In the design of buildings, one of the first and most important efforts to be undertaken by the design architect is to appropriately classify the building to one of the five regulated construction types as detailed in the building code.
Construction Type Determines Materials
The construction type of the building details the types of materials allowed to be used in the design and construction of the building. The construction type will detail two main attributes of the building elements: whether or not the materials are combustible or noncombustible (ex: wood versus steel frame), and the degree to which these building elements are required to be rated for fire-resistance (ex: fireproofed steel versus exposed steel).
There are five types of construction regulated in chapter 6 of the IBC, roughly summarized as follows:
Construction Types I & II
Types I & II construction are fully noncombustible, with exceptions noted in Section 603 (per the 2018 IBC). Sub-designations to both construction types (I-A, I-B, II-A & II-B) will determine to what degree the noncombustible elements require fire-resistance ratings. For example, Type II-B construction does not require any additional fire-resistance rating, and allows the noncombustible building elements to be exposed, where Type II-A construction requires the application of fire-resistant materials to be applied to these same building elements.
Construction Type III
Type III construction requires noncombustible exterior walls, and allows combustible interior construction. This construction type has been colloquially referred to as “ordinary” construction for decades, although this construction type was much more common in the second half of the 20th century and is not utilized as much nowadays.
Construction Type IV
Type IV construction requires noncombustible exterior walls, and requires the other building elements to be heavy timber. This construction type is commonly referred to as Heavy Timber construction, as it relies on the thickness and mass of the heavy timber elements to provide a degree of fire-resistance.
Construction Type V
Type V construction allows any type of material allowed per code. Typically Type V construction is used for wood buildings, where the exterior walls, roof, floors, etc. are all framed with wood. Sub-designations to Type V construction (V-A and V-B) will determine to what degree the building elements require fire-resistance ratings or not.
Why Construction Type Matters
Why does this matter? Because the construction type will limit how big your building is allowed to be, in both height and area. For more restrictive construction types composed of fireproof or noncombustible construction (for example, Type I or II), allowable building heights and areas are allowed to be greater, and for less restrictive construction types composed of unprotected or combustible materials, the allowable height and area will be lower. These determinations are important for the designer and the builder to understand and to follow in order to ensure that the resultant building is sufficiently safe for the expected number of occupants. Because unprotected and/or combustible construction is inherently more prone to fire damage and danger than protected and/or noncombustible construction, the code reasonably limits the maximum size of such less safe buildings. Other factors such as active fire sprinkler protection and building frontage (open space around the building) can allow additional area and height increases per code allowances.
Proper construction classification is one of the most fundamental and basic requirements regarding code-compliant building design and construction, and is one of the architect’s fundamental responsibilities in building design. The commentary to the IBC explains this in more detail:
“The purpose of classifying buildings or structures by their type of construction is to account for the response or participation that a building’s structure will have in a fire condition originating within the building as a result of its occupancy or fuel load,”
and:
“Correct classification of a building by its type of construction is essential. Many code requirements applicable to a building, such as allowable height and area (see Chapter 5), are dependent on its type of construction. If a building is placed in an incorrect construction classification (for example, one that is overly restrictive), its owner may be penalized by increased construction costs. On the other hand, when a building is incorrectly classified in an overly lenient type of construction, it will not be constructed in a manner that takes into account the relative risks associated with its size or function. The provisions of this chapter, coupled with Chapters 3 and 5, and Tables 601 and 602, establish the basis for the ‘equivalent risk theory’ on which the entire code is based.”
Therefore, if you are an architect or designer for a building subject to construction type classification, it is very important that care is taken to properly assign the correct construction type. If the construction type is mis-applied, major design and construction mistakes can follow.