Sister joists in bathroom

weeg

grumbler said:

Personally, I don't think that they are notched too heavily.  Joists are mostly stressed in the middle, not at the sides.

And I don't think that these extra pieces really 'reinforce' anything.  The strength of a joist is directly proportional to it's cubed height. Even if the height is 1/2, it's just 1/8 added, but it looks rather like 1/3 or 1/4, not 1/2.

That said, yes, good plywood screwed to the sides of the split joist will reinforce it.

This is right, as far as it goes, but isn't the whole story. Joists are mostly stressed in bending the middle, but shear stresses are highest at supports, and the design needs to account for both. In a joist the modes of failure are usually deflection, then bending, then shear.  And shear strength is directly related (ie not squared, or cubed) to height.

I'm not going to speculate on whether or not the joists are strong enough with those notches - I don't have enough info to look at the numbers, and nor does anyone else here. What I will say is that, those notches are significantly bigger than are permitted under the design codes. They shouldn't be more than 12.5% if they are outside the middle half of the beam.

Does that mean they should be reinforced? Yes, to comply with building standards. Is the floor unsafe? Probably not. But that horizontal crack is a bad sign. I'd advise spiking a 1m length 100mm deep to the side of each joist. Cheaper than steel angles, less cutting than ply. 2 rows of nails or screws at 75-100mm spacing should do the job.

danrv

weeg said:

grumbler said:

Personally, I don't think that they are notched too heavily.  Joists are mostly stressed in the middle, not at the sides.

And I don't think that these extra pieces really 'reinforce' anything.  The strength of a joist is directly proportional to it's cubed height. Even if the height is 1/2, it's just 1/8 added, but it looks rather like 1/3 or 1/4, not 1/2.

That said, yes, good plywood screwed to the sides of the split joist will reinforce it.

But that horizontal crack is a bad sign. I'd advise spiking a 1m length 100mm deep to the side of each joist. Cheaper than steel angles, less cutting than ply. 2 rows of nails or screws at 75-100mm spacing should do the job.

Thanks. 
The crack is in the middle so about 100mm up. Maybe ply cut to shape would be better.
Could then be notched the same as the joist and strengthen the copper pipe area.

danrv

weeg said:

grumbler said:

Personally, I don't think that they are notched too heavily.  Joists are mostly stressed in the middle, not at the sides.

And I don't think that these extra pieces really 'reinforce' anything.  The strength of a joist is directly proportional to it's cubed height. Even if the height is 1/2, it's just 1/8 added, but it looks rather like 1/3 or 1/4, not 1/2.

That said, yes, good plywood screwed to the sides of the split joist will reinforce it.

 Cheaper than steel angles, less cutting than ply. 2 rows of nails or screws at 75-100mm spacing should do the job.

Yes.
Small sheet of 3/4” inch ply, £20 from B&Q so quite expensive.
Was thinking ply cut to same height as the joist so to bridge across the split.
It’s not straight vertically though, about 8mm out from there upwards and not sure it will straighten.
Might then do as suggested with 100mm wide or maybe 3”x2” length of wood like the others.

Access for drill and screwdriver is a bit tricky so I’ll probably pick up a right angled drill adapter.
https://www.toolstation.com/keyless-angle-driver/p40354?store=SL&utm_source=googleshopping&utm_medium=feed&utm_campaign=googleshoppingfeed&gclid=EAIaIQobChMI76u25sHL8AIVQ7TtCh2owAJzEAQYBCABEgJREvD_BwE

danrv

weeg said:

grumbler said:

Personally, I don't think that they are notched too heavily.  Joists are mostly stressed in the middle, not at the sides.

And I don't think that these extra pieces really 'reinforce' anything.  The strength of a joist is directly proportional to it's cubed height. Even if the height is 1/2, it's just 1/8 added, but it looks rather like 1/3 or 1/4, not 1/2.

That said, yes, good plywood screwed to the sides of the split joist will reinforce it.

This is right, as far as it goes, but isn't the whole story. Joists are mostly stressed in bending the middle, but shear stresses are highest at supports, and the design needs to account for both. In a joist the modes of failure are usually deflection, then bending, then shear.  And shear strength is directly related (ie not squared, or cubed) to height.

I'm not going to speculate on whether or not the joists are strong enough with those notches - I don't have enough info to look at the numbers, and nor does anyone else here. What I will say is that, those notches are significantly bigger than are permitted under the design codes. They shouldn't be more than 12.5% if they are outside the middle half of the beam.

Does that mean they should be reinforced? Yes, to comply with building standards. Is the floor unsafe? Probably not. But that horizontal crack is a bad sign. I'd advise spiking a 1m length 100mm deep to the side of each joist. Cheaper than steel angles, less cutting than ply. 2 rows of nails or screws at 75-100mm spacing should do the job.

Thanks, getting on with this now.
Job is taking a long time as I’m doing my garage workshop as well.
I’ll probably go with a 130cm length of wood on the cracked joist (reaches up to electrical cables) and possibly on others if ok to access.

I was thinking that the long crack needs to be covered over to strengthen hence the ply.
Realize now that’s not really necessary.
Have bought a right angled drill adapter as gap is too narrow for a power drill.