Wood Cantilever Diaphragms

In modern construction, wood-framed mid-rise buildings tend to have larger and numerous window and door openings and irregular exterior wall layouts. This construction trend translates into reduced area for lateral force-resisting systems. In some cases where a remaining exterior wall area doesn’t meet the requirement, this wall cannot be used as part of the lateral force-resisting system, and the floor diaphragm above this wall is classified as a cantilever or open-front structure. When a diaphragm has a cantilever on two opposite edges, it is classified as a double open-front or corridor-only shear wall diaphragm. Cantilever diaphragms are also encountered in old existing buildings with mortared stone foundation walls where exterior walls cannot be efficiently anchored into the foundation. Due to its flexibility, wood cantilever diaphragm is susceptible to deflection caused by wind or seismic loads that can damage the building structure. Torsion can amplify this deflection if the building has an unsymmetrical layout.

In buildings with concrete foundation walls, placing steel moment frames or wood portal frames around some openings can solve the deflection issue of the cantilever diaphragm. However, in existing buildings with mortared stone foundation walls, engineers should use their creativity to come up with an effective anchorage solution. With a few new mid-rise wood-framed buildings designed and projects involving the remodeling of mid-rise wood-framed commercial buildings to residential buildings, Team Engineering has developed experience and creativity in solving cantilever diaphragm issues.

Blaise Imbembe, PE

Staff Engineer