Construction projects can be classified according to a number of criteria. Buildings may be categorized, to pick a few examples, according to their owners (say, public versus private), the materials used to build them, or their uses. The last in that list are also known as occupancy classifications and they are the basis of chapter 3 of the International Building Code (IBC) which divides structures into categories including assembly (both churches and restaurants), residential, institutional, and storage. (The IBC is a code that serves as the basis for local and state codes and helps assure that there are consistent standards within the United States.)
The five building types discussed here, however, describe the necessary level of fire resistance that a building should have—which is determined largely by the size of the building and its intended use or occupancy. A 30-story apartment or office building (Type I) has more stringent fire resistance standards than a warehouse (Type III). The different tiers are explained in detail in chapter 6 of the IBC.
The highest tier of fire resistance is reserved for large buildings with many occupants and the lowest for single-family homes. An owner is free to follow the fire resistance requirements of a higher tier, but it will raise the cost of construction. (On the other hand, if local building codes require that a building follow a higher tier of fire resistance, an owner or architect must adhere to them.)
These different types of construction matter most to firefighters, for whom they can be literally a matter of life and death. Knowing whether a building is tier I or tier III provides information that is essential when coming up with a strategy to bring a blaze under control and to stop it from spreading to other buildings. Being familiar with the different tiers is also essential to architects and builders who must assure that structures comply with the requirements of the respective tiers.
The IBC’s descriptions of the various tiers often refer to minimum fire ratings for different elements. Fire ratings are typically measured in terms of the time a structural element can be exposed to fire before it fails or collapses. A beam with a 2-hour rating can be exposed to fire for at least two hours while a wall with a fire rating of 0 will typically fail after less than an hour. Concrete (whether cast or blocks) and steel have high fire ratings, which can be further lengthened with protective coatings.
Wood tends to have the lowest (or shortest) fire resistance ratings, but these can also be lengthened if the wood elements are in a so-called fire-resistant assembly (when they might be covered with gypsum board or another material), if the wood is treated, or if especially large timbers are used (as charing will protect the wood).
Type I: Fire Resistive
The most stringent fire-resistive standards are applied to high-rise buildings, defined as those over 75 feet tall in the IBC (that would typically be a building with six or more stories). For these Type IA buildings, all the materials used in construction must be noncombustible (such as concrete or steel) and meet the very highest fire-resistance standards. The structural frame and exterior walls must have fire ratings of at least 3 hours and floors and ceilings of at least 2 hours. The next half-tier down, Type IB, includes mid-rise office buildings and some residential structures such as apartment buildings and hotels. These have slightly lower fire rating requirements: 2 hours for structural frames and exterior walls, 1 hour for ceilings and floors.
Type II: Non-Combustible
The second tier typically applies to school buildings and some smaller commercial buildings that don’t reach heights of 75 feet or taller. As with Type I buildings, all the structural elements in Type II buildings must be made of non-combustible materials however it is not necessary to treat them with fire-resistive coatings or otherwise protect them. A steel column or beam may be exposed in a tier II building while it would have to be protected in a tier I structure.
Type III: Ordinary Construction
As with all the tiers except IV, Type III (often referred to as “ordinary construction”) is divided into two subcategories, A and B. Type IIIA buildings are also called “protected combustible” structures. Their exterior walls are of noncombustible materials, often brick, while internal floors and roofs may be of combustible materials (like wood) that have been rated as fire resistant for up to one hour.
Type IIIB buildings, or “unprotected combustible” buildings have noncombustible exterior walls, while the floors and roofs may be of wood that has not been rated as fire resistant. This type of construction is found in many older warehouses.
Type IV: Heavy Timber
The next tier covers a form of construction which was the norm for many buildings into the 19th century and has been enjoying a resurgence of interest in recent years. Heavy timber buildings have structures built of wood but, given the large size of the timbers, they can withstand fires for longer than light wood frame houses. At a minimum the wood columns, beams, and girders of heavy timber structures must be eight inches thick while floor planks must be at least six inches thick. The greater structural mass of these wood elements assures that buildings will remain standing for longer–a fire rating of one hour is required for the structural frame. The exterior walls of Type IV buildings are made of noncombustible materials.
Type V: Wood Frame Construction
This tier which covers buildings that use light wood framing is also divided into two levels, A and B. Type VA buildings are known as “protected frame” constructions and include many newer, small apartment buildings. With these the exterior walls, structural frame, and floors, ceilings, and roofs must be fire-rated for up to one hour. Type VB covers most single-family homes and garages and is known as “unprotected frame.” With these structures both the exterior walls and the supports can be made of any materials, including combustible ones, permitted by the IBC.
Other Rules in Chapter 6
The IBC also lays out rules for a number of specific materials and architectural features such as acoustic insulation, millwork, and internal partitions. Before breaking ground on any project, a contractor should review the sections of chapter 6 that are relevant to that specific building and its tier on the IBC system.
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