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RE: scissor trusses - drift

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Getting with the truss engineer is the first step. You have to get their horizontal deflection/movement in their truss, which must be limited by you the EOR. Their system has to be compatible with your system. There is mostly likely going to be a gyp ceiling if it is a scissor truss with a vaulted ceiling, and another issue is you want to prevent movement to prevent cracking in the gyp ceiling. They may need to alter their design such as truss spacing or bumping up members to 2x6s, adding webs, etc, to conform to your requirements. This will depend a lot of what interior and exterior finishes you have, and if the drift is compatible with those materials...
I am sure they design the trusses as pinned and roller, so you don't have any horizontal restraint. That does not mean it won't move horizontally, but it does mean you don't have to resist it, just have to plan for it. And you don't want a horizontal slip connection because you have to transfer lateral wind loads from the walls to the roof diaphgragm...
I don't see you designing a wood wall to EVER cantilever up from a foundation, unless it is a balloon framed parapet wall that has a back span or something like that. Even doing a CMU wall to cantilever up from the footing would be very tough at 12 feet.
Andrew Kester, PE
Lake Mary, FL
N:Andrew Kester;PE
FN:Andrew Kester, PE