The notches cut into the extreme fibers of those joists, seriously weakens them.
Example;
I= bd3/12,... z= I/y
let max stress at the extreme fiber = WL/8z
Let W max = 1000lb
Let L = 160"
let d= 8", let b= 2"
I = 85.3 z= 21.3 stress mid span = 939 psi
Now cut a notch center span, say 1" deep;
now d = 7" and;
I = 57 z = 16.3 stress ms = 1227 psi
288 psi increase = + 31% stress increase!
Quite apart from deflection considerations:-
span/d originally = 160/8 = 20
Notched span/d = 160/7 = 22.8
The cut joists will now probably be outside the deflection envelope= floor bounce/ cracked plaster etc..
If the engineeer wanted 7" joists he would have specified them. The only safe place to cut a simply supported joist is on the the neutral axis, ie on center. It's imperative not to cut or drill any joists which are continuous, those with a centre support(s) or encastré, or those forming part of a triangulated-truss. The neutral axis, ( if it even exists! ) may not be on the center line!
To notch engineered joists, [OSB web/ softwood flanges] would be to court disaster, as most of the load is carried in the softwood; the manufacturers specify where and how big access holes in their products may be safely cut....
...and eventually, oh dear!, a 'DIY weekend warrior' is going to climb up into his loft, (fitted with engineered joists), with his axe.....
Alan
