Slide 51 of 67
Use the Proper Thickness & Number of Plies
- We usually assume that the limit of the shear wall strength is determined by the strength of the sheathing fastener
- During the Northridge Earthquake, some shear walls constructed with 3/8-inch plywood tore at their inner ply seam, instead of failing at the fastener
- This is the first time this type of failure was documented
- Result: We now recommend using thicker panels with additional plies in order to allow fasteners to reach their strength limit before the sheathing prematurely tears.
- The minimum recommended thickness for wood structural panels is 15/32 of an inch.
- Both four-ply and five-ply plywood panels are commonly available for this thickness.
- At least four-ply should be used..
- Five-ply panels may be required by the prescriptive standards for some purposes.
- The more plies and layers that are used in plywood, the more defects and inner ply seams are overlapped and strengthened.
- The following panel information was furnished by a major plywood wholesaler and conforms to UBC Standard 23-2 (PS 1-83).
- 3-ply panel construction is used on 3/8 inch Structural 1, 3/8-inch C-D Exposure 1 and ½-inch C-D Exposure 1.
- 4-ply panel construction is used on ½-inch Structural 1 (Southern Pine) and 5/8-inch C-D Exposure 1
- 5-ply panel construction is used on ½-inch Structural 1 (Douglas Fir) and 5/8-inch Structural 1.
- The panel thickness shown is nominal. Use 3/8 for 11/32, ½ for 15/32 and 19/32 for 5/8-inch thickness.
- Structural 1 plywood in ½-inch thickness is normally available in 4-ply (Southern pine) panel construction because it is less expensive than the 5-ply (Douglas fir). C-D Exposure 1 allows 3-ply in 5/8-inch thickness under UBC STD. 25-9 but manufacturers typically construct the panel with 4 plies to obtain the required thickness.