Performing Stress Analysis on Pole Bunk

This one is interesting, I suspect that setting the coeff. of friction to zero will be a conservative approach. There is an open question about impacts, how hard is the drop, what is the velocity of the pole when it hits? Largest size of pole? 100lbs per inch of height, I assume but over what width? It sounds high.

I need to think about this one, the biggest problem might well be when one pole falls on another, driving it to the wall, see the top left of your diagram, if the top one was dropped directly on the one below, there would be the weight of both poles plus the impact of one pole acting around the contact between the lower and the pole that supports it.

I wasn't so concerned about the impacts from when the poles are bing loaded, as they are usually placed on with a piece of equipment that limits them to a gentle impact, if any.

I'm more interested in the stress on the frame during transportation, cornering, and in a static state under its heaviest loads before the poles are tied down (the straps are passed over all of the poles, which bunches them all together, removing some of the weight on the vertical supports).

The heaviest poles are up to 4400 lbs each, but only 6 of those can be loaded at once. The smallest are 660 lbs each, and up to 50 can be loaded at once. The heaviest combination is when ~50 of the 1100 lb are loaded, so I think this is the scenario the design will focus on.

The 4400 lb poles are 24" in diameter, tapering down to 10" over 85 feet.
The 1100 lb (40 ft) and 660 lb (35') both taper from 10" diameter to 8".

Static loading is pretty straightforward. Beams...

The dynamic loading acceleration will depend on the road conditions and speed of the lorries over those roads. Could get quite a whip up under some conditions.

So what I ended up doing was looking at how about a third of the poles would not load the sides, and the remaining two thirds would apply a force to their respective sides. Assuming that these poles would naturally want to roll away from the center of the bunk, I looked at the weight of each pole as acting at a 45 degree angle towards the outside of the trailer, then combined all of the horizontal components into one equivalent point load.

From there on it was a simple beam loading question. The end result gave a fairly low maximum stress ( 7500 psi ) and left a Safety Factor of 5.17 before yield of the material.

Do you think this is reasonable for something that will be used on a semi-trailer? Does anyone know how large of Safety Factors are usually applied to transportation eqiuipment like trailers?

Anyway, thanks for the help guys. Just needed to get the ball rolling on this one.

It does not appear that you are preparing these calculations under the supervision of the P.Eng. If you are making tha calculations by your self and the P.Eng is stamping them, this is a violation of basic engineering ethics and if the Province Engineering Board was made aware the P. Eng would be sensured at least. Because you refer to the engineer as a P.Eng I gather you are in Canada, in the states we use PE.

It is interesting that there is a blog on this page discussing engineering ethics

That is not the case, John. I am simply trying to hone my design engineering skills by preparing these calculations on my own. This P.Eng always performs all necessary calculations for anything he stamps, then I compare notes with him to see how close my methods are.

Thanks for the information though. And yes, I am in Canada. Good attention to detail.

Eric, I used your max stress value and worked backwards. Just cause I'm curious!

I took your sketch and ran a quick analysis. Below is the result. My peak stress (excluding contact pressure = 22000psi) was about 9000 psi in the middle of the gusset near the edge. I modeled the tubes as 6 x 6 x .50 wall with length dimensions as per your sketch. I modeled the gussets as 16 x 16 x .38 thk with a 9.5in radius. My applied loads to achieve the peak stress mentioned earlier were very low at only 2000 lb in 2 locations. I applied these 2 seperate loads simultaneously at 2 different points on the vertical post. The loads were applied 6in from the top of the post and 30in from the top of the post. Below are the results. Note that the deflection you are looking at is magnified 250x for visual purposes only. I'm curious as to what load you used since your peak is lower than my result.

Your max stress will be in the gusset and your resulting safety factor will depend on what stress value you are required to use for your calc. In my industry (aerospace material handling) we are always on the conservative side and we require minimum 3:1 on yeild, 5:1 on ultimate and it's based on calculated peak stress. Typically this is based on a FEA results since accurately calculating irregular shapes (weldements, machined parts, etc) is difficult using traditional calcs.

Nice work, I really miss having access to FEA programs.

I combined a component of the weight of all of the poles that would be naturally prone to falling without the vertical supports in place, (worked out to 388 lbs per pole acting normal to the vertical support), added all of the vectors and took the weighted average of their location to find one equivalent point load (which worked out to 6222 lbs acting 13.25" above the top of the gusset).

And yes, those results make sense. I only looked at the support from the gusset going upwards because I thought the max stress concentration would be there, because I assumed (you know what they say about that word) the large radius on the gusset would remove a large portion of the stress concentration there. There is also a plate across the top of the gussets to further strengthen them, welded all around.

Do you know how I could accurately perform the hand calcs to find the max stress in a gusseted corner like that? I guess I would start by doing the beam bending stress as if the gussets weren't there, then I'm not sure how to proceed after that.

Thanks for your help!

Eric, we don't typically analyze a welded box section like that with hand calcs. It can get far too complex and time consuming to do accurately. We apply simple beam bending formulas like you did to get the rough sizing then go straight to FEA to check the details of the welded section joints. We'll also compare the simple bending calculations to verify the FEA results.