Beam made of back-to-back channels

[imcivilized]

Is there a standard procedure for analyzing a beam made of back to back channels?
I'm acutally working backwards. I know the I-beam size for what I need, but want to fabricate it out of channel so I can place hooks between the channel to make it a spreader bar..
Questions, comments, jokes?..

 

[SteveMort]

Compare the geometric properties of the back to back channels with those of the wide flange beam you designed. If the section modulus, cross sectional area, and moment of inertia are equal to or greater than the wide flange, the channels will be sufficient for bending, shear, and deflection. The caution is to assess the way that the channels are connected, loaded, and supported to ensure that they work compositely. Also, be certain to take into account the unbraced length of the compression flange of the built-up section to avoid lateral torsional buckling. If this is a spreader bar, the unbraced length is the full length of the beam and its allowable bending moment is reduced accordingly.

 

[imcivilized]

Steve;
Thanks, thats' how I started, then realized that my beam was a W610x372 (thats metric - w24x207 imperial)and I can't get channel that big.
I'll likely have to go with MC18 as heavy as I can get then do a lot of reinforcing.
Likely have a 1" x 24" top plate at the lifting point, with stiffeners at the sides. Its 50T capacity.

 

[frv]

A word of caution... channels are typically specified as A36 instead of A992. (Although this actually ends up helping with LTB)

 

[RareBugTX]

What are the spreader bars look like in relation to the channels?. Size?. Fabrication? Sounds like a lot of labor and maybe unnecessary analysis and design complication.

Can you detail the connection with a Standard Wide Flange Beam.

Best regards

Rarebug

 

[PMR06]

Don't forget to analyze the spreader beam with bending and the compression load due to the angled lifting cables (if applicable).

 

[JAE]

If the section modulus, cross sectional area, and moment of inertia are equal to or greater than the wide flange, the channels will be sufficient for bending, shear, and deflection.

Steve - I'm glad you then afterward mentioned lateral torsional buckling as this statement by itself is not true.

 

[JedClampett]

All comments above are good. One idea that comes to my mind is to castellate the channels to create a deeper member. I'm not sure how this affects the concentrated load resistance or the shear capacity, but it's more efficient in bending.

 

[CTW]

My $.02.
There is going to be a lot of time spent on your part to analyze this 2 channel configuration (possibly a lot of trial and error) and to determine how to reinforce it. Plus it's going to end up being a lot of material. Not to mention the amount of fabrication required. Quite honestly I doubt the channels will work without a lot of plate scabbed onto it.

Unless there is some specific reason for using the channels, other than the hooks, I would suggest staying with the W24 and using lifting lugs with hooks attached to lugs to achieve the lifting points. In fact, you could consider a smaller beam and weld vertical plates on either side to create a box. This will help with torsion.

 

[DTGT2002]

Or just use a HSS member.

We have switched to square and rectangular members for all lifting beams we design. The strength in both axes makes for an excellent member for resisting the primary bending from the applied load as well as resisting the commpressive forces of the rigging and the need to provide adquate strength ro resist buckling.

Coming up with a basic lifting lug detail out of plate can make a reproducible detail that can be cut in mass during fabrication.

We typically design for two lifting points on the top of the member and provide a continuous punched plate on the bottom with rigging points @ 24"o.c. for rigging and analyze accordingly.

Good luck!

Daniel

 

[imcivilized]

Thanks for the input.
It was a client spec (the two C-channels). I've recommended we use a smaller beam and box it in, and add attachment points underneath for the hooks.
We'll see if they accept the modifications..
Thanks again.

 

[DTGT2002]

If you are a member of AISC, search for "Design and Construction of Lifting Beams" by DAVID T. RICKER.

It is a free publication to members and is only $10 or so (US) for non members. It is a pretty good read for design and analysis of beam members.

I know your specific discussion was about using the as specified case, but this is a good article to have around.

I have struggled for years with some of the "built up" bolted members we use in construction. I was hoping this thread would have some good simple answers. Much of the data I use with our back to back channel beams comes from testing to support various conservative assumptions.

The newest AISC Steel manual came with a companion cd with some back to back angle design examples that i have considered working through to try to resolve the test and performance data with the codified building related info, but have not found time for the exercise.

Best of luck with the client. And do pick up the article.

Daniel

 

[IFRs]

I'd use a pipe, rig from the ends of the pipe to the hook with cable or chain, hang from plates welded to the pipe. Treat the pipe as a simple compression member. Easy calcs, simple construction. Double channels will have a much higher design and fabrication cost. And be weaker.

 

[WmacG]

I read some threads about spreader bars made with wide flanges where it was not appropriate to use the unbraced lenght as the length of the beam. This is due to the ends of the beam not being torsionally restrained. Search for "spreader bar lateral torsional buckling".