The National Academies

NCHRP 12-113 [Active]

Proposed Modification to AASHTO Cross-Frame Analysis and Design

  Project Data
Funds: $590,000
Staff Responsibility: Dr. Waseem Dekelbab
Research Agency: University of Texas - Austin
Principal Investigator: Dr. Todd Helwig
Effective Date: 6/1/2017
Completion Date: 9/30/2020

Developments in bridge design and analysis in recent years have created the need for improvements to cross-frame analysis and design for steel girder bridges. In the past, the configuration of cross-frame systems was generally based upon standard designs in which member sizes and layouts were dependent upon geometry and minimum member cross-section requirements. The opportunities for improvements to cross-frame analysis and design cover a variety of topics including: (1) improved definition of fatigue loading for cross frames in curved and/or severely skewed steel girder bridges analyzed using refined analysis methods; (2) implementation of stability bracing strength and stiffness requirements in the context of AASHTO Load and Resistance Factor Design (LRFD) bridge design; and (3) additional guidance for adjustment of the effective stiffness of cross-frame members in refined analysis models to reflect the influence of end connections on cross-frame member stiffness. Addressing these topics could result in a dramatic improvement in reliability and economy of cross frames for steel I-girder bridges.
The objective of this research is to propose modification to the AASHTO LRFD Bridge Design Specifications to provide quantitatively based guidance on: (1) the calculation of the fatigue design forces in cross frames in steel I-girder bridges; (2) the calculation of strength and stiffness requirements for stability bracing; and (3) the influence of cross-frame member end connection upon the cross-frame stiffness. Accomplishment of the project objective will require at least the following tasks.
PHASE I—Planning
Conduct a literature review of relevant domestic and international research, guidelines, and specifications including fatigue loading for transverse members. Document current design practices and how design software incorporates these design practices. (2.) Synthesize the results of the literature review to identify the knowledge gaps related to the research objectives. These gaps should be addressed in the final product or in the recommended future research as budget permits. (3.) Propose an analytical program to be executed in two parts as follows. Part 1, to be executed in Phase II, includes modeling and validation of three bridges as described in the field experiment in Task 4. Part 2, to be executed in Phase III, conducts comprehensive parametric studies to achieve the research objectives using validated models in Part 1 of the analytical program. At a minimum, the analytical program should consider the following:
    1. Analytical and loading studies (finite element analysis) to investigate appropriate fatigue stress ranges for evaluation of cross frames for right, skewed, and curved bridges, and Fatigue I and II;
    2. The influence of girder spacing, cross-frame stiffness and spacing (including staggered), and deck thickness;
    3. Development of stability bracing requirements for steel I-girder during construction and in-service extending available solution to include bottom flanges in compression in multi-span continuous bridges with non-prismatic girders. The analytical studies should include evaluation of how to combine stability bracing strength requirements with consideration of other loads such as wind load, construction loads, etc.; and
    4. Parametric modeling studies to investigate the effective stiffness of cross frames, including study of issues such as the effect of connection details and connection plate stiffness on cross-frame member stiffness reduction.
Propose field experiment, to be executed in Phase II, to achieve the project objectives. At a minimum, the field experiment should consider experimental verification of analytical models by instrumenting cross frames including one right bridge, one skewed bridge, and one curved bridge to measure cross-frame member fatigue force ranges under controlled application of live load. Same field experiment shall be repeated for in-service effective and maximum stress range. (5.) Identify existing articles of the AASHTO LRFD Bridge Design Specifications that require modification. (6.) Prepare Interim Report No. 1 that documents Tasks 1 through 5 and provides an updated and refine work plan for the remainder of the research no later than 4 months after contract award. The updated plan must describe the process and rationale for the work proposed for Phases II though IV.
PHASE II—Part 1 of Analytical Program and Field Experiment
(7.) Execute the Part 1 of analytical program that is related to field experiment only (i.e., three bridges) to finalize the field experiment work plan including a matrix of testing parameters and design details. Submit a task report for panel review and approval before conducting a field experiment. (8.) Execute the field experiment according to the approved Task 7 report. (9.) Validate the analytical program based on the results of the field experiment. (10.) Prepare Interim Report No. 2 that documents the results of Tasks 7 through 9 and provides an updated work plan for the remainder of the project. This report is due no later than 12 months after approval of Phase I. The updated plan must describe the work proposed for Phases III through IV.
PHASE III— Part 2 of Analytical Program and Proposed AASHTO Specifications
(11.) Execute the Part 2 of the analytical program as described in Task 3. Perform related parametric studies to cross-frame fatigue forces and live load. (12.) Based on the analytical and experiment investigations, develop specification and commentary language for proposed changes to the AASHTO LRFD Bridge Design Specifications supported with design examples and flowcharts. (13.) Prepare Interim Report No. 3 that documents and summarizes the results of Tasks 11 and 12 no later than 15 months after approval of Phase II.
PHASE IV—Final Products
(14.) Update proposed modifications to the AASHTO LRFD Bridge Design Specifications after consideration of Interim Review No. 3 comments and prepare ballot items for AASHTO Highway Subcommittee on Bridges and Structures consideration. (15.) Prepare a final report that documents the entire research effort.
STATUS: Research in progress

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