Lifting Beam 2

[eswar12]

I've a lifting beam that I'm trying to design. The lifting beam is 30ft in length, which utilizes a S 10x25.4, the top lifting lugs are 15'-0'' apart located centrally and at the bottom there a T section welded with holes spaced 1'-0'' O.C for lifting. I want to design the beam for one point loading, two point loading, three and four point loading. When I have just one load smack in the center I understand that the unbraced length is 15ft. When I use a two point load with loads placed atleast 8ft apart upto 28' apart, the unbraced length is 28ft. But when I use a three point load with loads spaced 8ft in b/w them max., I've one load say 1ft from left end, second load 14ft and third 28ft, in this situation will the unbraced length be 14'? or the distance between the end support which is 28'? .....similary when I space a 4point load spaced 8ft each will the unbraced length be 8ft? since there is load attached at the bottom and might help against lateral axis movement? Please respond Thanks

 

[kslee1000]

Please refer to steel textbooks to find out what is considered as a valid brace point for the situations yo described.

 

[kslee1000]

Please refer to steel design handbooks to find out what constitutes a valid brace point for the situations you have encountered.

 

[eswar12]

Well, any beam which has a support that is laterally restrained can be considered as a brace. But when you have a beam with multiple lifts as I've asked I'm not sure if the intermediate support would count as braces or if I should consider the full span.

 

[kslee1000]

Well, you got thar right, if there exists adequate lateral restrain at "the support". Now you should have the answers for the questions.

 

[eswar12]

what determines so said " Adequate lateral restrain at supports"

The only lateral restain is the lift load itself in a multi span support, for example is a 30ft beam with 15ft lugs has lift 28apart, the Lu of the beam is 28, but for a multisupport beam which has lift say every 14ft, can the lift load acting vertical be considered as lateral restrain itself, there will no other lateral restrains in a lift beam

 

[kslee1000]

- what determines so said " Adequate lateral restrain at supports"

That's why I refer to testbooks that tell you how to determine, or can you count on the lifted weight itsself as restrain/support.

Maybe I was confused over this issue. A sketch will help.

 

[eswar12]

Well Well Well

I've attached a lifting beam loaded as shown, just tell me what is the unbraced length if you can.

 

[CTW]

I would think you could consider the 8 ft spacing as the unbraced length. Although, make sure to check the beam two ways. One using the maximum moment between the 15 ft lifting point spacing on the top of the beam with Lu=15 ft, and the other using the maximum moment between the 8 ft lifting point spacing on the bottom of the beam with Lu=8 ft.

 

[kslee1000]

A brace in a beam system is to ensure the beam would yield prior to undergo lateral buckling due to compression. I could be wrong for your case.

 

[eswar12]

Awesome, that is what I was looking for. Yes, I checked the beams with a point load in the midspan that would create an Lu=15ft and other with a load at the extreme ends that would create an Lu=30ft , 10 and 8ft.

Thanks

 

[eswar12]

The problem arises when you have just two lifting points 28ft apart basically this makes the Lu=28ft which seems to be the worst condition. I've used ASD AISC9th F1-8 with Cb=1 since positive and bending moments are almost equal to approach Fb allowable. Lu=28ft allows a 19.3Kip-ft moment (with reduced Fb) which says you could have two point loads 2950lbs on each end. and the value increase as Lu decrease or as the load spread since the unbraced length decreases....I'm not sure if I am assuming this right

 

[steele6707]

Some design references for lift beam design:
1. AISC Engineering Journal 1991 4th Quarter, "Design and Construction of Lifting Beams" by David T. Ricker
2.ASME BTH-1-2008 Design of Below-the-Hook Lifting Devices
3. ASCE Structural Design and Construction Periodical May 2008 containing several articles by David Duerr.
I think the Ricker article has good information, however, some of this is superceeded by the ASME standard. One of the Duerr articles goes through a design example using the ASME code.
I thing the ASME standard uses an approach similar to AISC 9th edition (ASD) but applies factors to increase the factor of safety. Some parts similar to the AISC 13th Edition (2005 Specification) are also included.
It is a full design specification.