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.

 

[kslee1000]

trainguy:

Everything I said was guessing, unless a picture is shown. I think he wants to verify the strength (welds and channels) against side swing buckling caused by compression, not bending.

 

[WpgKarl]

Ok - this should help to clarify things a lot. The spreader beam is custom made to attach to a forklift and it is used for yarding hollowcore slabs. There is a hydraulic cylinder that extends the ends outwards to accommodate different slab lengths and then if the slab is much shorter, it can be lifted with plates in the interior portion of the main beam.

Want to verify capacities of strongback with different lifting locations and would like to know everyone's approach to analyze this section.

Thanks!

 

[WpgKarl]

Photo #1

 

[WpgKarl]

Photo #2

 

[WpgKarl]

Last photo....

 

[trainguy]

I believe the welds in question will only see appreciable shear if you get torsion. You may want to consider a percentage of your vertical load (at each lug)that acts horizontally, thereby creating torsion, and size you "puddle" welds accordingly, using very low allowables due to the poor nature of this type of weld.

You should also consider replacing these welds with additional groove welds between the existing intermittent ones, and only considering the new ones in the calculation.

tg

 

[DRC1]

I don't think the weld in the toe actually matters structurally. I can see for service purposes why the channels were welded toe to to toe, but typically for lifting beams, channels are set back to back. My point is that the toes are not welded for those beams and the section modulus is twice the value of a single channel. So I dont think reversing the orientation will make a significant difference. Also you have a lot of weld to brace the flanges to each othe and the system seems to transfer load equally to each angle. However, you do need some dependable weld a t brace points so system acts as a composite. In a back to back channel, it may be necessary to put spacers in to maintain brace points. Braced length of composite section would be as for as for cantilever, Q would be dictated by largest load lifted due to controlling momemt shear deflection or forklift capacity.
Weld size would depend on Q to insure adequate stength.
As far as the Hyatt, There were mutiple failures. The size of the rods were one the detail to use two instead of on rod was another. However there was also a detail that used a standard washer at the box/rod connection. A larger stiffer plate washer should have been used to transfer load out to the webs, instead of to a small area of the flange. This can be seen in one of the posted photos.

 

[trainguy]

DRC1,

Respectfully, it's important to differentiate between what "matters structurally" and "section modulus". If I had a nickel for every non-structural engineer (i.e. mech, electrical, etc) who believe all there is to structural engineering is Mc/I, or M/S, well, I'd have a lot of nickels.

Slenderness ratios, buckling, instability, connection strength, catastrophic failures, improper load paths, progressive collapse, none of these are directly related to section modulus, but I would say they matter.

Remember, structural design is not about just calculating the stress and comparing with yield - it's also about calculation of reduced allowables based on instabilities, connection strengths, etc.

tg

 

[DRC1]

Trainguy,

Although I agree that there is more to structuiral design than Mc/I, I am not sure what would change in my post.

 

[trainguy]

DRC1,

Neither am I.

I guess its quite obvious that my work load is a little light these days!

tg