General

One of the features that distinguish the AASHTO LRFD Bridge Design Specifications from the Standard Specifications is the introduction of the Modified Compression Field Theory (MCFT) for the prediction of ultimate shear capacity. The shear capacity provisions of the LRFD specifications have changed many times over the years. Changes have been made to simplify the application of the MCFT method and provide alternatives that more closely resembled the Standard Specifications.

WSDOT has issued three design memorandums relating to shear capacity analysis, establishing office policy that deviates slightly from what is described in the AASHTO LRFD. Listed below are the design memorandums. Each describes a method of computing shear capacity.

June 2001 Design Memorandum
Simplified computation of longitudinal strain by letting 0.5cot(q) = 1.0.

April 2007 Design Memorandum
Directs engineers to follow the June 2001 design memorandum and not apply the provisions of LRFD 5.8.3.4.3 (Vci - Vcw method)

August 2007 Design Memorandum
Supercedes June 2001 design memorandum and directs engineers to use, what will be known as the "General Procedure" when published in the 2008 interim provisions.

Load Rating

One of the unique features of the MCFT is that ultimate shear capacity is a function of loading. Even though load ratings are computed for individual vehicles the shear capacity is computed based on the live load envelope of which the vehicle belongs. For example, the shear rating for the legal load rating for routine commercial traffic may be controlled by the Type 3-3 vehicle, the shear capacity will be based on the envelope of all the AASHTO Legal Load vehicles. In most all cases, the vehicle causing the maximum shear in the live load envelope will be the same vehicle governing the load rating.