built-up section - steel spreader beam

huangyhg

built-up section - steel spreader beam
does anyone know how to design a built-up section for a steel spreader beam, using two channel section (toe to toe) to form a box section? this is an existing spreader beam, and there are welds along the seams (at the toes of the channels top & bot.) but we need to verify beam capacity.
i am concerned about lateral torsional buckling of the channels, which would induce longit. shear stresses on the welds? (i.e. take the section and turn it 90 degress and load it, the weld stresses are from vq/i at mid-depth of the box section.
beam is loaded with a point load at each end and supported by the crane in the middle.
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how is the point load applied? i wouldnt recommend a double channel spreader beam as you described. if a double channel was to be used they should be put back to back to put the load into the major axis of the channels. channels placed toe to toe are eccentric and unconventional. also depending how the point load is applied you have flange bending issues and possible pull through
and with toe-to-toe channels, things like this happen:
yikes, that's reaching!
a spreader beam is located in between lifting points (where loads are applied), it maily experiences axial compression. a box section, provided the welds were done properly, is more stable than the original form - either single or back to back channels.
jae,
you beat me to it...
tg
due to the hyatt collapse, with good reason everyone is scared using of toe to toe channels!  this failed for a number of reasons, but one of the main reasons was that the rod pulled right thru the joint.
in our case there is a thick steel end plate welded at the ends of the spreader beam (around the perimeter of the toe to toe channels) and there are puddle welds 2" long at 6" o/c running along the seam at the channel toes top and bot.  the point load is applied at the end of the beam thru the end plate.
if you look at shear flow (vq/i) in the spreader beam under straight vertical load and assuming elastic stresses, there would be no load in the seam welds (in my mind).  however, if the beam was to buckle laterally would there be shear stresses in the welds due to vq/i, using q of one of the 2 channels about the channel toes?  (this can be visualized by turning the spreader beam on it's side, and considering bending about the weak axis of the composite section.)
the question is to check lateral torsional buckling of this unbraced cantilever beam how do i check the welds along the seam at the channel toes for the composite section, for bending/buckling in the weak direction?
* i'm using a k=2 factor on half the length of the spreader beam, where l/2 = distance from load to center lifting point0>  this is based on k factors for steel columns in bending, k=1 is pin-pin, k=2 is fixed and free at the far end.  therefore my unbraced length for beam stability calcs. = l = entire spreader beam length.
i believe i read the same article as wpgkarl did, the failure initiated at the connection due to deviation from the original design, causing the box beam (c-c) load to double.
also,
"the investigation found out that both designs of the walkways were well below the required safety stress required by the kansas city building code."

i will prefer to see continuous full-pen weld even not required by strength, stitch welds are liken the dominos, one is gone, rest are done.
never done this before, but try:
image the 2 channels are independent but hold together by some clamping force at ends (use your fingers if you wish), now bend the channels [] side way, the tip of the left side flanges will be either longer, or shorter, than the right side flanges. to close this discrepancy, you will need to introduce compression on the longer flanges, and tension on the shorter flanges, these are the forces acting on the seams, and are required to make the 2 channels a composite (to check required welds to hold flanges together).
i think you can find better explanation in any of the "strength of material" text books, the question is what is the initial bending force. i will find p (fraction of the lift load), assume incidental eccentricity e=2tf/6 (or else), the m=pe.
the composite box will bend in random manner w/r to x or y axis, side sway buckling will not be critical if compressive stress kept low. it might rotate endlessly about longitudinal axis, but does not twist. since there is no rigid end support, therefore the rotation is immaterial from structural point of view. keep your focus on design of the end picking (lifting) point, which is a much more critical issue.
above are personal opinions on this matter. keep talk/listen to persons familar with the design of lifting devices.      
wpgkarl,
i would suggest you take care in your assumption that some sort of bearing plate will avoid splitting the seam on the weld.  in fact, edward phrang stated in his summaries of the hr collapse that even a plate like that wouldn't have helped all that much.
the reason, as i understand it, is that with a cover plate, all you are doing is slightly reducing the concentrated load from the rod directed right at the seam weld/hole.  with the plate, it is still bending in weak axis mode and will still bend, introducing stresses across the throat of the weld.  this is a very dangerous application of stress on a weld, especially a toe-to-toe "butt" weld that is very difficult to install correctly.
i believe they even tested mock-ups of a cover plated double channel box beam and it failed rather quickly.