I need to drill a bunch of relatively small holes in 2x10 floor joists
(for electrical cables, pex, etc). The holes will be 1 1/4" or 1 1/2"
diameter, well within allowable hole size for 2x10s that are 16" oc.
I have noticed the following.
Most drilling information I have seen says that holes should not be
drilled in the ends of floor joists and instead should be in the middle
1/2 of the span. Plus, there are rules for multiple holes and rules
for where holes should be in relation to top and bottom of joists.
Nonetheless, when I go look in the basements of many houses, I see
multiple holes near the ends of joists. I have seen many houses where
literally all the holes are in the ends of joists.
Why is this the case? Is this because codes are very conservative and
2x10 joists, say, are very robust to holes in their ends? Do some
plumbers simply drill like crazy, knowing that it does not matter that
Mid span experiences the highest bending moments and the ends, the greatest
shear. A hole at mid-depth has less effect on bending than on shear. Why
should holes in the shear zones be stronger than in the bending zones?
Because the "Shear" zone experiences compression.
If the holes are filled with pipe the holes
would have no effect. The holes in the middle
tend to pull apart. Filling them doesn't affect
that. Just my guess. But it is should be
immediately obvious that holes in mid span are the
worst possible place. You might want to test that
with two 8 foot pieces of 2x4.
Not unless the pipe is a perfect fit and is glued in to transmit
shear. Otherwise, it's still a hole.
Holes at mid-depth and mid-span experience relatively little stress.
In steel beams, holes must be in the middle third of the span and
near mid-depth unless they are reinforced. There are limits on the
size of the holes as well. I'm curious as to why the standards
for wood are so different.
Michael Daly ( firstname.lastname@example.org) said...
I believe it is shape, not material, that makes the big difference.
I suspect that if you used a steel bar (and not an I-beam), the hole
cutting issues would be a lot closer to using wooden joists.
Wood I-joists have hole cutting requirements/limitations that are closer
to steel beams than to solid wood joists.
"Never ascribe to malice what can equally be explained by incompetence."
You have it kinda backwards. Holes (of proper size and distance from the top
and bottom edges) are permitted anywhere along the joist, *but notching* is not
permitted in the central third of the joist. Here is a joist drilling guide
with nice illustrations:
You don't want to mess with the part of the joist that clamped between
the support wall/beam below and the load-bearing wall-plate above,
because there are resisted twisting, compressive, shear, and tension
loads on the joist at that point, and figuring out which way
everything is pushing/pulling in non-trivial. That is also,
incidentally, where you're likely to hit a 12-penny nail while
trying to drill.
The reason people do it anyway is because it's closer to where
they want the utilities.
This article talks about 'ripping' being unacceptable. Can someone explain
why? Isn't ripping essentially what the sawmill does?
To clarify my question, if I buy a 2x12 and rip it to the size of a 2x10,
why is this less acceptable than buying a 2x10?
Or am I misunderstanding 'ripping'?
The way I interpret the article is that ripping in-place joists (say, to get
thresholds to work out better) is a no-no, on the assumption that the joists
were already sized correctly (not a bad assumption, IMHO :-) ).
If you have money to burn, and want to rip down an uninstalled 2x12 to 2x10, I
doubt anyone is going to give a rip ;-)
Nope, they explain why, sort of. After logs are cut into dimension
lumber, the planks are then graded, based on twist, and the size,
number, and placement of defects. If you rip a plank, then the
ratio of the defects may change in either or both resulting planks,
and either or both of the resulting planks may twist, bow, or
(depending on which axis you ripped on) cup. So if you take
a #2 Fir 2x12, and rip it lengthwise, the result may or may not
be a pair of #2 2xNs. The previous grade/specification becomes
invalid, and you have to visually grade the plank(s) again,
and the standard assumes that the people doing the cutting
either cannot or will not do that reliably.
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