sling forces in 4 point lift

[Qluq]

Hi everyone,

what would the "usual" way to find the sling forces in a 4 point lift be? at the moment i am setting up a hand calculation to find them using the displacement method. it is turning out to be quite difficult and i am starting to wonder how other people have solved this.

for the time being i assume that one of the sling legs becomes slack and calculate the sling forces in the other 3 legs. then assume that one of the other sling legs becomes slack until i have 4 upper limits of which i select the maximum to size lifting lugs. but what would the actual exact solution look like?

thanks for your thoughts!
Mark

 

[Sjqlund]

Hi Mark

Depending on what code you follow, you might just distribute the load in 4, and the apply some additional safety factor.

E.g. according to this:

http://files.engineering.com/downlo...idelines_for_Marine_Lifting_Operations[1].pdf

See section 5.7. Here you are required to apply a "f_SKL (skew load factor)" of 1.25.

Regarding structural analysis - you need to post some more info if you need help on that.

 

[Qluq]

Hi Sjqlund,

thanks! i have made acquaintance with some standards now (DNV, Lloyd's, and indeed Noble Denton), but my question is actually concerned more with the process before adding safety factors...

if i understand correctly you mentioned to simply divide the sling load equally among the sling legs. but how would you calculate the sling forces when the sling hub is positioned directly over an off-centre center of gravity? in other words, when all 4 sling legs have different lengths? this is a statically indeterminate problem in 3 dimensions, and becomes quite confusing.

some standards (DNV for instance) indicate that deviations of the CoG must be included in a safety factor called PL (for Percentage of Loading). it is off course 1/(number of legs) in case of equal length sling legs and centered CoG, but has another value in other cases. again the problem of finding all 4 sling forces...

how do you guys handle this normally? it must have been done thousands of times before, right? i am the only guy in my company doing calculations, so i don't have anyone nearby to discuss this with :-(

 

[Sjqlund]

I do not have experience with that kind of load situation myself. However, if i had this problem i would probably use software to solve it. I imagine you could model it as a truss system in e.g. a FEA software, if you have that available.

As you said yourself, handcalculation will probably be quite difficult. And furthermore it will be difficult to know whether or not you get the correct answer, since you do not have anything to compare it to.
Another way to go, could be to find a book that treats statically indeterminate trusses in 3D and then do some of the book's problems/and compare with book results.

 

[desertfox]

Hi

Try this link below, it's based on the au motion the deflection in the kings are small and whatever you are lifting remains horizontal.

Desertfox

http://files.engineering.com/getfil...-4368-8826-a483768fa79a&file=crane_slings.pdf

 

[PersonalProfile]

In AU it is generally accepted to assume two (2) off only of the four (4) "slings" take the load.

Regards,
Lyle

 

[Ussuri]

You have 4 unknowns in your sling forces. You can get three of the equations you need to solve them from vertical and horizontal equilibrium. The fourth equation comes from your skew. If the skew is distributed between diagonal pairs (to cause a racking effect in the structure) then a 1.25 skew works out as 62.5% on one pair of slings and 37.5% on the other.

 

[Qluq]

Thanks Desertfox, lylebrown00 and Ussuri!

Ussuri: does 1.25 skew simply mean that one of the sling legs takes 25% more load?

 

[Ussuri]

Not if the load is distributed between diagonal slings. If the loads was distributed evenly then the load would be 50%/50% in each pair of slings. Applying the 1.25 gives 1.25 x 50% = 62.5% and (100% - 62.5%) 37.5%.

Some engineers will just multiply a calculated sling load by 1.25. Others take the approach given above.