During 1952 and 1953, the Timber Mechanics Section of the Oregon Forest Products Laboratory built and tested eight lumber-sheathed and two plywood-sheathed roof diaphragms 20 by 60-ft in size, in order to determine the strength and stiffness at various load levels when the following structural elements were investigated: (1) ⅜-in. plywood sheathing; (2) 1.-in. lumber sheathing used green and dry and applied in a longitudinal or diagonal orientation; (3) different nailing patterns; (4) variations in boundary member construction; (5) presence or absence of bridging and blocking; (6) different corner connections; (7) use of metal corrugated fasteners; and (8) presence or absence of skylight openings.
A pin-connected steel-and-timber truss was used in testing the diaphragms. Loads were applied by two 30-ton hydraulic jacks connected in unison. The load was applied in steps to a predetermined level, then released to a zero load; the same procedure was followed for each new load level. In some of the diaphragms, a repetitive loading procedure was employed to determine the load level where permanent set occurred.
In addition to deflection readings, buckling in the framing members, sheathing movement, and strains across joints and within members of the tension chord and endposts were measured.
The summary of test results indicates that the strength and stiffness of roof diaphragms can be appreciably influenced by altering any of the test variables included in this testing program.