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truss uplift

truss uplift

truss uplift

(OP)
This subject may have been beaten to death before (if so please show me a link) but I have some questions about an old thread. This was posted previously: thread1066-106836
My questions regarding that thread is about the cracking around the perimeter. My experience (and review of liteuature) is that cracking from truss lift occurs at the interior, not around the perimeter. Was this thread refering to something else?
Also, regaring truss uplift, speaking about winter seasonal uplifting of trusses that is, I have a question about the use of slotted angle brackets on top of the interior walls. Except to help stabilize a long straight wall, are these really benificial?
Also, how comon is it for the truss manufacturer to specify these? (I've never seen the truss maker spec them. and would expect it would be on the building designer to know whether or not they are needed.
Any experience with this? what are good references for this problem?

RE: truss uplift

Most ceiling sheetrock cracking is caused by truss movement usually traced back to Wind, Thermal, or Installation. Here are some abreviated conclusions which I have experienced first-hand during inspections;
1.
Wind uplift due to negative pressure on roof sheathing and/or attic (balloon) pressure through vents is causing excessive truss movement which the brittle sheetrock joints cannot withstand. This is confirmed by truss calculations of the longspan truss showing an L/107 deflection of the 15' bottom chord panel lengths.
2.
Wind exposure on the large porch ceiling is creating an uplift on the bottom chord and the toe-nailed connections have broken loose from the wall. Replace toe-nailed connection with hurricane ties at the exterior wall similar to those that were installed on the porch beam.
3.
Ceiling sheetrock was installed on green lumber truss bottom chords with a moisture content in excess of 19% on a hot summer day. Sheetrock panel-butt splices have now telegraphed through showing an 1/8" ridge in a 4x8 pattern on the entire garage ceiling surface. The same sheetrock installer did both the house and garage, but the garage is not insulated and therefor subjected to more extreme with the 9 degree winter temperature.
4.
The longspan scissor trusses at the great-room entrance will deflect (breathe) during snow and temperature changes. The same sheetrocker did both houses of the same model and only yours shows sheetrock cracking at the parallel interior wall and 4'oc cracks near centerline perpendicular to trusses. Because the other owner lives there year-round, and yours is a vacation home visited infrequently, the cracks are easily explained by thermal changes. When your heat is totally off while you are away, the attic/house temperature nears an ambient 25 degrees this week. When you arrive and heat the conditioned space to 75+ degrees, the truss bottom chord warms and expands causing a truss vertical deflection. This same deflection can happen when your roof snow load goes from zero to 3' of snow. The truss has been designed to carry the load, but deflects (breathes) when it is applied. This movement will break the sheetrock corner joint next to a stationary parallel wall.
5.
When the truss was installed reversed, it caused an interior bearing to occur in a mid-panel instead of the designed-for truss joint. The dead load truss deflection created a crown in the bottom chord at that bearing which made an 'S' wave in the entire ceiling.
6.
Cambered trusses were forced down 1/2" to interior bearings and toe-nailed to top plate in lieu of vertical slotted panel stabilizing clips. After 6 months, random sheetrock cracks occured at interior walls where the truss returned upward and pulled some toe-nails out.
7.
Trusses were toe-nailed to interior wall top plates. The alluvial soil beneath the slab settled and so did SOME walls creating up to 1" gaps between the ceiling and top plate, or between the sole-plate and slab. The trusses were too-strong and pulled the interior walls off the slab and broke the sole-plate (red-head) slab connection.

RE: truss uplift

I have found similar causes but if the sheetrock is installed as per the manufactures recommendation and the joint at the wall is not nailed but allowed to float cracking at the wall will be avoided.

RE: truss uplift

What are the uplift limits at bearing supports. ?
Can't find anything in TPI.

RE: truss uplift

Uplift is limited by the connections truss to bearing, bearing to foundation, and foundation.

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