Built-up section - Steel Spreader Beam 1

[WpgKarl]

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.

 

[STEVE97]

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

 

[JAE]

And with toe-to-toe channels, things like this happen:


Hyatt Collapse

 

[nutte]

Yikes, that's reaching!

 

[kslee1000]

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.

 

[trainguy]

JAE,

You beat me to it...

tg

 

[WpgKarl]

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.

 

[kslee1000]

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.

 

[kslee1000]

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."

 

[kslee1000]

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.

 

[JAE]

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.

 

[WpgKarl]

JAE - actually the plate is not sitting flat on top of the []'s. If you were to look at an elevation of the spreader beam, the plate is attached at the ends of the beam.

(See attached sketch in Excel)

This is an existing spreader beam that we have used in our precast yard for 20 years, and someone has asked us to verify the capacity.

Perhaps on Monday, I can scan the shop dwg and upload it to make it easier to see.

PS - JAE - I really value your opinion, (and everybody else's as well!)

 

[WpgKarl]

Sorry - didn't attach excel file

 

[JAE]

That sketch makes sense to me. Much different than what I was desribing.

 

[asixth]

I have always used rectangular hollow sections for spreader beams. I image the section would have a weakness such as a weld along a toe to toe channel connections.

JAE,

Good post about the hyatt collapse. There is also good reading in the Salvadori book "Why buildings Fall Down" regarding this structural failure.

I always thought that there was a mis-communication between the structural engineer and steel fabricator regarding the walkway collapse. The structural engineer designed an continuous threaded stud that supported all three walkways, so the rod has a maximum tensile force of the three walkways, and each nut only needs to support one platform.

What the steel fabricator provided was three individual rods, so the nut on the top platform needed to support three platforms, instead of the one that it was designed for.

That, and the fact there was people up there dancing to a beat would not have helped the matter.

 

[SteelMover]

With all of the above concerns by the time you get your beam design modified to your satisfaction, you could have specified an entirely new beam and put the money into material instead of design.

I usually keep the designs as simple as possible and use an omega of 5-6 depending on the failure mode. Makes everybody comfortable & more efficient in the field as well.

 

[kslee1000]

Another suggestion to verify the strength of the channels and the welds.

1. Determine end compression forces applied to the built-up box.
2. Assume the flanges take the full load.
3. Now look at one of the channels, it has end equal and opposite loads at ends, that the line of action is eccentrical to its geometric axix , from which you can calculate the resultant stress at the tip of the flange.
4. Check compressive stress against allowable, taking K=1, L=full length.
5. Check weld strength by multiply the resulting stress and weld spacing.

Hope this would help.

 

[trainguy]

WpgKarl,

What does the connection at the middle look like?

tg

 

[kslee1000]

From previous descriptions, the box beam was formed by placint two channels toe to toe with puddle welds. there are two end plates, from which the loads are applied.

It is important to know the lifting points and configuration above and below to identify the most critical situation.

My assumption was there are two cables above that forms triangle with the spreader, and two vertical cables below. The picking points above and below are aligned. If there is offset, the whole picture will change.

 

[trainguy]

kslee1000,

If what you're saying is correct, there is no bending, just axial compression. Look at th OP - it's different

tg