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

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Bruce,

The CMAA specifications are written for cranes only.  The impact arises from the hoisting motion of the 
trolley hoist, hence the percentage of the hoist speed.  The impact may come from suddenly braking the 
downward motion of a load or suddenly lifting a load.  You often get operators jogging a load to get it 
to a certain position- this can set up a harmonic action which makes things worse.  
The AISC spec and the Canadian standard and many others specify impact factors which are a reflection of 
the crane either lifting/lowering a load, moving down-aisle thereby hitting unevenness in the rail or 
splice points.  The lifting/lowering effects are generally absorbed by the crane.  The hitting of 
"potholes" is generally not too severe but the effects can add up after a while.  Studies (Milman, I 
believe) have indicated that the impact factor on a runway on a runway is only approximately 7%.  The 
various codes have used 10 and 25% for quite a while.  The lower value is for pendant or hand-operated 
cranes which are slow, the 25% being reserved for cab or radio controlled cranes which can be faster.  
Impact applies only to the runway beam as the deflection of the beam creates a magnifying effect.  
Consequently, impact does not apply to columns and foundations.  

Gary Hodgson, P.Eng.

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On 20 May 2003 at 15:44, Bruce Holcomb wrote:

> 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 3.3.2.1.3.4.2).
> 
> 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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