WBP vs fir or structural ply

[meow2222]

Hi

I'm puzzled re plywood deflection.

According to sagulator:

150lb centre load on 3'x6' of 1" fir ply supported at ends only gives 0.08" deflection, less than noticeable.

And 600lb distribbed load on 1" ply gives 0.16" sag, which is safe.

And 150lb centre load on 3'x6' of 0.5" structural fir ply supported at ends only gives 0.65" deflection, which I can use as a sanity check.

But... I looked at some 4'x8' 0.5" WBP and it deflected more than

0.65" under its own weight alone, making it nowhere near as strong as sagulator suggests, and meaning 1" of that isnt going to be anywhere near strong enough.

Is there really an enormous difference in strength between wbp and fir? Is sagulator miles out?

What I'm trying to achieve is the thinnest possible 6'x3' that will support 1 person without visible sag, and support 4 people safely, even if theyre a bit fat. So I'm using 150lbs per person to ensure it can perform under any realistic scenario.

Sagulator tells me I can do that using 1" ply, but a quick fondle of the ply shows otherwise (for want of a better word!).

cheers, Puzzled of Plywood

[dennis]

Sounds like it to me, wbp may be stronger but not much stiffer AFAICT.

I can't see ply giving much different results to any other timber so what does say 1" oak give?

You need a structural sandwich, ply, filler, ply like a door, a 6x3 door. ;-)

I am well over 150 lb and not particularly fat BTW.

[PCPaul]

Fir is much closer grained than the crappy stuff used for the middle layers of typical WBP.

IME 12mm ply is an awful lot more 'saggy' than 25mm. And 6mm will sag a foot or so under it's own weight - it's not a linear thing.

You need to actually try a thick sheet of ply.

I'm 200lbs and only slightly overweight at 6' or so. Designing your maximum load for the 'average person' doesn't work if there's ever a chance of, say, four blokes standing on it. Usually civil engineering etc works on 3x or more safety factors...

Depends exactly what you need it for, though, and what the consequences of a structural failure are.

If you just want it to 'look' slender, is the viewing angle such that you could add a couple of edge-on 2x1 braces hidden well underneath the 1" ply, which would make it *much* stiffer?

[meow2222]

I doubt I can.

Sorry, I wasnt thinking was I!

yes - 4 sitting on it is the max, and if theyre big then 4 wont fit on. Consequence is nothing disastrous, I just need to ensure it survives heavy loading.

unfortunately I need it to really be thin. If I knew how to calc the deflection for wbp or structural spruce I could use that & make it thicker, but sagulator and real life are so far apart I dont know what size or type to use.

NT

[Martin Bonner]

150 lb is less than 11 stone. I would use at least 200 lb (which is still less than 14 stone). 300lb is 21 1/2 stone.

[newshound]

There's really no substitute for "thickness" not necessarily in the massive sense but in the distance between the top and bottom planes. This is how I-beam girders work. It's surprising what load a sheet of A4 paper can take if you fold it like a concertina. If you have to use thin ply, can you really not add strategic battens?

[meow2222]

Thats more what I'm trying to avoid. By using flat sheet with no strips under it I can achieve the required strength with the minimum of total thickness. Thin sheet and joists would only make it thicker.

NT

[Rod]

Nanotubes, 'sobvious, innit?

Maybe some composite such as GRP - especially if you made it youself so you could optimise the properties?

But please put me out of my misery - I want to know what on earth you are constructing - and why?

[meow2222]

Unfortunately I know sd all about GRP.

A small removable semi-permanent platform that will be sat on & stood on. Clearance is critical.

NT

[dennis]

8<

That doesn't make sense, are the people a fixed height? If not, how much clearance do you have to play with?

[Tim Downie]

What thickness are you prepared to go up to? Is weight important? Must it be 6'x3'?

Tim

[meow2222]

Hard to put an exact figure, but 3" is too deep here.

no, as long as its not excessive, it needs to be lifted on occasion.

yes - and its to be supported either at the 2 ends only, so 6' span, or theres a possibility I may be able to get a point support halfway along one side. Maybe. Space is restricted so its not going to see

My latest idea is to make a plywood sandwich as before, but this time using planks as the middle layer & thin plywood as top & bottom layers. So the end result is a thick plywood sandwich with more timber oriented the wanted way than would be the case if I just glued ply sheets together.

NT

[Tim Downie]

As someone else has mentioned, your bog standard sapele door is pretty rigid. Two of those glued together would still be under 3" thick and leave room for a another veneer of ply or sheet ally to prevent foot penetration through the thin surface. Given their relative cheapness it might be worth a try. Don't seem to come wider than 2'9" though which might be a problem for you.

Tim

[meow2222]

Interesting idea - I'll see if one would fit. Would it survive the load though?

NT