The shear check is based on AASHTO Article 5.8.6, Shear and Torsion for Segmental Box Girder Bridges.

The shear check design request provides the following results:

• Controlling Demand/Capacity Ratio
• Shear Rebar Area/Length
• Torsion Rebar Area/Length
• Shear + Torsion Area/Length
• Longitudinal Rebar Area - shear causes tension in longitudinal reinforcement (see AASHTO LRFD 2007, C5.8.3.5)

Web Shear Resistance and Web Shear Forces

The assumption here is that the shear force and resistance act along the length of the web. The capacity is just proportional to the area, but the vertical force is increased by 1/cos(α) by a truss analogy to get the force along the web. However, since we are actually reporting in the vertical direction rather than along the web, we instead reduce the capacity by cos(α).

This is basically saying that an inclined web is not as good at resisting vertical shear as is a vertical web of having the same area.