Shaft/Fire Damper Issue for Ducts

[CarolinaPE]

I am a little confused on the shaft construction for ductwork serving more than two stories. Our state code and the NFPA code says that you can serve 2 stories without a shaft if you put a fire damper at the floor (I think it actually says you can penetrate only one floor & use a Fire Damper in NFPA). That I understand. If you serve 3 or more you need a shaft. Understand that too. My question is shaft construction. NFPA 90A, figure A.5.3 shows a supply duct in a shaft (in "9"). One of its branches leaves one shaft and then enters another shaft ("24") on the floor above, I was under the impression shafts had to be continuous from the lowest floor to the highest. Does anyone know how to best explain this or have a better diagram? Thanks.

 

[CarolinaPE]

Anyone? Anyone?

 

[walkes]

The diagram in NFPA 90A shows shafts that are continuous. There are fire dampers where the duct penetrates the fire rated enclosure. The fire damper maintains the integrity of the fire separation. Is that helpful at all?

 

[CarolinaPE]

Sort of, I would have thought shaft construction would require the shaft to enclose the duct from the lowest floor to the highest floor it serves. The diagram shows this not to be the case. It made me wonder the point of the shaft to have a duct leave the shaft and then go serve other floors in a separate shaft??? We essentially have a 3 story building and it would be best

 

[CarolinaPE]

I think I have this figured out. Where a duct penetrates a shaft, that duct can still then penetrate one floor with a fire damper at the floor, no shaft required at that type of penetration. At least that is what the diagram shows on 2 occasions, even though the "duct system" is for 3 or more floors. Confusing as heck, doggone codes!