is this strong enough?

[48Rob]

EDIT-Please disregard the first drawing, and use this one;



Trailer is 14' box.
Total length with tongue, 17'-4"

Main frame rails are 2 x 6 x 1/8

I will be welding square 2 x 2 x 1/8'" tube to the bottom of the 6" beam as a spacer, then welding a 2 x 2 x 1/4 square tube under that to carry the main frame rail load in the dropped floor/doorway area.

I will only remove a section of the main frame on the curb side, the other beam will not be altered.

Forward of the door opening there is plenth of room for weld, and strength.
Behind the door opening there is only 6" of space before the spring hanger.

Is that enough, with plates welded on both sides of the beam, and is 2x2x1/4 enough?

Overall weight of the trailer will be 3000#

Rob

[Forrest747]

ok if i looked at it right you are carrying the 2x2 from teh rear spacer to the front edge of the trailer at about 72 inches or so. with gussets and 6 inches at the rear solid weld

chrunching numbers you are looking at possbile 1452 lbs of torque before snapping.

[KCStudly]

In order to do a beam calculation we need to know the distance from the rear door jamb to the tongue coupler.

We can guesstimate the distance from the center of the forward spring eye to the edge of the fish plate. We can determine the theoretical static load at the front spring hanger (85 to 90% of 1/4 of the rated capacity, assuming that there is 10 - 15% on the tongue and that the spring pins are centered front to rear on the springs, i.e. each spring eye is bearing about 1/4 of the axle load). We can take half the tongue weight (assume 5 - 7.5% total capacity). Then we have an idea of the loads at the reaction points and can start looking at the non-uniform section beam and how it might behave to these reactions at those distances.

I don't see how this can be figured w/o knowing the distance from the front spring eye to the hitch ball.

Throw in some factors for dynamic vs. static loading and see what "looks about right".

This is kind of cheating because it would be better to look at all of the loads along the whole side rail (ball, front and rear spring load points, back of frame, uniform(?) wall/floor loading, and location of any major components (for point loading) then treat it like a non-uniformly loaded beam, but unless the numbers come up real marginal using the above approach, I think we could at least get an idea as to whether it needs to be looked at with more scrutiny.

Please, anyone with a differing opinion, speak up.

On the 6 inch 'beam', is that a built up 'I' section (such as S-Shape) or is it box tube?

I'm not sure how much I will be able to help (with any level of confidence), but the problem can not be solved if the loads and the distances where they are applied are not known.

[Forrest747]

i used 14 inches from teh end of the weld to the axle midpoint. used autofem for teh calculations.

[KCStudly]

hmm. Tell me more. Every beam calculation I have ever done uses the loads and their realtive distances to determine the moment and shear forces. These are compared to the moment of inertia of the shape and the cross sectional area (with consideration for the type of material) to see if the member is adequate. I realize that this gets very complicated when the member is not of uniform section, but...

You have done a torque calculation about some point assuming a static condition(?). How do you know that that is where the highest load occurs?

I don't mean to be cheeky, and I am by no means an expert on the subject, so if I can learn something new I'm all for it. Thanks.

I've done these types of calc's by hand and using Strucalc, but have not ventured into FEA.

[48Rob]

Thank you.

I've decided to use 2 x 3 x1/4 instead of 2 x 2.
I've also increased the gusset plates from 6" wide to 12" wide, but chose 1/4" thick, instead of 3/8.

I moved the axle back slightly and narrowed the door opening to 29" for a 28" door.

Now there will be 12" behind the door opening, and 15" in front for a total of 56"
I made it a little shorter since it will be hanging down an extra inch.

Distance from the front spring eye to the center of the hitch coupler is 134 inches.

The 6" main beam is a box tube.

14" from the end of the weld to axle center is correct...or was on the first drawing.
On the revised drawing, it is 16"



Rob

[Forrest747]

hmm. Tell me more. Every beam calculation I have ever done uses the loads and their realtive distances to determine the moment and shear forces. These are compared to the moment of inertia of the shape and the cross sectional area (with consideration for the type of material) to see if the member is adequate. I realize that this gets very complicated when the member is not of uniform section, but...

You have done a torque calculation about some point assuming a static condition(?). How do you know that that is where the highest load occurs?

I don't mean to be cheeky, and I am by no means an expert on the subject, so if I can learn something new I'm all for it. Thanks.

I've done these types of calc's by hand and using Strucalc, but have not ventured into FEA.

I made alot of intial assumptions. i went with 2x6 .120 thickness. i had a standard full length single pass of weld. I applied the entire 3000 lbs evenly across the frame. had stress points at the tongue and the hitch. i then set the conditions and measured the point at the joint. it was the highest stress point due to its smaller size.


Taking teh new drawing into account the new gussets and steel being used i came up with 6198.23 lbs psi.

[48Rob]

Taking teh new drawing into account the new gussets and steeling being used i came up with 6198.23 lbs psi.

That sounds strong enough!
If the road side wheel came off the ground while going over a bump, and all the weight came down hard on the dropped frame side, it should hold.

I'd guess in the next 20-30 years that I'll be using this new trailer, that I'll scrape on a few steep driveways and dips.
Lightweight and strong is great for the average gentle user, but that isn't me.
I'd much rather overbuild the foundation just a bit and never be concerned, than worry every time.

Any other opinions, things I've missed?

Rob

[KCStudly]

Well I bow to the superior/modern method applied by someone who appears to know the tools that they are using. Thank you Forrest for elaborating.

So is it safe to assume that the 6200 psi is in a static condition? I've always used 24,000 psi yield for steel (36,000 x 2/3 = 1.5 SF) so that's almost a factor of 4 to account for dynamic loading (road undulations, pot holes, bumping a wheel into a curb or two). Is that enough?

I have to admit that the 2 x 6 by itself is probably way overkill. My car trailer is double axle 7000 lb gross and has main rails of 2 x 4 x 3/16, with a second smaller rail running along the inside edge of the diamond plate stringers where the hauled cars tires travel that is 2 x 2 x 3/16. The deck is something like 16 ft long and it does have a bit of flex, but there is absolutely no triangulation or side rail truss to stiffen it up, and it is certainly more than adequate in that application. With rigid cabin walls, I certainly think you could drop down to 2 x 4. The weak link is that drop down doorway, for sure.