Commentary on AISC Sections F12 and H3 Unsymmetrical Shapes

In our opinion Sections F12 and H3 need to be rewritten or not used for crane girder design. It is Structural Design Corporation’s experience that crane girders are subject to catastrophic, sudden failure if not properly designed. Our opinion is summarized on our Homepage. Crane girders are very unique structural elements of crane runways. The only discussion regarding the design of unsymmetrical shapes is found in Sections F12 and H3 of the AISC Steel Construction Manual.

F12 does not address shear center issues with unsymmetrical shapes which along with fatigue are the root causes of the catastrophic failures. Section H3 states that non-HSS members subject to torsion need the critical buckling stress determined by analysis. The F12 commentary does mention beams that do not have an axis of symmetry and that stress distribution must be determined from the principles of structural mechanics without mentioning shear center. The H3 commentary specifically mentions crane girders where the required stress needs to be based on established theories of structural mechanics.

AISC Design Guide 9 is noted as a source which provides a complete discussion on torsional analysis of open shapes. However, Design Guide 9 does not mention the need to determine the shear center and principal axis of the unsymmetrical shape for a complete analysis. Determining the shear center location is critical to any crane girder design as all crane loads rotate about the shear center. The shear center can be a few inches to several feet from the top of the crane rail where there are typically four (4) wheels with 125K vertical and 25K lateral loads for a 50T crane. It is our additional opinion that AISC’s stated position on unsymmetrical shapes is not realistic.

SDC spent many years to develop our proprietary CRANE GIRDER PRO analysis and design software to perform a complete stress analysis of any unsymmetrical shape. The program along with our extensive field experience allows SDC to answer the question, Why Do Crane Girders Fail?

Footer