Perhaps I should cut directly to the chase! I'm wrestling with this question because I'm engineering the joinery for a timberframed garage/loft design I have been working on for... Oh, it seems like a really, really, long, long time now. It's a relatively simple frame: 24' x 28', principle rafter/common purlin, 12/12 pitch roof, kingpost truss bent plan with a 24" kneewall.

I've been using the TF Guild Joinery & Design Workbook as well as other texts, to educate myself on just what forces are at work within my frame, and making calculations to assure that my joinery can take the forces that my building codes require. Everything has been going great... that is until I started trying to resolve the forces being carried by the knee braces. While I can easily calculate the roof forces and their distribution through my rafters, purlins, posts and struts... I'm at a loss when I try to resolve what portion of total load is being carried by my kneebraces. Short of an computer-generated FEA analysis, is there any way for me to determine these forces on my own? The method for determining the load on the kneebraces seems to change, depending on who you ask.

I've already had an engineer stamp my frame based on my the layout and sizing of the beams, but it's up to me to make sure that the joints are up to the task. I've got most of the joints worked out and within acceptable values... with the exception of the kneebraces and attached members. I'd like to have some accurate numbers to deal with, rather than rely completely on good juju!

I'm sure that the lawyers would advise me never to rely too heavily on good juju.

Specifically, my design uses a strut between the principal rafter and tie beam to reduce the roof load at the end posts. This strut, in worst case scenario on the center bent, transfers 8721 lbs of the roof load to the 8" x 10" tie beam at a distance of 3'4" from it's end, and offset 8"OC from the knee brace below. I have been told that the entire 8721 lbs should be transfered directly to the brace... which sounds to me like it may cause some problems with bending stresses on the post, as well as requiring one honkin' big brace. I've seen this strut solution used on many larger frames without abnormally large braces so I think that this approach must be in error.

That being said, my trees have been chopped and delivered to the sawmill... (I got pretty excited when I got my engineering stamp), and I'm trying to prepare my final cut-list for the sawmill this week.

My engineer tells me that he thinks the frame will be fine, (I added the strut AFTER his analysis, to reduce the end loads) and honestly I don't think the frame will ever experience forces anywhere close to those required for engineering (the stick-framed walls and roof absorbing much of the load). Still, I'd like to design the frame by-the-book as this is my first, but definitely not my last, timber frame!

[This message has been edited by bartpop (edited 10-25-99).]