Hi Mo & Mark,

This is an interesting question and as you would expect from me no straightforward answer.

It would be important to look at the degrees of freedom possible for the components that are attached to both ends of the brace and to consider what might happen if excessive thrust or tension was developed in the brace.

For example, in a classic crossframe (bent) where a pegged jowl is present then any upward levering force developed by the brace due to the application of wind forces would tend to try and lift the tie beam off its seat. To keep the joint together would require that the upward component of the force developed in the brace would need to be resisted by the peg. Where no jowl or peg is employed then the tie beam could possibly be lifted off its (dovetail) seat and would instead need to rely on the downward application of dead load as applied by the roof to help keep the tie beam firmly on its seat. The possibility of levering up an unsecured tie beam is very real.

Early English and later American timber frame design attempted to get round this problem by using dropped (i.e. enters the post under the plate) tie beams that employ housed joints and this is quite effective at resisting vertical forces generated by braces.

Medieval houses rarely employ 45 degree braces and instead attempt to align the forces generated in the brace more closley to the component to which the brace joins thus typically one might see curved braces being introduced which would also introduce an element of springiness into the equation due to the bending moments that would now be generated in a curved brace.

In reality we can observe that people do not tend to live in open frames and instead employ wall infill or cladding systems that take over much of the work thought to be done by the braces.

Many old houses are seen to have lost a significant portion of their original braces or have them cut through to make door openings and no great damage seems to result provided the wall that remains is still sufficiently stiff to resist racking.

Braces don't just come in the Jack Sobon garden shed variety - they can be highly decorative and include such standards as ogee, passing, foot, arch type and decorative quatrefoil panel braces all of which will introduce their own design peculiarities when employed in combination with any particular type of frame design.

Finally some structures don't have any overt form of bracing. Check out the Minka farmhouse currently being dismantled in Japan which is an earthquake zone.

Go figure !

Regards

Ken Hume P.Eng.
http://www.kfhume.freeserve.co.uk