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CASE CN7 - Rotate Structures on Portal and Floating Cranes, P. Malone
Problem:  Stability of Rotate Structures on Portal and Floating Cranes for
handling critical loads.
Symptom:  Some of the rollers that provided support under all loading
conditions lifted off the supporting structure during load test and
Solution:  A finite element model of typical portal frame was analyzed. The
results of this analysis indicated that the portal frame was flexible and that
the rollers carried load in proportion to the stiffness of the supporting
member.  The rotate structure (upper structure) housed all of the machinery
including the power plant.  Because of the requirement to maintain alignment
of installed equipment, the rotate structure is constructed very rigid and
therefore when loaded, does not deflect to the same degree as the portal
Based on the above information and varied field trips a procedure was set up
to evaluate existing portal and floating cranes with key points of evaluation
as follows:
1.  Park the crane on the level section of the track with the boom
parallel to the track (this provides the most stable conditions for
stability analysis).
2.  For NAVFAC designed portal cranes with the 125% test load,
check rollers of the least loaded quadrant for contact with the
rotate structure and portal frame. If rollers make contact with
both structures, stability is satisfactory.
3.  If
rollers do not make contact with the rotate structure or the
frame check the center steadment nut for contact with the
frame.  If clearance is maintained between steadment nut and
frame, stability is satisfactory.
4.  If contact is made between center steadment nut and portal
frame, an engineering evaluation is required to determine the
suitability of the crane for service at the existing rated load.

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