TO: All Design Section Staff - 47340 FROM: Chuck Ruth DATE: July 13, 2000 SUBJECT: AASHTO LRFD Article 126.96.36.199, Longitudinal Reinforcement Requirement
The AASHTO LRFD Specifications, Article 188.8.131.52 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 184.108.40.206 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 220.127.116.11 shall be checked for continuous structures or structures made continuous for live load and superimposed dead load.
Article 18.104.22.168 of the LRFD Bridge Design Specification states that Equation 22.214.171.124-1 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 126.96.36.199-1 becomes a simple expression of ultimate moment capacity which is easily satisfied. However, the inclinations of the cracks are limited to 43° (See Table 188.8.131.52.2-1) in the AASHTO LRFD criteria. This limitation prohibits the cotq term in Equation 184.108.40.206-1 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
cc: J. A. Weigel - 47340
E. H. Henley - 47340
H. L. Coffman - 47341
F. Higgins - 47340
Z. Fahoum - 47340