The Skewing Forces
To achieve a lateral rail movement, allowed by floating rail installation, · magnitude of the lateral horizontal wheel force (skewing or straight side thrust forces) shall be larger than the wheel vertical load times the coefficient of friction between the rail base and the top of crane runway girder (Figure 7).
The analytical determination of the crane skewing forces is the "grayest" area among the present design codes and specifications.
AlST Technical Report No. 6 (TR 6) and CMAA Specification #70 propose oversimplified approaches.24 TR 6 considers only lateral forces at the four corners of the bridge frame due to dynamic asymmetry. CMAA Specification #70 provides a skew force factor as a function of ratio of the crane span to the wheelbase.
None of these documents provide determination of the steering force. which basically creates the crane rotation in the horizontal plan and causes friction (grinding) between the wheel flanges and the rail head.
Skewing forces determined by using the above documents, in most cases, do not exceed the value of crane vertical load multiplied by the coefficient of the friction, which mean that no lateral slide of the rail should be expected and there is no reason to make floating rail installations. However, the grinding of the rail head and wheel flange is observed due to friction of skew-positioned wheels during the crane motion, regardless of the "fixed" or floating" rail installation.
AIST Technical Report No. 13 (TR 13) does not address the skewing forces at all.1 It is hoped that the extremely conservative TR 13 straight side thrust provisions would cover the effect of skewing force. There is no evidence of the structural failures of buildings designed per TR 13 , except broken rail clip-to-girder connections found during periodic crane runway inspection, which show a significant magnitude of skewing forces.
A more accurate determination of the skewing forces is offered by title German Standard DIN 15108.9
If the crane takes a skew position relative to the crane runway, a contact (steering) force is produced on the front wheel flange, and the remaining crane wheels develop reactive lateral forces to resist the crane rotation in the horizontal plane. (Figure 8).
The magnitude of the steering force is a function of the skew angle. The larger the skewing angle, the larger the coefficient of contact friction of the wheel angle against the rail head. This coefficient of friction fluctuates from 0.09 to 0.30. The steering lateral force at the front wheel, determined in accordance with this approach, could achieve a significant magnitude, which would be large enough to move the rail in the lateral direction, if it is permitted. Figure 8 shows a skewing force diagram for the 16-wheel, 250--ton ladle crane, developed using the DIN 15018 approach. The steering wheel force of 121 kips is large enough to cas e the lateral movement. of the rail.
The field observations of the floating rail installations confirmed the rail lateral movements and this creates a valid concern about the accuracry of the skewing force prediction byTR 6 and CMAA.