Precast/Prestressed Girder introduces a powerful new feature that will enable bridge designers to compute and subsequently use more accurate live load distribution factors in the design and analyses of prestressed concrete bridge beams.

By default, the live load distribution factors in Precast/Prestressed Girder are computed using the formulae presented in the bridge design specifications (either AASHTO Standard or AASHTO LRFD) as mentioned in Theory. However, there are certain situations as described below in which the user may wish to compute the load distribution factors using more accurate methods.

• When the parameters are outside the range of applicability specified in the code.
• When working with the older approximate distribution factors in Standard Specifications.
• When skew angles are varying for the beginning and end of span.
• When more accurate DF computation is needed during the bridge rating process.
• When an alternative to the conservative Lever Rule approach is desired. Currently the Lever Rule method gives very conservative results for the design of the exterior beam (when used with an LRFD multiple presence factor of 1.2 for single lane).
• When a non standard gage width truck load rating is required.

The analysis engine that computes these distribution factors is that of a rib-stiffened plate (for solving skewed cases) or a plane grillage model (for solving non-skewed cases). The program generates the appropriate finite element model based on the geometry input into the program and then generates the various load case responses and calculates the corresponding load distribution factors. The detailed theory and background of the modeling techniques used in the computation of the refined distribution factors can be obtained from Reference 1.

Sufficient flexibility has been provided for using this option withinPrecast/Prestressed Girder. When this option is selected the program computes distribution factors, for positive moment, for negative moment and for shear, along the tenth points of the span compared to a single distribution factor computed by the equations for each beam. In keeping with the requirements of LRFD Art. 4.6.3.1, which states that when using refined method of analysis, a table of live load distribution coefficients shall be provided to aid in permit issuance and rating of the bridge, the program outputs distribution factors for all tenth points along the selected beam. You can also use different methods for different beams, i.e., you can apply the refined method to the exterior beams and use the code equation method for the interior beams, etc.