The type of construction refers to the ability of the building to resist fire. Each building or structure is to be classified into one of the five possible construction classifications (and further classified into the appropriate sub-classification): Type I, II, III, IV, or V. The sub-classifications are A or B. The type of construction will dictate limitations and building element requirements for fire-resistance ratings, such as structural frame and bearing walls (exterior and interior).
The type of construction and subsequent code requirements limit the allowable height and area and egress facilities, depending on the correct classification of the type of construction. Then there are several allowances in the code and it gets pretty confusing. This can be complicated for architects, particularly with mid-rise building that may have an option to lower building height or lessen floor area to accommodate a less stringent type of construction. For example, a condo building that is 65 feet in height and with a floor area of less 16,000 square feet per floor would fall into a Type III building per table 503 of the International Building Code. Type III requires non-combustible exterior framing rated for two hours. Wood framing may be used if the wood is fire-treated.
However, if compliant automatic fire sprinklers are used throughout the building, the code allows an additional 20 feet increase in height limitations. This would make the building a Type VA and allow standard wood framing and a one-hour fire rating. While most professional designers frown on “trade-offs,” they are often too tempting to pass up when trying to make a job fit the budget.
HOW TO DETERMINE
Type I is the most fire restrictive when it comes to what materials are allowed to be used (primarily non-combustible only) and thus have almost unlimited height and floor area allowances. Type V buildings can use any materials allowed by code, and therefore have limitations on height and floor area. Type III buildings fall somewhere in the middle; both combustible and non-combustible materials may be used in certain areas of the building, and the height and area limitations are more than the Type V and less than the Type I.
Here are some basic examples of each type:
• High rise buildings are a Type I
• Single-Family homes are a Type V
• Mid-rise buildings can fluctuate from III to V
• Heavy timber buildings are a Type IV
Caveat: Municipalities can, and often do, make unique provisions and alterations to the IBC. Always contact the local building department to verify the changes with regard to types of construction as compared to the current IBC.
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