Load Distribution in Multipoint Rigging Using Lifting Beam

[phamENG]

Good morning, all. I'm trying to improve my rigging design skills and I'm having some trouble finding good information on verify load sharing/distribution through a multipoint rig. I'm considering a lifting beam with two slings coming off of each end to provide 4 lift points to the load. I know enough about rigging to know there are methods/devices out there to ensure equal (or, at least, near equal) loading on all four lines, but I also know that it's possible to get an imbalance resulting in two loaded lines and two "slack" lines. If anyone is able to give a basic rundown or point to a good reference (or both), I'd certainly appreciate it.

Thanks.

 

[robyengIT]

hope it can help

 

[robyengIT]

and this too

 

[robyengIT]

some info on unbalanced loads

 

[phamENG]

Thanks robyengIT. That's quite helpful. The first paper is one I've heard of but never sought out - it'll be useful for other projects in the future, I'm sure.

The last one you posted is interesting. "A perfect static weight distribution is achieved by using a spreader beam and two symmetrical pairs of anchors." In theory I agree, but in practice it seems dubious. If one sling is just a bit shorter than the other due to manufacturing tolerances or one has been used more and has stretched ever so slightly, the load will rotate and redistribute the reactions unevenly. In a proper setup this should be pretty minor. Are those sorts of tolerances considered in the OSHA safety factors, or is that something that should be accounted for separately?

 

[robyengIT]

I don't know about OSHA because I am Italian (European Standards ruling), but as far as I know You can almost settle the matter if You use adjustable slings (i.e. adjustable chains) that You can adjust during lifting trials. In any case chains or wire rope have (usually) a factor of safety = 5 or 6

 

[phamENG]

Yeah - OSHA minimum safety factor for components is 5. But it's also not the kind of safety factor one wants to "dip into" to account for known unknowns. I've never had much luck determining how the OSHA factors of safety are determined or what they account for, so it's tough to make the call. It seems like an additional load factor to account for minor variations in the load balance on the slings may be appropriate.

 

[robyengIT]

personal design rule : total load lifted by 3 slings (even if the slings are 4)

 

[r13]

Safety factor starts at least 4 for lifting devices, it covers the mishandlings/tolerances you had in mind, but OSHA also requires thorough inspect the devices and equipment before each use, as it can't estimate the extent of material damages. For the design of the item to be lifted, the designer shall evaluate these tolerances, and take into design consideration, as some mishandling may affect the distribution of loads in the lifted item that might cause failure.

 

[robyengIT]

some other useful docs. Google :

1 - NCCCO rigger reference manual
2 - Brookhaven basic rigging workbook
3 - DOE-STD-1090 hoisting and rigging
4 - CSAO hoisting and rigging - safety manual