The method for computing the effective flange width changed with the AASHTO LRFD Bridge Design Specifications, 4th Edition, 2008. The effective flange width is now taken to be the tributary width of the slab. See LRFD 4.6.2.6.1. This method of computation is only applicable for bridges with skew angles less than or equal to 75 degrees, L/S greater than or equal to 2.0 and overhang widths less than or equal to 0.5S. In unusual cases where these limits are violated, a refined analysis should be used to determine the slab effective width.

The effective flange width settings allow you to ignore these limitations and compute the effective flange width in accordance with LRFD 4.6.2.6.1 regardless of the bridge geometry.

NOTE: Ignoring the limitations on computing effective flange width is generally NOT RECOMMENDED. Ignoring the limitations should only be done for preliminary analysis or in cases where you have verified the validity of the computational procedure for your particular structure.