USDA Truss Design, your opinion ?

[Arky]

The following link is a picture of a 24' span nailed truss design from the Department of Argriculture.
What is your opinion of this design for a home built nailed truss to span the indicated 24 feet ?
(you may have to open the image in Windows picture viewer to see details)

Truss (http://images43.fotki.com/v1328/photos/1/1404952/6773112/Truss-vi.jpg)

Thanks,
Arky

[Don_P]

Attached is a similar design that I think is better. It is a glued and nailed truss of the same Fink style. The truss is designed with the nails doing the work, so there are enough to bear the entire load but in reality the glue holds it together stiffly and the nails are backup. It then doesn't suffer as much from the problem of nails possibly loosening over time.
This is from "Design of Building Trusses" by James Ambrose, maybe check through interlibrary loan. I got a used copy off the 'net. It does not give a detailed assembly drawing like the usda prints but does give the engineering behind sizing one. The USDA plans to my knowledge were done on an early truss program, Perdue Plane Structures Analyzer. You need to check loads carefully on the print. They also seem to me to have eccentric joints in many cases, which may be fine for the loads but I wonder if they will split members as the joints rotate a little if loaded heavily. I'm, just a nail banger, don't know. A true truss in theory rotates freely at every connection on frictionless pins. In reality we build with some amount of "fixity" as long as that doesn't tear the joint apart no worries. As the connections become more eccentric, off of lines drawn down the axis of each member, the potential for that type of leverage increases. If its for a residence I'd get an engineer to look the design over or design one for you.