gpoli111
PE
That's essentially what the solution does though. It takes the pressure over a reduced area
Errors in NCEES practice exam
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The solution assumes a triangular pressure distribution with a reduced footing area. The CERM, like other references I've seen, assumes a uniform pressure distribution over the reduced area.
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geomane
Well-known member
Here is the equation (eq. 3.38) from one of my geotech books. I get 2.72ksf.
geomane
Well-known member
ptatohed
Licenced to Spell
I have it. I have the 2008 errata and 2011 but for 2011 I only have the AM and the Transpo PM. Feel free to PM me your e-mail address.
What section of the CERM are you looking at for if located outside of B/6? The CERM that I have does not go into detail about e greater than B/6?
jijir83
Expert in everything useless
Mmm.... Ok. Yes, your markings reduce to the same. So you're bugged by being misled. You have to remember that this is about taking the test and passing and not about our OCD engineering tendencies. The pipe "must flow full" means and is an indication to "use the full pipe equation". The solution is "most nearly" means "We, test writers, are too lazy to give you an exact number so pick the closest value". It's just another way, if you don't know your stuff, to throw you off and make you think that you made a mistake. Next thing you know, you might use a different equation and might pick the wrong answer if you lack confidence.
Part of the exam is learning to take the exam. One of the first things that a review course instructor, like the ASCE ones will do, is bring your attention to these "most nearly" and things (i.e. assumptions to make unless otherwise stated) that are strictly for exam sake. For all intended purposes, that 16-in pipe is flowing full. If they gave you 14, 15.56, 16 and 17, you better pick 15.56. If they give you 14, 15, 16, 16.7, you better pick 16 and not 15 or 16.7. Your goal is to pass within the confines of this test.
I don't know what the CERM is doing but my instinct would say to use the method that I mentioned. Seems like others have worked it out the same way. I don't do foundation design anymore but, in determining dimensions of combined rectangular footings (one strip with two columns as is shown), uniform soil pressure is used. In making structural design of the combined footing, non-uniform soil pressure is considered. In this question, the footing is not being designed. So, part of the clue of what to use is in the question itself. If there are multiple ways of doing something, the methods have to be ones that give the same solution, or they either have to tell you what to use, or it is an inherent known fact which assumption to make. If you're not well versed in the topic, you won't know which way to go.
I hope you figure out those quirks. The exam in April was loaded with things like that and the afternoon geotech was all about these quirks that could through you off. If it's like that again this cycle, everyone needs to be sure to study with and have a good foundation/geotech reference. They were sneaky at linking structural (became foundation design) and water (became seepage) back to geotech in the morning.
Good luck on the test!
Do you need some kind of special access to view this? It is telling me it's blocked.
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Perhaps I'm misunderstanding what the CERM is saying, but it sounds like there are two different methods described: one for eL/6 which is uniform and reduced footing dimensions. It's on page 36-9 in the 2013.
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geomane
Well-known member
This isn't about being OCD. This is about them being wrong. There's a difference between making a problem tricky and making a problem intentionally misleading and contradictory. I admit i am not well versed in water resources, but this isn't something that should ever be on a test (probably why it's not anymore). It's designed to get people stuck even when they have the right answer. And on that, i think a question like this is more likely to confuse someone who actually knows this stuff well, rather than someone like me who doesn't. I can do the structures problems in my sleep, but when i run into the structures equivalent to problems like this, which there are plenty of, i still sit there and question myself because i know what I'm doing, but my answer doesn't make sense. I've come to realize this test has nothing to do with technical knowledge. It's like a series of riddles with a timer. How does that prove anyone is a qualified engineer? It doesn't. I know plenty of people with PEs that i don't think should ever have been given a license. But whatever, I'm willing to play this game to get my license. Its not exactly news to me that this test is all about tricks and time management. But in order to make sure i can beat it, i need to understand it. It's hard to do that when the questions and solutions are contradictory or flat out wrong. Look at my other examples. These mistakes shouldn't happen.
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Sorry, the board seems to have cut off my post. Maybe they don't like the greater than and less than signs? Or greater than or equal to? Anyway, what i was trying to say is that there appears to be two different cases in the CERM.
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That is the same section and page I was looking at. To me it does appear to go into detail about the method for e greater than B/6 other than stating negative soil pressure will result. Either way, I believe eccentric loading of a footing like this would always have a triangular load.
geomane
Well-known member
I get B, 98 feet. You are using an equation that doesn't take into account the downhill gradient.
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Thinking about this problem logically, do you think you can stop your car on steep slope as fast as you can on level ground?
JHW 3d
Here's Johnny...
On 10/15/2016 at 10:19 PM, rfehr613 said:
I haven't done the math, but agree the approach in pencil marks is wrong. The problem statement only shows decel rate for downhill portion, not for level portion.
Update:
I calculate 70ft (A).
a_flat = 11.2 + 32.2*.06 = 13.13 ft/s^2
s_flat = 10,540/(2*13.13) = 401.4 ft
s_downhill is 470.5 ft (your calculation)
Diff = 69.1 ft (70 ft)
What's the solution list as the answer?
geomane
Well-known member
The NCEES solution is B.
Ok, well that is a horribly worded question then. It reads as both having said deceleration rate, not one of them. I hope the real test isn't this much of a disaster.I haven't done the math, but agree the approach in pencil marks is wrong. The problem statement only shows decel rate for downhill portion, not for level portion.On 10/15/2016 at 10:19 PM, rfehr613 said:
Update:
I calculate 70ft (A).
a_flat = 11.2 + 32.2*.06 = 13.13 ft/s^2
s_flat = 10,540/(2*13.13) = 401.4 ft
s_downhill is 470.5 ft (your calculation)
Diff = 69.1 ft (70 ft)
What's the solution list as the answer?
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JHW 3d
Here's Johnny...
What is their reported answer?Ok, well that is a horribly worded question then. It reads as both having said deceleration rate, not one of them. I hope the real test isn't this much of a disaster.Sent from my VS980 4G using TapatalkI haven't done the math, but agree the approach in pencil marks is wrong. The problem statement only shows decel rate for downhill portion, not for level portion. Update:On 10/15/2016 at 10:19 PM, rfehr613 said:
I calculate 70ft (A).
a_flat = 11.2 + 32.2*.06 = 13.13 ft/s^2
s_flat = 10,540/(2*13.13) = 401.4 ft
s_downhill is 470.5 ft (your calculation)
Diff = 69.1 ft (70 ft)
What's the solution list as the answer?
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