Static equivalent hoist load

[henerythe8th]

While in a discussion regarding the analysis of some portable skids that will be lifted with a crane (about 8ft X 30ft) the subject of "hoisting loads" was mentioned. While the hoisting should be performed in a "smooth and consistent manner" it seems logical that a "dynamic load" will be induced. Without doing some dynamic calcs based on a HEAP of assumptions relative to the hoisting maneuver is there a "rule of thumb" or code driven (unable to find in ASD manual, IBC, UBC, etc.) static equivalent that the structure can be evaluated to?

 

[IFRs]

Unless you are hoisting in a non-normal set of circumstances and will be jerking the load deliberatly, my guess is that the safety factors built in to the proof load design and the difference between working load and breaking strength are there for the exact purpose you are worried about - the difference between paper calcs and the real world.

Having recognized a potential safety issue, you could upsize every part of the system to increase your factor of safety.

 

[dbuzz]

Australian Standard AS 1418.1 applies a dynamic partial load factor the hoisted load. The factor is dependant on the hoisting acceleration and velocity and ranges from 1.1 to 2.2.

If you think about it, a 100-tonne crane can exert a much greater "shock" on a 2-tonne load than say a 10-tonne crane.

 

[corus]

Also refer to the british standard on rules for the design of cranes, BS 2573, or whatever it might be called now. The maximum impact factor in lifting is 2.

corus

 

[henerythe8th]

Thanks for the info! Just to clarify. I am fishing for information regarding the evaluation of the skid structure, not the crane structure. A commercial, portable crane will be used for placement and relocation of the skids.

 

[steve1]

HenryVIII,

I regularly engineer skids for a manufacturer of natural gas metering devices. These skids are up to 60 foot long and may contain multiple metering "runs".

I have found that the largest stresses occur during the lifting and setting operations.

For these skids (composed of tube steel for the base frame and various other shapes for pipe and equipment supports) I base the design on the maximum deflection during the lift. Working with the manufacturers engineers we have determined that for our purposes the maximum deflection allowed is 3/8". If the calculation indicates that we exceed that value then we simply add more lifting locations. Since stiffness is more important than stress we very seldom obtain stresses beyond 10 ksi in the base steel.

 

[henerythe8th]

Thanks steve1!

Do you apply a percentage "adder" to the weight of the skid to account for the hoisting? Or do you just design the skid as if it were just hanging by its lift points?

henerythe8th

 

[SlideRuleEra]

henerythe8th - For the conditions you describe "smooth & consistent manner", a reasonable assumption could be based on the AISC Manual of Steel Construction, 9th Editon, ASD, page 5-29. Impact load equals 100% of the live load. (Of course the total load to evaluate is then the sum of live load + impact load).

 

[JStephen]

Based on the crane handling I've seen, big objects tend to be handled pretty gingerly, and I would assume that acceleration forces were negligible. The exceptions might be if it was to be handled by container cranes or loaded on a ship or something else out of the ordinary.

Normally, crane load charts don't require you to add acceleration to the weight of the object, if that helps.

 

[steve1]

I have to agree with JStephen. I've witnessed these skids being lifted a number of times. Hard to see much acceleration.

I just use dead weight (very carefully calculated) for the lift, but I do add in a horizontal component at the lifting points due to the sling geometry.

 

[henerythe8th]

Thanks for the info, will make for an even better discussion with my cohort!

henerythe8th

 

[dpa]

I agree with SlideRuleEra. The navy standard for testing cranes and booms was a 200 percent static load with the load being placed on the hook by an even larger crane. Or just as he said impact load equals 100 percent of rated load.

DPA