Since crane girders fail due to torsion and fatigue, industry needs new design standards. The traditional flexural analogy design only applies to double symmetric crane girders. This is proven by SDC’s technical paper on AIST Torsion design theory. Singly symmetric and unsymmetrical cross sections need to be designed by performing a full, detailed torsional and fatigue analysis. All loads including the dead weight of the crane rotate about the girder shear center. This is how tension forces are created in what is normally the compression zone of a girder. Other methods such as finite element analysis do not work. This is the reason SDC developed CRANE GIRDER PRO.
Crane girders can also fail due to serviceability issues. A torsional resistant girder needs to be proportioned so that is falls in the area of mixed torsion. Mixed torsion is the area between St. Venant torsion (Pure Torsion) and Warping Torsion. Crane girder design includes limiting the amount of rotation or twist.
SDC recommends the use of girder tie-backs to connect the girder top flange to the building column. Some tie-backs have spherical bearings to allow the girders to rotate. The tie-backs need to be designed for both the lateral and torsional loads including crane rail offset. SDC supplies tie-back manufactures with the design loads that are doubled to address stress reversal. All girder elements need to be designed for stress reversal including the building and crane column connections. Since the tie-back structural design is proprietary to the different suppliers, we doubt that they are designed for stress reversal. SDC has also learned that the tie-back bolts need to be fully tightened to meet AISC Steel Design requirements. Otherwise the bolts can become loose or fail. It is likely that calibrated hydraulic torque wrenches will be required to generate the bolt tension.
Fatigue resistant crane girders require the minimum use of welds. This is accomplished by eliminating the intermediate web stiffeners and specification of a thicker web plate. Maintenance Management should make it clear to all personnel that no welds on cranes or crane girders should be make unless approved by Engineering. For a complete print out of the computer model and stress report for a W40x324 girder with a 3/8″ thrust plate and W14x30 back-up beam, go to Crane Girder Pro Calculation Output. A summary calculation is shown in the Crane Girder Analysis Summary Calculation for a similar girder. These types of girders are not specifically addressed by AIST Technical Report No.13 but are found in every steel mill.