on shake tables at UNR.  The objective of the study to is to generate test data on interaction among various components and connections under biaxial simulated earthquake.  Approximately 300 channels of data are collected on this bridge.  The scope of the current project allows only for design, shake table testing, processing of data, and preliminary analysis.  Extensive analytical studies of the bridge model are essential to maximize the benefits of the shake table tests and translate the results into practical design methods.

Objective:
The purpose of the current project is to address the analytical modeling and design aspects related to the shake table testing of a two-span ABC bridge with steel girders.  The preliminary analytical studies of the bridge model were conducted under an ABC-UTC Cycle 2 project.  Specifically, the objective of this phase is to extensively analyze the bridge model using program OpenSEES.  The analytical model will be first evaluated and refined as necessary based on comparison of the calculated and measured data.  Critical measured seismic response data that reveal the global and local performance of the bridge system, its components, and connections will be compared with the analytical results.  Subsequent to satisfactory agreement between the calculated and measured results, the analytical model will be used to study several analytical variations of the bridge subjected to different earthquakes including near-fault and long duration earthquakes.  The purpose of this part of the study is to determine the sensitivity of the seismic response to changes in the bridge model and the input ground motions to identify refinements that are necessary in seismic design of ABC bridges, their components, and connections.  The results will then be translated into design methods for ABC systems.

Scope:

  • Task 1 – Update literature search on analytical studies of seismic performance of prefabricated bridge components, connections, and systems:  An in-depth literature search will be conducted to identify the most recent analytical modeling methods and results on dynamic load studies of prefabricated bridge elements and their connections.  The search will include any analyses of ABC bridge systems subjected to seismic loading.  Included will be precast deck panels and their connections to girders and to other panels.  Under Task 1 of the proposed study, the literature search will be updated and expanded to identify any new information that could potentially enhance the menu of analytical models for different earthquake-resistant ABC elements and connections.
  • Task 2- Identify critical macroscopic and microscopic bridge model response parameters and extract measured data for use in analytical studies:  The accuracy and acceptability of analytical modeling methods may be assessed at two levels: global response simulation and local response simulation.  The global seismic response consist of forces and displacements and relationship between these parameters that define stiffness and its variation as inelastic deformation in steel and concrete develop.  Other important global parameters consist of the effective stiffness and damping factor of the bridge.  These two parameters will be determined based on the white-noise tests that will be conducted in between shake table runs.  Local responses of importance are curvature and rotations in addition to strains in superstructure steel girders, steel reinforcement, and concrete at various critical sections of elements and connections.  The curvature and rotation data indicate the extent of section nonlinearity, while the strain data will help explain some of the visible damage that will be documented in the shake table tests.  Although SDCL, the cap beam, the superstructure, and the footing are designed to be capacity protected, the measured data will be streamlined for correlation studies with the analytical model and determine if these elements were indeed capacity protected and, if so, the margin against being damaged.
  • Task 3 – Conduct analytical studies of the bridge model:  The most likely analytical tool to be used will be OpenSEES, which is a proven software package for earthquake analysis of structures including bridges.  Fig. 8 shows a preliminary sketch of the OpenSEES model of the bridge.  The adequacy of analytical modeling methods to capture the inelastic response history of the bridge models will be evaluated under this task with respect to correlation between the measured and calculated data.    The correlation studies will address both global and local response data.  Based on experience of the PIs, reasonably detailed analytical models developed in OpenSEES are able to capture the global response of bridge columns and piers tested in the in-plane direction.  The work of Task 3 will determine if the same holds true for bridge systems with multiple components that are subjected to biaxial earthquake excitations.  Correlation studies at the local level will determine if standard element models incorporated in OpenSEES are able to replicate the measured response and the extent of any discrepancies between the measured and calculated results.
  • Task 4 – Refine the analytical model and conduct parametric studies:  Refinement of the analytical model will involve attempting different concrete and steel elements embedded in OpenSEES and identify those that lead to the best correlation between the measured and calculated results.  Once an acceptable correlation is achieved, there will be sufficient confidence in the analytical modeling techniques, and the model will be ready for parametric studies.  The parameters to be studies will be carefully selected to address issues that could be affected by a system response rather than component response.  The ultimate target of the parameters to be studied will be to generate information that could be incorporated in ABC seismic design guidelines.  An example of parameters to be studies in the effect of biaxial earthquake motions.  Past experimental studies on bridge piers have focused on the in-plane response of the piers, and conclusions have been reached.  One of the parameters to be studied will be to apply only transverse motion to the bridge and compare the results with those from biaxial motion studies.  Stresses that will be developed in the cap beam, the columns, and the superstructure connections are expected to be different.  To determine the effect of biaxial motion on SDCL connections, the bridge model will be analyzed only in the longitudinal direction of the bridge and again the strains at the connection will be compared to those from biaxial earthquake simulation.  These are only a few examples of the extensive parametric studies that are envisioned under this task.
  • Task 5 – Summarize the investigation and the results in a draft final report:  A final report describing the details of different tasks will be prepared and submitted to the ABC-UTC steering committee for review and comments.  Upon addressing the review comments, the report will be finalized and made widely available for dissemination.

Research Team:
Principal Investigators:  M. Saiid Saiidi, Ahmad Itani
Research Assistant:  Elmira Shoushtari

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