Exam question assistance please

An interesting discussion and a good example of how the practical doesn't always agree with the theoretical. Years ago when wooden 5 bar gates were made of real wood in reasonable cross sectional areas the brace was (from memory) invariably in compression. Nowadays with gates made from paper thin sections, putting them in tension stops any droop caused by bowing sideways of the invariably too thin cross brace or unseasoned wood. However if you look at the current best gates when the brace is in tension the hinge post is usually extended upwards and the suspension point of the brace is above the top rail thus providing some additional support for the top rail as well. Alternatively they are triangulated with the first half of the gate span braced in compression and the outer half in tension.

Just observations of the practical application, I'm not getting involved in the boring theoretical stuff. Of course it does highlight the crap that is set as exam questions these days; perhaps it is to find out if the train driver knows which one to aim his train at in an emergency - they have got steering wheels on trains haven't they :-)))

Keith

Reply to
jontom_1uk
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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.

Reply to
Ray

Because mortice & tenon joints are not particularly strong in tension, but work well in compression.

Richard

Reply to
Richard

You have some damn big Hamsters :-)

Mark, I was as sure as you that the answer they were looking for was C but in fact they want B for the answer. I agree that B is hung incorrectly. All the gates on my hobby farm are hung like B and most in the area are also. The thing is, the question doesn't ask which is hung properly, it asks which gate is the strongest, and I still say B & C are identical. If you take them both off the hinges and away to a testing facility, they are going to come up the same under any strength test. Agreed? or not? If I don't pass this test, (It is today) then I am going to challenge the validity of the test questions.

Cheers, Phill.

Reply to
Phill

I should add that these are quick-quiz type questions, we are given 30 seconds for each one. :-)

Cheers, Phill.

Reply to
Phill

"Phill" schreef in bericht news:fhei5v$4qf$ snipped-for-privacy@aioe.org...

I would say B. Own weight is less, so it can take op more weight, assuming all three are evenly strong. Dirk

Reply to
PG1D/PA-11Ø12

Sounds like you're going to have to put your Maxwell Hutchinson head on when answering these questions ... bite the bullet ....and just give answers what they want.

no wonder bridges are failing .

stadium roofs are collapsing under weight of snow.

if they design things through theory and not look at every eventuality .

all the best.mark

Reply to
mark

However, with modern farm gates being made from welded steel tube, it probably makes more sense for the brace to be in tension.

Regards, Tony

Reply to
Tony Jeffree

Imagine the gates with all the members removed except the two running from the top and bottom hinges. In the case of B that's a diagonal running upwards and meeting a horizontal and in C vice versa. Those two are the primary elements of the gate and in fact could be a gate on their own in some cases.

Now imagine a person swinging on the end of the gate. In the case of B the horizontal member has almost zero strength on its own but as it tries to rotate it transmits the force into the diagonal putting it into almost pure compression. The diagonal is bracing upwards against the downwards force being applied to it. The horizontal is being put into tension at the same time which makes it very strong.

Now do he same with C. As the diagonal tries to rotate it also tries to pull the horizontal down with it. The force isn't being transmitted into the horizontal at anything like such an advantageous angle. It is also going into compression to some extent but is experiencing a greater bending moment than the upwards diagonal in B.

The question seems perfectly fair to me. All exam questions require the candidate to make certain common sense assumptions and it's reasonable to assume a gate isn't being designed to resist upwards forces. It has to withstand its own weight and anything or anyone swinging on it. B is clearly the much stronger design. Yes it's the same gate as C but hung the other way up but that's the whole point isn't it? Doing things right instead of wrong.

Reply to
Dave Baker

...finally got to look at the two questions. There simply isn't enough info given to be able to answer the questions, regardless of how long you are given. I suppose that was the emerging consensus. Can you tell us who set these 'questions'? (The answer page won't come up) Peter

Reply to
Peter J Seymour

Thanks for that great explanation Dave. It makes sense to me at first reading, however, can I put this to you.

Using you idea of just the two main elements, ...assume a very very long gate made of material that is too light for the job. In gate B the diagonal would have a slight bend in it because of its own weight, then someone swings on the end and the diagonal gets bent further and fails.

In gate C, the diagonal would have the same bend from its own weight, then someone swings on the end and the diagonal will straighten out and all is well. Am I wrong in this assumption?

The only doubt I have about my reasoning is that if this scenario was that heavy enough material was used and the gates were tested to destruction, wold the diagonal under compression outlast the diagonal under tension? In other words, does it take a greater force to concertina a given tube (or rod) than to stretch it to breaking point??

Cheers, Phill.

Reply to
Phill

Peter, I'll try and re-submit the answer paper to the web site.

It was set "FOR" Australian Railways Group, but by whom I'm not sure.

Cheers, Phill

Reply to
Phill

Sounds ok :)

Given that a steel tube or bar is always going to be slightly distorted from manufacture, I think it would be /far/ stronger in tension (gate C best). But a wooden gate would be stronger in compression as it pushes the joints together (gate B best).

Just to stir the pot: the hinges are offset a bit from the top & bottom cross-beams of the gate, so I reckon it would probably bend or break close to them, 'cos they're fulcrums.

Guy

Reply to
Guy Griffin

In gate C it would also be the horizontal that would be bent under it's own weight and would fail further with extra weight on it. In either case type B would be stronger.

Relatively thin struts will always be stronger in tension than compression but any braced structure like this will rely on compression somewhere and that's where it will fail. With wooden gates the upwards diagonal needs to be fairly sturdy to take the compression loads. The rest can be weaker as they just provide extra bracing. In fact most of the rest of the gate other than the two main members is only there to completely close off the opening in question. To an extent the lighter they are the less strain the gate is under from its own weight. All the parts underneath the diagonal are only there for the ride. The bits above the diagonal help brace it.

Reply to
Dave Baker

That's also why you need to go to suspension bridges for very long spans. Braced structures operating in compression fight a losing battle against their own weight eventually.

Reply to
Dave Baker

Reply to
shane smith

What an appalling set of answers, this is an 'engineering' test set by someone who doesn't have the first clue about basic engineering principles.

You've got to love question 4......

"A cog turning clockwise, will turn a cog next to it anticlockwise"

Ok so far.

"Cogs connected with a straight belt will move in the same direction" They're not bloody cogs then are they!! They're pulleys!

Reply to
SimonJ

Sorry gents, but my ISP bit the dust for a couple of days.

To the person who mentioned the offset hinges, that was something I noticed a few days back also. If you wanted maximum strength the diagonal should be welded to the upright at the point where the hinge is mounted. (Pedantic :-)

To the person who mentioned cogs v's pulleys, :-) :-) Spot on!!

I am still amazed at the variety of answers and explanations, but I am still going to hang my farm gates as in C because I would rather have the grandkids straightening out the diagonal rather than bending it.

Thanks to all who participated in this discussion, it's been a hoot :-)

I have re scanned the answer sheet and will post the link with the heading ANSWER SHEET.

Cheers everyone.

Phill.

Reply to
Phill

## answer sheet is here##

formatting link

Reply to
Phill

A (7.36 MB) jpg ...!!!!!!!!

all the best.mark

Reply to
mark

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