huangyhg

scissor truss horizontal movement requirements
if a scissor truss comes back from the fabricator with a requirement for horizontal movement of say 0.5", is it ok to allow the top of the wall to translate out 0.5", assuming that this movement can be accomodated. if this movement occurs is the horizontal reaction then released?
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if the movement is accomodated then there is no outward force at the support.
if it is a stud wall, no problem, a brick wall may have issues.
is the truss manufacturer saying that you have 0.5" at each support or 0.5" total? if total, does that mean that you have 0.25" at each support?
check the height over deflection ratio and compare it with the allowable movement for various wall construction material.
connect2
i think i would be careful about allowing a scissor truss to horizontally deflect the top of a wall especially if the wall is masonry. there will be horizontal delection particularly if the truss is light and flexible. to ensure the truss does not spread when loaded it would have to be very stiff and rigid. i think the top of the wall would deflect and then continue to take horizontal force.
if the truss is stiff enough to ensure it will not spread then everything is ok. if things are potentially flexable and causing spread deflections you could consider using a concrete or steel ring beam around on the top of the wall perimeter. this ring beam would support thr horizontal loads from the truss.
another option would be run a beam under the apex of the trusses which would support the tusses from spreading.
simpson makes scissor truss slip connections. you can put one of these on one end of the truss, load up the roof with all the dead loads, allow the truss to spread, then install the screws or nails into the connector. this way at least your dead load deflection will not push the wall out laterally. you still need to accomodate the live load deflections however.
i would not worry about 1/4" deflection at the top of each wall, whether it is a wood stud wall or a cmu wall. 1/4" is not much movement at the top of an 8'-0" or taller wall. however, if the supporting wall is very short--like a stub wall that sits on roof framing, them perhaps this is a concern.
daveatkins

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