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The National Academies

NCHRP 20-07/Task 306 [Final]

Revision of Strut-and-Tie Provisions in the AASHTO LRFD Bridge Design Specifications
[ NCHRP 20-07 (Research for AASHTO Standing Committee on Highways) ]

  Project Data
Funds: $76,000
Additional $40,000 (FHWA contributed $20,000) for Phase II
Research Agency: MDC Research, Inc.
Principal Investigator: Denis Mitchell
Effective Date: 4/22/2011
Completion Date: 2/28/2013

BACKGROUND
Strut-and-tie modeling (STM) for the design and detailing of concrete members subject to geometric and load discontinuities was first incorporated within the AASHTO LRFD Bridge Design Specifications (LRFD) 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 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 AASHTOLRFD 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.

OBJECTIVE
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.

PHASES

Phase I—Recommended Specifications

  1. Comprehensive review of the strut-and-tie modeling procedures recommended by TxDOT Project 0-5253 and prominent international design codes including: NCHRP 20-07/Task 217, Eurocode 2, ACI 318-08, CAN/CSA A23.3-04, and CEB-FIP Model Code 1990.
  2. Synthesize the modeling recommendations into a straightforward design procedure suited for routine design of concrete structures.
  3. Develop specification language, commentary and associated graphics for inclusion within future editions of the AASHTO LRFD Bridge Design Specifications.
  4. Make a presentation on the strut-and tie revisions to the AASHTO SCOBS Technical Committee T-10 Concrete Design for review and comment.
  5. Revise the recommended specifications and commentary in accordance with T-10 feedback.

Phase II—Examples

  1. Explanations of the strut-and-tie model and important behavioral aspects
  2. Explanations of specific changes to the specifications
  3. Develop detailed design examples illustrating the recommended strut-and-tie provisions of the AASHTO LRFD Bridge Desing Specifications
  4. Develop additional detailed design examples illustrating the application of the strut-and-tie provisions of the AASHTO LRFD Bridge Design Specifications to prefabricated bridge element technology
  5. Revisions to design examples based on comments from project panel
STATUS: Agency final report was submitted to AASHTO Technical Committee on Concrete Design (T-10). (NCHRP Staff: W. Dekelbab)

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