Spreader Beam with Different Sections

[Lion06]

I have to analyze some spreader beams that are adjustable by sliding HSS sections of different size into each other.

I have ASME B30.20-2013, but this doesn't deal with the design of spreader beams. I'm trying to determine what radius of gyration I should be using as the spreader beam is extended and half has an r of 1.2 while the other half has an r of 2.2.

Every spreader beam reference I can find deals with a single section. Does anyone know of a reference for different size sections being used together?

 

[rb1957]

you want radius of gyration for compression allowable load ?

you can get the allowable for a stepped column, but I'd just use the smaller section.

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

 

[AUCE98]

There is also a BTH-1-2011 that was put out by ASME which has better guidance for structural design.

 

[BAretired]

For elastic buckling, you could use a Newmark Numerical analysis or similar numerical procedure. An example is shown in "Theory of Elastic Stability" by Timoshenko and Gere.

For inelastic buckling, it becomes a bit more difficult, but I suppose you could still use a numerical approach...don't know, haven't tried that.

 

[dhengr]

Lion06:
That’s a tough problem to solve long hand. You have two different strength/stiffness, telescoping sections, with some play in their mating region which has the potential of exacerbating the buckling problem, if not handled properly. Newmark’s numerical integration methods provide one means of approaching a buckling problem where the compression member varies in stiffness over its length. Of course, FEA would be another approach. The fortunate thing is that the compressive stress/buckling problem can usually be accounted for with a slightly larger size (pipe dia.) for little added material cost. And, the material cost is usually not the major cost driver. This is usually true unless the lifting sling angle is to small. What do these spreader beams look like and how are they rigged from the beam up the main hook?

 

[CANPRO]

If you have some initial trial sizes for the two sections, the first thing I would do is check the buckling strength based on the smaller section...if that works out and the section sizes are reasonable I would leave it at that.

If you need to refine your answer, I 2nd BA's suggestion for "Theory of Elastic Stability". As he says, they have this exact problem in the book. I've used it successfully in the past to check adjustable pipe braces. If you set up your solution in a spreadsheet you can easily check for multiple combinations of lengths and stiffness.

 

[KootK]

While it might sound a bit daunting, this would be a good application for the direct design method. Particularly because the response will be dominated by the large-ish kink that you'll have at the sleeve. I would:

1) Fire up your favorite FEM program.
2) Model the the spreader as two moment connected columns with a conservative kink at the joint.
3) Split the two tubes up into four or so members each .
4) Modify your stiffnesses according to DAM and run a non-linear P-delta analysis.

I took AISC's night school course on stability recently. Apparently, DAM is becomeing quite ubiquitous for rigging applications.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Lion06]

Thank you all for the responses.

rb - this is for existing spreader beams that a rigger has on site and is trying to get reliable calculations and capacities for. He has some random capacities from about 20 years ago, but the markings have become unclear.

AUCE98 - I will look into that reference right away. That may be just what I need.

dhengr - You're exactly correct that they're telescoping. They are square HSS sections one side a full inch larger than the mating side with a thicker wall. I could be conservative and use the smaller section only, but I don't want the rigger to look at me cross eyed because he's already lifted more weight multiple times with these beams. The weight is slung directly below the lifting hooks, so the only bending is the self-weight with the play where they mate and the P-delta from the high compression force.

kootk - I think that would be a good solution, but could be time consuming having to do 10 models for each spreader beam to capture each length that each one can achieve with the telescoping.

 

[KootK]

At full extension, what proportion of the span does the smaller section constitute?

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Lion06]

At full extension, about 50%.

 

[CANPRO]

if you have 10 different scenarios to run then writing a spreadsheet based on the solution provided in "Theory of Elastic Stability" is the way to go in my opinion. Its a handy spreadsheet to have for future use as well.

 

[CANPRO]

this thread may also be of use to you...

http://www.eng-tips.com/viewthread.cfm?qid=278129

 

[rb1957]

my 3rd attempt to do this (effing computers and Explorer) ...
with 1/2" thk steel sections, I'd be worried about the load transfer from one to the other.

I've got a crummy graph from a text ... gives allowable in terms of the smaller section.
if I1/I2 = 0.5 and a = 0.5*L, then m = 0.6, and Pcr = pi^2*E*I1/L^2/0.6

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

 

[dhengr]

Lion06:
We don’t know any of your loads (load ranges), member sizes, thicknesses, beam lengths, end fittings and details, etc. When your licence is on the line, and you should design to some reasonable set of standards, you should be very careful leaning to heavily on a statement like ‘the rigger has already lifted heavier loads with these spreader beams.’ The way some of those guys work, if it hasn’t started to fail during this lift, it still has much to much factor of safety built into it, so they can lift a heavier load with it the next time. Many of them are very good at what they do, and I’ve worked with a number of them, but riggers pay high insurance premiums to cover their type of work, and many/some of them are kinda cowboys about the chances they will take, or the way they will eat into margins of safety. I would check the telescoping mechanism and its locking/pinning means to minimize any play in that part of the system. Finally, inspect the entire beam for wear, nicks, cracks and the like. I believe that ASME BTH-1 requires that these beams be load tested to 1.25 or 1.5 times their load rating, before actual use.

 

[SAIL3]

I would also use the capacity of the smaller section full length.....on the other hand, IMO, this is not a good concept in the long term as it leaves it up to the rigger to connect the two sections properly each time they need a different length bm.As dhengr pointed out, most riggers are knowledgeable and qualified but some are truly not.

 

[Lion06]

dh - I didn't mean to imply that I would base anything I do or my calculations on what they may have lifted in the past. I meant that I didn't want to come back with a lifting capacity of, say, 20k, when they have used that rig to lift 60 and the only reason for the discrepancy is that I was too conservative at every juncture in the analysis. I'm comfortable with the numbers working out to whatever they do and communicating that information to the rigger, but I want to be reasonable and rational in my analysis.

The beams are typically small (4" - 8") square HSS or round pipe with thicknesses around 3/8".

all - Thank you for your input. I ended up setting up a spreadsheet using the approach in Theory of Elastic Stability and accounted for P-delta by determining the maximum "kink" that can occur in the beam when fully extended.

 

[rb1957]

if they've lifted 60k in the past, and you show it good for 90k, then everyone's happy. if you show it good for 60k, then the rated load is 40k, and when they complain you say ... before you weren't safe (meeting code), now you're safe (and legal). if you show it good for 20k, then you need to sharpen your pencil!

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

 

[BAretired]

Lifting devices require a greater factor of safety. OSHA requirements may govern.

BA

 

[rb1957]

I agree ... to use it for 60k, you need to test to 90k and show it good for 240k (300k?, 360k?) ...
my point was if they said "90k on it ? are you crazy!?" then you know that it's not rated at 60k.

and of course it can have different rated capacities for different extensions.

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