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Crane impact factors in AISC / Building Codes / ASCE 7/ CMAA

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Does anyone know why there is a big difference between the vertical impact factors in the CMAA "Specification for Top Running ..... Cranes" (publications 70 & 74) and the vertical impact factors in the 9th ed AISC "Manual of Steel Construction", IBC 2000 and ASCE 7-98?  

CMAA indicates the vertical impact to be 0.5% of the hoist speed in ft/min but not less than 15% nor more than 50% of the rated capacity (section

AISC, ASCE 7-98 and IBC 2000 (as well as other model building codes) indicate that vertical impact is 10% of the wheel load (for electric, pendent operated bridge cranes) and 25% for cab-operated bridge cranes.

Does the CMAA impact apply only to the design of the crane while the 10% applies to the building design?  Am I missing something?

On a related note, does the impact factor apply to columns and footings or only to the runway beams and connections?

Bruce D. Holcomb, PE
Butler, Rosenbury & Partners
300 S. Jefferson, Suite 505
Springfield, MO 65806-2217
ph (417) 865-6100
fax (417) 865-6102

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