notching joists

K5 factor. Modify the calculated max shear stress by beam depth after notching/beam depth.

On Mon, 17 Aug 2009 11:49:31 -0700 (PDT), a certain chimpanzee, sm_jamieson randomly hit the keyboard and produced:

Rafters can be birdsmouthed, and up to one third is a rule of thumb, so I assume joists are the same. I know I've had floor joists & steel beams which have been chamfered to ridiculously small depths and still passed calculations. I'm no engineer, but AFAIK, longish lengths fail in deflection or bending long before they would fail in shear.

Don't assume. Rafters are largely in compression. Joists are pure bending moment.

However having said that joist are way too STRONG in order to meet deflection/loading targets.

So you can probably notch massively,.

I know I've had floor joists & steel

Yes.

Definitely worth checking. All the out-buildings around here (barn aside) have the rafters cut to about half their depth where they meet the walls, and it doesn't seem to cause any issues - but then maybe that's due to the way they're anchored (and anything toward the outer edges beyond the cut doesn't exactly have to support a lot). As you say, joists probably have quite different loading characteristics, particularly at the ends.

Probably, assuming they're not all like that. My gut feeling* - and that's all it is - is that I wouldn't want to take more than about 20% of the total height though, if I could help it. But then surely more than that suggests a brick structure below which has some major issues of its own? :-)

• I tend to massively over-engineer stuff - and if it's around long after I'm gone, well that's generally no bad thing...

cheers

Jules

Think Sydnney harbour bridge, upside down. That's the shape where there stresses are on a joist. The ends are pretty irrelevant. Its not shear that will get you: its actual wood splitting..

I wouldn't need to notch anywhere near 20%, so no issues then. Cheers, Simon.

Simon The rough rule of thumb is 1/8th the depth. The joist is in tension at this point. if you want to be purist drill a hole at the requisite point and cut to the hole so as not to cause a stress point That having been said I have seen plenty of joists cut deep and they fail or not depending entirely on the span and load Failure mode is splitting along the middle (near the neutral axis) by the top of the cut Chris

OK thanks, I'll drill a hole for the "corner" of the notch. I only need to notch a small amount. Simon.

If rafters are in compression what keeps the walls from spreading out?

Usually the ceiling joists to which the rafters are fixed or collar ties. Except for an old barn I have responsibility for, where there are no ceiling joists and the collar ties are not substantial enough. The walls are spreading out.

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Mostly it's down to the fact that the walls are thick enough and heavy enough that the thrust line doesn't go outside the walls. The tension in the joists can help, or you can add flying buttresses, or the walls can fall over after a while.

... but rafters /are/ in compression.

Joists?

Perhpas one of us is using the wrong term..the joists are horiozontal tension members and the rafters are the slopey bits?

Probably. I have only had dealings with trussed rafters.. In those all the peripheral members are in tension on one side and compression on the other. The inside bits tend to be in compression AIUI.

Tie beams! (aka ceiling joists often - which is why if there is a joint in the middle over a partition it needs nailing or strapping and why it is important to tie into the wall plate_ chris

Yup. Thats why a truss is a truss. No (few, low) bending stresses anywhere.

Ok. ill settle for tie beams!

That's been done to our garage... badly :-)

Evidently the walls started spreading at some point, so they just punched holes in either side wall at the top halfway down the length, and ran 3/8" cable across.

It's a really sloppy job, but the cable must be under a hell of a lot of tension (and I don't know how weak the walls would be without it) - so I'm not inclined to try and tidy it up. I just put "rebuild garage" on the to-do list instead as that's a much more interesting DIY project ;)

cheers

Jules