Shear Strength of the fasteners will control the design. The plywood need only mobilize the fasteners. You can check the distributed load at any fastener against published values of the plywood strength; however, I don't know of any published data showing the strength of plywood parallel to the plies as this is not a typical loading mode for which tested data are sought. If the load is high enough to mobilize a tensile load in the fasteners, then enough deformation has occurred such that all bets are off with regard to stress distribution anyway.
In short, determine the shear load and design using 1/2" nominal plywood thickness with enough fasteners to handle the shear as well as any safety factor needed. [sig][/sig]
The "Canadian Wood-Frame House Construction" book published by the Canadian Mortgage and Housing Corporation (CMHC), phone (613) 748-2367 provides standard details for wood gussets for trusses of various span and loading.
Current codes use truss fasteners (metal) and the provisions for plywood gussets are obscure. There is a lot of information with regard to maximum stresses in plywood and some references to gussets such that the gusset can be sized, but the nailing requirements are obscure and are not clear.
Another source is the Council of Forest Industries of B.C. (COFI). COFI has a lot of plywood literature, which was free (in 1986) the phone number in 1986 was (604) 684-0211.
I recommend telephoning the Canadian Wood Council for advice, the CWC is very helpful and may better direct you. CWC tel: (613) 731-7899 or toll free 1-800-463-5091.
for downloadable manual "Plywood Design Fundamentals" which will help you size the gusset.
For fastening requirements I recommend the Canadian Wood Council (CWC) book "Wood Design Manual". Section 7.2 of the manual"Nails and Spikes" provides design guidelines for fastening of the gusset.
Essentially the design process is two fold. First size the gusset according to strength requirements. Then using the CWC manual determine the nailing requirements. The nailing requirements are based on the minimum thickness of plywood, nails, and truss member grade. The Wood Design Manual will provide all the information required for designing wood trusses with either metal truss plates or plywood gussets. [sig][/sig]
Hi. Once you know the values of the loads at the intersecting members you are planning on "gusseting", you can compute the number of nails or fasteners that will resist those loads following the requirements on the NDS; these values will be governed by various factors, of which, the thickness of the plywood will come into play. You must also take into consideration, that the size of the plywood gusset in each direction will be governed by the shear capacity of the material in plf's. You may use the tables for vertical diaphragms of the UBC to see what the allowable plf is for various plywood/lumber combinations. Also, keep in mind that nails placed too close together may split the lumber they are supposed to join.
