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Sizing for floor strength

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siggy_7

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I'm planning to board up the rafters in my garage workshop at the weekend, and I'm trying to work out what material would be suitable. This is just for storage of items of varying weight (think up to a loaded toolbox, say 25kg) and also to take the weight of a person crawling around on the boards at the same time. The garage rafters are 32" between centres, which I think rules out quite a lot of material including most chipboard. I was looking at 18mm thick OSB, but I'm struggling to find much in the way of information on what materials take what loads, so I tried some beam calcs instead. I wanted to find 25mm thick OSB but builders merchants don't seem to stock it, and 25mm plywood seems a bit overkill financially being 4 times the cost of the OSB.

I found the following properties for structural grade OSB/3: 16.4MPa bend strength parallel to span, 8.2MPa perpendicular to span, 1MPa planar shear. Do these figures sound about right?

I did some calcs per m width (parallel to span). Using basic beam theory, for a uniformly distributed load of 150kg/m^2, the max bending moment is wL^2/8 = 123Nm. The second moment of area I is bd^3/12; for d=0.018m this gives I = 4.86x10^-7m^4. Maximum distance from neutral axis y = 0.009m, therefore the maximum bending stress = 123 * 0.009 / 4.86E-7 = 2.28MPa, which is well within the bending strength of the board. The maximum shear force is wL/2 = 608N, for a cross-sectional area of 0.018m^2 the shear stress is 608 / 0.018 = 33.8kPa, again well within the material strength.

If my calculations are correct then 18mm OSB should comfortably take a 150kg/m^2 load with plenty in reserve. I read somewhere that wood can creep if it's continuously loaded above 50% of its strength but again I think I'm within this limit. Any thoughts on whether my calculations are reasonable and what I'm considering is sensible much appreciated.
 

Chrispy

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Can you reduce the spans by fixing lots of noggins in between the joists? 25mm thick flooring is going to weigh quite a lot on its own even before you load it up..
 

Lons

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If you wanted to beef up the OSB, could you not put in 2 layers of 12mm screwed together, pref with joints staggered, or even 1 @ 18mm plus a layer of 9mm.

Just a thought although Chrisps suggestion of fitting noggins is valuable advice.

cheers

Bob
 

MickCheese

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32" seems like a large span for any board.

Is there any way of fitting an additional joist between the existing ones then they will be 16" on centre and much easier to span with just flooring grade chipboard.

Mick
 

siggy_7

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I'm not sure how easy it would be to fit extra joists between the trusses, I haven't really looked at the side walls in detail and I can't picture it I'm afraid. I'd rather not have to fit noggings as there are some cables running on the side of the joists that I will need to move (I'm lazy!) but could do that if necessary. The builders merchant just down the road doesn't stock OSB but sells 18mm ply and for the quantity I need doesn't make much difference on the price, so for the hassle of transporting 8x4's 15 miles on a roofrack I'm thinking of using that instead, which will make the floor a little stronger. I may have to go down the bracing route, was interested to see if I could get away with not having to do the extra work as it's just for storage, maybe my calcs are wrong somewhere. Thanks for all your replies.
 

Dibs-h

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siggy_7":22zwydg5 said:
I'm not sure how easy it would be to fit extra joists between the trusses, I haven't really looked at the side walls in detail and I can't picture it I'm afraid..
You could bolt timber to the wall, bit like a wall plate and then place the extra "joists" onto the top face, instead of into the face.

Dibs
 

siggy_7

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Thanks for all your advice. In the end I fitted extra 4x2 joists, was actually quite easy to rest them on top of the wall although I needed to cut them into two sections to install them and then bolt together. At 16" between centres I then just used chipboard flooring. Given how it all felt during installation I reckon I could have gotten away with 19mm ply at 32" between centres, but the 4x2 beams would probably have struggled a bit. Thanks to you all for contributing.
 

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