cables through joists

[Stephen]

Hello,

I was reading the on site guide, kindly provided by someone here (thanks), about drilling through joists. It says holes should be drilled between 0.25 and 0.4 x the joist span and notches should be cut between 0.1 and 0.25 x joist span.

Now I am a bit confused about this. Why can you notch 0.1 x the span but not drill until 0.25 x the span? Surely if it is structurally safe to do one, it ought to be safe to do the other? I would have thought drilling a small hole is always better than notching because notching reduces the effective depth of the joist?

I notice that these limits keep the notches and the holes in different zones and that must be a good thing. Is that the reason they ask for holes to be drilled further along so that no "plant pot" drills a hole beneath a notch and seriously weakens the joist?

Why can't you drill halfway along the joist? If you can drill at a point 0.4 times the span from each end, you are pretty close to the centre already, so why is dead centre kept free? I knew from posts I had read here not to cut at the ends of joists but I never knew the centre had to be untouched. Why?

Thanks, Stephen.

[The Natural Philosopher]

The answers to all these questions lie in the stress distribution withing the beam under assumed normal loading.

The tensile and compressive forces increae from teh center of the beam vertically where they are zero to a maxiumum at the top and the bottom. This is why holes through the vertical center are best,.

The actual horizontal stresses increase *along* the beam from zero at the walls, to a maximum at the center, which is why you dont want holes or notches in the middle. Likewise any Euler instability type vertical compressive effects peak at the center. In this sense teh center later of the beam functions to keep the top and bottom bits apart: it doen't need to be THAT strong, but it does need to be REASONAbLY strong. Also, a floor nailed on top shifts the effective beam center pretty high up the beam. so the center for a well nailed/screwed floor over a beam is actually somewhat above the beam center line.

That is why notches are best near the walls.

Now as to why *holes* near the walls are not ideal, I suuspect that is becaus there is a huge VERTICAL compressive force near the supports. This doesn't impact on a notch on top, but a hole in the middle could collapse, dropping the beam end somewhat.

Since all of this analysis is well beyond teh average builder, sets of 'rules' are developed that basically identify the optimal places to put notches and holes: follow them.

[<me9]

It's a while since I did beam theory, but the stresses vary along the span, bending stresses are greatest in the centre of the beam in the top and bottom surfaces, so no notches allowed there. Shear stresses are highest nearest the supports, so nothing taken out there. I can't remember if the centre should be a special case, someone else can comment?

[The Natural Philosopher]

Further to my original post, which basically says what you are saying, I have been thinking about a beam with a chip floor screwed on top..

Here, if the floor is well screwed, the best structural solution is a notch up to about 10% of the span in from the walls..Ive been doing a bit of crude analysis for stressed skin over beams for a model aircraft design..;-)

Wood is generally up to three times stringer in compression than tension BTW, which is why a beam generally breaks apart somewhere in the middle area.

Compressive limit stress across the grain are comparable to tensile along..

[Spamlet]

While you are at it, check to see if the joists actually reach the walls. When I went to move the rads in our old (100y) house, I was 'amused' to find how many of the ends had rotted off, and how those that had ended in the fireplaces had the ends charred away (That's springy man.). In effect, it was the floorboards that were holding it up, so you may find, that in an old house, when you lift boards to make your holes or notches, you may experience a sinking feeling... I had to prop up a ceiling and suspended ceiling below, whilst bolting on joist extensions. Fun fun fun, just to move a rad!

S