Skew Load Lift Analysis: 2

[MoreAC]

I am doing a lift analysis using SACS software. I have modelled the diagonally opposite slings attached to two coincident nodes. One of these nodes is fixed in three degrees of translation, the second is free to translate in the vertical direction. A load equal to 25% of the total weight is then applied to the node free to translate in the opposite direction to gravity. This forces the remaining 75% onto the other slings.
What is the significance of this analysis and what should be the interepretation of the analysis.
By doing this analysis, due to absence of the vertical restraint on one of the hook points, the deflections are like 2 meters at some point and the stress ratios are above 4. in some cases. If somebody could help me on this topic.

 

[Ussuri]

The purpose of the analysis is to take into account potential tolerances on the sling lengths. The slings will not be exactly the same length. So when you lift your structure the effect is to twist the structure causing a racking effect. This twisting will increase your member and joint stresses above that if you assume the structure does not twist. Also the unequal length of the slings will mean that the load will not be distributed evenly between them.

A 75%/25% load split is pretty conservative. Normally a 1.25 skew factor is used which equates to 62.5%/37.5% load split.

You dont say how large your structure is, but is your deflection of 2m relative to the origin or relative to the other members. What I mean by this is, if you largest deflection is 2m and your smallest deflection is 1.9m, relative deflection is only 100mm, it just means the whole structure has moved when lifted (possibly due to the properties you have assigned to the slings, they have a lower apparent E values than steel).

If the relative deflection is 2m then it suggests your structure has very little resistance to twisting. In which case UR's of 4 might be correct.

 

[MoreAC]

Thanks A lot USSURI. The structure is a overboard platform (kinda steel deck) 24m x 13m in plan and height is 1.0m. The deck is fitted with grating and no deck plating. THe cross bracing is not so strong.
I did the lift analysis using one point hook and the deflections are (relative) 10cm. And the stress ratios are well below 1.
But, when i do skew load analysis as you told in your response (32.5%-67.5%, the relative displacements are 800cm ...
I was doing the skew load analysi for the first time, and couldn't interpret, learn from the analysis, in absence of any senior people here.

If you need i can show you the plan of lift.

I will appreciate, if you can suggest some reading on cross/lateral bracing requirement.

Thanks a lot.

 

[MoreAC]

One more question.
I do the static lift analysis with Weight and COG contingencies including consequence and DAF's and check that my structure is okey if lifted horizontally 'NO TILT" and I am happy with 10 cm relative displacement.

Now, While doing Skew load analysis do we need to add Weight contingency, CoG contingency, Consequence factor and DAF to the base weight? (Plus we have 67.5-37.5% distribution) Then the members will fail for any structure i believe.


Regards,
Anurag ...

 

[Ussuri]

If the item you are lifting is flat then it is going to have a tendency to twist. If your structure is not stiff enough, and is overstressed then you basically have two approachs. One stiffen the structure up or two justify a lesser lift case. 24m x 13m is a fair size.

Depending on which code you are looking at, there are different consequence factors for different parts of the structure. For instance the lift points will have the highest consequence factor, where a minor member will have a unity factor.

Regardless of whether you are assuming a perfect lift or taking the skew into account you have to include your contingencies and DAF for both. If it fails, it fails.

If the lift is just ok for a pure static case, then of course it is going to fail once you include the dynamic effect.

Options:

1) A three leg lift does not have the skew effect
2) Justify using less skew by controlling tolerances and reducing the effect
3) Use a spreader frame. You can make this as stiff as you like and then just use droppers to pick up your structure.

 

[Ussuri]

Or alternatively include temporary bracing for the lift alone.

 

[MoreAC]

Thanks USSURI.

The reply was really useful and it helped a lot.

Well, in my present case, I am going ahead with the static analysis (including all load factors ie. weight, cog contingency, consequence and daf)using one hook. In which, i have deflections of 12 cm and stress ratios less than one. SO i assumme that the structure can be lifted without any twist.

The lift will be done in the yard and we will make sure that the sling lengths and rigging is properly chosen to maintain horizontal position of the deck while lifting.

With two hooks, the twisting occurs. More realistically, if i adjust the load split to maintain 75mm difference between two hooks, which is as per API specification, the structure gives similar results to static analysis.

THanks a lot once again.