The five types of building constructions according to the IBC (International Building Code)
Why separate structures into five types of constructions?
The first purpose of the IBC (International Building Code)
when classifying buildings into these five types is to separate each kind of construction according to its response in the presence of a fire. Materials used in the building process and their physical properties are taken into consideration to classify each building into Type I, Type II, Type III, Type IV, and Type V, as well as the occupancy it serves.
Type I represents the highest fire-resistant rating, whereas a Type V would represent or require the least amount of fire resistance. They usually also have a maximum height, where the higher a building is, the more fire-resistant it must be, which is understandable safety-wise. The types are defined then, where you can, once you know precisely the characteristics of the construction you are building, categorize it into these types to better understand the requirements according to the IBC.
Why is it important?
The importance of this system is precisely informing the standards you will have to abide by when building a new construction based primarily on its occupancy. This way, when someone builds their home or a small business, which represents no significant fire hazards and won´t be occupied by too many people, they won’t have to follow the same standards a commercial building with multiple floors would.
Someone building their house, for example, can still follow a safer construction standard if they would like to. Even though it would make it way more expensive than it would have to be, there is nothing that prohibits it, as long as you are using a higher standard than the minimum required for the construction type. This cannot happen the other way around.
This ends up working in favor of all building occupants. Suppose a building is incorrectly classified into a lower category. In that case, it will not be constructed as required for its intended use and will result in a relative risk to all occupants it serves.
These types are better explained in the IBC, along with the maximum dimensions of the buildings and their occupancy classification
. Still, at this time, we will be focusing on the construction types.
IBC’S Five Type of Construction
Before getting to the types themselves, we should define an essential key concept: Fire-resistance rating
. The building code describes it as: “The period of time a building element, component or assembly maintains the ability to confine a fire, continues to perform a given structural function, or both, as determined by tests or methods based on tests as prescribed in section 703.”
With this in mind, all structural elements in a building must attend to a specific fire-resistance rating to be framed into a particular type of building. This concept will be brought up several times during the definition of each type as follows:Type I: Fire Resistive
This is the most stringent fire-resistive standard. It is applied to high-rise buildings (Over 75 feet tall according to the IBC), typically defined as buildings with six stories and above. This type can also be subdivided into two other subcategories: IA and IB.
For Type IA, all the materials in the construction must be non combustible (i.e., concrete or steel), except as otherwise listed in section 603, 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 other subdivision, 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 and 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 apply to Type I (meaning less than 75 feet tall). Following the norms of the Type I buildings, all structural elements must be made of non-combustible material as well. Still, unlike the previous type, those structural elements do not have to be treated with fire-resistant coatings or protected in any way. These elements can be exposed, unlike in buildings from tier I.Type III: Ordinary Construction
These constructions are characterized by exterior walls made of noncombustible material, often bricks, and the interior building elements are of any material allowed by code. There are also two subdivisions for this specific type: Type IIIA and Type IIIB.
In the IIIA category, also called “protected combustible”, the internal elements may be of combustible material (often wood) as long as it has been rated as fire-resistant for up to one hour. In contrast, in category IIIB, there are no such fire-resistance conditions for these elements.Type IV: Heavy Timber
Heavy Timber is technically a combustible structure, but one in which a degree of fire safety is attained by placing limitations on the sizes of wood structural elements and thickness and composition of wood floors and roofs, counting also on the avoidance of concealed spaces under floors and roofs.
That means that they can withstand fires for longer than light wood-frame houses. The greater structural mass of these wood elements ensures that buildings remain standing for longer (a fire rate of one hour is required for the structural frame). This type of construction was the norm for many buildings into the 19th century and has been enjoying a resurgence of interest in recent years.
This is the building type with more subdivisions in the IBC, so for a more in-depth reference of each subtype and standards, check the International Building Code, Chapter 6, section 601.Type V: Wood Frame Construction
This tier, as mentioned before, englobes the most straightforward constructions fire-resistance-wise. It is also divided into two subtypes: VA and VB. Type VA, known as “protected frame”, must have all structural elements fire-rated for up to one hour, while Type VB, or “unprotected frame”, the structure can be made of any material permitted by the IBC, combustible or noncombustible.
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