According to Superbeam 4, a uniformly distributed load is giving a higher deflection than the same load as a point load dead centre.
That doesn't seem right???
50x150mm C16 timber, span 2.8m supported each end, sample load of 3kN.Point load at 1.4m is 17.75mm deflection, Uniform load gives 30.73mm deflection.
What have I overlooked? Is the Uniform load (U option) kN per metre rather than total load?
Not using Superbeam, I reckon central point load should give about 1.6 times the deflection of a uniform load. Are you sure it's central?
Cheers
Handy rule of thumb ^^^
Well the answer was in a help file:
"All loadings in kN for point loads and kN/m for other loads..."
Now, next "issue":
3.35m span, assuming C16 50mm wide, 100mm deep joist, 1kN point load (me) centre (1.7m near enough) = deflection of - wait for it...33.65mm !!!
But in reality - I've stood on most of the old ceiling joists, including dead centre and they do not move perceptibly at all. And no ceilings cracked.
And these are joists that have not been tied to any new floor joists except by a sheet of plasterboard.
What gives? 34mm vs maybe a few mm?
Is my old wood just "really really strong"? Even C30 couldn't account for that.
Tim
Gives:
100kg centre load, Redwood (old growth), 3350mm span, 50mm depth front2back, 100mm thickness:Floating bearing points: 19.95mm Fixed bearing points: 4.99mm
The latter seems more like reality. Wonder if it's to do with the end being fixed to the rafters and the other end actually continuing through the house over the centre wall (so it is a triple point supported single beam of 2 x 3.35m spans.
Ow - mechanical engineering breaks my brain.
Steel beams are sized to minimise deflection, not on what they can stand.
BTW you should put a support under the centre of a long beam before you load it up. Remove when cement has completely set. This minimises deflection.
Having the ends effective built in will make a hell of a difference - as will the plasterboard, which will help to spread the load across several joists.
Why not just use the building regulations charts?
Oh - interesting.
Do you know where I can find them? Thanks - Tim
I guess...
It might be instructive to measure the *actual* deflection - taping a ruler to the light rose which happens to be dead centre on the ceiling under this and running a fixed laser line over it whilst a coupld of people go stand on stuff...
I have concluded that I am going to put the stud wall back where it was rather than 400mm further out to discourage excess loading on a weak floor - but, in conjunction with the tie beam that was full of woodworm, I have ordered some 150x50 C24 which I will put next to the tie beams (or whats left of one of them) and bolt through, and screw angle iron brackets between that and every ceiling joist along the mid span.
To boot, it can get screwed into the 1st 8x2" joist at the edge of the alcove.
That will:
a) be no worse than it was for the last 40 years.
b) distribute the load in a more convincing way over 8 ceiling joists (2.6m span) and also couple it to the first floor joist.
That should tie the whole lot together rather better and I can do it in all 4 corners of the roof.
As the BCO who's seen all this crap said: "it hasn't fallen down in 40 odd years but you'd do well to add a bit of wood as you see fit" (more or less).
I'm thinking ahead to when the kids hold parties up there in a few years with much bouncing around! I like my bed (underneath) free of plaster pieces!
In a word, yes ;-)
A typically floor load on a joist would be 0.8 kN/m, with the joists calculated in a load sharing arrangement.
i.e. in cantilever - there is an option for that.
So long as your body does not break the beam, its fine ;-)
IIRC you need an old approved document that had span tables....
Part 'A' 1991 version (incorporating later amendments) seems to have them, though I'm sure Tim will be OK with superbeam now he knows what units to feed it!
Hi John,
I know how to do cantilever with overhanging end - but not with
Support --- Span --- Support --- Span --- Support
Did I miss something?
Aye - and that's the saving grace - but I would like to have a number to give me an idea of max load on a permanent basis - ie don't go piling books there - that sort of thing :-|
Anyway - have the 6x2" C24 on order to strap all of them together and act as a tie beam extension. At least that way the load is guaranteed to be shared, more or less with all 8 joists.
Just need some round hole punched angle iron to screw between the joists and the new beam.
Cheers, Tim
Thanks - that would explain why I could not find it...
Well, if I could figure out an option for 3 support fulcrums[1], or "fixed at one end" "free support at the other" I'd probably get a more representative figure...
Is that something SB4 can do?
[1] ceiling joist is some 7m long and is supported at both ends and over the mid wall - possibly why it does not bend as much as you think it would...
I note you can enter a negative point load... but not sure if that is the right way to do it.
Might be best to ask Tony himself... he seems to respond on his forum regularly:
I *think* that is for an overhanging cantilever - or maybe I'm wrong...
Thanks - I'll try that. I might even buy a copy, it will be useful for designing a workshop where I actually want the floor joist to cantilever so as to be able to build near a sewer whilst keeping the supporting walls a good metre back..
Cheers,
Tim
Looks like he answered, but assumed you meant a steel beam rather than a timber joist.
Thanks John - I thought it might email me - but I have replied...
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