6m 'bridge' using decking materials?

[Tim Nicholson]

Hello all

As a sometime lurker and infrequent poser of questions to this knowledgable group, the time has come once again to ask for your advice.....

I want to build an 19' bridge across a burn leading to a river at the bottom of our garden, and I need to know what size joists I'll need to use. I understand that 2x8's have a maximum span of around 13ft, so I'll probably need something bigger (2 x 10 perhaps?). I'll want to use two joists spaced about 3ft apart, (might I need three, to get midsection support for the crossmembers?) and each end will be secured to railway sleepers embedded or sitting on concrete foundations.

Anyone know whether there's a downloadable calculator that I can use to check whether this is ok?

Sorry if the above description doesn't give you a clear mental picture of what I'm trying to achieve, but any assistance will, as always, be gratefully recieved.

Tim

To respond by email, try tim, then the at sign, then intergrowl, then the dot com bit at the end.

[Colin Wilson]

Completely unrelated, but I recently came across a fun "game" called Pontifex II, where you have to build bridges, and it simulates weight loading etc - you just tell it what material to build spans in (up to your budget) then test it out :-}

[Grunff]

3 lengths of 3"x9" will be plenty for foot traffic.

[Peter Crosland]

It is worth taking account of the fact that these are litigious times and if the structure failed your insurers would decline liability unless you could show that it had been properly constructed. Bridge engineering is not something for the amateur so you would be well advised to consult a structural engineer. In any case if you need building regulations approval they will insist on proper calculations.

[Andy Hall]

Are you sure that that's adequate, Peter?

I would have thought the OP would have been better off getting Ove Arup in to do the consulting, McAlpine for the construction and Balfour Beatty to paint it in their spare time when not working on the Forth Bridge........ . .andy

To email, substitute .nospam with .gl

[Grunff]

And even then you'd still one to throw in a full public liability insurance package, which may well require 24hr cctv monitoring of the installation so that if anything were to happen the exact circumstances would be known.

[Tim Nicholson]

Funnily enough, this bridge is intended to improve OUR private access to the riverbank to view wildlife etc, and it may well therefore end up with a webcam looking at it! I already have a camera equipped birdbox, and am keen to expand on that.

Oh - and many thanks for the other, helpful advice Grunff - I'll be ordering the joists tomorrow (hope it stops raining by wednesday)

Thanks again all - mainly for the humourous content. (they *were* intended to be funny, weren't they? - I sometimes can't tell in this group)

Tim

[CRB]

on my Windows 2000 computer and it caused the computer to crash. On re-booting, Windows 2000 would not load, but just hung up on the initial loading sequence. Fortunately I have the Go Back program, so I simply reverted to an earlier setup, but without Go Back (or equivalent) I would have been faced with a complete reload of Windows

2000.

CRB

[Colin Wilson]

Eek - I have a similar problem at the moment with a relatives` machine - they installed some spyware infested shyte, and following a complete reinstall from scratch we can`t get her cable modem working...

Computers can be a pain in the ass at times :-/

[Danny Burns]

In message , Tim Nicholson writes

Tim don't know about a calculator but the building regulations have tables for floor joists, this may help give you a rough idea.

The nearest one I can find for 19' (5.8m) is 75 x 220 which will span

5.42m with 400mm centres. (This is for a maximum loading of not more than 0.25 kN/m2)

You could space the timbers a bit closer together if you wanted to be sure.

Hope this helps

[Danny Burns]

Oh forgot to mention this is using SC4 timber.

[Tim Nicholson]

[Tim Nicholson]

Thanks for that Danny (please disregard the last reply - a misclick sent the post before I'd finished......)

Could you translate 0.25kN/m2 into real world numbers for me? is that one, ten or a hundred people standing on the bridge at once? Also, please excuse my ignorance, but I don't know what SC4 relates to when talking about timber, but no doubt a quick google will enlighten me!

Thanks again

Tim

[Abdullah Eyles]

There is a very good design service here, try it and see what you get! (intended for others following this thread, I guess you've started already!)

To respond by email, try tim, then the at sign, then intergrowl, then

Ouch! that taxed my brain too much - try the group instead!

[David Hemmings]

Damn that must be an old calculator, SC4 doesn't and hasn't existed for a long time.

SC4 (strength class) is equivalent to ss (special structural) or c24 grade

[Danny Burns]

In message , Tim Nicholson writes

Haven't a clue about the physics of it I'm just a thick, hairy arsed builder. ;-)

I do know that most domestic dwellings ask for a load of between 0.25 and 0.5 though.

As someone has already posted elsewhere the SC4 grading for timber has changed, I've no idea what it has changed to but I always ask the timber merchants for SC4 and they know what I mean.

You can look at the table on the governments website here

still quote SC4 timber here as well.

Hope this helps

[stefek.zaba]

By happy coincidence, 1 Newton is more or less the force exerted by one apple under Earth's gravity: near as damn it 100g, or 4 of your earth ounces. So 0.25kN is equivalent[1] to 25kg, or 60lbs: a bit under half a Standard Adult's weight, or (blush) a quarter of a Stefek. So it's not a massively heavy loading. On the optimistic side, though, the beam sizing tables for interior construction are all about keeping deflection (bending) down to acceptable limits, so the floor doesn't feel too bouncy and any ceiling underneath doesn't crack. For an outside all-wooden footbridge, you can get away with a lot more deflection: breaking loads are (from memory, and I'm *not* any kind of structural engineer, mind!) about 8-10 times bigger than the "that'll be too much bending" limits.

HTH, Stefek

[1] "Equivalent" under earth-surface gravity, OK? All the pedants ready to whine about slapdash mixing of force units with mass units can just get back into their box and discuss the finer points of how to cleverly and to maximim phrasal effect split participles, *now*.

[Grunff]

Also worth noting is that the 0.25kN/m^2 figure is exactly that,

*per square metre*. That means the beams can take that as an average loading for every single square metre of the room without deflecting significantly.

[David Hemmings]

SC4 is now SS = C24 SC3 is now GS = C16

[The Natural Philosopher]

Any chance of using bolts and making a braced girder type structure? I.e. a top and bottom rail, with vertical and diagonal struts bolted between...such a structure is far more resistant to deflection and thus far more suitable for long spans such as you have. IUn addition it will need to be restrained and made stiff horizintally, otherwise teh top spars will buckle under load. This is done by linking each side of the bridge with horizontal members, both straight and diagonal.

THis osrt of problem is usually given as an example in first year structural engineering courses - was on mine anyay - suggest that you find local uni dept and/or firm of structural engineers to rough out the calculations on absolute strength and deflection under load for a simple braced girder end supported beam.

If its for foot traffic only, a gut feeling says that you could probably build a decent box gorder structire out of 6x2 and get way with it. To carry farm vehicles over that span tho, I'd say steel is required or very substantial timbers - maybe 12" or more.

A couple od substanntial steel I beams might anyway be a simpler solution, with cross pieces welded betweeen..