STABILITY BRACING DOUBLE LIFTING BEAM

[nevsam12]

STABILITY BRACING
I am designing a sort of Double Lifting Beam. Actually, they are a pair of Parallel Beams 4 ft apart. The beams are 60 ft long and W8x31. The beams are connected in the transverse direction by W8x31 beams as well. Flanges and Webs welded like a moment connection. (All beams at same level) These transverse beams are spaced at 15 ft oc. There are 4 lifting points on top of the longitudinal beams symmetrically placed at 50 ft oc. There will be bar grating welded on top of the beams. What should be the lateral unbraced length? 15 ft or 50 ft? Thanks for your help.

 

[dhengr]

Nevsam12:
It’ll act as a braced frame in the one direction, with a 15' unbraced length on each of the side beams w.r.t. their weak axis. And, each beam will act as a beam/column with 50' unsupported length w.r.t. the beam’s strong axis, with a cantilever end moment applied at each end. There may be some optimum other than 50' which balances the cantilever and interior span lengths. And, the length of the upper slings, btwn. the hook and your lifting frame comes into play here too, in that this determines the compression in the two beam/columns.

 

[paddingtongreen]

Are these vertical pulls, a long way sling or two short way slings? Please clarify.

I would certainly try to locate cross beams at the lift points.

Whatever, it looks as though it will be bouncy.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[nevsam12]

Hi paddingtongreen,

The pull angle will be 60 to 75 degree to the horizontal - the cables will actually be fastened to a Lifting Beam above this sort of a platform. I do not know the vertical distance from the platform to the lifting beam above and will be checking it with the customer. I am told that the platform could be pretty high - 200 or 250 ft above ground level. Something to keep in mind - loads are rather small. Total load on the platform may be around 1500 lb. I plan to use a FS = 3 to 5. There are hanging loads from the platform - The loads are attached fastened to the underside of the platform. I will check for a maximum deflection of L/600. With this information - lighter loads and tight deflection, you still feel that the platform could be bouncy. Thanks for your input.

Hi dhengr,

Thanks for your recommendation. First I will do some hand calcs to get a feel of the structure followed by STAAD. I believe STAAD will fail the structure for the full unbraced length in the strong axis considering it a compression member. If that happens, I plan to lift the platform vertically, thus eliminating any column action in the platform beams. It appears I am on the right path. But suggestions from you folks are extremely valuable in confidence building.