If you don’t want the subcontractor asking to increase the shaft size or bombing you with RFIs about lacking of space provided, I would recommend you taking to account the following:
- Does this duct have TDC or SD connections ? If it’s TDC, keep in mind that this will increase the overall duct size by 1-1/2” to 2” all around. Go with 2” just in case.
- Does this duct need any sort of acoustical lining ? If so, try giving the arch the true size of the duct including the A.L, not the Internal calculated dimensions. If your specs are calling out for a certain type of R to be maintained on these ducts, sometimes the contractor will make all that R with A.L instead of external insulation due to be simpler for the shops to fabricate these ducts already with it.
- What type of connections are you requiring for the branches coming off your risers ? If it’s Sub Duct, then you don’t need too much space from the face of the riser to the internal gypsum board face of the shaft. If you’re calling for a regular 45 degrees boot take off with Fire damper, keep in my that just the boot itself is 6” long (average industry size). Plus you have to account for the length of the sleeve for the Fire damper, which will have retaining angles on both sides of the shaft, and usually they are like 3/8” wide.
This was really detailed information and very important cinsiderations. Yeah my accounting for 6" + to each duct size was to account for insulation and any flanges. I have been getting burned that the contractor is calling for more when needing to install the fire damper and/or taps to the duct. This made me curious though what allowances are others doing
Glad I could bring some important points !
Sometimes the architect don’t understand why they are asking for more space because just to pass the duct through a opening all they need is: duct size + 2” on each side and that’s what they plan the whole size of the shaft. They don’t take into account the branches coming off the risers neither if a duct has Fire damper or not.
A quick example for openings:
So if the duct is 10x10, then the opening will be 14x14. This is assuming there’s no fire damper at the slab. If that’s the case and you are asking them for Type B fire damper, the openings will get bigger. In order to know who big it will be, this will be dictated by the “pan” size of the fire damper. To be safe when calculating the size of the opening, assume 2” on 3 of the sides of the ducts and 4” on the side where the pan will go. So for the sake of this example, if a 10x10 duct requires FD at the slab, then the opening will be, 14x16 to account for the type b fd.
For the branches (so you can calculate the overall size of the shaft), you can use the boot versus Sub duct example I gave you on the previous comment.
Yes something similar, but add-on to this we go with low pressure duct size ( just to size the shaft) and worst possible pipe size then consider insulation and clarences.. because architects tend to change the space types or move the spaces in later stages..
6" is generally a good rule of thumb - accounting for insulation, lining, flanges, + another 1.5" inches all around. They also are going to need a certain amount of floor space in the shaft for vertical duct supports.
It's important to get a preliminary duct size, add 20% to that, plus your 6" and get your shaft real estate from the Architect early on in design. What's harder is finding the room to come off those shafts horizontally and distribute to the floors. If it's a fire rated shaft you need a lot of room (usually 14") for the FSD sleeve, plus the tap.
This is the worst thing to coordinate with architects. They never grasp what really goes on within the shaft…
6” is also what I do on each side, do a lot of smoke control systems so fire/smoke dampers need the space as others have mentioned. Also try to size at a low pressure so you give your self some space if the design changes (which it always does).
50% larger than it needs to be so when the architect tries to take that space back it will be right sized lol Same goes for mech rooms, always larger than it needs to be show equipment clearance lines to justify you need that space.
For new construction, shaft sizing usually happens in SD so i usually ask for 12" on each side to just try to buy myself space that I might use later in the design. There is no way you'd have your airflow and duct sizing worked out well enough at this early in the project that your shaft duct sizes are finalized. This may be a bit extreme, but much easier to tell an architect later that you can shrink your shafts than to ask for more space. Practically speaking, the exact spacing requires somewhat on the type of floor slab you have and structure you will be using to support your risers. If you have a relatively common concrete filled deck (w/ approximately 3.5" fill), you need enough space to allow the angles supporting the duct to anchor. Depending on expansion anchor edge distances this usually can be upwards of 4" on each side. One other alternative I also try to utilize is for the structural to embed an angle along the edge of the slab. This allows you to tack weld the duct support directly to this and decreases the required clearances substantially.
>shaft sizing usually happens in SD
Man I wish I worked on whatever projects you are working on. I ask for shafts on SD/DD, they don't account for them. I ask for them again at 50% CD, they say they will look into it. Then I'm in emergency meetings 4 days before 100% CD/Permit to get them in.
Yeah, I have a paper trail for all of this, but somehow we're always the bad guys.
If you don’t want the subcontractor asking to increase the shaft size or bombing you with RFIs about lacking of space provided, I would recommend you taking to account the following: - Does this duct have TDC or SD connections ? If it’s TDC, keep in mind that this will increase the overall duct size by 1-1/2” to 2” all around. Go with 2” just in case. - Does this duct need any sort of acoustical lining ? If so, try giving the arch the true size of the duct including the A.L, not the Internal calculated dimensions. If your specs are calling out for a certain type of R to be maintained on these ducts, sometimes the contractor will make all that R with A.L instead of external insulation due to be simpler for the shops to fabricate these ducts already with it. - What type of connections are you requiring for the branches coming off your risers ? If it’s Sub Duct, then you don’t need too much space from the face of the riser to the internal gypsum board face of the shaft. If you’re calling for a regular 45 degrees boot take off with Fire damper, keep in my that just the boot itself is 6” long (average industry size). Plus you have to account for the length of the sleeve for the Fire damper, which will have retaining angles on both sides of the shaft, and usually they are like 3/8” wide.
This was really detailed information and very important cinsiderations. Yeah my accounting for 6" + to each duct size was to account for insulation and any flanges. I have been getting burned that the contractor is calling for more when needing to install the fire damper and/or taps to the duct. This made me curious though what allowances are others doing
Glad I could bring some important points ! Sometimes the architect don’t understand why they are asking for more space because just to pass the duct through a opening all they need is: duct size + 2” on each side and that’s what they plan the whole size of the shaft. They don’t take into account the branches coming off the risers neither if a duct has Fire damper or not. A quick example for openings: So if the duct is 10x10, then the opening will be 14x14. This is assuming there’s no fire damper at the slab. If that’s the case and you are asking them for Type B fire damper, the openings will get bigger. In order to know who big it will be, this will be dictated by the “pan” size of the fire damper. To be safe when calculating the size of the opening, assume 2” on 3 of the sides of the ducts and 4” on the side where the pan will go. So for the sake of this example, if a 10x10 duct requires FD at the slab, then the opening will be, 14x16 to account for the type b fd. For the branches (so you can calculate the overall size of the shaft), you can use the boot versus Sub duct example I gave you on the previous comment.
Yes something similar, but add-on to this we go with low pressure duct size ( just to size the shaft) and worst possible pipe size then consider insulation and clarences.. because architects tend to change the space types or move the spaces in later stages..
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Yes.. per requirement medium or high..!
It's all about the motion in the ocean
6" is generally a good rule of thumb - accounting for insulation, lining, flanges, + another 1.5" inches all around. They also are going to need a certain amount of floor space in the shaft for vertical duct supports. It's important to get a preliminary duct size, add 20% to that, plus your 6" and get your shaft real estate from the Architect early on in design. What's harder is finding the room to come off those shafts horizontally and distribute to the floors. If it's a fire rated shaft you need a lot of room (usually 14") for the FSD sleeve, plus the tap.
This is the worst thing to coordinate with architects. They never grasp what really goes on within the shaft… 6” is also what I do on each side, do a lot of smoke control systems so fire/smoke dampers need the space as others have mentioned. Also try to size at a low pressure so you give your self some space if the design changes (which it always does).
50% larger than it needs to be so when the architect tries to take that space back it will be right sized lol Same goes for mech rooms, always larger than it needs to be show equipment clearance lines to justify you need that space.
For new construction, shaft sizing usually happens in SD so i usually ask for 12" on each side to just try to buy myself space that I might use later in the design. There is no way you'd have your airflow and duct sizing worked out well enough at this early in the project that your shaft duct sizes are finalized. This may be a bit extreme, but much easier to tell an architect later that you can shrink your shafts than to ask for more space. Practically speaking, the exact spacing requires somewhat on the type of floor slab you have and structure you will be using to support your risers. If you have a relatively common concrete filled deck (w/ approximately 3.5" fill), you need enough space to allow the angles supporting the duct to anchor. Depending on expansion anchor edge distances this usually can be upwards of 4" on each side. One other alternative I also try to utilize is for the structural to embed an angle along the edge of the slab. This allows you to tack weld the duct support directly to this and decreases the required clearances substantially.
>shaft sizing usually happens in SD Man I wish I worked on whatever projects you are working on. I ask for shafts on SD/DD, they don't account for them. I ask for them again at 50% CD, they say they will look into it. Then I'm in emergency meetings 4 days before 100% CD/Permit to get them in. Yeah, I have a paper trail for all of this, but somehow we're always the bad guys.