"Coherers" wrote in message
...
"dundonald" wrote in message
news
I'm laying flooring [1] in my loft. The wiring is simply thrown over
the
joists rather than a hole drilled centrally through the joist and the
wiring
passed through the hole. Is it good practice to cut a little nick out
of
the joist, enough to lay the wiring for the flooring to flush with the
joist
over the top? Is this detrimental to the structure?

I tried searching the archive of this group for the answer, but after a
couple attempts trying to guess what others might call what I call a
'nick'
[2] I gave up.


[1] chipboard variant
[2] crescent type cut out

--
IEE Wiring Regs 16th Edn:

"Where cables are installed through timber joists in the ceiling or floor,
they shall not be less than 50mm from the top or bottom of the joist,
unless
mechanical protection that will stop the cable being penetrated by nails
or
screws etc. is provided. "

Sometime, I have to get around to drilling the joists and re-running all
the
lighting cables above a flat of mine for similar reasons. You only need
diddly holes if they are lighting cables.......


Beam theory says that a loaded beam ( loaded from above ) has the top
part in compression and the bottom part in tension. These forces are at a
maximum at the top and bottom surface of the beam and decrease linearly
as the centre of the beam is approached: thus for material that has the same
strength in tension as well as compression, there is a 'neutral line'
running through the beam along its length, exactly halfway between top and
bottom faces.
My back of fag packet calculations say that a notch taken out of the
surface of a beam reduces its strength pro-rata ( the stress reditributes
itself either side of the notch, so a notch 10% of the thickness of the beam
effectively reduces it to a beam with 90% of the original strength at that
point. A hole 10% the height of the beam drilled through the middle causes
very little stress to be redistributed, as it is on the neutral line, and a
reduction in strength to 99% occurs. If the hole is 20% of the beam height,
a reduction in strength to 94% occurs. A 30% height hole reduces strength
to 82% of original.
I wouldn't rely on these calculations since I haven't used the proper
beam formulae, but I reckon that a hole drilled dead-centre in a beam is a
lot less weakening than a notch taken out of the surface. The weakening
effect of the hole increases as the square of its size, so keep it small.

Andy