The structure of a home is surprisingly capable of resisting hurricane-force winds as long as its exterior perimeter remains intact. If the windows and doors give way, though, the wind is free to rage through the home’s interior, where it can lift the roof off and destroy the structure completely. In the last decade of the twentieth century and the first decade of the twenty-first, it became common for homeowners who found themselves in a hurricane’s cone of probability to tape their windows from corner to corner in the hope it would help hold the shattered panes in place and save their home. A practice that may have made the homeowner feel better but didn’t accomplish much more.
Today, however, there are a range of hurricane-resistant windows and doors that are tested against the impacts of wind and windblown debris. They receive a hurricane impact window rating based on the maximum wind pressure they are expected to resist. Though the ratings can be difficult to interpret as there are a wide range of different wind zones based on proximity to the coast. Choosing the doors and windows that are right for you means understanding these wind-loading zones and how these ratings relate to them.
For a quick reference, we have outlined the impact design pressure, pounds per square foot the window will be able to withstand, and the approximate wind speed of a storm that will produce that amount of pressure.
In the following sections, we will discuss hurricane-impact windows, ratings you need to know when researching, and where to purchase beautifully crafted French Steel hurricane impact windows and doors.
How Hurricane Impact Window Ratings are Determined
The American Society of Civil Engineers (ASCE) defines minimum design loads and associated criteria in its standard 7-22. This is a comprehensive standard that makes considerations including but not limited to:
- Exposure Category: Based on the location of the building and surrounding terrain, the building is classified as Exposure B, C, or D. Exposure B represents urban and suburban areas with many closely spaced buildings. Exposure C represents flat, open terrain with scattered obstructions. Exposure D represents coastal areas or other areas with high wind speeds.
- Wind Speed: The basic wind speed for the location must be determined based on local wind maps or other sources. Then, the wind speed is used to calculate the velocity pressure. The number is based upon a statistical worst-case scenario for twenty-five years.
- Location and Surrounding Terrain: The location of the building and surrounding terrain must be considered when determining the exposure category and the wind speed. Exposure categories B, C, and D represent urban and suburban areas, flat open terrain, and coastal areas or other areas with high wind speeds, respectively.
- Building Height: The height of the building affects the exposure coefficient and the topographic factor, which are used to calculate the velocity pressure.
- Building Use: The function of the building and its contents affect the importance factor, which is used to adjust the design pressure. Buildings with high occupancy or critical functions may be required to calculate for higher design pressures to ensure they remain usable in critical scenarios like a hurricane.
Each of these considerations provides a variable that will be used in calculating the final wind load that a building must withstand in pounds per square foot (PSF). The actual equation for a low-rise building—the category residential homes fall under is:
P = qh[(GCpf)-(GCpi)]
- P = Wind pressure on the surface (in pounds per square foot or PSF)
- Qh = Velocity pressure based on the wind speed and building height
- GCpf = is the external pressure coefficient
- GCpi is the internal pressure coefficient
There are additional formulas for enclosed, partially enclosed, high-rise buildings, and many others. The formulas laid out by the ASCE form the basis from which building codes across the US are derived, but not the actual codes themselves. There can be a great deal of variation from area to area as to what modifiers are applied to create the final pressure level that a building code will demand doors and windows must meet at the state and even city level. There are far too many of these to provide a comprehensive list in a single blog post. However, the general rule of thumb is that an exposure area that is less at risk, like Category B, an urban or suburban zone, will be multiplied by a number less than one (for example, 0.89) that slightly reduces the required wind pressure rating from that specified for the wind zone from that specified by the ASCE. While high exposure zones like coastal Category D areas will be multiplied by a value above one (1.11, for example) that increases the required pressure rating. This final figure is the design pressure for the building and what a window or door needs to meet in order to be suitable.
The most demanding state and city building codes are found in South Florida’s High-Velocity Hurricane Zone (HVHZ), which is exclusive to Miami-Dade and Broward counties. This specialized code is what provides the criteria for hurricane impact window ratings. The code specifies that the tested window sample must be impacted by a 2×4 piece of timber traveling at 50 feet per second (34 miles per hour) twice. Then it has to be cycled through wind pressure loading dictated by the Florida Building code, which modifies the ASCE wind pressure ratings according to a table of zone multipliers outlined in the preceding paragraph. If three test samples pass this test without creating a crack longer than 5 inches that allows air to pass through it, then it is deemed a hurricane impact rated window for the design pressure at which it was tested. It is this final test that determines the design pressure for hurricane impact-rated windows and doors.
Choosing Hurricane-Resistant Windows and Doors for Your Property
Most properties will not be exposed to the full and intense fury of a hurricane. The wind load drops off sharply within just a few miles from the coast. However, homes and businesses built along the Gulf and Southern Atlantic shores will likely face a major hurricane sometime in the next few decades. High-strength doors and windows with high hurricane impact window ratings will play an important role in protecting the property. Homes and businesses located further inland may not need the top-of-the-line, but they can still benefit from hurricane-resistant doors and windows. To know what is necessary for your property, it is essential to understand design pressure and choose windows and doors that meet the maximum expected risk for the area (see the chart above).
The strongest category of hurricanes, Category 5, begins when a storm’s wind speeds exceed 157 mph. In reality, most storms don’t make landfall with such high wind speed, and recent storms like Hurricane Harvey made landfall with wind speeds in excess of 130 mph, and even devastating storms like Katrina made landfall with wind speeds of only a little more than 140 mph. In most locations, hurricane window impact rated at a design pressure of 45 will be rated to withstand anything that your property is likely to face. Only along the coast is more really called for, and only along a very specific section of the South Florida coast is more mandated by building codes. In either case, French Steel’s classic impact line of windows and doors offers options that suit your needs.
Hurricane-resistant windows and doors for your property
The French Steel Company’s Classic Impact line of doors has been tested and qualified to a design pressure of 45, or capable of resisting the damaging effects of major Category 3 and 4 storms. Re0cently this line of strong, elegant doors with quality American-made hardware and anchors has been tested up to design pressure 65 and is capable of protecting homes in major Category 5 hurricanes.
Contact us today for more information on hurricane impact window ratings or to get a quote for your property. We look forward to working with you.