The National Academies

NCHRP 12-80 [Final]

LRFD Minimum Flexural Reinforcement Requirements

  Project Data
Funds: $200,000
Research Agency: PBS&J
Principal Investigator: Jay Holombo
Effective Date: 2/14/2008
Completion Date: 11/30/2009

The AASHTO LRFD Specifications prescribe minimum reinforcement in flexural concrete members to safeguard against brittle failure following flexural cracking. There are indications that the current provisions for minimum reinforcement--which are based on a factored cracking moment, an over strength limit, and a recently increased modulus of rupture of the concrete--may cause designers to use excessive amounts of flexural reinforcement, unnecessarily increasing the cost of the structure. There is no commentary on the applicability of the provisions to recent advances in concrete design including but not limited to materials, continuous cellular cross-sections, unbonded tendons, or the Strength II load combination.  Furthermore, there is concern that the current method may not produce consistent reliability.
Provisions for the design of minimum reinforcement must be established that are suitable for all structure and reinforcement types covered by the specifications. These provisions need to achieve an appropriate and consistent reliability. Commentary must accompany these provisions to explain their application and inherent assumptions.
The objective of this research was to develop recommended revisions to the AASHTO LRFD Bridge Design Specifications and Commentary for rational design of minimum reinforcement to prevent brittle failure of concrete sections.
Products Availability: The contractor's final report is available online as NCHRP Web-Only Document 149. The contractors draft final report can be downloaded: 

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