Question: Raise the roof? Dorf Vignette 2

Building Section Vignette and Multiple Choice

Question: Raise the roof? Dorf Vignette 2

Postby mantoupug » Sun May 04, 2014 1:37 pm

Hi, I'm wondering what people think about Dorf's sample Vignette 2, where a duct is going from the 2nd floor into the double height space. Dorf says to raise the roof slab to accommodate the 24" duct and 24" joist assembly beyond. However the section doesn't show the bottom of roof slab lining up with the dimension witness. Am I missing something? In theory raising the roof makes sense to me, but I wonder if this level of detail will be required in the exam. Has anyone come across this before? The first time I took this exam, I thought that I had this situation 10 minutes from the end of the vignette. I had just enough time to frantically raise the roof, and no time to check. I'm not sure whether the raised roof was a minus or if I had made another fatal flaw elsewhere. Coach, what do you think?
Last edited by Coach on Tue May 06, 2014 7:57 pm, edited 1 time in total.
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Re: Question: Raise the roof? Dorf Vignette 2

Postby AREcuact » Sun May 04, 2014 1:54 pm

So, are you saying that Dorf's solution for the second floor needs to have 8" more in the interstitial space in order to accommodate the 24" duct and clear 8" for light fixtures in the lobby ceiling?
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Re: Question: Raise the roof? Dorf Vignette 2

Postby mantoupug » Sun May 04, 2014 2:42 pm

I think it should be higher, because otherwise there would be two different levels of roof for the 2nd floor.

Dorf says "once the (lobby roof) elevation is determined, the roof height for the rest of the building reflects the minimum vertical dimensions required to get the 24" duct from the 2nd floor ceiling into the lobby ceiling".

In other words, top of the 2nd flr roof should be at 30'-0" (25' + 8" + 24" + 24" + 4"). That's 1'-8" higher than what's being indicated...

I'm testing tomorrow and this is throwing me off a little... :(
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Re: Question: Raise the roof? Dorf Vignette 2

Postby AREcuact » Sun May 04, 2014 3:30 pm

Not sure if this helps but shouldnt the 24" joist beyond dimension be to the underside of the second floor roof slab not top as shown?
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Re: Question: Raise the roof? Dorf Vignette 2

Postby mantoupug » Sun May 04, 2014 3:41 pm

Yeah, that's exactly what I think!
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Re: Question: Raise the roof? Dorf Vignette 2

Postby AJD » Sun May 04, 2014 8:19 pm

I agree. I spotted this a few days ago and had the same questions. The first edition of the Ballast CD's exam question book has a similar problem with a similar solution. I asked two licensed architects and they both agreed that the given "solutions" were incorrect. Very frustrating.
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Re: Question: Raise the roof? Dorf Vignette 2

Postby LNT » Sun May 04, 2014 8:42 pm

I haven't tried Dorf's second vignette yet, do you mind sharing the DWG?
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Re: Question: Raise the roof? Dorf Vignette 2

Postby AJD » Sun May 04, 2014 8:59 pm

I can't post tonight but will be happy to do so if nobody else has posted it the next time I check in.
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Re: Question: Raise the roof? Dorf Vignette 2

Postby laurstar » Tue May 06, 2014 7:39 pm

why isnt the largest joist/duct combo for the 2nd floor 30"(j)+24"(d)? am i not reading this correctly? what usually happens in corridor conditions? which joists govern these weird interstitial spaces?
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Re: Question: Raise the roof? Dorf Vignette 2

Postby mk2383 » Thu May 08, 2014 8:16 am

What I think Dorf has done is draw the double height space first (Lobby-right side of section): 24” duct and 60” joist. Now that 24” duct is coming in from the corridor behind the lobby (behind seeing it in section or beyond). That corridor has a 24” duct and 24” joist. He then uses that as the roof height for the whole second floor (raised roof!) and works downwards from there in the section of the second floor part (left side of section). From that roof height he draws 30” for joist and 16” for the duct (which is where the section is cutting). The ceiling height doesn't change so therefore there is plenty of space to allow for the 24” duct beyond.
My feeling is that when the mechanical room is in front of the section line you have to raise the roof, depending on if the ducts can go through double height space to second floor space? But when it’s behind the section line you don’t have to worry about it. Any thoughts?
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Re: Question: Raise the roof? Dorf Vignette 2

Postby zerna30 » Thu May 08, 2014 7:40 pm

It took me a while to figure this one out (DORF Sample vignette 2): Since the duct is coming into the upper lobby goes through the corridor behind (North) first, the upper lobby ceiling must be lowered to allow the duct to continue running at the same level without having to bend up into a higher clearance space. Notice that the joists sizes over the mechanical room are different than those where the section cut runs through... these are not obvious at the line cutting through the section.

Once I realized this, I was embarrassed on how long it took me to understand what was happening here.:oops:

I just received my results today and it was a pass! Fortunately for me the test vignette was closer to the NCARB practice example than it was to DORF. But I have to admit that reading through DORF helped me pay closer attention to all dimensions on the floor plan.
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Re: Question: Raise the roof? Dorf Vignette 2

Postby appletree » Sun Jun 01, 2014 6:05 pm

im testing tomorrow, so i realize i'll only get more comments to this after the fact...but this is driving me nuts! i looked at the Dorf Vignette 2 a million times and I decided that since his dimension string isnt complete in the solution, he may actually have drawn it correctly, but where he adds up the slab + joist + duct = 50", that is where an error is...shoudl say 24" duct, b.c in the corridor there is a 24" duct, in the same zone as 30" joists. its been driving me insane that it could be so unclear, in a book known for its clarity!
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Re: Question: Raise the roof? Dorf Vignette 2

Postby robert07 » Thu Jun 05, 2014 11:24 pm

So after staring at it for some time thinking the roof joists in the west section need to be raised by 8", I realized Dorf did indeed draw the correct solution (to his own problem, imagine that). As the location where the section is cut has 16" deep ducts, but also has plenty of clearance between the lights/ceiling for the 24" deep duct in the corridor. Once the roof system changes to 24" joist, the duct can easily bend upwards to be snugly below the 24" joists. This means the limiting condition is the bottom of the 24" joists that need to align with the bottom of the lobby's 60" joists to allow the 24" duct to pass between the corridor and the lobby.

Hope this helps and makes sense as it sure was initially confusing to me!
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Re: Question: Raise the roof? Dorf Vignette 2

Postby georginal » Sat Jun 13, 2015 9:38 am

Not sure if anyone found the solution to this problem yet.
I agree with the fact that if the ducts have to run below the joists and there should be 8" clearance there is no other way than:
24"+30"+8".
What am I missing?

Did anyone run into this problem recently?
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Re: Question: Raise the roof? Dorf Vignette 2

Postby kikstear » Wed Jul 26, 2017 10:54 am

I've also been really confused by this vignette...
Originally I thought the west 2nd floor part of the section should allocate for the 30" joist and the 24" deep duct in the corridor, despite the fact that the section cut is through the 16" deep duct (looking for the biggest duct/joist combination).
After reading this thread and taking another look at the vignette, it looks like the corridor joist is actually 24" as it originates at the North bearing wall (where all other joists are 24")?
However, even if this is the case, the space needed for the joist and duct would still be 24"+24" = 48" which is even larger than the 30+16 combo...

I test next week and this has really confused me :shock: :shock: :shock:
Should the plenum for the 2nd floor be 24"+24"+8"? or 30"+24"+8"? or as Dorf shows 30"+16"+9"?
I have been looking at it so long and am not sure what I am missing! Any ideas?
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