Research for AASHTO Standing Committee on Highways. Task 306. Revision of Strut-and-Tie Provisions in the AASHTO LRFD Bridge Design Specifications

Strut-and-tie modeling (STM) for the design and detailing of concrete members subject to geometric and load discontinuities was first incorporated within the American Association of State Highway and Transportation Officials (AASHTO) Load and Resistant Factor Design (LRFD) Bridge Design Specifications in 1994. The provisions are based on research that dates back to the 1980s. Research conducted over the last two decades has provided far greater insight into the design of concrete members via strut-and-tie modeling. Particular deficiencies of the current provisions have been commonly referenced within the literature and recognized by design professionals: (1) Overly Conservative: An assessment of the AASHTO LRFD strut-and-tie provisions was conducted as part of Texas Department of Transportation (TxDOT) Project 0-5253. The current provisions were found to be inaccurate in comparison to alternate code procedures. Strength estimates consistently typically exceeded the measured values by 50 to 500 percent. (2) Ambiguous: Strut-and-tie modeling is limited to components with a shear span-to-depth ratio less than 2. At this limiting ratio, the shear strengths calculated using Strut-and-Tie Model (LRFD § 5.6.3) and Sectional Design Model (LRFD § 5.8.3) can differ greatly; unrepresentative of observed behavior. (3) Difficult to Implement: AASHTO Subcommittee T-10 has acknowledged the design community's long-standing confusion and difficulty regarding implementation of the strut and- tie procedures (see NCHRP 20-07,Task 217: Verification and Implementation of Strut-and-Tie Model in LRFD Bridge Design Specifications). The mandated use of these procedures (effective October 2007) further highlights the importance of a revision. Updates to the strut-and-tie provisions within AASHTO LRFD will reflect the most recent state of knowledge within the bridge design community. In particular, the STM provisions will benefit from unique data and knowledge gathered during TxDOT Projects 0-5253 and 0-6416. Large-scale tests of over 50 specimens (up to 75" deep) have provided unprecedented insight into the behavior and design of directly (e.g. rectangular bent caps) and indirectly (e.g. inverted tee bent caps) loaded concrete members. Ultimately, revisions to the strut-and-tie provisions within LRFD will allow simple implementation of STM for the design of safe, serviceable structures. The objective of this project is to update the AASHTO LRFD Article 5.6.3 Strut-and-Tie Model to reflect the most recent state of knowledge regarding the behavior and design of concrete members subject to geometric and load discontinuities.

Language

  • English

Project

  • Status: Completed
  • Funding: $76000.00
  • Contract Numbers:

    Project 20-07, Task

  • Sponsor Organizations:

    Federal Highway Administration

    1200 New Jersey Avenue, SE
    Washington, DC  United States  20590

    American Association of State Highway & Transportation Officials (AASHTO)

    444 North Capitol Street, NW, Suite 225
    Washington, DC  United States  20001

    National Cooperative Highway Research Program

    Transportation Research Board
    500 Fifth Street, NW
    Washington, DC  United States  20001
  • Performing Organizations:

    MDC Research, Incorporated

    ,    
  • Principal Investigators:

    Mitchell, Denis

  • Start Date: 20110422
  • Expected Completion Date: 0
  • Actual Completion Date: 20130228
  • Source Data: RiP Project 38697

Subject/Index Terms

Filing Info

  • Accession Number: 01549634
  • Record Type: Research project
  • Source Agency: Transportation Research Board
  • Contract Numbers: Project 20-07, Task
  • Files: TRB, RiP
  • Created Date: Jan 6 2015 1:00AM