Design for Wind Resistance
Buildings overturned. Roofs lifted. Structures separated from the foundation. Entire buildings levelled. The aftermath of a major storm is a sobering sight, and there's no denying that catastrophic weather events, such as EF-5 tornadoes, are incredibly destructive.
The good news is that these events are statistically rare. In fact, 95 percent of tornadoes are weak enough to be withstood by a well-built structure. With tornado season starting in spring and hurricane season right behind it, wind-resistant construction is a timely topic for designers.
Impacts of the most common high-wind events are easily mitigated by a few wind-resistant construction techniques. "A wind-resistant home costs a little more than a code-minimum home, but it can be several times stronger at resisting wind forces," says Bryan Readling, PE, of APA. "These building shell reinforcements typically cost less than $1000, including labor, for an average-sized home."
Wind as Lateral Force
Correct lateral design is essential to wind-resistant construction. Wind can act on the structure from any direction, so the design must withstand lateral forces in two directions at right angles to each other.
Wind-resistant structures are designed to handle lateral forces acting along both the length and the width of the structure, as well as an uplift vertical force, in addition to the obvious downward vertical load path. The forces, acting in all directions, act on every element, and every connection between elements, of the structure. (For more information on lateral design, see Introduction to Lateral Design, Form X305.)
Fully sheathed exterior walls and roof and floor diaphragms of plywood or OSB that are properly connected to each other and to the foundation provide strong and reliable resistance to lateral forces exerted by high wind activity.
As Strong as the Weakest Component
Wind-resistant design is dependent upon all of its combined factors. Attention to correct design and installation of framing, foundation, shear walls, diaphragms, and inter-element fastening details is the key to a wind-resistant structure.
Good connections are especially important. A shear wall or diaphragm is not effective on its own in resisting lateral load effects but must be a part of a system. A roof diaphragm must transfer lateral loads to shear walls, and the shear walls must transfer this load into the foundation.
Common Vulnerabilities to Consider
High wind places stress on connections, so it's not surprising that poor connections are the single most common reason for failure during wind events. During tornadoes, inadequate roof-to-wall and wall-to-foundation connections are among the most frequent causes of structural failure. With hurricanes, it's slightly different: loss of roofing and sheathing due to improper connection detailing, including inadequate fastening of sheathing to supporting members, very frequently causes structural failure.
Another common vulnerability: breaches in the building envelope. Once a breach has occurred, the entire structure is vulnerable to failure due to internal pressure. Notorious weaknesses in the envelope include gable-end walls lacking structural sheathing, large openings like windows or garage doors, and any area where inadequate or poorly attached sheathing, non-structural sheathing, and/or low-strength cladding systems are used.
General Recommendations for Wind-Resistant Construction
Designing a wind-resistant structure does not require significant labor or cost increase over conventional construction. "It is more about getting the details right for providing load-path continuity than adding significant product costs," explains Readling. "A few more strategically placed nails, and properly lapping structural wall sheathing over joints and other discontinuities in the wall framing, can drastically improve the chances a home will survive severe wind events."
Ten Techniques for Wind Resistance
- Nail wall sheathing with 8d common (0.131 inches X 2-1/2 inches) nails at 4 inches on center at all edges of plywood or OSB wall sheathing and at 6 inches on center in intermediate framing for enhanced resistance to negative wind pressure.
- Avoid staples because they offer less resistance to blow-off than nails. A larger number of staples is required in order to achieve a level of wind resistance that is offered by the use of nails.
- Use deformed shank nails to improve the resistance of sheathing to negative pressure.
- Sheath gable end walls with plywood or OSB. Tie gable end walls back to the structure.
- For roof framing to wall connection, use a light-gauge metal uplift connector attached on the exterior (sheathing side) of the exterior walls.
- Nail roof sheathing with 8d ring shank or deformed shank (0.131 inch X 2-1/2 inches) nails at 4 inches on center along the edges of plywood or OSB roof sheathing and 6 inches on center along the intermediate roof framing.
- Nail upper story sheathing and lower story sheathing into common wood structural panel Rim Board®. The most effective way to provide lateral and, in some cases, uplift load continuity is to attach adjacent wall sheathing panels to one another over common framing.
- Continuously sheath all walls with plywood or OSB, including areas around openings for windows and doors.
- Extend wood structural panel sheathing at the bottom of the wall to lap the sill plate. The connection of the wall sheathing to the sill plate is important because this is where the uplift forces are transferred into the sill plate and into the foundation through the anchor bolts.
- Space 1/2-inch anchor bolts 32 to 48 inches on center with 0.229-inch X 3-inch X 3-inch slotted square plate washers at the wall to sill plate intersection.
For more detailed technical information on designing for wind, consult APA System Report SR-101: Design for Combined Shear and Uplift from Wind, Form SR-101.
More on Wind Resistance
Other factors to consider include building size and shape, exposure category, roof slope, openings in the building envelope, and design wind speed.
Large openings: Use windows and doors that are rated for high wind and impact damage. Picture windows, sliding glass doors, garage doors, and other large openings are extremely vulnerable to damage in high wind events. Not only are they easy targets for wind-borne debris, but their large surface area is acted upon with a commensurate large force during wind events.
Reinforce vulnerabilities: When storms are imminent, reinforce or protect large openings and weak areas. For more information, consult APA's Hurricane Shutter Designs.
Use hip roofs: Hip roofs perform better than gable roof styles during high-wind events. This is because hip roofs are more aerodynamic and do a better job of supporting the top of the exterior walls as compared to gable roofs.
Include safe shelter: Consider adding a safe room or a basement for sheltering building occupants from dangers associated with hurricanes and tornados.