BACKGROUND

Most bridges respond inelastically to large seismic demand. However, seismic design using the AASHTO LRFD Bridge Design Specifications and the AASHTO Guide Specifications for LRFD Seismic Bridge Design methodologies relies on an elastic analysis to estimate the inelastic demands using the “equal displacement approximation.” The equal displacement approximation, introduced over 60 years ago, has not been fully validated.

Advances in numerical and physical methods suggest that the equal displacement approximation underestimates the “real” seismic displacement demands. Consequently, bridges may not perform as intended when using the elastic analysis methods used for most designs. The potential underestimation associated with the AASHTO provisions was verified by the recently completed NCHRP Project 12-106, “Proposed Guidelines for Performance-Based Seismic Bridge Design.”

Research is needed to examine the equal displacement approximation and, if necessary, develop simple, reliable adjustment factors that can modify the elastic displacement predictions to better match the “real” inelastic displacement response. The research outcome should provide the basis for design recommendations to better predict seismic demands resulting in safer and more reliable bridges.

OBJECTIVE

The objective of the research is to develop robust yet simple procedures for calculation of inelastic displacement demands in bridges subject to earthquake demands.

Accomplishment of the project objective will require at least the following tasks.

TASKS

PHASE I – Planning

Task 1.  Conduct a literature review of relevant research related to nonlinear bridge response and the impacts of damping models and other factors on displacement demand estimation. The review shall include published and unpublished research conducted through the NCHRP; FHWA; and other national, international, state, and pooled-fund sponsored research.

Task 2.  Synthesize the results of the literature review to identify the knowledge gaps related to the research objectives, and the limitations of current analysis techniques. These gaps should be addressed in the final product or the recommended future research as budget permits.

Task 3.  Compare current techniques that use elastic response spectrum analysis for estimating inelastic displacement demands against the results of existing nonlinear response history analysis and existing experimental data for single degree of freedom and multiple degree of freedom bridge systems.

Task 4.  Propose a research plan, to be executed in Phase II, to achieve the project objective. At a minimum, the research plan shall include:

• The development of an improved methodology to calculate inelastic displacement demands from elastic response spectrum analysis;
• The development of recommendations for the appropriate damping models when nonlinear response history analysis is to be deployed;
• A process to validate the results using existing experimental data; and
• A list of design examples to demonstrate the use of the methodology. The design examples shall cover a range of bridge geometry and seismic zones. 

Task 5.  Prepare Interim Report No. 1 that documents Tasks 1 through 4 and provides an updated work plan for the remainder of the research no later than 6 months after contract award. The updated plan must describe the process and rationale for the work proposed for Phases II through III.

PHASE II - Methodology Development 

Task 6.  Execute the proposed research plan according to the approved Interim Report No. 1. It shall include (1) the development of the methodology to calculate inelastic displacement demands from elastic response spectrum analysis; (2) the development of recommendations for the damping models when nonlinear response history analysis is to be deployed; and (3) validation of the methodology and the recommendations using existing experimental data.

Task 7.  Prepare Interim Report No. 2 that documents Task 6 and provides an updated work plan for the remainder of the research no later than 19 months after the acceptance of Phase I. The updated plan must describe the process and rationale for the work proposed for Phase III.

PHASE III – Final Products

Task 8. Revise the proposed methodology to calculate inelastic displacement demands and the recommendations for the damping models described in Task 6 after consideration of the panel’s review comments.

Task 9.  Develop design examples according to the approved Interim Report No. 2.

Task 10.  Prepare and submit draft language that may be considered by the AASHTO Committee on Bridges and Structures for inclusion in its next update of the AASHTO LRFD Bridge Design Specifications and Guide Specifications for LRFD Seismic Bridge Design for NCHRP review and approval.

Task 11. Prepare final deliverables including: (1) a final report that documents the entire research effort; (2) a methodology for accurately determining seismic displacement demands; (3) requirements for the modeling of damping in nonlinear response history analysis; (4) design examples; (5) draft language that may be considered by the AASHTO Committee on Bridges and Structures for inclusion in its next update of the AASHTO LRFD Bridge Design Specifications and Guide Specifications for LRFD Seismic Bridge Design; and (6) a stand-alone technical memorandum titled “Implementation of Research Findings and Products” no later than 6 months after approval of Phase II.

STATUS:  Research In Progress