Beam design and deflection

[davesen]

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....

 

[rb1957]

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 ?

 

[davesen]

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?

 

[desertfox]

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.

 

[BAretired]

Why not use a hollow rectangular section?

BA

 

[davesen]

Attached is a sketch showing better what I am proposing along with the new ideas. The situation has a tight clearance and the reason for the channel shape requirement, otherwise I would just use an I beam.

Hopefully the quick sketch comes through as readable.

 

[desertfox]

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

 

[rb1957]

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 ?

 

[PittEng88]

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.

 

[davesen]

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.

 

[BAretired]

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

BA

 

[desertfox]

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.

 

[JAE]

Be sure to meet the ASME BTH-1 Below the Hook Lifting Devices.

Check out Eng-Tips Forum's Policies here:
faq731-376

 

[rb1957]

my chicken scratch ...

don't know why you can't close off the bottom of the angles (and make a box section ?

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

 

[canwesteng]

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

 

[MikeE55]

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.

 

[davesen]

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

 

[canwesteng]

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.

 

[PittEng88]

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

 

[davesen]

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?