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?
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...
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
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
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.
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
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
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
Well, if I could figure out an option for 3 support fulcrums, or
"fixed at one end" "free support at the other" I'd probably get a more
Is that something SB4 can do?
 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
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
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..
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.
On Wednesday, 10 February 2016 14:08:06 UTC, Tim Watts wrote:
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.
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