March 03, 2017

Wind Resistant Building Design

The lateral (horizontal) and uplift forces caused by thunderstorms, tornadoes, and hurricanes attack the integrity of buildings and can damage structures with flying debris. A wind resistant building design protects a structure by transferring the lateral forces that attack the walls and diaphragms (roof, floor, and shear walls) towards the foundation and ultimately into the ground. Wind resistant design also prevents damage to the exterior of the building from flying debris.

A structure's ability to resist lateral and uplift loads depends on a building having a continuous load path. A continuous load path uses wood, metal connectors, fasteners and shear walls* to connect the roof, walls, floors and foundation. The continuous load path ensures that when a load or force attacks a building, the load will move toward the foundation and into the ground. Without a strong continuous load path that holds together the roof, walls, floors and foundation, a building can fail or collapse during extreme winds.

Wind resistant building must also consider the specific elements along the continuous load path. The roof, walls, and floors must individually have the strength to handle all the vertical and lateral loads during severe weather. A wind resistant building design will ensure the integrity of the building, and the safety of those inside, during hurricanes, tornados and severe thunderstorms.  

Roof Construction of a Wind Resistant Building Design

Building failures during high-wind events often begin with damage to the roof.  Shingles or tiles blow from the roof sheathing, the roof sheathing rips from the roof framing, and the roof framing tears from the supporting walls. A roof’s main function in a continuous load path is as a horizontal diaphragm that transfers the loads imposed by heavy winds to the supporting walls below.  The roof sheathing is the first structural component in the load path between the roof system and the foundation. The sheathing works in conjunction with the roof framing to transfer lateral loads to the building’s shear walls. According to FEMA’s Building Framing Systems and Best Practices, common nails can be used to connect sheathing to supporting components in regions where basic wind speeds are less than 100 mph. Ring-shank nails are required in higher-wind regions. Wood nails are recommended in the eaves and corner zones of the roof, where winds can create large uplifts. Roof framing is the next building element found within the load path.The rafters of a roof’s frame must be sized to resist the weight of the roof system, and also the loads caused by wind and snow. The roof framing must also transfer lateral loads to the shear walls below. It is essential in wind resistant roof design that the roof sheathing and framing are constructed and sized for the potential wind forces of the specific region.

Wall Construction of a Wind Resistant Building Design

Exterior walls of a building must resist wind and provide stability for the entire structure. For a continuous load path design, the walls must be anchored to the foundation. According to FEMA, in regions impacted by heavy, dangerous winds, walls constructed from reinforced concrete or concrete masonry units (CMU) are common. However, concrete and masonry walls lack the thermal performance required by the IRC and IBC, and typically need added insulation.  A better option over concrete and masonry walls are insulated concrete block wall sections. Insulated concrete block wall sections contain thermal and structural features within a single, reinforced concrete-wall section.  The Bautex Wall System insulated concrete blocks have the thermal performance along with the strength to resist heavy winds. The Bautex Blocks also meet the Federal Emergency Management Agency FEMA 320 and FEMA 361 guidelines in storm zones with possible wind speeds up to 250 miles per hour (Zone IV, southeastern states).  

Flying debris is also a threat during thunderstorms, tornadoes and hurricanes. It can damage the exterior of a structure, and injure the occupants of the building. The Wind Engineering Research Center at Texas Tech University recently concluded that concrete wall construction is preferred over frame walls for reducing damage caused by flying debris. The Bautex Block insulated concrete wall system has the strength and mass to resist the impact to wind driven debris at speeds greater than 100 mph. Tests on the Bautex Blocks were done at the Wind Science and Engineering Research Center Debris Impact Test Facility and the National Wind Institute at Texas Tech University in Lubbock, Texas. The Blocks were tested for horizontal debris impact. The Bautex Blocks single concrete integrated assembly met or exceeded the FEMA standards for debris impact**.

Floor Construction of a Wind Resistant Building Design

A building’s floor is a platform for the building’s occupants. The floor system is also part of the continuous path that transfers the loads to the shear walls in the stories below or, in the case of the lowest floor, to the foundation.  Floor framing typically consists of dimensional lumber, or floor joists, spanning an open space.  Floor joists must be sized to resist the loads of the entire floor system along with vertical loads. The floor of a wind resistant building ensures the loads meet their final designation - the ground.

A wind resistant building design will guarantee the integrity of the building and the safety of those inside during dangerous winds.  A structure's ability to resist lateral and uplift loads depends on a  building having a continuous load path from it’s roof down to it’s foundation

*A shear wall is a structural system composed of braced (shear) panels that counter the effects of lateral load acting on a structure

**Tests were conducted on Bautex Blocks in accordance with the debris impact guidelines of FEMA P320/P361 (2015) and ICC-500 (2014) for hurricanes and tornados. ICC-500 includes a hurricane building envelope standard that is required by Florida Building Code and Texas Department of Insurance Windstorm Resistant Construction Guide.