Multipoint lifting - Symmetrical circular points 1

[Megamoule]

If multiple lifting points are used for circular, symmetrical load (with the CG at its center), is it viable to consider that more than 3 points share the load?

Let's say I want to lift with 8 equidistant lifting points, would each point take 1/8 of the total load?

This is, of course, without any equalizing system in place.

I haven't found any litterature on the subject.

My learning of rigging taught me that if you lift a load, you have to consider that only 2 lifting points are considered, 3 if they are equidistant. What about more than 3?

 

[Ussuri]

I would suggest that 1/8th of the load is the best you could possibly get. i.e equal load distribution.

If you consider that the slings are fabricated to a length tolerance, then the slings will all be different lengths, resulting in a non equal load distribution. For a four sling lift a skew load factor of (1.25-1.5) is applied to allow for this variation. The skew also has a tendency to twist or rack the item being lifted.

I have not come across an eight sling lift before, but the principle would be the same. The difference is how to consider the load distribution effects. You could conservatively assume only 4 slings carry the load and apply a skew load factor.

 

[Megamoule]

Do you have any sources on the skew load factors?

For 4 slings, I always used the principle that 2 of them did the work (i.e. take 50% of the load each)

 

[robyengIT]

in design of column-press (4 columns)usually I considered only three columns for the total pressing load, but with a rigid structure connecting the 4 columns

 

[Ussuri]

My references are relate to offshore lifting, which may not be of any help to you.

DNVGL-ST-N001 gives 1.25 of you can meet certain tolerance limitations (ie 62.5% load on one diagonal pair of slings/37.5% load on the other pair)
API-RP-2MOP and ISO 19901 give 1.5 (ie 75% load on one sling pair/25% on other sling pair)

 

[Geoff13]

We've done multi-point lifts, similar to what you seem to want to do, many times. Neither the rigging nor the assembly would survive with only 2, 3 or 4 "loaded" slings. The picture is a smaller one of these lifts.

You will need some method of adjusting each sling (turnbuckles used in picture) to "equal" tightness. Hopefully you also believe your load / rigging will flex enough to redistribute the load to something close to equal loads.

Good luck.

 

[WARose]

If multiple lifting points are used for circular, symmetrical load (with the CG at its center), is it viable to consider that more than 3 points share the load?

Let's say I want to lift with 8 equidistant lifting points, would each point take 1/8 of the total load?

That's what John McLaughlin use to call "rosy scenario". (I.e. that's what you want to happen.) You have to prepare for a bit of unbalance and redistribution. I witnessed a ring (I designed the lift frame and lugs for) being put into place. And sure enough: several [of the dozen or so] cables kept alternating between taut and slack. You have to account for a minimal amount doing the job.

 

[rb1957]

1/8th per sounds optimistic ... the best possible world, all cables equally taut. But maybe you can get there by carefully controlling the lift, and adjusting turn-buckles to even out the load.

1/3rd per sounds unduly conservative ... sure it is the minimum number of cables.

if your applied load includes all your safety factors ... ok, you've now got a highly factored load (as opposed to the real one), so 1/8th may not be unreasonable.



another day in paradise, or is paradise one day closer ?

 

[Denial]

Two comments on rb1957's comments, made with my tongue partially in my cheek.
1.[ ] If the eight attachment points are reasonably regularly spaced, the minimum number of active cables could be two rather than three.
2.[ ] Safety factors provide a margin to accommodate unknowns.[ ] Be careful if applying them to cover unknowns they were not intended to cover.[ ] (Rumsfeldian unknown unknowns??)

 

[kingnero]

Is lifting through an intermediate device (similar to a spreader beam, but circular) an option?
It would be easy/easier to have all equal length chains for the "bottom" lifting part, and using a three-leg lifting chain for the top would guarantee a better-than-50% load per lifting eye.

 

[Megamoule]

We've done multi-point lifts, similar to what you seem to want to do, many times. Neither the rigging nor the assembly would survive with only 2, 3 or 4 "loaded" slings. The picture is a smaller one of these lifts.

You will need some method of adjusting each sling (turnbuckles used in picture) to "equal" tightness. Hopefully you also believe your load / rigging will flex enough to redistribute the load to something close to equal loads.

Good luck.

As I mentionned I was enquiring about non equilibrated loads.

I am actually working with something designed by someone else, and with the WLL of the rigging elements, it seems that the initial engineer considered that the load was equally spread between all 8 points. Considering that he uses chains with no adjustment, I'm pretty sure the design is inappropriate.

 

[human909]

If multiple lifting points are used for circular, symmetrical load (with the CG at its center), is it viable to consider that more than 3 points share the load?

If you are considering worst case then it can tend toward 2 points taking the load. If you are using a single chain to each point. You need only 2 for resolving the forces, 3 for stability which might only take 5%.

Let's say I want to lift with 8 equidistant lifting points, would each point take 1/8 of the total load?

Depends how it is rigged. Geometry can go a long way in allowing you to get reasonable equalisation without the pretensioners.

This is, of course, without any equalizing system in place.

Let the rigging do your equalisation.

Here is a 3D isometric of lift I did with 8 lift points. (Ignore the lines coming in from the side they are just dimensioning lines. This was done with chain with equal length chains on each side of the splits.

Each point is "equalised" with triangular geometry. Each triangle will mostly self equalise for minor differences in chain length. It needed alot of rigging headroom but that was necessary anyway for this vessel. It lifted perfectly. Which I'm glad as vessel was in poor condition and being removed for replacement. For scale the item was about 6.5m round. The rigging is 12m high.

(If you don't quite the lift mechanics, consider the behaviour of any on of the end points having zero or minimal load. This will go only so far and some intolerance to your chain lengths and you'll see some variation in loads. As long as the angles can't change much though you are reasonably good.)

 

[rb1957]

wouldn't that top bay make the whole thing very unstable ? (a 2 point lift)

another day in paradise, or is paradise one day closer ?

 

[human909]

There is still 8 lift points on the object and lift point on the hook.

Yes, if you centre of gravity well outside the centre then it will shift and adjust awkwardly as it attempts to distribute the load. But we are talking about a symetrical load with its centre of gravity in the centre. With all chains equally loaded and the lift point directly above and much higher than the CG there are no stability issues.

 

[Denial]

I'm not suggesting it is applicable in this case but sometimes the stable-versus-unstable dichotomy is not all that intuitive, especially to the uninitiated and especially when spreader bars are used.[ ] See, for example, figure[ ]4 in

https://www.liftingmatters.com.au/blog/2020/10/16/load-stability

 

[human909]

No spreader bars used in my example above. That would defeat the purpose of the rigging in this case. The separation geometry relies on equal spaced rigging points and equal or close to equal length chains. With some tolerance for non equal length chains or a slightly shifted C.G. If you get the chain lengths badly wrong then yes you could readily end up with only two chains loaded and the others mostly slack.