Exam question assistance please

Well, I am a at least not alone. I thought C. because of the fact that the triangles sides are more vertical, so less side stress,
but the answer paper says A.
I would really like to get a professional engineers opinion on this one.
Thanks for the input. It is interesting.
Phill.
Nick Mueller wrote:

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from Phill

I wasn't going to offer an opinion because (to me is seemed so simple) but the variety of anwers has amazed me.
Without looking at the diagrams I deduced that the Equilateral had to take the most weight since the virtical and sideways stresses will be equal and be directed down the length of the side of the triangle - putting the material in compression - with the obtuse the vertical stress component will be greater than the sideways component and in the acute the sideways stress will exceed the vertical.
J (nearly failed HND mechanical engineering but was reassessed ;) ) G
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Well, you only "nearly" failed. I haven't even sat the test, so I'll take your word for it.
Thanks for your help.
Phill.
JG wrote:

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wrote:

Another way to look at is the load distribution per unit area. If we are assuming that the amount of material in the triangular supports is the same in all cases, and the load is evenly distributed across the top surface, then there will differing apex loadings at the lower surface.
The top surface support is across the base of the triangles and will concentrate the load across 2, 4, or 7 points on the bottom. Therefore there should be a lower force per area with the acute triangles.
As such, perhaps the failure mode would be on the lower beam, rather than the triangulation?
Peter
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wrote:

I'm a professional engineer, unfortunately not a mechanical one, so I don't really have a clue. :)
I'd suggest that in reality, if the weight was evenly distributed, there would be no difference in their load bearing capability. The bigger issue is the load bearing capability of the ground it is resting on - those pointy bits could do no end of damage!
Sorry, I told you I wasn't a mech. eng.
Mark
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I am, and I don't understand the question.
--
Charles Lamont

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On Wed, 14 Nov 2007 23:12:41 +0000, Charles Lamont

So am I and I think the question is appallingly badly set. How is the load applied, how is the girder supported and how is it constructed? As it stands it may as well have read 'Are apples better than monkeys?' or some such sureal gibberish.
Richard MIMechE
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uk.rec.models.engineering wrote:

Sadly this sort of test question is becoming all to common. Acceptable in a Sunday night pub fun-quiz for a prize of a few pints of beer, but not if the test is to relevant to employment, etc. I've seen some horrors in the amateur radio test questions, electricians' trade test stuff and even in the driving licence written test. Intentionally, no comment on recent A-level maths and science papers, it would be very bad for my blood pressure.
Regards,
David P. C.Eng, MIEE (or whatever the IEE calls itself now.)
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Nick Mueller wrote:

Still am convinced of my answer*). But that question is quite stupid, because girders don't look that way. If the triangles weren't solid (but tubes) it wouldn't be a girder but a structural frame**). And then, a) would be right.
*) with one assumption on how it is supported. Can argue in detail if you want.
**) Looking in the dictionary, I see that a structural bridge is called "girder bridge". I thought that "girder" is an I-beam. Don't you distinguish between the two or is my dictionary wrong?
Nick
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Nick Mueller wrote:

I think they are using the term as a structural element, it can also be a structure in itself though.
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Phill wrote:

So girder can be an I-beam, but also be something more general? To be more specific, is "structural girder" a valid expression? And an I-beam zigzag-cut/shifted/re-welded is called a honeycomb beam/girder?
Nick
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wrote:

...Of course there was the one about the Irish labourer that was asked the difference between joist and girder...he said "Joist wrote Ulysses and Grider wrote Faust"...<G>
Regards, Tony
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wrote:

I was taught "castellated beam/I-beam/RSJ".
Regards,
David P.
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I am flabbergasted.
I am applying for a job as a Trainee Locomotive Driver, and this is one of the Sample Test questions sent to applicants to familiarize them with the entrance examination. (This by the company that runs the rail system in this state)
If Engineers can't understand the question, what hope have hope-fulls like me got?
Another question that would have to fall into the category of "Very Poorly Written" surely:-
The question is, "Which gate is the strongest?"
It is at:- http://www.mediafire.com/imageview.php?quickkey=dpsljlwgy93&thumb=4
I would suggest that both gates B. and C. are identical in strength, as they are identical in construction, only the method of mounting is different. However, that is not what the question is in my view.
Another point is that a gate is presumed to be for the purpose of keeping something in or out. This being the case, the direction of force requiring strength would be presumed, in the absence of contra-indication in the question, to be parallel to the ground and at right angle to the face of the gate. This would also result in A. & B. being equal in strength would it not?
I would suggest that the question, based on the given answer, should have read "Which gate is mounted in the strongest way?"
The answer sheet providing their logic is at:- [Question (8)] You will need to click on LARGE at mediafire:-
http://www.mediafire.com/imageview.php?quickkey=6tvb0lmnmn9&thumb=4
It also gives their logic to the girder question (8)
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wrote:
Phil,

I can't respond to your particular points since I can't raise the image on the web site, but one major force present in a gate is its own weight. In a long gate, like a farm gate, the moment of the weight about the hinges can be significant. So I believe that the preferred way of building a gate is to place the diagonal struts(s) so that it(they) are in compression rather than tension under the force of the weight.
Jim.
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Jim Guthrie in uk.rec.models.engineering wrote:

Depends on how the gate is made. An old fashioned 5 bar all timber gate with dowelled, morticed & tenonned corner joints would have a compression brace - no need to tie the brace to the frame corners. A welded metal gate frame would have a thin tension brace welded at the corners.
Ledgered and braced sh*t-house doors are often hung upside down, the braces should be in compression, transferring the load to the ledgers. If you have one hung wrong, with a corner dragging on the floor, the easy fix is a tension brace - a loop of steel wire between top hinge side and bottom lock side, fixed by two bolts, then tighten using spanish windlass method and secure.
Regards,
David P.
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On Thu, 15 Nov 2007 07:28:53 +0000, Jim Guthrie wrote, in impeccable English,..

Quite. And it's surprising just how many newly made wooden gates you find the wrong way up. These are usually to be found at "wildlife" reserves complete with over elaborate hinge and latching devices, most of which fail to work as designed due to poor installation/tuning/maintenance. Lovely wood though.
--
Ray
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Phill wrote:

Oh sh*t! Now this is really stupid. The question wasn't asked well (leaving out constraints) and the construction shown wasn't very clever (none of the three) and then they ask that a loco driver? I bet they didn't give an explanation why a) might be the solution.
Try to make the pilot's exam, their questions should be more logical. :-)
Nick
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You would have to hope so....:-)
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Clearly B is the strongest. A has no diagonal bracing at all and C has the brace running in the wrong direction.
--
Dave Baker - Puma Race Engines



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