Under body framing reality check . . .

[TaliaDPerkins]

Does anyone have general unease with the use of 1"x1"x1/8" angle steel (carbon steel) framing for framing storage under the body of a bus? I am putting in vertical members both at the ends of floor cross members going down no more than 4' apart along the length of the sides, and vertical members going from the frame rails to either side of the center line at no greater spacing. Permanent diagonals are at either end of the platform I'm putting in underbody, supplemented by bolt on diagonals in the middle of the length to permit tanks to be placed/removed.


ASTM A36 steel has a tensile strength of 56kpsi to 80kpsi, with a load to yield of 36kpsi. Throwing in a safety factor of 4 to start permits each to be loaded with 9,000lbs. As long as I have horizontal pieces carrying load to the vertical, I -- famous last word -- feel like I can't go wrong. I'm replacing all the sheet metal from the floor down between the wheels basically with big empty box.


I'm about to pull the trigger on a $5,200 order of metal here for that and the roof raise before tariffs and am hoping for a reality check.

[Ronnie]

Your vertical load on the uprights will be fine, but spanning 4ft with one inch angle is pushing it. Downward deflection is mostly carried by the vertical web. I am doing similar to yours but with 2" by 1/8" on my Thomas HDX.


I am just southwest of you in Virginia, and pass through Clarke county regularly.

[Albatross]

I generally use 2"x2"x1/4" for any angle-iron support members I use for stuff like that, specifically so I know that it's almost always over-engineered, and will hold whatever I put it through without complaining. Not only that, but it can even be stored outside and basically ignored, and should still be more than safe 20 years from now.

[s2mikon]

Here is something else to consider when you "engineer" this, is this going to be able to take the forces of running over truck tire caps, wild animals, small farm animals and other things you may roll over in your travels? I'v run over plenty of things over the years and ripped off tank steps, tank fairings a fuel crossover hose to name just a few. The funniest one was on I-40 going through New Mexico when a pickup carrying some furniture in front of me had a dresser full of clothes fall out and I dead centered it. When I stopped in Arizona to refuel there was a pair of jeans wrapped around the trailer landing gear. So make it sturdy enough.

[AlphaHare]

IMO - not nearly big enough, especially if you stick with the same section for horizontal parts.

Without getting too deep, it's the modulus of elasticity of a material that's important for beam deflection. ASTM 36 has a modulus of 29,000 kpsi. I did a quick calculation with an online beam calculator, and a horizontal 1x1x 0.125 angle has a moment of inertia of 0.022 and a horizontal 4 foot long piece will deflect about 0.36 inches with a 100 lb centered point load. Obviously a lot less than 9000.

Any reason not to go with 2x2 angle or 2x2 sq tube?

A 2x2x 0.25 angle has a moment of 0.343, or roughly 15 times as stiff (Albatross)

A tube 2x2x 0.063 (16Ga) has a moment of 0.303, or roughly 13 times as stiff, and uses 1/4 of the material of a 2x2x0.25 angle. Steel is mostly cost in pounds, not section.

Calculate section moment of inertia here:
https://amesweb.info/section/second-...alculator.aspx

Calculate beam deflection using that moment of inertia and modulus of elasticity here:
https://www.omnicalculator.com/const...eam-deflection

[TaliaDPerkins]

Thank you, I am looking at a rectangular section tube for the lengthwise horizontals. I will post what I end up with.


I've used this with good success. https://app.clearcalcs.com/freetools/beam-analysis/us


and these


https://amesweb.info/section/unequal...alculator.aspx


https://www.cim.mcgill.ca/~paul/HollowStruct.pdf


Trying to look at how a point load like the corner of an attached tank is affected by a pothole or speed bump is complicating things.


Oh well.

[turf]

i think you're ok!

and your math is probably good too.

a couple of years ago, i flipped a bus. it was a holygrail kind of bus. the understorage was made from surprisingly small tubing, 1" i think.

maybe it gives in an accident? but strong enough is.... strong enough.

good luck!


pics of the flip and understorage

[TaliaDPerkins]

That. Storage. Is. Very. Nice! 8 ^ )

[turf]

the spare tire carrier on my bus is made from sheet metal and spot welds. it hangs from a couple of j hooks and the siding.

[AlphaHare]

Originally Posted by TaliaDPerkins

Thank you, I am looking at a rectangular section tube for the lengthwise horizontals. I will post what I end up with.


I've used this with good success. https://app.clearcalcs.com/freetools/beam-analysis/us


and these


https://amesweb.info/section/unequal...alculator.aspx


https://www.cim.mcgill.ca/~paul/HollowStruct.pdf


Trying to look at how a point load like the corner of an attached tank is affected by a pothole or speed bump is complicating things.


Oh well.

Yes, consider dynamic loading and design for that, static is easy. Big pot holes, sudden braking. For non-safety stuff a reasonable estimate is probably just to double the weight (2G) for usual driving events. NFPA 1192 requires 8Gs in any direction for propane tank mounting specifically. 30Gs is not unusual in a head on collision....

Steel is relatively cheap. You're not building the lunar lander where they had cash prizes for each once of weight saved. IMO go with tubes (better all round structural properties - there's a reason roll cages are made from tubing and not angle), and anything bigger than 1" will be easier to get a good weld as well. You could make a fun truss frame from 1" if that's your thing, but it'll be a lot more work.

[TaliaDPerkins]

Originally Posted by AlphaHare

You could make a fun truss frame from 1" if that's your thing, but it'll be a lot more work.

Roger that.

[Mekanic]

That is exactly what I used on my truck conversion.
You will see it will need triangulation on the sides.
It will work quite well.

[TaliaDPerkins]

Did you post pictures of your build ? Thank you.