Shandy Heil, Skinner, Patricia | 1/8/2007 9:17:34 AM
Walls need to overcome uplift, sheer and over-turning. While design can be important in reducing these forces under for any given wind-speed, wind resistance will depend on proper spacing of stude, as wells and use and proper installation of connectors and shear panels during construction of the exterior (and some interior) walls.
For wood frame construction: The wood species used for framing should have a specific gravity greater than 0.49. Examples are Southern Pine, Douglas Fir-Larch, Hem-Fir, Red Oak and mixed Maple. Lumber 2x4 wall studs should be spaced 16 inches on center (o.c.); 2x6 studs can be spaced 24 inches o.c.
Plate to foundation connection: The sill or bottom plate of walls should be secured to the foundation with 5/8-inch anchor bolts (and 3x3-inch washers), embedded 7 inches in solid concrete foundation or 18 inches in a block stemwall. Bolts in stemwalls with frame floors should be spaced every 4 feet (or to code tables) and at all corners and wall openings. For slabs, bolts should be spaced closer, typically 18-24 inches apart.
Stud to plate connection: Each stud must be connected to the anchored bottom plate and to the top plates to resist the code specified uplift load; this may be achieved with hurricane hardware or with specially sized and installed structural sheathing that extends continuously from bottom plate to top plate.
Story-to-story connection: Upper and lower stories of wall framing must be tied together. This is typically done with long metal strapping used every 4 feet or specially installed and nailed sheathing.
Structural sheathing and shear walls: Exterior walls and interior load bearing walls need sufficient reinforcement to resist wind shear (racking) and lateral forces on the house. Full structural sheathing of exterior walls is recommended; typically 7/16-inch or 15/32-inch plywood or OSB. Blocking is required to provide framing behind all panel seams; nail spacing of 6 in. may meet shear loads, closer when the sheathing is used for both shear and uplift loads.
Narrow wall segments next to large openings (particularly garage doors) may require commercial engineered shear wall segments for increased lateral resistance. When designing your home, try to avoid placing window and doors near corners.
Corner hold-downs: Additional hardware called “hold-downs” are needed at the ends of each wall segment (typically at each outer corner of the house) to prevent tip-over and resist the higher forces created when wind flows around a corner. If installed on a properly nailed 3-stud corner, one hold-down can serve both walls.
For masonry construction: Concrete block walls should be connected to the foundation with full-height steel rebar reinforcement at least every 4 feet. Vertical rebar reinforcement is also needed at all corners and both sides of openings.
For SIPS, ICF, ISPS and other building systems: The manufacturer or an engineer should verify that the building system is engineered and attached for your design wind speed. As with any building system, installation according to manufacturer’s or design specifications is essential.