PDD - inacuracy in the handbook

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    Scott Barber

    Not sure what's wrong about this...

    You're calculating the load on the ridge beam, therefore you need to determine what load it's carrying - which would be half of the distance from the stud walls and the ridge beam in both directions. The ridge beam doesn't carry any of the load from the overhang. 

    I didn't experience any "inaccuracies" on the actual exams either.

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    Evgeni Pavlov

    Scott, 

    The overhang slightly relieves the ridge support. If you draw the static diagram and write both equations will see that support at the cantilever takes the greater part of the load. If the cantilever was 10', you`d have 0lbs in the ridge support. If it was 11'', you`d need something to hold the joist down.

    Am I doing something wrong?

    P.S. Sorry for the photo. I don`t have a scanner at home. 

     

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    Derek Mason

    Don't over think it. 

    Do you have Building Codes Illustrated? or Building Construction Illustrated? Ching does a great job explaining these. If you want, read the section in the 2015 IBC regarding structure. NCARB's response/rational to the practice question even calls out the IBC. 

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    Kurt Fanderclai

    Hi Evgeni,

    I've passed all 5.0 exams -- the only thing I'd suggest that you're doing wrong is maybe that you're not taking the question at face value.

    Here's the thing to remember -- the exam poses questions based on little snippets of reality with very limited and defined contexts.  Don't attempt to expand on the "givens".  In other words, for the exam, if the question says "ignore the cantilevered ends", then that is exactly what you must do.  Don't worry about the reason why the cantilever is to be ignored.  Whatever the reasons -- which are not given -- for the question at hand, you must ignore the cantilevered ends.

    That's the important part.

    Separate from the question -- your thought process is not wrong, and I think you appear to have ample understanding of the level of structural calcs you will see on the exam.  You just need to concentrate on answering the questions based only on the given parameters of the question.

    Good luck with your next exam!

     

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    Scott Barber

    Similar to Derek and Kurt's advice, these exams are pretty straightforward. Questions will often have more information than is required so you'll need to narrow it down to the relevant information.

    In this question, look at it more simply. Identify the tributary area, do the multiplication, and there's your answer. A lot of study material gets more detailed than necessary for structural aspects of the actual exams (Ballast in particular - waaaay too detailed), so be aware of that when studying, too. 

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    Evgeni Pavlov

    Guys, 

    Thankfully this is just an example question but it is supposed to represent the real exam scenario. Look at the question. No one says "disregard the overhang" nor "look at it simply" nor "snow doesn`t fall on overhangs" :) ... they are just asking us how much it is. This is an objective thing. It is like "what is the color of the sky?" - "blue" - "wrong answer = fail, disregard this and that; look at it simply, it is purple". Further, this is not super high-level engineering, it`s a very typical situation that appears in day-to-day life. If they are asking me to look at it simply, why don`t just ask me a simple question? I am not in their heads to know what they think the right answer is. 

    Yes, I have BCI and IBC and all that stuff. Nobody says "ignore all overhangs" :) 

    I may be wrong but... do you really find this being OK? I am really curious to see what an NCARB official (hopefully they read) has to say. 

    ...wha`rever :) 

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    Scott Barber

    In my experience (including my structures class while getting my master's degree taught by well-known David Thaddeus), the way NCARB shows the calculation is how I've seen it done. Find the tributary area (half the distance between the structural members), calculate the length times the load, and that's it. Architects aren't engineers, so we can look at it a little more simply for the most part (in my opinion), and omitting the overhang doesn't seem like a drastic tolerance - the ridge beam will be slightly oversized but that's not a bad thing...Is there a book or study resource that you're looking at that shows the calculation the way you drew it out?

    I think the discussion in previous comments about a question's simplicity is a broader discussion and will be more pertinent in other questions, though with this question including the roof pitch and overhang length there is some superfluous information.

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    Kurt Fanderclai

    Scott -- the way Evgenie has diagrammed the loads is certainly not necessarily wrong, given his assumption that the rafters are continuous beyond the exterior walls.   As for your request for a source for this, it's a simple "teeter totter" moment around a point diagram that can be found in any number of books.  If the rafter is continuous, adding load to either side of the pivot point would definitely affect the ridge beam.  So, I don't think you've yet necessarily answered him satisfactorily.  But I'll bet you'll continue to try...  ;-)  

    Evgenie -- 

    I took your original post as what the question actually said.  So, I was under the impression that the question actually said "ignore the cantilevered end...".  Looking at the page you've now reference, the question does not say this -- and so, maybe a little bit ironically -- your post was a bit misleading.  ;-)

    But...  I could be inclined to agree with you that this may not be the best question ever written.  

    That said, however, I could argue that you're probably still not quite correct in your assessment of this question: 

    One could easily argue that the overhang clearly would not affect the ridge beam load,  since the rafters aren't -- by typical drafting standards -- continuous past the exterior walls.  Look at the "blocking" in the given diagram.  It's not really blocking, but rather a continuous 2x member, and the overhang must be connected in some other way.  Would this be an odd way to create an overhang?  Probably, for anything longer that about a foot of overhang.  But... all we have is the given question and diagram.  By longstanding drafting convention, the "X" notes a continuous member.  If it were actually blocking, it would instead show a single diagonal line.  So, based on the diagram, the overhang snow load does not affect the ridge beam.

    Format issues, such as a few potential ambiguities, have not historically been a cause for alarm.  Candidates have indeed pointed out a few over time in the handbook/study materials -- and it's not unwise to take note of them.  But, all of them combined are essentially negligible in the larger picture.  

    The questions on the exams themselves were -- in my experience - quite clear. 

    Evgenie, you appear to have a solid understanding of the nature of such questions -- based on my experience, I'd suggest that is what will get you through the exam(s).   

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    Evgeni Pavlov

    Kurt, 

    Thank you for the good words. I see how your professional curiosity starts to tickle :) Are you an NCARB official?

    Continuous and discontinued members behave essentially the same way. That`s why you connect them. 

    If the overhang was braced diagonally down to the wall it would be totally different story. However, now it acts as a pure cantilever.  Make the experiment - take two pencils and bind them together with two elastic bands (to create the discontinuous condition). Then, model the cantilever beam with your fingers. You will notice that continuous and stiched members behave the same way. 

    Further, why would you use glue-lam if you will split it into pieces? 

    I think that the question is fine. Just the correct answer is wrong. 

    I agree with Scott that oversizing of ridge member is not that bad, but this would mean under-sizing of the member at the wall (thankfully bearing wall in this case). 

    Apart from this putting a liiiiiittle doubt on the "correctness" of each "correct" answer, it is like the typo on my dissertation - not a big thing but yet made the auditorium laugh :)

    Professional tickling is a good and positive thing which leads us to progress....hopefully. Cheers. 

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    Kurt Fanderclai

     Evgeni -- all good.  When I was taking the exams, several such discussion happened -- always enjoyable to participate in them.   

    Further, why would you use glue-lam if you will split it into pieces? 

    Hey, I'm not following this.  Is this related to my comment on the "blocking"?  The ridge beam appears as an engineered member, but the "blocking" is shown on the diagram as dimensional lumber -- and, as I've pointed out per drafting conventions, it's not blocking -- this is a continuous member running along the top of the wall.  The overhang is structurally separate.  

    Continuous and discontinued members behave essentially the same way. That`s why you connect them... 

    If you like experiments, you might try informing your structural consulting engineer that "continuous and discontinu[ous] members behave essentially the same way".  :-) 

    Jabbing aside, this statement would not be generally true.  It can actually be quite difficult to connect discontinuous members so that they will truly perform as a continuous member -- especially in wood.  More to the point and for the question at hand, no such connection is defined for the discontinuous condition pictured in the question, so such assumptions can't be made.    

    However, now it acts as a pure cantilever.

    Again, I'd argue that this statement is not accurate, since the diagram in the question indicates that the rafters are not continuous at the overhang -- due to the continuous member at the top of the wall.  It's like a subfascia condition with a discontinuous overhang extension somehow connected.  Weird?  Yep, but still not continuous.

    This is where the given question and diagram may not be the clearest, but it's not possible to justify that the rafters are continuous.  So, the given answer would seem to match the given question.

    Enjoy, and good luck -- what exams remain for you?

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    Ariel Jamison

    Evgeni,

    Consider the prompt to ignore the overhangs a gift.  Basically, redraw the example without the overhangs and then do your calc.  You don't even need to draw the actual load diagram for this problem, just know that the diagonal slope of the beam is also ignored, and then I'm assuming, just like calculating floor joists, only half the load on either side is considered.  It seems like you understand the formulas for calculations really well so this next bit is maybe over explaining:

    Take the length of roof structure (5'0" + 5'0") x snow load (since they want the linear load only 60 psf) = 600

    This is not so weird to simplify these questions when you think about it.  It is the same when the practice exams ask for the general plumbing heights based on psi/ft and they say to ignore friction.  In real life, you can't ignore friction otherwise you could easily get the wrong answer. Luckily to be an architect you don't have to know all the calculations an engineer does, so only a general understanding "rules of thumb" is required. 

    Hope that helps! 

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