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Our Experience with Preassembly
Pre-Assemble Roof Truss Test
We pre-assembled 7 roof trusses together on the ground. Then we cut one bottom chord of the truss. We lifted the pack of trusses about 6” off the ground and had several people stand on the bottom chord. The cut truss did not separate or fail. The people were removed and the pack of trusses was installed on the building. An engineering repair was made to the bottom chord.
A properly braced pack of trusses provided redundancy to the roof system. Each pack of trusses is braced.
Strong Wind Test
By coincidence we had about 18,000 square feet of roof trusses installed on Friday afternoon. The trusses were on top of a four story building. Nearby, another contractor had 11 single story residential units (about 1800 sq ft each) with roof trusses in the same condition, installed without plywood sheathing. Over the weekend the site experienced 60 mph winds. On Monday morning we examined our truss system and found a single broken 1x4 brace. The other contractor had collapsed roof trusses on 4 buildings and two building in which the trusses were blown off the buildings onto the ground. The wind pressure would be about 25% higher on the 4 story building than single story structures.
In1996, we had 8000 sq ft of erected and braced roof trusses, with no plywood sheathing on a 2 story (23” high off the ground to truss bearing) commercial building. The building was out in an open area. Hurricane Fran came The building steel structure was found to be almost 1” out of plum due to the wind forces but no trusses were damaged of blown out of plumb.