Cracks form in structures where tension forces or sometimes shear forces exist. In a simple beam the top flange is in compression while the bottom flange is in tension for normal gravity loads. Crane girders are unique since cranes generate both lateral and longitudinal loads as they bridge and trolley down a runway. These additional loads applied at the top of the crane rail can be up to 50% of the wheel loads and make crane girder design very complex. SDC has inspected, designed, and worked with contractors to repair and replace crane girders for over 30 years.
Most crane girder failures start near the intermediate stiffeners near the top flange of the girder. The fillet welds between the flange and web have had a long history of cracking issues. Base metal cracks also develop in the girder web at the corner where the stiffeners abut the top flange. The question is how do weld and base metal cracks form in the compression zone of a crane girder from a very heavy crane?
SDC has studied this issue for many years. AIST Technical Report 13 is a steel industry standard that addresses both mill building and crane girder design. Technical Report 13 is an excellent source for determining the lateral and longitudinal crane loads imposed on a crane girder. These loads are unique since they are applied at the top of the crane rail which induces torsional stress into the girders. Technical Report 13 has a method to determine torsional stress by using the concept of flexural analogy However, Section 5.8.1 in the report states that “An exact analysis and design solution is complex and beyond the scope of this document”.
SDC’s CRANE GIRDER PRO performs an exact structural analysis of a crane girder. The exact analysis determines the location of the shear center for the cross section. All loads including the dead weight of the crane rotate about the shear center. This is how tension forces are created in what is normally the compression zone of a girder.