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Beam design and deflection

Beam design and deflection

Beam design and deflection

(OP)
I need to fabricate a small beam to lift a load with center point loading.

A channel shape works best for the beam design due to space constraints. In my situation, I would utilize a channel shape with the 'C' pointing to the ground, lift at the center bottom to lift a equal load on each end at the top.

Looking at moment of inertia and weight per foot, I would be much better off using 2 unequal leg steel angle iron sections to form a channel shape versus using a common steel channel. Obviously the longer legs would be in the vertical direction to increase strength.

My question is how to weld the channels, and would this arrangement result in strength comparable to the sum of the moments of inertia of the two angles? I believe welding a 1" deep weld each foot length along the entire length of the angle iron assembly would be sufficient for this application?

Thanks for any suggestions....

RE: Beam design and deflection

the longest leg you can, in the direction that's developing bending stresses (ie the vertical leg). just add the I from the two angles, each angle would support 1/2 the moment.

maybe C-channels connected to the web plate.
I'd like to have the load carried in shear, like having a C section wrap around the beams, and weld on the vertical webs. or C in the middle and run the beams on the outsides ... lot's of variations.

size of weld depends on size of load ...

another day in paradise, or is paradise one day closer ?

RE: Beam design and deflection

(OP)
Thanks, great ideas! I was thinking about welding a bar on top of the two unequal angles also to increase strength, I guess by roughly the shear strength of the bar..

Anyone know approximately how much weld conservatively needed to match up to the load, say 2000# load how much weld to hold this together?

RE: Beam design and deflection

Hi

Any chance of a sketch, I can see how you are going to connect to angle sections together to form a c channel, what I cannot see is how you get a 1" deep weld on the thickness of the angle toes.

RE: Beam design and deflection

Why not use a hollow rectangular section?

BA

RE: Beam design and deflection

Hi
Why not just weld some vertical ribs to increase strength of the original channel section you proposed.
I'm not a big fan of stitch welding particularly on lifting equipment I prefer continuous welding and from your sketch your weld can only be as deep as the section thickness

RE: Beam design and deflection

strongly support that suggestion ... there isn't a good shear path from the load to the angle legs. another option would be to turn the angles around ... mount them back-to-back. maybe back-to-back with one flange at the top, and one at the bottom (like a Zed ?

another day in paradise, or is paradise one day closer ?

RE: Beam design and deflection

My vote is with BA, I would try and utilize a HSS rectangular section.
1.) You will get a much stiffer section than you would with just two angles.
2.) You can avoid the welds. Personally, I am not a big fan of applying concentrated point loads to welds, as you have shown.

RE: Beam design and deflection

(OP)
Thanks for the feedback! I can't use a rectangular section due to the clearances...

Desertfox- are you saying to weld outboard vertical stiffeners to a standard channel to increase the moment-like in using bar stock (I think this makes sense)?

As noted the point load would be only on the channel web, was wondering about this type of mod considering rb1957's comment about the shear path.

Actually I thought rb1957's suggestion as relating to my 'B' option sketch is the most robust reinforced design, and assume just add up the total vertical moments of the 3 pieces.

RE: Beam design and deflection

The lower ends of the vertical legs shown in the sketch are stressed in compression and may buckle sideways before reaching flexural yield.

BA

RE: Beam design and deflection

Hi davesen
Yes that's exactly what I'm saying, just weld some vertical ribs onto the back of the channel to increase the depth.

RE: Beam design and deflection

Depends how you lift/ support the beam, but I could maybe see bending stresses developing in the weld, which is not a good idea. Any of the suggestions here are a better idea from that point of view

RE: Beam design and deflection

Be careful with the welding procedure. Your original sketch looks like a no-throat fillet weld. Maybe a bent plate in the shape of the two angles would work better, and eliminate the welding.

RE: Beam design and deflection

(OP)
Thanks to all for the help, many good suggestions!!!

I may just go with a channel that is heavier than what is needed to be conservative. In this application the 'C' shape will be pointed down and the single point lifting load will be under the center (inside the 'C'). The load will be distributed over approximately 1/2 of the internal width of the channel. The load I am picking will be at the top at both ends, also in the center of the channel width and distributed over about 1/2 of the total channel width.

I will be using a beam with calculated deflection ~50% of the 1/240th of the beam length (I have read 1/240 is acceptable for beam deflection).

One last question: Is there any issue with the loading on a channel being centered on the web, creating a problem with stress tranfer to the vertical members in this configuration?

I can sketch if anyone wants to look at the actual configuration... Thanks

RE: Beam design and deflection

Potentially there could be an issue there, but probably not. Sketch some yield lines for failure modes of the web as it transfers load to the flanges and see what you're demand/capacity looks like. Maybe two way shear of the web as well. I don't think either of these are likely to govern over weak axis bending.

RE: Beam design and deflection

I would definitely be sure to check the local buckling of those flanges since they will be in compression.

RE: Beam design and deflection

(OP)
Yes the flange buckling under compression is the potential issue I worry about. I am not structural and do not have the expertise or software to evaluate the loading simulation.

I was considering testing the selected channel and measuring deflection as I apply the lifting force. My thought was that if the deflection matches the 50% level as described above I should be fine??

Could there be catastrophic failure if the deflection is tested to be conservative?

RE: Beam design and deflection

Yes, there could be catastrophic failure if local buckling occurs. Deflection is not related to that type of failure.

BA

RE: Beam design and deflection

Let me elaborate a bit more on my last response:

According to the AISC 2005 spec, yielding will control for most of the channels currently manufactured with 36 ksi steel. So typically, flange instability is not an issue since most of the channels have stocky flanges. However, there are a handful of them that are classified as slender with 36 ksi steel. Therefore it would be worth your while to check to make sure the one you are using does not fall into this category. Table 1-6 of AISC will tell you which ones are slender. Also Section F6 of AISC will give you more insight into channels bent about their weak axis.

If the flanges were to buckle, than yes that would be considered a "catastrophic failure." However, as I mentioned above, most channels will yield before this happens. Still worth checking though.

Load testing the channel couldn't hurt, especially if you are limiting the deflection to half the allowable. However, I'm not really sure what your ultimate goal is here with this beam.

I would also check into any special loading requirements you may need to include in your design i.e. impact loads, etc., since this is a lifting beam.

RE: Beam design and deflection

I agree that local buckling is not a factor if you are using a standard channel; sorry, I thought you were talking about using the double angle shape.

BA

RE: Beam design and deflection

Hi davesen

Just a thought if you are going to use the existing channel why can't you turn it upside down so the vertical legs are in tension instead of compression.

RE: Beam design and deflection

(OP)
Thanks again gentlemen! I would be using a standard channel with relatively thick flanges. I will look at turning the 'C' up but I think my space will limit this option.

I am looking at rectangular tubing also, the extra height makes things more difficult but sounds much less risky!!

RE: Beam design and deflection

Some additional thoughts:

1) Lateral torsional buckling is a concern. Using a tube section would address that. So would using a a channel or a pair of angles so long as the composite section is bent about its weak axis. If you use a channel or angles, I would ensure that bending is about the weak axis.

2) Whether you use angles, a channel, or a tube, I would highly recommend using a stocky bearing plate at the points of load and support to distribute the loads out to the vertical, shear resisting elements of the cross section.

3) Don't be spooked by the flange buckling business. You don't need software or voodoo mathematics to evaluate it. You only need to ensure that the width to thickness ratios specified in the AISC manual are not exceeded.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Beam design and deflection

"Is there any issue with the loading on a channel being centered on the web ..." ... do you mean placing a tension bolt into the web ? or instead using a bolt (or similar) spanning the two legs, and shackle onto this (that way you directly load the stiffest part of the section).

another day in paradise, or is paradise one day closer ?

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