Tuesday, March 22, 2011

Hanging from the Rafters...

OK! That is how I would describe the fit and function of the rafter assembly fixture. Looking at the pictures you get the idea that there are several pieces that must first fit together.  Then realize that  when assembled they must fit over the top truss chords and inside to the cleat and seat that will brace the roof to the truss.

Above -  Primary and Secondary Rafters meeting at the peak (Collar Tie not installed yet). The Secondary Rafters act as knee braces from the roof trusses to the bridge truss to control racking from side to side forces.
Above -  All the truss pieces assembled in the fixture. The large squares represent the top chords of the trusses that the notches in the horizontal Cross Brace must fit. At the lower ends of the inside, Secondary Rafters, you can see blocks that represent the cleat and cleat seat. Point where these will be fastened to control racking from side to side forces. Also,  there is a 1/2" Carriage boltsfastening the Cross Brace where it crosses over each rafter piece.
Above - And here you have it! The Collar Tie, at the top, keeps the butt joints of the Primary and Secondary Rafters lined up. In  this picture you can also see mortise slots in the horizontal Cross Brace. These are where the Lateral Braces will be installed to provide strength in the length wise direction of the bridge.
Above - The roof system is complex to say the least. See the Lateral Braces installed into mortises. Look close at the end of the piece right here and you can see the 90 degree shoulder that is cut to stop the piece in the mortise. Tricky! The roof rafters are 8 feet on center. So how long would each Lateral Brace be? By the way these are 1" x 6" boards.

Above - This is how the rafters will be installed and mated to the Lateral Braces. The Primary Rafters are removed and added later.

Puzzler - Why do the Primary Rafters need to be removed and installed later?

Tech Vocab - Lateral Brace - Racking - Mortise - Knee Brace

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