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Calculation of flange stresses for underhung trolleys
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- Subject: Calculation of flange stresses for underhung trolleys
- From: Ahltomjo(--nospam--at)aol.com
- Date: Sun, 26 Dec 1999 20:41:36 EST
The December 1999 issue of Modern Steel Construction's Steel Interchange (page 9), had a question concerning the design calculation of the lower flange loading capacity of a steel beam to be used to support an underhung crane. It further asked if there are any published ASD or LRFD design procedures. The answer given by David T. Ricker of Javelina Explorations of Payson, Az., investigates capacity based on tensile stress and on bending stress. The capacity based on tensile stress in the web is determined by using 3.5 x k times the web thickness as the effective area of the member. This seems like a reasonable way to proceed. The capacity of the member would then be the effective area times the allowable AISC tensile stress. He further investigates the capacity based on flange bending using the section modulus of an "equivalent cantilever" beam whose effective width is 2xe (e is the eccentricity of the load measured from the k1 point as presented in the AISC Manual) and whose depth is equal to the thickness of the flange. The allowable moment is equal to the section modulus times the AISC bending allowable. The length of the "equivalent cantilever" beam is the eccentricity plus the edge distance, although it does not enter into the calculation. The "equivalent cantilever" beam is considered to have free unsupported longitudinal edges. The moment is equal to the local load times the eccentricity. The stress calculation results in e cancelling out in the numerator and denominator and is thus independent of the eccentricity. In other words, as the eccentricity decreases, the effective width also decreases. The result is a very low capacity and seems to be unrealistic and very conservative. It would seem that taking into account the continuity of the longitudinal edge of the beam (instead of assuming it to be a cantilever slice) would dramatically influence the capacity calculation. It would also seem to be more realistic to use a greater effective width of the "equivalent cantilever" member. Possibly, something more like the 3.5 x k value used in determining the tensile effective area. Is there a more realistic assessment of the flange bending capacity which accounts for the continuity of the "equivalent cantilever" beam's edge restraint? More importantly, is there a prescribed ASD or LRFD procedure? Tom Ahl
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