Determine load rating of a lifting beam 2

[1437n6]

I have a lifting beam that is detailed in the attached sketch (5/8" thick flat steel plate, 4'-1" long, 4" high, with 1 Lug Plate in the center). I to determine what the load rating would be on this. Any help with calculating this would be great.

 

[bridgebuster]

try this

 

[bridgebuster]

and this

 

[nicam]

I don't mean to be rude but do you have any ideas on how to start this?

 

[1437n6]

What confuses me is that it is not laterally supported in any way. If I was rating a standard beam (monorail for example). I would first determine the estimated lift that I needed to pick and then verify if the current beam was adequate. In this case, it is 4000 lbs. I would add the dead load of the beam and hoists, add an impact load of 15%, and then base it on a load test of 125%, therefore a beam that needed to lift 4000 lbs would have to satisfy nearly 6,000 lbs or 6 kip. I would then determine the Max Compression (Fb=12,000,000/(Ld/Af) and compare this to my allowable compression (fb=0.6x36 ksi = 21.6ksi). I would then check tension, calculate moment M=Pl/4, solve for fb=M/S and verify it is less than allowable 21.6ksi. I would then check deflection =Pl^3/48EI and make sure it is smaller than the allowable l/600. I would finally confirm lateral is within 20% of the total load.

 

[rb1957]

sounds like you're answering your only question ...
the beam does look narrow, "tippy" ... from your sketch i take it that the load is reacted at a pair of the grooves ? i guess you could consider the beam has fallen over (so the load is applied across the weak axis of the beam ...

 

[Lion06]

Look in F11 of the 13th edition manual for yielding and LTB. It should be noted, however, that all AISC equations assume that both lateral and torsional bracing is present at the member supports.

 

[drewholbrook]

How do you determine the tearout force of crane hook on the lifting eye?

 

[1437n6]

The article by David Ricker that Bridgebuster referenced above addresses the lifting lug and any tearout forces. However, I am still trying to calculate the beam only being a piece 5/8" flat steel (without any flanges).

 

[kelowna]

My two cents!

Without doing any calculation, that beam does not look right. An I have seen a few lifting beams in my travels. Having said that, the loads you talk about (4000lb) is punny.

The beam has very little stability, and since the grooves are on the top of the beam I would assume that the load is applied above the CoG and hence de-stabilising. Moving the application point of the load to the underside of the beam would help.

Is the load secured to the lifting beam? The grooves seem to indicate that the load is just hanging from the beam, saddled in one of the grooves. In that case a sudden movement, unequal loads or beam defflection would make it fall. Not a good thing.

 

[civilperson]

I calculate the lifting plate can handle 470 pounds when using A36 steel.

 

[1437n6]

Kelowna, you are correct. The gooves on the top is where the load is applied (above the center of gravity). The load is not secured to the lifting beam, they are supported by shackles that rest in the grooves. Knowing the instability, and the type of beam and loading, how can I calculate the actual allowable load this "beam" can handle.

 

[Dinosaur]

This is a fairly straight forward LTB problem. I recommend you locate a copy of Timoshenko, Theory of Elastic Stability, or something like that, and compute the buckling load of the top of the beam.

 

[kelowna]

I am with Dinosaur. Your limiting factor is lateral torsional buckling of an unrestrained beam. Somebody will correct me, but the length to consider on the calculations is actually larger than the distance between loads.

I highly recommend modifying the detail (even if structurally you can make it work). Holes drilled on the bottom part of the beam allow you to put the shackles and ensure that they will not move

 

[rb1957]

"The gooves on the top is where the load is applied (above the center of gravity). The load is not secured to the lifting beam, they are supported by shackles that rest in the grooves." ... that doesn't sound very stable to me, won't it be tricky to keep the beam level ?

 

[1437n6]

I agree. However, I need to show reasons why (via calculations) that it is unstable to prove that other measures need to be taken. Can anyone help with calcuating the torsion on this beam?

 

[Ussuri]

I would change from a single point lift to twin bridle lift and move the lift points out to the edge of the lifting beam. The have your droppers as close to this as possible thus effectively using the beam as a spreader in compression to keep the two legs of the bridle apart.

Single point lifts are very unstable, if the beam tilts as it is likely to do when the equilibrium of the lifting arrangement changes so the COG of the lifted object is below the lift point. The tilt may force your slings out of the grooves on the top.

For there to be any torsion generated in the beam there must be some torsional restraint, otherwise the beam will just rotate. Your loads are all applied in a line on the top flange. I cannot see how you have any torsion.

Also, are you lifting the object (ie attaching to it) above or below its COG? If you are lifting below the COG then any misalignment may cause the load to topple (depending on geometry).

 

[rb1957]

you show what happens when you shackle up two slightly different loads. i like usurri's approach, though it may not work for you.

 

[miecz]

The theory for stability of bars in bending can be found in Article 39, page 202 of Timoshenko's Strength of Materials, Part II, 3rd Ed. However, This material has been incorporated into F11 of AISC's 13th Edition, as mentioned earlier by StructuralEIT. It also appears in the Structural Stability Reasearch Council's Guide to Stability Design Criteria for Metal Structures.