Analysis of X-Shape Lifting Device

[RaptorEIT]

Have an existing X-Shape lifting device that I have been asked to determine a rated capacity. The device appears to be of the type where someone in a shop through some shapes together and used it to lift something in the past without ever being engineered. I say that because of how the device was fabricated. The box shape is made from welding two 3x3x3/8 angles together. I have attached a picture of the device as well as a FBD sketch below that I have made in preparation for the analysis.

My primary concern when I first saw the device was the torsion that the side members of the X would exert on the longer, single member of the X. But, it looks like from a global standpoint that the torsion forces cancel each other out assuming balanced loading. The torsion can be resolved into a force couple on the single member of the X. The bad part is that the force couple creates a large compressive and tensile load on the local section coming from each side.

I'm thinking the local effects are going to be the limiting factor (local buckling of bottom angle leg, local bending of top and bottom vertical angle legs, and local bending of top angle leg). I'm using ROARK 7th ed. Table 11.4 Case 1c for the local leg bending, and ROARK Table 15.2 Case 1a for the local buckling. For the local bending of the vertical elements, I am going assume the moment is the Torque moment and then calculate the plate section to compute max stress.

I'll check the overall member strengths and pad-eye strength too, but those items are relatively straightforward. The welds framing the two angles together are all continuous so I'm not worried about them.

Am I missing anything, is this approach correct? I want to make sure I evaluate the cross section a "D" correctly.

[URL unfurl="true"]https://res.cloudinary.com/engineering-com/image/upload/v1560529233/tips/FBD_cbgkpf.pdf[/url]

Thank you

 

[CANPRO]

I do this type of work often, I have a few thoughts on this but not much time to type it out, so I'll leave you with this for now:

[ul]
[li]Before you give them a maximum capacity, ask them how much they need it to lift. Its a lot easier to check it for a target capacity than it is to try to find the maximum[/li]
[li]You have a single lift point in the middle, so any net unbalance in the load will not create torsion in the steel, it will create an instability in the lift[/li]
[li]That looks to be a very heft lug in the middle with a large weld to the built-up box section. I don't think the 3/8" wall can match the lug capacity, I would take close look at the wall of tube[/li]
[/ul]

 

[JStephen]

If this needs to comply with the ASME below-the-hook requirements, there will be testing and labeling requirements also.
Not sure what buckling you're talking about, but if you weld angles into a box-shape, you don't have the normal style of angle buckling.
Do check the welds. If you don't know how they were welded, you don't really know the adequacy of them either.

 

[dhengr]

RaptorEIT:
It would really be helpful if you put some dimensions on you plan view sketch. What sort of loads “P” are you talking about, and what is the actual piece being lifted? It is pretty unlikely that all four of the “P’s” will be equal, so the whole system will tip and swing until the C.G. of the load is directly below the hook. This complicates the analysis. Show a section through leg CDD, since you talk about a ‘single member of the X,’ is it a box section too, or not? How is it attached to the other leg at the center, weld sizes, details, etc. You have to know the weld sizes and details much better than you seem to, at the moment, they are critical. The welds btwn. the angle toes are probably more like a flare-bevel than fillets, and the welds at the center joint have the potential of being downright dangerous, since the load path passes through several welds. What is the detail and welding at the top center pick point (pad eye)? I don’t have the 7th ed. of Roark’s book, and I’m not going to dig to see which table and case is applicable from my ed., but if you need Roark’s book to get started on this problem, you should probably have one of your superiors watching over your shoulder on this, to keep you and the company out of trouble. You should get a copy of ASME BTH-1, “Design of Below-the-Hook Lifting Devices,” and dig out your Engineering Mechanics and Strength of Materials text books, along with the AISC and AWS specs. and manuals, they have a wealth of info. needed to address this type of problem. What type of steel and welding rod did they use?

 

[Compositepro]

The proper name for this contraption is a scissor lift.

https://www.google.com/search?client=firefox-b-1-d&q=sissor+lift+design

 

[CANPRO]

RaptorEIT, I have to blunt with you - your sketch is no good. Compositepro thought you were designing a scissor lift because that's kind of what you show in your sketch. I opened your attachment and saw the picture of this contraption so I knew what you were talking about, but showing vertical loads in a plan view like you did is just down right confusing. Not meant to be malicious, just fair constructive criticism. On to the problem...

Aside from the actual engineering work here, let me share my experience with this type of project. You state "The device appears to be of the type where someone in a shop through some shapes together and used it to lift something in the past without ever being engineered". This is often the case, build now and engineer after. Not ideal, but it happens. Based on the picture, this thing has been used successfully in the past so it must have some allowable capacity. This is where it pays to really listen to the guys who made it/use it and understand how they use it and what they really need for a capacity. In my experience, these types of devices tend to be over-built...maybe I've just been lucky in that aspect.

Now a few notes about the details you've provided:

[ul]
[li]I believe the torque you're referencing in your OP is just the outrigger load multiplied by the length of the outrigger. For analysis purposes, I would assume the load is off balance and design for maybe 0.4*R at any given extension.[/li]
[li]The reference to a 1/4" weld doesn't tell us much. It can't possibly be a fillet weld based on the geometry noted. I agree with dhengr that it is more like a flare bevel, but the toe of angle probably has a fairly tight radius so I don't know how effective the flare bevel would be. You should do some reading on the effective throat of a flare bevel weld[/li]
[li]You need more details on the other welds as well, particularly where the two outrigger arms are welded to the through-member[/li]
[li]You're going to have to take a close look at the top wall of the box beam where the lug is welded. At this location, the wall is in tension due to bending in two different directions, plus the out-of-plane loading from the lug in tension. There is a lot going on there and you need to evaluate the net stress at this location[/li]
[/ul]

If you can't get enough details to feel comfortable putting a rating on this, you can always load test it and apply a healthy factor of safety.

 

[Craig_H]

In addition to what has already been mentioned here, you may want to consider the following:
[ul]
[li]The ends of the outriggers appear to be designed to have your slings drape over the beams. The edges of those sections appear to be rather sharp. I would consider softening these edges with round HSS or pipe sections to prevent sling damage. Polyester roundslings are very susceptible to cutting, and wire rope is easily damaged (and loses strength) over small radii.[/li]
[li]If the outriggers are designed to have slings draped over top, consider a small torsional applied load from rotation of the sling and it takes load. Original misalignment will likely cause the sling to slide around the outrigger, generating a frictional applied torque.[/li]
[li]Be very cautious using this device with slings that are attached to the bottom of a load. Your system will become unstable should the load's center of gravity lie above the sling attachment points by a distance greater than the distance from the top of your beams to the center of your main lug hole. I would draw you a FBD of this effect, but do not have a scanner with me at the moment. This is a global stability failure mode (IE the load tips under the crane hook) rather than a material or local stability issue.[/li]
[/ul]

 

[robyengIT]

looks like a lifting beam for big bag :

http://www.dlhonline.co.uk.

For me it's the only safe use, because, more or less, the load is almost balanced. I'd never used as a spreader (central hook is unsafe)

 

[BAretired]

The sketch is a bit confusing. There are two points marked "D". If we change the name of the junction point to point "E", then Member ADB becomes AEB. The other member now marked CDD becomes Member CED.

Member CED is substantially weaker than Member AEB at Point E because it has no vertical wall at the point of maximum moment and maximum shear, i.e. for the width of Member AEB. This should be considered in the analysis.

I'm thinking the local effects are going to be the limiting factor (local buckling of bottom angle leg, local bending of top and bottom vertical angle legs, and local bending of top angle leg). I'm using ROARK 7th ed. Table 11.4 Case 1c for the local leg bending, and ROARK Table 15.2 Case 1a for the local buckling. For the local bending of the vertical elements, I am going assume the moment is the Torque moment and then calculate the plate section to compute max stress.

Roark's Table 11.4 Case 1c does not take into account tensile stress for Member AEB and two of the edges are not simply supported. A Yield Line Analysis could be considered with correction for the tensile stress.

Roark's Table 15.2 Case 1a does not take into account compressive stresses in two orthogonal directions and two of the edges are not simply supported. Table 15.2 Case 2a is a little better but still assumes simple supports on all four edges.

An exact analysis is not a straightforward calculation. Engineering judgment would be required, but as CANPRO suggested, it's better to ask the client how much they need to lift before giving a rating on the assembly.


BA