Joist strength question
I have a question about the strength of a floor joist.
My friend was working on a job installing floor joists.
The joists were 2×10 about 6 ft long.
He was cutting one end (notching) about 2″ to fit and the other end 3″ to fit.
I asked why he didn’t just use 2×8 joists instead.
He felt they were stronger, and I thought that the strength was the same.
No big deal, but I am curious how this works.
If he notched from the bottom, he now has a 2x7 on one end.
"Put your creed in your deed." Emerson
"When asked if you can do something, tell'em "Why certainly I can", then get busy and find a way to do it." T. Roosevelt
I think your friend is somewhat right. Suppose he cut all but 1 1/2" off each end. Compare that to a 2x2 joist. I have to think it would be better. Well until the ends split off and the floor caved in. Which is the rub, but until the tension shears the joist, I believe it is stronger.
Well, you could park 4 tons right in the middle of that 6 ft 2x10 (#1DF or SYP) and it would not break. Aint gonna deflect much either.
If your buddy glued a piece of 3/4 ply to each side near the notch so the 2x10 would not split (biggest strength reduction from notching) , you could notch that puppy down to about 4" left. Without the split prevention ply - can't do that calc in my head or without looking up some split factors, but down to 7" should not reduce ultimate strength.
notching on the end doesn't subtract all that much from the strength. You see it all the time when ledger strips are used - say like when joists are attached to the face of a girder. 3" is kinda excessive though.
OTOH - with a 6' span it really doesn't matter - 2x8 or 2x10 unless there is something else going on there - like a water bed, hot tub, etc.
I have no idea what's going there.
I do know he glues plywood decking to it normally and he's been doing this stuff for a long time.
I guess I'll have to admit he's probably right and I'm wrong.
@ a 6 foot span the control is shear and depending upon the species of wood, the 2 x's allowable load would be somewhere around 600 plf for a 2 x 8.
IMO he is wasting time using 2 x 10's.And of course deflection would be a non factor.