STAAD Pro - simple lifting analysis 2

[Z_Z]

Hi all,

I am trying to do a lifting analysis for a skid with some pipes, fittings, valves, heaters etc. I am trying to figure out how I can calculate the best location of the hook for the cables to be attached to avoid bending of the skid structure (picture 1) and how to calculate the size of the lifting hook which will be attached to the cables (picture 2).

I attached an actual picture of the skid being lifted as well to get a better understanding of my question.

I am kind of stuck at the moment and would greatly appreciate any help!

Thanks
Z

 

[XR250]

Can you send a file type besides .RAR?
My Mac does not like it

 

[Z_Z]

Ya for sure.

The uploaded file is the STAAD pro file and below are the three pictures.

Picture 1:

https://files.engineering.com/getfi...3-411a-b829-7d2d6e09f6ec&file=SKID_PIC_1..jpg

Picture 2:

https://files.engineering.com/getfi...1-401d-89bd-9e4626b814b9&file=SKID_PIC_2..jpg

Picture 3:

https://files.engineering.com/getfi...f-47b9-9d88-af210f43fb41&file=SKID_PIC_3.jpeg

Thank you so much for taking the time

 

[RPGs]

Is that an example skid you are using as a reference for a new skid with different equipment? or is that the actual skid? Looks like that skid is working fine.

 

[XR250]

I'm curious if you are a Structural Engineer?

 

[JoelTXCive]

I can't help with question 1 (best position of attachment points), but I can help with question 2 "size of lifting hook"

People's terminology can be different, but I refer to the attachment point at "padeyes" or "lifting lugs." There is a decent amount of design info online for padeyes. Attached are two document I have found and used.......

 

[rb1957]

you minimise bending of the skid by lift near the weights.

can you draw a FBD of the skid, with the weights, and with the sling loads applied ?

I would also worry about the in-plane (compression) loads from the slings. Maybe have some temporary reinforcement beams ?

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

 

[JoshPlumSE]

How to calculate the size of the lifting hook which will be attached to the cables.

This should be pretty easy. A lifting lug is a really simple structural calculation. For the cables / hooks that attach to them, it should just be a matter of looking at the rating of those items that you're buying and making sure that they exceed the reaction you calculate would be in each of those lifting lugs.

If you are not a structural guy, you probably will need to have one sign off on your work. So, that should be the person you should be getting to answer these questions.

I am trying to figure out how I can calculate the best location of the hook for the cables to be attached to avoid bending of the skid structure
How to calculate the size of the lifting hook which will be attached to the cables.

First, you cannot avoid "bending" of the skid. No matter how big or strong the skid is irrelevant. You will always get bending. The question is what do you want to limit? Do you have a deflection limit that is causing problems? What is that limit? Do you need to calculate the strength of the skid members and make sure they are not exceeded?

Lastly, there is another aspect of "lift design" that you did not mention.... getting the center of lift to correspond to the center of gravity of the skid as lifted. And, what happens (i.e. how unstable is it) if the center of lift and center of gravity do not fully match up.

 

[Z_Z]

I appreciate you all so much for replying.

@RPGreen "using as a reference for a new skid with different equipment?" Yes precisely

@XR250 I have no background as a structural engineer (chemical/project junior engineer here).

@JoelTXCive I finally got the name of those "lifting lugs" thank you so much . I will look into it. I actually had the impression that I can obtain this through STAAD but all my search on this matter has been in vain, any idea if that is true or other methods are required? Thanks!

@rb1957 Yes I get that lifting near weights minimizes bending, however while lifting it is important that the skid remains fairly parallel to the ground (just like the picture provided) hence the placement of those hooks are key as well as the lifting lugs size.

@JoshPlumSE We are a small company and do not have a structural guy (this is the only thing related to structures that we have) hence I am trying to learn how to do it, because most probably we will have a 3rd party do it this time, but I want to be able to do it in the future specially that it will always be a repetition of the same stuff.
Regarding bending of the skid, I want the skid to carry the loads and to be lifted with no major bending that could cause the skid any damage.
I checked the Center of Gravity of the structure from STAAD as well as the center of mass of the structure with all the loads, in the case I attached, it is almost fully match up

Thank you
Z

 

[CrabbyT]

Oh man, be careful. I recently designed a below-the-hook basket and it was a hard learning experience.

The hook is the easy part. Usually, hooks come with a listed load rating. Just figure out how much weight you need to lift and then buy a hook that meets that requirement. Alternatively, just reach out to a hoist vendor, give them the loading criteria, and see if they can give you a quote.

In short, you want the loaded skid's center of gravity to be located directly below the lug, or directly between two lugs if you're using a spreader bar. By loaded skid, I'm talking about the skid itself plus the contents.

In my case, the location of the load in the basket was highly variable, and it ended up being really unstable depending on where the load was placed. We had to go back to the drawing board and get a double hoist with synchronized chains to keep the load balanced. JD Neuhaus makes one called the BBH 2000. Alternatively, I recommended installing guide rails but the client didn't like the idea of having guide rails taking up space on their floor.

Using math to calculating the center of gravity is kind of a fool's errand, especially if you're not sure where the center of gravity will be once the skid is loaded. It's easier to use a doctor's scale mechanism to "figure it out" on the fly. For instance, check out this double drum lifter from Morse (Model 86S-2D). They have one fixed lug located dead center in the spreader bar, but they also have an adjustable lug that slides up and down the spreader. They can slide it back and forth, figure out where the center of gravity is by lifting it and seeing if it's balanced, and then lock it down once they find the sweet spot. Imagine if the doctor had to calculate your center of mass to take your weight... it's technically possible, but not at all practical.

 

[Z_Z]

@CrabbyT I appreciate this so much!

"In short, you want the loaded skid's center of gravity to be located directly below the lug, or directly between two lugs if you're using a spreader bar. By loaded skid, I'm talking about the skid itself plus the contents."

I will have 4 lugs, one lug on each side of the skid (Attached picture). The loads remain constant so I am able to get the loaded skid's center of gravity. Having the hoist at the center of those 4 lugs (above the loaded skid center of gravity). Is this right?

Also I am curious to know if STAAD pro gives any suggestions regarding size of lugs? (cause I thought it did, might be false information, hence would be great if you can clarify that)