Exam question assistance please

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.

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

You would have to hope so....:-)

It's related to the second moment of area, no?

-- Nige Danton

And Barnes Wallis and the Wellington Bomber! Or for the really wheelies-- the bird cage construction on cars.

Mark is usually right-- cornflake packets or

A HOUSE OF CARDS!

Well?

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.

Nick

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

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

...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"...

Regards, Tony

I'm not counting them as complete triangles

because top and bottom you got beams ...

this is supposed to be a girder ...not a bunch of triangle sections stitched together

I'm counting them as diagonal spars ...

on "c" you have 14 diagonal spars

on "a" you have 8 diagonal spars.

the top and bottom of the triangles is already there in both cases as the beams ...

so you got two girders to compare

one with 8 spars ... and one with 14 spars . welded to the top and bottom beams .

thats how i see it .

all the best.mark

The triangles were solid. Question during the test: "Sir, are these triangles *really* solids?" And if the answer is "Yes!", try to look as stupid as possible. :-)

Nick

I thinks thats it exactly. Isn't it bd squared over 12 or something like that?

Peter

I was taught "castellated beam/I-beam/RSJ".

Regards,

David P.

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.

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.)

Well if you don't want to answer the question that was actually set there's probably little left to discuss.

Clearly B is the strongest. A has no diagonal bracing at all and C has the brace running in the wrong direction.

its another stupid question, strength depends on what use the gate is put to ..

but as a farm gate ...

C is how most of them are made ....and would be the answer examiner is looking for .

C is least likely to buckle or sag when someone swings on the end of it

and is economical compromise over metal used.

on c the brace acts like suspension bridge

where as A would survive a charging rhino, full on, into it.better than C...it has not the same abilities to survive someone swinging on it...or the test of time over sag.

B is just bad design with brace running wrong way.

no rhinos in the uk .....so C is best.

all the best.mark

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