On 28/05/2017 04:49, Bill Wright wrote:
On 27/05/2017 00:28, John Rumm wrote:
On 26/05/2017 21:24, Bill Wright wrote:
On 26/05/2017 18:06, John Rumm wrote:

[1] e.g. like this one I made earlier:

http://wiki.diyfaq.org.uk/index.php/...and_brace_door


In which the braces are the correct way!

Indeed. On that type of construction, it is the "right" way. For the
type pictured in the mechanics paper, it does not matter.


What's the difference?

In a framed construction, the brace is inline with the other timbers.
This means that its not easy to make a joint at the brace ends that
would work well in tension unless you are going to opt for more
difficult to construct angled M&T joints. So the expedient option is to
ensure this brace is oriented that it acts in compression like a gallows
bracket, since this puts little stress on the fixing points of the brace.

With a picket style construction like that shown in the paper, all the
joints are simply planted on top of each other and fixed with a bolt
through the faces (rather like you were bolting together three lengths
of Meccano). Unlike M&T joinery, there is no inherent ability of the
joints themselves to resist racking, everything comes down to shear
loads on the bolts caused by the triangulation. Some will be in
compression and some in tension.

You can argue in the latter case that the brace used in the "wrong"
orientation will have a failure mode with the fixing tearing out of the
end of the brace (or the bolt shearing - depending on whether the bolt
or gate material is stronger). This would be true, but in the
circumstance with the "right" orientation, you simply get the same
failure at the end of the top cross member instead. The engineering
approach will often opt for the tension brace in circumstances like this
since it can be much thinner and needs no inherent stiffness. (as
someone else mentioned, even a cable would function here). The result
being a lighter (so less self loading in the first place) and cheaper to
construct gate.



--
Cheers,

John.

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