Have you ever wondered why hurricane shelters protect you better than your home in the event of a hurricane? One of the main reasons that municipal buildings and hospitals usually fare better through a wind event is that they are built under a stronger building risk category. A hospital needs to not only make it through the wind event but it should also keep the occupants safe and maintain facility operations through the wind event. While single-family homes should perform to the same level, the risk is not as great for occupants of a home. There are four main categories, as outlined by ASCE, that define levels of performance for wind uplift and this article will address them, out of order, for a very clear reason.
Building Category IV is the highest level of performance for wind uplift considerations. This is the big one. Category IV buildings and structures are “essential facilities.” Hospitals, hurricane shelters and other buildings and structures that must maintain integrity of the building envelope are in this category. In addition, buildings or other structures that could pose a substantial hazard to the community if they fail, are also in this category. This includes emergency services and operations buildings. In addition, buildings or other structures that contain a toxic substance in quantities that would pose a threat to the public if released are included in Category IV. There are some other definitions but this establishes the theme for Category IV. Basically, anywhere that can put multiple people at risk for loss of life or life-threatening injury would be covered here. In prior revisions of ASCE 7, this category would add 15 percent to design requirements. This is where you want extra safety and robust performance for the building over time. Someone on life sustaining oxygen in a hospital or a toxic material storage facility needs the highest level of wind uplift performance.
Building Category III is the next highest level of performance for wind uplift considerations. While not as severe as Category IV, these buildings and other structures require extra consideration. This category is mainly for large occupancy considerations and toxic or hazardous materials. Think of schools, jails and chemical storage and distribution buildings or any building that would have toxic materials in lower quantities where the entire community is not at risk but a localized area would be. There are more defined parameters for the definition of this building category but any ties to hazardous or toxic materials to people is the consideration. This could be a small chemical manufacturing building or an explosives storage building. You get the idea. In previous ASCE 7 revisions, this category also added 15 percent to the design requirements.
Building Category I is the lowest level of consideration and includes buildings or other structures where failure is not a risk to human life. This is a category for buildings like garages or barns, where people are not likely to occupy the building during a wind event. In prior revisions of ASCE, this category would reduce the design requirements by 13 percent from Category II.
Building Category II is the last consideration and the largest and most used category. In the definitions, this is called out when the building or other structure does not qualify for Building Category I, III or IV. This category includes single family homes, apartment buildings, offices, retail and commercial buildings. Based on this definition, it makes sense to define this category last. It’s the catch-all category.
These definitions are handy for everyone to review from time to time. While prior revisions of the American Society of Civil Engineers (ASCE) Standards accounted for this with something called “importance factor,” the current revision, ASCE 7-22, uses separate wind speed maps to account for the building category considerations in the uplift calculations. As Floridians, the threat of hurricanes, tornados and strong thunderstorms are always in the back of our minds. If nothing else, please consider these categories when weighing options on where to seek safe shelter in the event of a wind event. Do not go into garage buildings or warehouses to ride out the storm. Instead, look for municipal storm shelters, large occupancy buildings or even hospitals if you have health considerations.
For anyone considering building a new home with increased performance, you can ask your design professional regarding the construction differences to classify your building as an essential facility or Building Category IV level building for the purposes of the design. Not only will the roof be upgraded but you’ll also quickly see how a 15 percent design increase will fortify all the components of the building to perform when the storm rolls in.
Riku Ylipelkonen, Owner, Standard Building Advisors has been in the roofing industry for 15 years working for Polyfoam Products. When Polyfoam Products was acquired by 3M, the name was changed to ICP Building Solutions Group. Riku worked at ICP as Technical Services Manager until March of 2023, when he left to begin his own company. Riku is an engineer and is working as a consultant with FRSA. He is a member on FRSA’s Codes Committee, Codes Subcommittee, Tile Committee and on the FRSA-TRI Manual Rewrite Committee. Riku is also a member of the American Society of Civil Engineers (ASCE).