In hurricane and earthquake country, remodelers and designers are faced with creating structures that can withstand high wind and seismic forces. Although building codes are established to help regulate the design process for these types of structures, many of the recommendations are the minimum requirements — leaving homeowners in high-wind zones left with picture-perfect homes, but disasters when faced with hurricanes and earthquakes.
According to The Engineered Wood Association (APA), a nonprofit trade association that focuses on helping the industry create structural wood products, remodelers need to have a basic understanding of how lateral loads act on wood framing systems and how construction detailing and fasteners affect the ultimate performance of a structure. Remodelers and designers can use these principles and design recommendations to ensure strength, quality and safety in residential construction located in high wind and seismic zones.
Strenghtening the ‘weak‘ spots
Over the past few years, there has been a rise in education on how to design and build structures to withstand these types of conditions. With this, manufacturers are producing products that coincide with this trend. Simpson Strong-Tie has stated that a good way to avoid major damage in a hurricane is to design or retrofit a home with connectors at common stress points. Solid connections at stress points such as the foundation, walls, floors and roof, provide greater safety through better structural support and improved resistance to the damaging effects of storms, high winds and hurricanes.
The Simpson Strong-Tie connectors are steel components designed to connect and strengthen the joints within the frame of the house. “Joints that use connectors carry a much higher wind load than traditional joints that have been nailed,” says Kristin Lincoln, vice president of marketing for Simpson Strong-Tie. “A toe-nailed connection has a load capacity of 300 lbs., while a connector load capacity can handle up to 1,055 lbs.”
Including a system of these connectors helps absorb and resist the pressures exerted by high winds, earthquakes, floods or snow and transfers external forces from the frame through the foundation. The redistribution of outside forces is known as the continuous load path.
Continuous load path
A load path is made up of a chain of structural components such as roof truss members, metal connectors, nails, anchor bolts, wall studs and floor joists to name a few. When a home is hit by a hurricane or tornado, wind loads are applied to the exterior walls and roof.
These wind loads must be transferred from structural component to structural component until they are transferred into the earth. These various structural components make up a chain creating a continuous load path.
If the load path chain remains continuous and unbroken, no structural damage will occur. If there is a break in this chain, damage will occur.
Wind creates sliding, overturning and uplifting forces on a structure. Uplift forces, in particular, cause much of the damage seen in windstorms.
Damage can happen anywhere along the chain of structural components. All it takes is one “weak link.” These “weak links” are more likely to occur at the connections between components, not within the components themselves. The design and construction of these connections must be considered carefully, keeping in mind the basic concepts of how the load path chain works.
There are several major connection points common to houses. There are:
• Foundation to floor system
• Floor system to exterior bearing wall system
• Connections within exterior bearing wall system
• Exterior wall system to roof structure system
• Roof structure system to roof deck
Because its effects are not immediately seen, designing and building for wind uplift takes more thought. The concept of a continuous load path must go into the design and construction of each and every connection when the structure is located in an area with the potential of high winds.
“When designing a home for wind uplift in a high-wind zone, fully sheathing it is key to a well-engineered structure,” says Ed Keith, P.E. senior engineer for the APA.
The need for a structual engineer
“The building of a home in a high-wind region should require the aid of an architect or structural engineer,” explains Keith. “These areas that are effected by the proposal made to the International Code Committee to reduce the trigger from 110 to 100 mph in hurricane-prone regions will dictate when the IBC (design) must be used as opposed to the IRC (prescriptive).” Keith goes on to add, first, the phrase “hurricane-prone” is not defined by the code proper so it will be up to the state/county/local jurisdiction to translate just what that means. Ostensibly, it would be the Southeast, Atlantic coast, the Gulf coast, and Hawaii. Actually, politics will determine which areas are hurricane prone and will have to spend the additional monies to build safer structures.
“The extra dollars required to build the structure if designed will put the purchase of a home out of the reach of some people and that has big political impact,” comments Keith. “So far we have opted to support equally with research and development.”
According to FEMA, homes that are designed without regard to proper engineering will produce a reasonable wind-resistant structure. However, it is unlikely that the home will withstand severe windstorms without moderate to severe damage. The advantages of including wind-resistant design methods are to assure that the home can withstand more than the average wind conditions without damage.
“Conventional construction is not an engineered solution and was not developed around the concept of total building wind resistance; therefore, technical details will most likely be missing,” adds Keith. “This is why we (the APA) are pushing for a reduction in the wind-load requirements.”
Structual engineers can also help design the stud walls for gravity, wind-load conditions and assure sufficient stiffness in the walls to avoid deflections. The engineers will design the overall building stability system to resist failure from sliding or racking of the entire framework for wind load conditions.
Structural wood panel wall sheathing, whether plywood or oriented strand board (OSB), is a major element in a high-quality, engineered wood home. OSB consists of wood strands bonded with waterproof adhesives to form a mat which is then pressed under high pressure and temperature to create the finished product.
OSB is widely used as construction sheathing and as the web material for wood I-joists. A house with walls sheathed with OSB or plywood will provide much greater resistance to high wind and seismic conditions and will be four times stronger than one with only foam sheathing — providing a shear capacity of 350 lbs./ft.
There are many other advantages of a fully sheathed home including design flexibility, meeting code requirements and providing an excellent noise barrier (see pg. 39 for detailed advantages).
The new Steel Strong-Wall by Simpson Strong-Tie is an engineered composite of steel and wood and it features loads that are between two and three times higher than the original strong wall. “The unique design resists distortion of the center section and results in sustained vertical load capacity,” explains Lincoln. “Input from framers and installers was important to the development of the new shearwall, which now includes several features to simplify installation.”
The design includes pre-attached wood studs to attach interior and exterior finishes, two easily accessible anchor bolts, fewer top-of-wall screws and numerous predrilled holes for mechanical needs.
Made of Southern pine plywood, Georgia-Pacific’s Plytanium Sheathing is APA-rated for limited exposure to the elements during construction.
According to Georgia-Pacific these panels should be installed with an 8-ft. dimension perpendicular to supports, and in compliance with panel span rating. Space panel edges 1/8 in. apart to prevent buckling.
Narrow wall bracing method
The APA is always in development of better ways of designing and building. One technique that the association is fully supporting is the narrow wall bracing method. Designed to deliver a better built home, especially in high-wind areas, this method also gives design flexibility for the remodeler and homeowner.
“All residential remodelers are familiar with corner bracing,” says Keith. “What they may not realize is that Section R602.10.4 of the International Residential Code requires 4-ft.-wide bracing segments near the corners of buildings and at prescribed intermediate points.” He goes on to say that with the rapid adoption of the new IRC, enforcement of this requirement is becoming widespread.
This code also requires a 48-in. bracing wall next to garage openings. But many home designs show wall segments adjacent to the garage opening that are as narrow as 16 in.
“When the home is fully sheathed with OSB or plywood and the narrow wall segment is properly installed, braced garage wall sections can be as narrow as 16 in. and holddowns are not required. Keith adds that, “It’s a whole-house solution that delivers strength at the garage door openings, and all the way around the house.”
The two primary components to this method are to fully sheath the exterior walls with structural wood panels and install a header that extends beyond the garage opening to the corner framing.