Explain please!

[Are09]

I am confused with this sentence in the vignette part :
"the largest joist- duct combination?...
Can anyone explain?

[TroyW]

In order to determine how much space is required above the ceiling.
You look at the drawing with both the mechanical and structural layer turned on for a particular floor.
Usually at the riser location for the mechanical is where the duct will be its deepest, but not necessarily where the structural joist will have the deepest section.
So you need to find out where on that floor the structural member and the duct running below it requires the deepest combined clearance.
Hence largest duct-joist combination.

[Are09]

Thanks TroyW! Let me ask u this too: in your posted vignette in the fourm, this is something that I am not sure off, you show the duct size in the 1st floor corridor area draw as 24x16. Why it is not 8x8?????

[TroyW]

The section cut lies right at the portion of 24x16 ductwork right before the junction where that run becomes an 8x8

[Are09]

Ok great thanks a lot .