Shear membranes in wood frame party walls

Abstract

On a recent project, Brown Strachan had recommended a standard party wall construction consisting of 2×6 wood studs @ 16" o.c., separated by a 1" air space, with fibreglass batt insulation in the stud cavities, and one layer of 5/8" gypsum wall board (gwb) on each outside face. This wall has a typical sound transmission class rating of STC 57. Late in the design stage, however, the structural consultant added a requirement for 1/2" plywood on both inside faces of some walls. The plate spacing was increased to maintain a 1" clear space between the two inside plywood faces. A Gypsum Association test shows STC 57 with 1/2" Type X gwb on the inside face of studs @ 24" o.c. On the basis of this test, the architects concluded that the change was acceptable, as the Code required a STC 50 rating. Geiger & Hamme had tested a similar wall consisting of 5/8" drywall on the inside of studs spaced 16" o.c., and obtained a rating of STC 45. Obviously, the bending stiffness of the inside face had a major effect on the net stiffness of the air trapped between the two inside layers, spaced 1" apart. Plywood is about eight times as stiff as gwb, and since the stiffness of the inside plate is the key factor in determining the stiffness of the air, a problem was clearly indicated. Potential treatment options included resilient channel, extra layers of drywall, and holes in the plywood membrane which would act to vent air in the 1" airspace to the fibreglass filled stud cavity. A mathematical model was used to study the effects of the various treatment options, and field measurements were made to determine the treatment results