Please explain - engineers?

supersleuth · Oct 24, 2023 2:10 pm

Hi. The floor joists were cut out to allow ducting through as per photo below.

But I dont understand how this cut-out does not completely destroy the load bearing capacity of the joists. It was approved by engineering but I'm just too dumb to understand why the floor doesn't collapse?

https://imgur.com/a/QBXjFtQ

Ashington Homes · Oct 24, 2023 2:29 pm

supersleuth

Hi. The floor joists were cut out to allow ducting through as per photo below.

But I dont understand how this cut-out does not completely destroy the load bearing capacity of the joists. It was approved by engineering but I'm just too dumb to understand why the floor doesn't collapse?

https://imgur.com/a/QBXjFtQ

have you seen this guide?

https://www.dindas.com.au/wp-content/up ... yer_FA.pdf

supersleuth · Oct 24, 2023 2:41 pm

Ashington Homes

have you seen this guide?

I think the builder showed me something similar.

I believe it is within approved standards, but it just defies me how it can hold weight when only 3mm either side of hole is left. Why not just make the joists 6mm thick? Because gravity.

My question is more around how can the joist hold any weight when its almost complete cut through. And its not just the one joist so that adjacent joists could pick up the load - is the whole area with all joists cut leaving almost nothing.

I think the answer may lie in that the joists would have to tear apart at the top and bottom ridges around cut-out for it to fall, So at the point where the hole is, the remaining bits just need to hold from tearing apart - not hold any weight. Kind of does my head in thinking about it.

Ashington Homes · Oct 24, 2023 4:37 pm

supersleuth

Ashington Homes

have you seen this guide?

I think the builder showed me something similar.

I believe it is within approved standards, but it just defies me how it can hold weight when only 3mm either side of hole is left. Why not just make the joists 6mm thick? Because gravity.

My question is more around how can the joist hold any weight when its almost complete cut through. And its not just the one joist so that adjacent joists could pick up the load - is the whole area with all joists cut leaving almost nothing.

I think the answer may lie in that the joists would have to tear apart at the top and bottom ridges around cut-out for it to fall, So at the point where the hole is, the remaining bits just need to hold from tearing apart - not hold any weight. Kind of does my head in thinking about it.

I guess the circles must act like an arch. I note that you can't do it with squares.

Either way I personally wouldn't allow that in one of my builds. We always run bulkheads above the kitchen cabinets.

Pulse · Oct 24, 2023 9:19 pm

I- joists strength is in the top and bottom chords, the OSB web just keeps them apart. If you are worried add an extra ply face fixed to the side of the joist.

Ardo · Oct 24, 2023 11:47 pm

To me those holes are not round. You need to consider them to be a larger hole then cut. So the distance between the two sides is the actual hole size. By my reckoning not in spec.

chippy · Oct 25, 2023 12:20 am

Ardo

To me those holes are not round. You need to consider them to be a larger hole then cut. So the distance between the two sides is the actual hole size. By my reckoning not in spec.

Oval holes are permissable. Those holes have been cut in the factory so will definitely be engineered from the factory and not just cut randomly on site.

building-expert · Oct 25, 2023 8:48 am

supersleuth

Hi. The floor joists were cut out to allow ducting through as per photo below.

But I dont understand how this cut-out does not completely destroy the load bearing capacity of the joists. It was approved by engineering but I'm just too dumb to understand why the floor doesn't collapse?

https://imgur.com/a/QBXjFtQ

I cannot give engineering opinion but I completed structural enginnering subjects In the School of Engineering as part of my building degree.
The holes in your joists are at mid span where there are maximum bending stresses in top and bottom chord, but shear stresses are zero, that is why it will work.

supersleuth · Oct 25, 2023 10:23 am

Pulse

I- joists strength is in the top and bottom chords, the OSB web just keeps them apart. If you are worried add an extra ply face fixed to the side of the joist.

Thanks that kinda makes sense.

Im not worried about it, just always wondered how it works and forgot to investigate further during build.

The home is 3.5 years old and very happy with the overall quality of the build.

supersleuth · Oct 25, 2023 10:24 am

chippy

Ardo

To me those holes are not round. You need to consider them to be a larger hole then cut. So the distance between the two sides is the actual hole size. By my reckoning not in spec.

Oval holes are permissable. Those holes have been cut in the factory so will definitely be engineered from the factory and not just cut randomly on site.

Yep well spotted - they were definitely cut in factory and delivered that way.

supersleuth · Oct 25, 2023 10:27 am

building-expert

supersleuth

Hi. The floor joists were cut out to allow ducting through as per photo below.

I cannot give engineering opinion but I completed structural enginnering subjects In the School of Engineering as part og my building degree.
The holes in your joists are at mid span where there are maximum bending stresses in top and bottom chord, but shear stresses are zero, that is why it will work.

Thanks. I wish there was a youtube video explaining this concept.