Subject: cut into joists
From: "andrewpreece"
Date: 24/04/04 02:06 GMT Daylight Time
Message-id:

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

An excellent summary of beam theory and quite correct. Running cables through
the centre of a beam is by far the best way to go albeit more time consuming
than hacking a slice out of the top.


Dave Baker - Puma Race Engines (www.pumaracing.co.uk)
I'm not at all sure why women like men. We're argumentative, childish,
unsociable and extremely unappealing naked. I'm quite grateful they do though.