roof thrust problem

I am building a 20 foot wide barn with a gambrel roof. The trusses are specially engineered so that no chords are needed (almost like a scissor truss). This allows me to use the top of the barn as a loft. But, I want to raise the height of the walls on which the trusses are supported by 18 to 24 inches so that I have extra head room on the loft level of the barn. Normally, the floor joists would stop the wall spreading caused by the thrust of the roof, but since I am raising the wall height, that is not the case anymore. Will I have a problem if I don't add any more supports elsewhere (I am not about to use flying buttresses on my barn). One potential solution I have is to use some cross bracing from the top of the wall down inward to the floor joists at an angle every so often along the wall, like:
/ / / |\\ | \\ |----\\--------- |
That way I get the extra head room, and the braces won't be in the way. Any thoughts?
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Personally I would do it just like your little illustration. I would nail the brace onto the rafter itself and down to the joist.. Only 20' wide is not much more than a small shed and the loads have to be small. I'm sure someone will come up with a more complicated way though.

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We had a large two story barn that was constructed exactly as your illustration. It was built about 1900 and is still standing very nicely. We used to load up the second story with a lot of stuff that seemed to do no harm.

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I am encouraged that a barn build this way over a hundred years ago is still standing. Do you remember how often along the wall that the braces were placed? Was it from every rafter (or truss) or more separated. I would think doing it from every rafter would be a bit overkill.
bitternut wrote:

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When we specify raised tie trusses we specify that they sit on glide shoes. These let the trusses spread without inducing lateral loads on the top of the wall.

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I don't know what glide shoes are and couldn't find a good explanation on the net...the trusses I am using are made onsite and use plywood gussets at all of the intersections. A picture can be found here:
http://www.barnplans.com/trusses.html
It's the top picture, although you can't see the truss very well at that angle. Other pictures on the site show a better picture.
Darryl Bailie wrote:

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I am using are made onsite and use plywood

your link answers your question "Engineered trusses are designed to transfer any lateral forces directly downward onto the side walls. This eliminates the need for upstairs post and beam supports"
kickstart
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You must be able to get something like this in the States
http://www.alpeng.co.uk/publications/RoofTruss04.pdf#search=%22cullen%20truss%20clips%22
Darryl

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Darryl Bailie wrote:

The knee walls have to be able to carry the lateral loads from seismic and wind loading. It's not just a question of the truss and truss connections. Balloon framing is probably the easiest way to go.
R
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In a previous post snipped-for-privacy@gmail.com wrote...

One option is to balloon frame the walls to the truss support point.
If the walls are already in place then I suggest you contact a local engineer for a solution. The building inspector, if he is alert, will most likely require an engineered solution anyway.
--
Bob Morrison, PE, SE
R L Morrison Engineering Co
  Click to see the full signature.
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