Offshore structures are code checked to the AISC 9th edition for W-section. In the case where there are no beam failures for Cb = 1.0, there is no issue.

However, where there are beam failures and the application of Cb is used to enhance the beam moment capacity, Cb calculated to the AISC 9th edition only applies to moment diagrams that consist of straight lines - a condition that is rare in beam design [see extract taken from AISC 360-10 commentary attached].

The AISC 360-10 provides a Cb equation that gives a more accurate solution for a fixed-end beam [see extract taken from AISC 360-10 commentary attached]. As such, having the option for a user-defined Cb will facilitate a more accurate Cb value is applied to beams for code checks to the AISC 9th edition.