Designing wooden structures using maths - where to start ?

[Simon]

I'd like to build a small wooden platform for use in a scaffolding tower. It will be a mixture of plywood and planed timber. I will be standing on it, so I want to make sure it's strong enough.

Rather than following my usual approach of using great big bits of wood, which has been 100% successful in the past, I'd like to try to calculate the sizes of the bits of wood required. Websites such as

answers some of questions, but I'd like to learn how an engineer would approach this. I realise this will involve some maths.

Can anyone recommend some websites or traditional paper books I could look at?

[dom]

I don't know how in depth you want to be, or the strength of you maths skills - but I've been very happy with Kermani's - Structural Timber Design.

It's aimed at professional engineers so it assumes reasonable algebra and trig.

[Simon]

that's probably the sort of thing I'm looking for. I don't mind trig. and a bit of calculus. Would it cover fairly simple cases like a sheet of plywood supported along 2 edges and with a load in the centre, as well as the more complex structures shown on the cover? I reckon at present I'm designing stuff in the same way as a novice medieval carpenter would, and I'd like to learn about some of the stuff that's been discovered since then.

[dom]

Sorry - I linked to the newer, more expensive book by Kermani - this is the one I have:

sheet of ply could (probably) be treated as a simple beam supported at both ends with a point load - assuming it's much longer than wider. If it is wide enough to be distorting both between the supports and across its width, a more sophisticated treatment might be necessary - not sure about that one.

The book linked to above has lots of worked examples and tables of typical performance characteristics. These were the most useful things about the book to me - along with explaining standard correction factors (I values) used by structural engineers).

If you buy the newer, more expensive book - be sure to post a review.

[mail]

I have a timber engineering degree from IC. I was a biochemist at the time and only signed on to remain in statute pupilare for sporting reasons in 1975/6 but it was the most intersting degree I have done. It did teach me that unless you are pretty proficient at maths you are wasting your time trying to design anything from first principles but instead just use some of the many tables available. You do not state what your objectives are within your overall objective. Is it to get the slimmest beam possible or the strongest one for a certain size etc etc. It is not appropriate to treat ply as a simple beam as the product is anisotropic and obviously desinged to be so. If, hypothetically, you treat it as a beam then you need to ensure that the comrpession and tension zones actually have the fibres in the line of the beam and in tension at the bottom and compession at the top! Also most of the "strength" of the beam comes from the separation of these two zones (look at the formula for a beam and work it out) ie the "depth" of the "joist", so even 1" ply will not span much! It would be more elegant to use 4mm ply glued and stapled to two

2" battens top and bottm to make an I beam as per the manufactured beams using this principle. It is is ply that you need to use for the platform then it will need stiffening under with mini joists suitably fixed. My suggestion is to find out the span, work out the maximum load you are likley to need in mid span (? pile of bricks, mortar and tools plus a fat contoller telling you what to do (smile) ) and then use the strongest material you can to make it as small as possible. this is likely to be a platform of high grade ply surmounted and glued and screwed to a very strong timber such as greenheart. Is it really worth it? Why not just make a bigger softwood beam on shuttering ply? Chris G