Building Construction Types
Building materials are, unsurprisingly, often at the center of discussions about fire damage, as we saw earlier this year following the destruction of a mid-construction apartment building in Raleigh, North Carolina.
It makes sense that the more fire-resistant a building is, the larger and taller it is permitted to be. In that same vein, a building made entirely out of combustible materials must be smaller. So how do we determine the fire-resistance rating and codes associated with a structure? There are five different building construction types, each with fire-resistance ratings (the duration for which a passive fire protection system can withstand a standard fire resistance test) that apply to the structural frame, bearing and nonbearing walls, floor, and roof.
The International Building Code spells it all out for us in Chapter 6: Types of Construction. It’s a hefty chapter with various charts and footnotes to review, so we’ve put together the very basic characteristics of each building type here:
Type I (IA and IB)
The most stringent of building types when it comes to fire-resistance rating requirements, a Type I building, including its roof, must be composed of noncombustible materials like concrete and steel.
Type II (IIA and IIB)
With noncombustible steel or concrete structural framework, walls, and floors, Type II is similar to Type I, but requires lower fire-resistance ratings. It’s a very commonly used construction type and Type IIB has no fire resistance requirements for any of the building elements, provided that there is adequate fire separation distance, as laid out in Table 602.
Also known as a brick-and-joist structure, a Type III building has exterior walls built out of noncombustible materials (like masonry or concrete) and the floors, roof, and structural framework can be made of any material permitted by the code (like wood).
Buildings constructed out of heavy timber (HT) are designated Type IV. With noncombustible exterior walls and interior elements made out of solid or laminated wood, a Type IV building cannot have a wood column any less than 8 inches thick or a wood girder any less than 6 inches thick. While similar to a Type III, Type IV instead relies on the fire-resistant nature of the large dimension lumber in place of a prescribed fire-resistance rating.
The most combustible of the five building types and the only one that allows for combustible exterior walls, Type V buildings allow both the exterior walls and interior framing to be wood. It’s a common construction method for single-family homes.
When combined with the occupancy, building construction type is a driving factor for many code requirements. It’s one of many details that must be considered during the design phase of a construction project, especially when determining the overall goals and ultimate use of the building.
For more information, check out Chapter 6 of the 2015 International Building Code here.