Understanding Potential Damages from Hurricanes

Category One: Very dangerous winds will produce some damage. Well-constructed frame homes could have damage to roofs, shingles, vinyl siding, and gutters. Large branches of trees will snap, and shallowly rooted trees may be toppled. Extensive damage to power lines and poles likely will result in power outages that could last a few to several days.

Category Two: Extremely dangerous winds will cause extensive damage. Well-constructed frame homes could sustain major roof and siding damage. Many shallowly rooted trees will be snapped or uprooted and block numerous roads. Near-total power loss is expected, with outages that could last from several days to weeks.

Category Three: Devastating damage will occur. Well-built framed homes may incur major damage or removal of roof decking and gable ends. Many trees will be snapped or uprooted, blocking numerous roads. Electricity and water will be unavailable for several days to weeks after the storm passes.

Category Four: Catastrophic damage will occur. Well-built framed homes can sustain severe damage with loss of most of the roof structures and/or some exterior walls. Most trees will be snapped or uprooted and power poles downed. Fallen trees and power poles will isolate residential areas. Power outages will last weeks to possibly months. Most of the area will be uninhabitable for weeks or months.

Category Five: Catastrophic damage will occur. A high percentage of framed homes will be destroyed, with total roof failure and wall collapse. Fallen trees and power poles will isolate residential areas. Power outages will last for weeks to possibly months. Most of the area will be uninhabitable for weeks or months.

Why Windows Fail

Windows and doors play a vital role in any structure. They act as centerpieces for design and architectural style, while also capturing natural daylight, allowing ventilation, and providing views.

Photo courtesy of the U.S. Navy/Interior Communications Electrician 1^st Class Jason Stephens

Windows in a high-rise building in downtown Houston were shattered by winds from Hurricane Ike after the storm came ashore on the Texas Gulf Coast early Saturday morning, September 13, 2008 as a major hurricane. The storm left broken windows, flooded streets, and millions of Texans without power.

Under assault from a hurricane, however, improper windows go from enhancing a building to becoming its weakest link.

As hurricanes sweep onshore, they bring an arsenal of destructive forces with them. When a hurricane makes landfall, it often produces a devastating storm surge, or an abnormal wave of water sweeping inland at high speed. A hurricane’s high winds torque, twist, and flatten structures, and pick up and launch debris, creating wind-borne projectiles. Hurricane winds can spawn tornadoes. Finally, torrential rains cause floods and landslides, destabilizing what remains standing.

There are three main reasons why windows fail during a hurricane or severe storm event:

1) Glass breaks from wind pressure or more likely from windborne debris
2) Whole windows, frames and all, can be blown into the structure
3) Water infiltrates the joints of windows or leaks around the frames and into wall cavities

Wind-borne debris, or airborne projectiles that cause glass breakage and other damage to buildings during severe wind events, are one of the major causes of damage during a hurricane.

When a window is broken during a hurricane, water, glass, and debris are blown immediately into the structure. Flying glass and debris can cause serious injury and death. In addition, a failed window opens up the building to the full consequences of the storm. Depending on the location of the breached opening, with respect to wind direction during a hurricane, broken windows in the exterior envelope allow strong winds to rush inside the building. These trapped wind forces create extremely high internal pressures on the roof, sidewalls, and leeward wall that exceed the total wind pressure resistant limitation of the building’s structural elements. Consequently, the internal pressures can ultimately result in severe damage to, if not total destruction of, the building.

Mitigating damages from debris is a principle priority in hurricane-resistant windows and doors.

Beyond broken glass from high winds and projectile impacts, entire window systems may fail from wind forces on the assembly. In older buildings, if the window is not anchored into the wall well enough to resist high wind pressure, the entire window may be lost. While a 50-mph wind pushes on a window or door with about 5 to 7 pounds of force per square foot, a 100-mph wind applies 20 to 28 pounds per square foot. A 130-mph wind applies 34 to 47 pounds per square foot. An average 3-foot by 5-foot residential bedroom window, when subjected to a 100-mph wind, has between 300 to 420 pounds of force applied to the window and the frame.

Windows and doors that have been tested and rated for high levels of water resistance will perform better and allow less or no water into the structure.

After being confronted full force in 1992 with just how much devastation hurricanes could cause, codes and standards were dramatically revised. Codes for windows in any zone with a hurricane threat now address:

• Higher wind loading: Coastal wind load charts were redefined significantly higher.
• Negative pressures: Building components are required to withstand high wind loads; greatest impact realized from negative pressure loads.
• Impact and cyclic testing: Cladding and building components must be tested and certified for compliance.