Shear Bracing Alert

(originally circulated on paper January 25, 1994)

Recent interpretations of the British Columbia Building Code requirements for shear bracing of wood frame multi-family residential dwellings are putting the acoustical isolation of party walls at jeopardy.

A common party wall design using 16mm gypsum wallboard on the outside face of two rows of wood studs insulated with R12 batt and separated by a 25mm airspace (Type 27 wall in Table A-9.10.3.A of the B.C. Building Code) with a nominal rating of STC 57 is being compromised by the application of plywood on the inside faces of one or both rows of wood studs. Tests performed by ourselves and others indicate that this practice can reduce the field acoustical performance of this partition to as low as FSTC 44, well below the Building Code minimum of STC 50 (Section 9.11).

The acoustical ramifications of this practice are that occupant complaints will rise dramatically.

We believe that it is important to advise our fellow design professionals of this problem. Design modifications must be developed to implement seismic bracing and other code requirements without reducing the acoustical isolation between dwelling units that occupants have come to expect.

This noise isolation problem must be addressed by our industry. Call or e-mail us for assistance in developing design alternatives.

Douglas J. Whicker, P.Eng.