February 2002, 66 pages
Roof ties can go a long way to keep the lid on.
But mind the nails.
This report investigates the systemic load path of roof framing connections in conventional, light-frame wood-roof residential construction. Research results are reported completed on the performance of heel joints, full-scale roof-to-wall connections, and individual toe-nailed roof-to-wall connections.
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- The performance of pneumatic nails is superior to common nails. This effect is primarily attributed to the nail polymer coating that adheres nail surface to surrounding wood.
- Attachment of the heel joint to the wall top plate with toe-nails improved the heel joint resistance.
- Use of light-gauge steel hurricane clips doubled the shear transfer capacity of the system to about 560 lb/joint without use of blocking between the trusses. However, the resistances of toe-nails and hurricane clips are not necessarily additive due to different stiffness characteristics of the two connection types.
- In moderate- to high-hazard areas of the United States, use of simple roof ties without additional blocking or detailing can significantly improve the shear transfer through roof diaphragm systems into shear walls in conventional residential construction and engineered wood-frame construction.
- The resistance of a toe-nailed connection in a system of metal plate connected (MPC) trusses is as much as 80 percent lower than that of an individual toe-nailed connection. This reduction is attributed to the decreased end distances in the truss heel joint that can result in premature wood splitting at the beveled end of the bottom truss chord.
See page 57 of 66 for a detailed summary of the results and conclusions.
- Conventional construction requirements for heel joints specified in current building codes should be reevaluated using joint capacity as the design basis and a minimum safety factor of 2.0.
- A prescriptive connection schedule should be developed for attachment of MPC trusses to provide lateral resistance equivalent to conventional roof systems.
See page 58 of 66 for the full set of recommendations.