"new at this" need some help with floor design

[yeto]

Hi to all and thanks for the great website,

I want to build a 5' wide x 10' long x 6'6" high cargo trailer on a wooden frame. I am in the process of designing the floor.

I read the following quote in the member designs>ultralight design thread. I was hoping that someone could explain the following comments in simpler terms (the following quote relates to the floor).

Thanks,
Yeto

beware of the phrase 'torsion boxes' as the 'torsion' bit doesn't really apply to any of this, except to stop the beams tripping over. What you are describing is many beams sandwiched between two skins and they still just behave, and carry load, like beams. Contrary to what 90% of people think, a 'grid' of beams (ie, at right angles) inside a sandwich structure is an inefficient way of carrying a load - unidirectional beams across the shorter dimension are more efficient (and a lot easier to build).

All this talk of oak makes me think you guys are trying to build a steel frame out of wood. Why? Design it to be made in wood (most probably plywood), using wood's natural strengths rather than working out how to make it look like a steel structure.

But, most of all, everyone stop thinking you're building a house with foundations, a floor and some walls. This is a monocoque trailer we're talking about and the walls are what holds everything else up.

[mikeschn]

Hi Yeto,

Welcome to the forum.

Think of a teardrop cross section as a huge piece of channel. The wall being the web of the channel, and the floor and roof being the leg of the channel. All the strength is in the wall.

So for example, if you are going to bolt an axle to a wooden teardrop, bolt it to the underside of the wall, not the floor!

Mike...

[yeto]

Hi Mike,

Thanks for taking the time to reply.

Would it still not be an advantage to make the floor a torsion box for no other reason than to have a sturdier floor?

My current plan for the floor-top to bottom is--1/8 plywood--1 x 2s across the short length of the floor--2 x 4 long length of the floor with the 1 1/2" side vertical--more 1 x 2s across the short length of the floor--last 1/8 luan. In between the top an bottom 1 x 2s will be a 1 1/2 inch gap(the thickness of the short side of the 2 x 4). I plan to place a 1 1/2" aluminum square bar in this gap as the tongue. In the gaps I will use blue Styrofoam board for insulation.

Also, could you go into some extra detail on the following [quote]unidirectional beams across th shorter dimension[end of quote]

Thanks,
Yeto

[mikeschn]

Oh a torsion box floor is always good. It gives you a place to put your insulation, and makes the floor more rigid.

But I am going to defer the remainder of the questions to Andrew. He's currently on vacation. I believe he'll be back next week.

Mike...

[angib]

Yeto,

I think you need to give quite a bit more thought to this project as you've got some very different strengths in the bits you're describing:

- a 1-1/2" aluminium square bar (solid bar or hollow tube?) isn't big enough as the tongue for any roadgoing trailer, however small - even if it's solid, when it may be strong enough, it won't be stiff enough to stop whip;

- a 1/8" top skin is too light for almost any load, unless the 1x2s are nearly touching each other - waay too light to walk on, for example;

- two layers of 1x2s one way with a layer of 2x4s the other way in between seems a heavy and very complicated structure for a light load.

If you're trying to make a fairly conventional cargo trailer (say, with a capacity of 2000+ pounds) on a wooden frame, I would say you made a wrong design choice. It can be done (there's a very nice all-wood racecar transporter with extensive web pages somewhere on the internet) but you'll be using 2ft of that 6-1/2ft height to make the wooden frame strong enough.

Andrew

[yeto]

Hi Andrew,

Thanks for taking the time to reply.

The weight of the cargo will be approx. 300 lbs. and I would like to keep the trailer at 700 lbs.

I am thinking the tongue will need to be approx. 6 feet in length.

- a 1-1/2" aluminium square bar (solid bar or hollow tube?) isn't big enough as the tongue for any roadgoing trailer, however small - even if it's solid, when it may be strong enough, it won't be stiff enough to stop whip;

.05 x 1000 x 72 = 36,000 (looks like I need maybe 3 x 3 x .125 alu. square tube)

- a 1/8" top skin is too light for almost any load, unless the 1x2s are nearly touching each other - waay too light to walk on, for example;
- two layers of 1x2s one way with a layer of 2x4s the other way in between seems a heavy and very complicated structure for a light load.

What would be the best way to "sandwich" the 3 x 3 alu. tongue into the floor? Also, how can I make the floor strong enough to walk on but still keep it lite?

Do you think that 1/8 luan outside x 1/8 plywood inside with 1 x 2 frame will be strong enough for the walls and ceiling?

Is there any chance I can keep this trailer under 700 lbs.? I am guessig the axle and fenders will come in at around 200 lbs. but that is just a guess.

Thanks for all your help so far,
Yeto