2 Point Center Lift vs 4 point corner

[MikeC88]

Hello All,

I am hoping to draw on someones expertise on lifting ductwork.

I have a piece of duct work to lift offshore ~ 3.7m in length, 1.8m wide, and 1.2m deep: Mass ~1000kg, which for intents and purposes can be treated as a cube with a reasonably central COG.

My original design utilised 4x lifting lugs, designed with permissible stresses in line with BTH-1 with Nd=6, with a vertical lift. The 4x lugs are located towards the 4 corners of the structure, on the upper surface (COG below lugs). One assumption made was that in reality, the weight would be taken by only 2 of the lugs.

To help with installation we have installed an additional set of lugs, based on the same design as the 4x corner lugs at the center of the duct work, roughly in line with the COG.

It has now been queried whether we could lift the structure from the 2x central lugs to a single point. It has been proposed that the 4 corners are then attached to the single point not to weight bear, but to assist with keeping the duct work level during the lift.

The mechanical design limits the lifting angle well below any value at which permissible stresses are exceeded, however I am hesitant to recommend this lifting arrangement, but am unable to justify why not other than the increased consequence of 1x lug/strop failing, and potentially stability issues?

Can anyone provide any pointers as to whether this type of lift would be permissible, whether it is recommendable, or any potential pitfalls that I need to be aware of/investigate.

Thank you in advance for any help offered, I look forwards to hearing peoples thoughts.

Mike

 

[MJCronin]

Suggest that you post in the Structural Engineering General discussion ...

MJCronin
Sr. Process Engineer

 

[MikeC88]

Will do , Thanks MJCronin

 

[CWB1]

So long as you're not overstressing the part, rigging, or the lifting device I wouldn't see an issue using 1, 2, or 4 pick points. All three are common in industry.

 

[MintJulep]

So the 2-point lift would actually require 6 drops.

Vs the 4-point lift that would require 4 drops.

 

[JJPellin]

We make lifts like this all the time lifting compressor and turbine covers much heavier than your piece. I would normally rig them to lift from four points. I would design the rigging so that any two of the four legs could take the load with an appropriate safety factor. I would pick it up and check for level. If it is hanging level enough for the need, I would proceed. If not, I would add chain-falls as needed to level the piece.

Since you have already installed the two additional lugs, you could pick from those two lugs and add come-alongs or chain-falls to two corners to level as needed.

This compressor cover weighs 30 tons. We picked it with four matched straps to the four corner lugs.

Johnny Pellin

 

[MikeC88]

Just for reference I've attached a sketch to show the required lift.

Impression I am getting is that 2 point center with stabilising strops (non load bearing) to 4 corners would be acceptable.

Key check is to ensure that the structure of the ducting is sufficient to take the increased loading due to change in lift plan.

Mint Julep just to confirm have you had auto correct change strops to drops?

 

[MintJulep]

Drop = strap, chain or cable between the crane hook or spreader and the load being lifted.

 

[MikeC88]

Thanks MintJulep

 

[SnTMan]

Why are the two lugs not on the longitudinal CL? Straddle the CG.

Edit: Top plate too thin / flimsy maybe?

Regards,

MIke

The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[MikeC88]

Hi Mike,

My apologies on that I should have marked the CoG onto the sketch,

The two center lugs are in line with the CoG which is located roughly in the center - Pretty much 1.85m down the 3.7m length, and very slightly offset on the width dime, ~0.85m on the 1.8m width

Just to note the red lines denote the requested lift (no spreader bar/green lines).

Lugs are welded to two I beams that run down length, so hopefully not too flimsy.

I realise now, that I should have sketched the two lift plans separately for clarity (with CoG's)

regards, Mike

 

[Geoff13]

Numbering the stabilizer lines going clockwise, I'd think 1 and 3 would suffice.

Chances are that the two stabilizer lines to an end would be slightly different lengths or initial tensions. Only the shorter/tighter stabilizer line would end up doing all the work, so why install the second one?

 

[SnTMan]

MikeC88, see attachement for what I had in mind.

The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[MikeC88]

Hello Guys,

Thank you for your input, at the time I pushed through taking advice from above.

Just a quick, and belated thank you for the advice rendered.

 

[chicopee]

Looking at your sketch, I would not have the slings in a diagonal configuration connected to your duct since there will be compressive forces developing on the duct. these compressive forces combined with the duct weight will noticeably bend the duct primarily lengthwise. I would rather have the four slings in a vertical position to the duct attachments which means you'll need an additional lifting frame for the sling attachment then another set of four leg spreader for the lifting frame to the crane hook. Or have a lifting frame supporting the duct then the four leg spreader could be in diagonal configuration attached to the lifting frame and the crane hook.