Joist-Duct Combination

Building Section Vignette and Multiple Choice

Joist-Duct Combination

Postby smew » Sun Apr 20, 2014 3:58 pm

I'll be retaking CDS in a few days after failing it 6 months ago due to a level 3 vignette. As no explanation is received from NCARB, I have no idea what I did wrong but have a strong suspicion that it's due to joist-duct combination. After reading through most of the forum postings on joist-duct combination, I think I have a general idea about how it's suppose to work. But still, with the fear of failing it again I just want to make sure I am having it down correctly. Attached PDF has 2 hypothetical scenarios of modified NCARB practice vignette with various joist-duct combinations. Please review and let me know if I am approaching the problem correctly. Many thanks for your help!!
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Re: Joist-Duct Combination

Postby lbgold19 » Sun Apr 20, 2014 5:16 pm

I passed CDS about a month ago. This was the rule of thumb I applied that helped me greatly:

1) Is the ceiling height uniform for the entire floor?
Apply this to all floors identified in the vignette

2) If the answer is yes, then locate the largest duct in each room the sections cuts through
While the section may not be cutting through the largest duct you have to accommodate for the largest duct because the
ceiling height is a uniform

Hope this helps, and best of luck. I posted my example drawing on this site a while back around 3.21. I got great feedback and the advice was very on point. Also referencing the Dorf vignettes is a great resource for drawing technique.
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Re: Joist-Duct Combination

Postby Coach » Sun Apr 20, 2014 10:28 pm

smew wrote:Please review and let me know if I am approaching the problem correctly.

Yes, that's how it's done.
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Re: Joist-Duct Combination

Postby Coach » Sun Apr 20, 2014 10:31 pm

lbgold19 wrote:While the section may not be cutting through the largest duct you have to accommodate for the largest duct

Of course, the solution must accommodate the largest duct; however, the largest duct is not necessarily used to determine the space required.
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Re: Joist-Duct Combination

Postby smew » Mon Apr 21, 2014 12:30 am

Thank you very much, Coach and lbgold19!!
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Re: Joist-Duct Combination

Postby caitiemg » Mon Apr 21, 2014 4:50 pm

Coach wrote:
lbgold19 wrote:While the section may not be cutting through the largest duct you have to accommodate for the largest duct

Of course, the solution must accommodate the largest duct; however, the largest duct is not necessarily used to determine the space required.


It may me because I've had a long day at work that I'm not understanding this, but Coach, can you explain why not? Also, in my solutions I hadn't looked at the plan on zones, and I have just been using the largest duct on the floor for the space allowance. Is this incorrect because of the beams?

I thought I had the vignette down but realizing now, I may have missed something major! Testing in a week, so am happy to get clarification now!

Thanks!
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Re: Joist-Duct Combination

Postby Coach » Mon Apr 21, 2014 4:59 pm

caitiemg wrote:
Coach wrote:
lbgold19 wrote:While the section may not be cutting through the largest duct you have to accommodate for the largest duct

Of course, the solution must accommodate the largest duct; however, the largest duct is not necessarily used to determine the space required.


It may me because I've had a long day at work that I'm not understanding this, but Coach, can you explain why not?

You have to understand the concept of the largest joist/duct combination. Unfortunately, the 4.0 vignette doesn't give you a good experience in dealing with it, so it lulls you into a false sense that as long as you find the largest duct and use that number, you're good --- NOT NECESSARILY TRUE!

For a better practice experience, do the 3.1 vignette. If you look at the document smew posted above, you should begin to understand.
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Re: Joist-Duct Combination

Postby caitiemg » Mon Apr 21, 2014 6:27 pm

I understand now! If you have different joist depths, you'll need to look for the deepest joist/duct combo! Glad I got that clarified. Thanks!!!
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Re: Joist-Duct Combination

Postby ztg1224 » Mon Apr 21, 2014 8:33 pm

Super helpful-- thanks for putting together that PDF, makes the concept very understandable. I test in two days. So glad I found this!
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Re: Joist-Duct Combination

Postby mekmek » Tue Apr 29, 2014 9:34 pm

looking at your logic pdf in scenerio 1 level 2. you list the joists as 20" for zone A when infact they are 24"
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Re: Joist-Duct Combination

Postby mekmek » Tue Apr 29, 2014 9:35 pm

ah i think i see what you did, you changed the numbers. the pdf was helpful. thanks.
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Re: Joist-Duct Combination

Postby A.Monica » Mon May 05, 2014 5:39 pm

Thanks, this is great.
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Re: Joist-Duct Combination

Postby designyoyo » Tue Feb 10, 2015 2:13 pm

just bumping this up because it was very helpful for me.
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Re: Joist-Duct Combination

Postby ZHAOYANG » Fri Feb 13, 2015 2:01 pm

Thanks a lot!
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Re: Joist-Duct Combination

Postby designyoyo » Fri Feb 20, 2015 8:19 am

no problem! good luck! Just found out I passed my test! (after 2 previous attempts....)
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Re: Joist-Duct Combination

Postby mateo1877 » Sat Feb 21, 2015 10:22 am

Question just to clarify...

We keep ALL ducts below the joists, right? Even if a main trunk line is running parallel to the joists?

I understand that if they're running perpendicular to each other they need to be completely below, but if the main trunk line is parallel, the top portion of the air supply line could potentially go just above the joists in reality. Coach, can you confirm?
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Re: Joist-Duct Combination

Postby mateo1877 » Sat Feb 21, 2015 10:50 am

Also, just to clarify...

A rule of thumb might be:

If the joists in zone A and zone B of the second floor are the same size, go with the largest duct size.
If the joists in zone A and zone B of the second floor are DIFFERENT size, go with the largest duct/joist combo

Would that keep us safe?
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Re: Joist-Duct Combination

Postby Coach » Sat Feb 21, 2015 1:51 pm

mateo1877 wrote:Question just to clarify...

We keep ALL ducts below the joists, right? Even if a main trunk line is running parallel to the joists?

I understand that if they're running perpendicular to each other they need to be completely below, but if the main trunk line is parallel, the top portion of the air supply line could potentially go just above the joists in reality. Coach, can you confirm?


Read the program again.
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Re: Joist-Duct Combination

Postby Peng67uins » Mon Mar 02, 2015 9:57 am

This has now confused me. I have been taking the duct that comes out of the mech. room as the largest possible duct in the areas that arent the 2 story part. Im not following the logic of the "zones" it seems to be an arbitrary position to just pick a spot and cut it into zones. Please someone explain what I am missing.
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Re: Joist-Duct Combination

Postby Patt » Thu Mar 05, 2015 10:19 am

If a ceiling height is uniform throughout the level, then one MUST design for the largest joist depth added to the largest duct depth on that level regardless of location of section cut line.
No?
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Re: Joist-Duct Combination

Postby Coach » Thu Mar 05, 2015 12:56 pm

Patt wrote:If a ceiling height is uniform throughout the level, then one MUST design for the largest joist depth added to the largest duct depth on that level regardless of location of section cut line.
No?

No. A joist/duct combo is where a joist and duct are in the same location. Use the largest combo, which is not necessarily the largest joist + the largest duct.
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Re: Joist-Duct Combination

Postby Patt » Thu Mar 05, 2015 1:41 pm

Need to cogitate on that.
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Re: Joist-Duct Combination

Postby Patt » Thu Mar 05, 2015 7:38 pm

Just to think out loud here.
1.) So, if that is the case then it means that a certain smaller size duct can pass under the deepest joist and the solution can be satisfied,
2.) and it means that a smaller joist may cross over the largest duct
3.) If, and only if, the largest duct and the largest joist do NOT cross ...or occur in the same location.
4.) If the largest duct and largest joist occur in the same location then, obviously, that is the largest combination.
5.) If they don't occur in the same location then some other combination must control.
6.) The combination that controls is the largest combination that actually occurs.

What is meant by "actually occurring in the same location" ?
7.) Well, when a duct and joist cross at right angles that definitely qualifies as same location.
8.) Is there ever a case where the joist and duct run parallel and co-planar? That too would qualify.
9.) But if the joist and duct run parallel yet out of plane, that does not qualify as same location.

So there are two possible types of locations ( 7. and 8.) and all of them must be found on the Vignette plans and scanned for the largest combination.

Do I get it or am I not seeing something?
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Re: Joist-Duct Combination

Postby Coach » Fri Mar 06, 2015 4:34 pm

Patt wrote:9.) But if the joist and duct run parallel yet out of plane, that does not qualify as same location.

What do you mean by "out of plane"?
It's irrelevant since the program tells you that ducts are always below joists.
You're making it a lot more complicated than it needs to be.
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Re: Joist-Duct Combination

Postby Patt » Sat Mar 07, 2015 6:02 pm

Right. Forget all that "out of plane" stuff.
So it seems to me now that "same location" refers to joist size. An area framed by 24" joists is not the same location as an area framed by 32" joists. But, a large area of 28" joists will be all the same location. So one must find the largest duct depth for the "same location" as defined by an area of all the same joist size.
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Re: Joist-Duct Combination

Postby Coach » Sat Mar 07, 2015 6:12 pm

You're talking yourself into circles. It's not complicated.

Look at framing, then see what ducts are below. Find tallest duct under each joist size, add it to joist, and that's the combo. Use the largest combo.

If ceiling heights vary under a floor or roof, it's a little more complicated, but I doubt you'll encounter that on the exam.
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Re: Joist-Duct Combination

Postby Patt » Sat Mar 07, 2015 6:28 pm

Beautiful.
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Re: Joist-Duct Combination

Postby Ferdox » Tue Mar 10, 2015 2:08 pm

Coach,"
Could you help resolve this:
On what basis does one ignore the largest joist-depth combo for eg in Dorf Solutions book (6th ed. March 2007) on page T-11
the section uses 16" joist even though the Lower level plan on Page T-9 shows there is a 20"' joist .Does anyone know why that is not accounted for in determining the plenum depth.
Please enlighten
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Re: Joist-Duct Combination

Postby Coach » Tue Mar 10, 2015 3:25 pm

Ferdox wrote:Coach,"
Could you help resolve this:
On what basis does one ignore the largest joist-depth combo for eg in Dorf Solutions book (6th ed. March 2007) on page T-11
the section uses 16" joist even though the Lower level plan on Page T-9 shows there is a 20"' joist .Does anyone know why that is not accounted for in determining the plenum depth.
Please enlighten

The largest duct under 20" joist is 12". 20+12=32.
The largest duct under 16" joist is 36". 16+36=52.
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Re: Joist-Duct Combination

Postby mono » Wed Dec 09, 2015 7:44 pm

Coach,
I understand what is happening in smew's PDF example above, but is there a reason why zone A and B cannot have different roof deck heights at the second level? I am not seeing a requirement except for the parapet (which might be why). Either way I'll do it as shown in his example, but just out of curiosity I thought I'd ask if I am missing something. Thanks.
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Re: Joist-Duct Combination

Postby Coach » Wed Dec 09, 2015 8:08 pm

mono wrote:Coach,
I understand what is happening in smew's PDF example above, but is there a reason why zone A and B cannot have different roof deck heights at the second level? I am not seeing a requirement except for the parapet (which might be why). Either way I'll do it as shown in his example, but just out of curiosity I thought I'd ask if I am missing something. Thanks.

Read item #1 of the program.
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Re: Joist-Duct Combination

Postby mono » Thu Dec 10, 2015 11:31 am

Coach wrote:
mono wrote:Coach,
I understand what is happening in smew's PDF example above, but is there a reason why zone A and B cannot have different roof deck heights at the second level? I am not seeing a requirement except for the parapet (which might be why). Either way I'll do it as shown in his example, but just out of curiosity I thought I'd ask if I am missing something. Thanks.

Read item #1 of the program.


I guess I just figured that meant they weren't sloped. Flat to me doesn't necessarily mean you cant have a difference in height between perimeter parapets separating zones. I think NCARB could word that different by saying roofs with the same ceiling height shall be on the same plane or something to the effect. Thanks for the clarification.
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Re: Joist-Duct Combination

Postby Analise87 » Sun Feb 07, 2016 11:09 am

Peng67uins wrote:This has now confused me. I have been taking the duct that comes out of the mech. room as the largest possible duct in the areas that arent the 2 story part. Im not following the logic of the "zones" it seems to be an arbitrary position to just pick a spot and cut it into zones. Please someone explain what I am missing.



I had the same question. It is separated in to Zones by Joist Size (and orientation). Someone - please correct me if I'm wrong.

Thank You!!
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Re: Joist-Duct Combination

Postby Raja » Sun Apr 16, 2017 3:58 pm

Coach wrote:
smew wrote:Please review and let me know if I am approaching the problem correctly.

Yes, that's how it's done.


This explains why the NALSA paper vignette #1 had a different joist/duct combo than me. I was doing the largest duct and largest joist on the 1st flr. But it's when structure changes depth, then what is deepest duct in that structural bay, where they occur together.

Super helpful diagrams Smew, I hope you've passed all the exams by now.
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