I am having to shift my kitchen. The waste pipe which I believe has to be
35mm is going to have to go under a joist (or through).
If go under the joist, I lose the slope required to take away waste.
If I go through it do I breach building regs ? The joist runs parallel to an
interior wall (old tenement flat) about 10cm away. I'm in Edinburgh dont
know if regs vary with location.
Thanx in advance.
Apologies for raising a concern, but surely it isn't acceptable to
chop a 32mm hole in a joist? Wouldn't that seriously weaken the joist
to the point of possible failure?
Please note that I am not basing my query on knowledge or fact, it
just seems unlikely to take out that much material without there being
an adverse effect.
I think square corners were the cause of the first jetliners
mysteriously and randomly responding to gravity back in the 50's.
Eventually they figured that cracked were raking out from the window
corners, destroying the aircraft frame.
With Liberty ships the key problem was the previously unknown
problem of brittle fracture: normal steel becomes brittle about
freezing point and then if overstressed will fracture rather
The Comet problem was to do with metal fatigue, with the square
corners windows causing concentration of stresses.
To get back on topic, good practice for notching joists is to
drill and cut, though of course it depends how much one is
taking out the joist.
Tony Bryer SDA UK 'Software to build on' http://www.sda.co.uk
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Fatigue and stress concentration. Relevant to a metal stressed skin
aircraft - less so to a wooden beam.
Most issues with square holes are to do with introducing stress
concentrations, but I don;t think its seriously significant in te
current case. A couple of holes and a pad saw will make a decent slot
for two pipes, but arguably two separate holes are better.
I think it was Comets that suffered from this particularly, messing up
what was otherwise seen as a pretty sucessful aircraft.
Although it discusses ships and hatch shapes rather than airplanes the
book 'The New Science of Strong Materials: Or Why You Don't Fall
Through the Floor' by J.E. Gordon covers this behaviour, called
Griffith cracking, IIRC. The 'magic' of these things is that once you
reach a critical crack size catastrophic failure is inevitable.
One story in there is about a ship that had a crack developing. One
of the galley crew (because this is where the crack was, IIRC) took to
marking the progression of the crack by dating the tip of the crack
Eventually the ship split and sank. Luckily (for science) the
recovered the half of the ship with the marks on it. These marks
become the first record of a Griffith crack 'in the field' and showed
that the ship had carried on sailing for years whilst the crack was
below the critical size, but once it reached the theoretical failure
point the next heavy seas took her down.
I may have elements of that wrong, but I think the gist is right. The
book quite a good read for other material stuff you've probably always
wondered about too.
Yes, it weakens it. Its not necessarily serious tho, because joists are
rather crude atructural devices. They are engineered not to not break,
but not to bend too much. As such they are well over strength for loading.
Putting a hole through them at one spotonly slightly reduces their
stiffness. It DOES reduce strenght, particularly in teh center of a long
span. but on a short span or at the edges, its not serious.
Now that's what I thought, but a joiner told me to try and keep a hole of
that size nearer the top of the joist. I didn't ask him to elaborate to
much on it at the time, I just did what he said because I tought he knew
better. I might just go and ask him why now.
Yep, also what I thought, but then, when the joiner told me to put the holes
nearer the top of the joist, my thought was that maybe the floor board being
screwed back down across the space would have something to do with giving
the joist some reinforcement. But I have to admit that I to thought the
holes were better in the middle of the joists.
I have to admit to being quietly amazed by this discussion!
A joist might be what - 100mm in height? And you drill out 32mm, thus
removing about 30% of the strength of the joist. That joist might have
an inconvenient knot around about that area to further weaken the
structure. And even if the joist was 200mm in height that would still
represent a 15% loss of strength overall - which in my book sounds
Doesn't add up for me, however I am happy to bow to the combined
expertise of others on this matter!
Building regs state:
Holes should be no greater diameter than 0.25 times the depth of the joist;
should be drilled at the neutral axis; should be not less than 3 dimeters
(centre to centre) apart; and should be located between 0.25 and 0.4 times
the span from the support.
BTW The neutral axis means the centre of the joist. Notches may only be
0.125 times the depth of the joist.
Gut feel is OK if like many many craftsmen, you have been using the
materials for years and have experienced failures in them.
Otherwise, stick to someone esses rules, or do the (very complex)
calculations, or take the engineering route, and put the structures in a
machine and stress them till they break.
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