What are the Lb and Lcomp fields on the Hot Rolled Steel Design Parameters screen?
These fields are the unbraced lengths for a member and control the AISC code checking that RISA-2D/3D can perform on AISC hot rolled steel shapes. For a detailed description, see the appropriate section in the program manual. In a nutshell, the Lb values (unbraced length) control the calculation of the allowable axial stress (Fa), while the Lcomp value (unbraced compression flange length) controls the calculation of the allowable bending stress (Fb). These fields are provided because in many cases the actual unbraced lengths are different than the member lengths in a structural model. For example, a column with K-bracing framing in on one side only would need to be composed of two model members, yet the unbraced length out of the plane of the bracing would be the full column height.
