Nailed Fir Plywood Gussets for Sawn Lumber Three Hinge Arches

Glued laminated wood construction is economical, versatile, and easy to erect for farm buildings, and fir plywood is an excellent finish material for arch rafter buildings, but competition from other materials has limited its acceptance. In this study a 3-hinge arch structural system using fir plywood’s mechanical properties for the transfer and distribution of loads was developed. To transfer the bending moment around the haunch, fir plywood tension gussets were used, because plywood is stronger in tension than in compression, and because tension gussets simplify framing and fabrication of the arches. Since gluing requires dry lumber and experienced workmen, nailed construction was chosen. Based on the expected snow load on the Ontario prairie, a total design load of 30 psf was considered reasonable. The arches to be spaced 2 feet apart were designed to carry a uniformly distributed load of 60 pounds per lineal foot. To obtain balanced construction, a gusset plate was placed on each side of the framing member. An estimate of bending moments indicated that 5/16 or 3/8 fir plywood could readily carry the tension loads. Six haunch sections were tested. Assembly of the frames was done by a student with no carpentry experience using No. 2 spruce dimension green lumber with numerous knots and 1 I/2-inch galvanized common nails for attaching the gusset plate. Five sawn lumber arches and one glulam arch were erected by three men using a test jig to prevent lateral buckling of the specimen and to absorb the thrust from the loaded frame. The six arches tested were 18, 24, 32, and 38-foot sawn lumber arches designed for use in farm buildings. All arches tested failed under loading in the lumber members; all but one of the failures were due to bending in the lumber members. The maximum bending moment in the arch occurs at the junction of the leg and spanning members. Thus the strength of the arches tested was determined by the quality of the lumber used, not by the nailed gussets. Common lumber is not stress-graded; however, the use of common lumber will be acceptable in this type of construction as long as the fabricator follows directions in the Plywood Manufacturers Association pamphlet for selecting lumber. This preliminary work indicates that the concept of nailed plywood gussets for sawn lumber 3-hinge arches is practical and competitive with types of framing for farm buildings such as turkey shelters.

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