I see that you miss the important thing Andy, and that is not breaking load but floor stiffness. BTW I assure you that Engineers do understand structures, whereas the others tend to follow various rules, a bit like the OSG! If you stand on a floor with 600 mm spacing between the chipboard sheets you will notice that it moves significantly as you move, and you may be able to crack the chipboard by jumping up and down or add heavy items carelessly like a bed or big bookcase with small leg diameter. Next time you have one handy get a scaffold board and support it either end on a couple of bricks. Stand in the middle and note the deflection (several inches). The board does not break but is reasonably close. A floor joist could have a similar deflection without breaking but they are much stiffer, so that the deflection is reasonable. Some modern designs have very poor floor stiffness and it is horrible, but then it saves a few quid. Now you should understand the design criterion. I prefer concrete floors upstairs because as well as the stiffness they have much better sound insulation, but again it is a matter of a few pound notes rather than ultimate building quality.


BTW you might like to try drilling a few holes in a beam on the centre line and verify that it makes little difference to the stiffness or deflection, a useful lesson.

I see that you miss the important thing Andy, and that is not breaking load but floor stiffness. BTW I assure you that Engineers do understand structures, whereas the others tend to follow various rules, a bit like the OSG! If you stand on a floor with 600 mm spacing between the chipboard sheets you will notice that it moves significantly as you move, and you may be able to crack the chipboard by jumping up and down or add heavy items carelessly like a bed or big bookcase with small leg diameter. Next time you have one handy get a scaffold board and support it either end on a couple of bricks. Stand in the middle and note the deflection (several inches). The board does not break but is reasonably close. A floor joist could have a similar deflection without breaking but they are much stiffer, so that the deflection is reasonable. Some modern designs have very poor floor stiffness and it is horrible, but then it saves a few quid. Now you should understand the design criterion. I prefer concrete floors upstairs because as well as the stiffness they have much better sound insulation, but again it is a matter of a few pound notes rather than ultimate building quality.


BTW you might like to try drilling a few holes in a beam on the centre line and verify that it makes little difference to the stiffness or deflection, a useful lesson.