It is my understanding that braces are the primary members in any timber frame that allow it to resist racking. Most often from wind. To do this effectively braces are framed in tandem, opposing each other. Thus, when the wind blows on the north gable wall (bent 1) of a frame it would be the braces on the south gable wall (bent 4) under compression that resist the racking effect. Braces were sized and framed to work in compression. Wood is strongest under compression. There are exceptions to this rule but the over whelming majority are meant for compression not tension.

Pegged or non pegged braces. Nailed vs. nonnailed. tenoned vs. untenoned. It seems these demonstrate regional building styles as well as historical transitions. Braces that are nailed throughout a barn/home (often with wire nails) represent the transition between timber framing and "stick" framing.

Why peg the braces if it doesn't work in tension? Because it helps to square the frame up during and after raising. But there are examples of barns where the braces have only stub tenons (I'm thinking of a barn from Ontario I have worked on) I imagine the builder saying:

Forget this there are just two many braces
were not going to mortise out 4 1/4" just 2
1/4" and leave it at that. No need to peg
them, they won't fall out because as soon as
one loosens up the opposing ones "catch" them.

I would thing it would be an interesting study to look at braces, purlin systems, bent spacing, etc. to discover different regional differences. through out the U.S. Ovisouly my experance is with barns from the northeast.

P Smith