Crane Issues
Cranes create girder issues from the perspective of duty cycles and crane operation. The most damage to crane girders is found in slab yards, coil storage buildings and shipping buildings where the duty cycles are very high. Many crane operators receive incentive pay based on the amount of steel they can move. The operators lift, trolley, and bridge down the runway simultaneously creating larger lateral loads. Operators also stop cranes bridging down the runway at top speed (5 points) by reversing the drive motors. This practice of “plugging” the crane to a stop can cause crane misalignment. Crane misalignment causes crane rail wear which can transmit even larger lateral loads to the crane girders. Cranes need to be surveyed for “square” and be realigned. The interaction between cranes and crane girders is complex but girders cause more damage to cranes than cranes cause to girders until the crane runways and crane rails become misaligned. Then damage to other crane runway components such as girder tie-backs, diaphragms, and crane/building columns also occurs.
Runway Issues
The combination of both crane girder design and crane operation create crane runway issues. SDC designs the crane girders to be independent of the building structure. Crane girders are connected to the crane columns and flange of the building columns. The traditional practice of connecting thrust plates or lacing angles to backup beams and column webs make the runways too rigid. Lateral and longitudinal loads are transferred to the most rigid building components until they fail. The assumption that the lateral bracing only restrains lateral movement is not correct. The crane girder becomes an unsymmetrical cross section that dramatically changes girder behavior.
Misaligned crane rails and girders create additional damage to both the crane, crane girders, and other elements of the crane runway. Crane girders can become misaligned due to torsion stresses from the unsymmetrical girder cross section. Columns can tilt due to heavy slabs or coils stored too close the columns overloading the footings.