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....

 

[BAretired]

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

BA

 

[PittEng88]

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.

 

[BAretired]

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

 

[desertfox]

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.

 

[davesen]

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!!

 

[KootK]

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.

 

[rb1957]

"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 ?