Memorandum

TO:	All Design Section Staff - 47340
FROM:	Chuck Ruth
DATE:	July 13, 2000
SUBJECT:	AASHTO LRFD Article 5.8.3.5, Longitudinal Reinforcement Requirement

The AASHTO LRFD Specifications, Article 5.8.3.5 for the longitudinal Reinforcement Requirement, unexpectedly results in more shear reinforcement in the middle part of the girder than the end regions. This requirement is primarily intended for the confinement of the sections subjected to shear and shall be primarily applied to the end regions of the girder where shear demand is higher. To avoid this inconsistency, the application of the AASHTO LRFD Specifications Article 5.8.3.5 for the longitudinal reinforcement shall be limited to the regions of simple span concrete girders where Mu/ffdv is smaller than (Vu/f-Vp)cotq. The provisions of Article 5.8.3.5 shall be checked for continuous structures or structures made continuous for live load and superimposed dead load.

Background

Article 5.8.3.5 of the LRFD Bridge Design Specification states that Equation 5.8.3.5-1 must be satisfied at sections not subjected to torsion. Towards the center of a span the inclination of the cracks should approach 90° as flexure becomes the dominant effect. At the center of a span, Equation 5.8.3.5-1 becomes a simple expression of ultimate moment capacity which is easily satisfied. However, the inclinations of the cracks are limited to 43° (See Table 5.8.3.4.2-1) in the AASHTO LRFD criteria. This limitation prohibits the cotq term in Equation 5.8.3.5-1 from approaching zero, thereby resulting in overly conservative shear reinforcing requirements.

If you have any questions, please contact Richard Brice at 705-7174 or Bijan Khaleghi at 705-7181

CCR/bk

cc:     J. A. Weigel - 47340

        E. H. Henley - 47340

        H. L. Coffman - 47341

        F. Higgins - 47340

        Z. Fahoum - 47340