Mr Whippy
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| posted on 23/4/07 at 02:49 PM |
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To the engineers, which is stronger
and if you know, why?
genuine reason for asking, ta 
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flak monkey
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| posted on 23/4/07 at 02:53 PM |
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I assume you mean stiffness. In theory a as the triangulation helps to directly transfer the load down to the supports.
[Edited on 23/4/07 by flak monkey]
Sera
http://www.motosera.com
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Mr Whippy
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| posted on 23/4/07 at 02:58 PM |
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true, but in both cases the struts are both in tension and compression so is there any difference between the two? in A the bottom two are under
tension and fewer struts are sharing the load, in B most if not all are now under a load therefore one would expect that each strut is taking less
than in A.
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DaveFJ
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| posted on 23/4/07 at 02:58 PM |
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I believe that A would be stronger - in a theoretical world.....
In A both the centre upright and the 2 diagonals are doing work, the top horizontal member and side uprights are redundant.
However in B only the centre upright and and left diagonal are working, the right diagonal becomes redundant in supporting the load. however if you
consider the load path for the right hand support then it gets messy - putting tension on the right diagonal and is 'probably' much
weaker
that , however, is all theoretical! in practice the structure would be unlikely to only ever have a point load vertically over the centre member with
no shear forces. at this point things get a whole lot more interesting because the left diagonal might be in compression whilst the right diagonal is
in tension ....
or maybe I am just talking b*ll/*cks
I am sure someone will be along to soon to tell me so! 
[Edited on 23/4/07 by DaveFJ]
Dave
"In Support of Help the Heroes" - Always
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Fred W B
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| posted on 23/4/07 at 02:58 PM |
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Are you considering that as a pin jointed frame, or be we have to consider bending moments as well?
Fred W B
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Mr Whippy
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| posted on 23/4/07 at 03:07 PM |
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no just a welded tube frame, pure compression and tension.
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02GF74
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| posted on 23/4/07 at 03:10 PM |
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what exactly do you mena by strongere? i.e. the one that can support the bigger weight?
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Mr Whippy
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| posted on 23/4/07 at 03:15 PM |
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yip, which will go bang first
I also notice that on truss bridges there is a mixture of both, so if A is clearly stonger than B, why do they use both?
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JAG
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| posted on 23/4/07 at 03:18 PM |
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quote:
no just a welded tube frame, pure compression and tension
If the joints are welded then there is no pure compression/tension. You'd have to have pinned joints for that to work.
I agree that A is the stronger.
It's already been explained, but ignoring torsional stiffness at the joints (as would be introduced by welding), then the two diagonals are both
in compression and are loading the bottom bar in tension - forming a triangle.
The other sections are simply stabilising the load and allowing the other parts of the structure to dissipate the load.
[Edited on 23/4/07 by JAG]
Justin
Who is this super hero? Sarge? ...No.
Rosemary, the telephone operator? ...No.
Penry, the mild-mannered janitor? ...Could be!
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mookaloid
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| posted on 23/4/07 at 03:19 PM |
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An old engineering adage "if it looks right, it probably is right" and the converse are usually true.
So for that load I would say A because it looks right and B doesn't 
Cheers
Mark
"That thing you're thinking - it wont be that."
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Alan B
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| posted on 23/4/07 at 03:20 PM |
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In theory if pin jointed they'd be identical.
Alan
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mcerd1
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| posted on 23/4/07 at 04:02 PM |
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I'd go for this (classic 'N' truss)
with much smaller diagonals (spelling) as they will only be in tension
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Confused but excited.
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| posted on 23/4/07 at 04:22 PM |
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I wonder why they build roof trusses the other way up?
Tell them about the bent treacle edges!
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JonBowden
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| posted on 23/4/07 at 04:49 PM |
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Won't it depend on the material.
If the diagonals are made of a material that is stronger in compression (eg concrete) then A will be better. However if they are made of a material
better in tension (eg string) then D will be better.
Now, problem here is that we are presumably talking about steel tubing - I don't know if it is better in tension or compression - I guess it
will depend on the design of the tube - ie a thin tube might tend to colapse under compression
Jon
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tks
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| posted on 23/4/07 at 05:06 PM |
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A clearly when they build a bridge they don't waste the money in the reversed T section...
sow the lowest bar is eliminated and also the vertical one..(in the middle)
then you get the tipically bridge shape..
Tks
The above comments are always meant to be from the above persons perspective.
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omega 24 v6
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| posted on 23/4/07 at 06:03 PM |
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As per flak monkeys original reply the loads are more directly fed to the supports. As you assume in the left hand diagram that the c.o.g of the load
is acting directly downwards due to the force of gravity then there is no need for the top bar or the two side bars (if you see what i mean) therefore
less material is needed to acheive the support.
If it looks wrong it probably is wrong.
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Mr Whippy
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| posted on 23/4/07 at 06:38 PM |
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Well thanks very much guys for all the responses and I see the voting went totally as expected
It wasn't a quiz and I don't have the answer in an envelope Truth is I as yet don't know but have a feeling that really
there's no difference between the two. I might make a model to find out and let you know.
I didn't think about the welds themselves adding to the stiffness and assumed that it would be the same whether pinned or weld but I think your
right there. It wasn't so much about what is the most efficient but simply what would break first. The whole reason I asked is that I'm
doing a large trailer based on a good old space frame and wasn't sure how to link the diagonals. Since it's for selling and not one of you
went for B I know which way to do it  .
Cheers, dudes.
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britishtrident
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| posted on 23/4/07 at 07:10 PM |
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A lot depends on the slenderness of the members --- as one or two others spotted buckling of a member in compresion is the most likely failure mode;
which is why roof trusses are the other way up.
[I] “ What use our work, Bennet, if we cannot care for those we love? .”
― From BBC TV/Amazon's Ripper Street.
[/I]
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mcerd1
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| posted on 24/4/07 at 11:33 AM |
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Tell you what - if you give me the dimensions, sizes and loads I'll run it through my structural design package (assuming its metal of course)
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Mr Whippy
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| posted on 24/4/07 at 03:08 PM |
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thanks for the offer but why not find out the answer first?
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