Type IV construction—historically called ‘heavy timber construction’—is being reinvigorated and has gained a lot of momentum in recent years. Building and fire officials have long understood the enhanced fire performance characteristics of Type IV structures; building designers are now considering the effects of new technologies and the associated benefits of its use. In fact, the 2015 IBC is the first edition to recognize new mass timber products such as cross-laminated timber (CLT). Due to the structural capabilities of mass timber, wood design is better able to take advantage of the greater building height and area permitted by IBC. As a result, interest in, and advocacy for, mass timber technologies in tall wood buildings—those exceeding prescriptive height limits and requiring special permission—is steadily growing because of the aesthetic, environmental, economic, and cost-related wood benefits. Heavy timber is recognized to have inherent fire-resistant properties due to its mass. When accompanied by additional protection from gypsum wallboard or other membranes, it can achieve high fire resistance ratings while also limiting the effects of the additional fire load created by the massive wood.
Type V construction permits the use of wood or other approved materials for structural elements, and is further identified as Type VA (one-hour fire-resistance rated) and Type VB (no fire-resistance rating required). Often, Type VA is practical since a one-hour rating is relatively cost-effective to achieve for roof, floor, and wall assemblies. Additionally, there are no special restrictions on materials used in exterior walls, as with Types III and IV construction. Type VB, on the other hand, offers the most flexibility for commercial wood frame structures since there are no fire-resistance ratings required for the structural frame—however, there may be fire-resistance ratings required for certain building areas such as means of egress.
Other than meeting fire-resistance requirements, there is no limitation on the use of concrete or steel if chosen for portions of a Type III, IV, or V building. Adding noncombustible elements to a Type V or any other combustible construction type is permitted as long as it does not exceed the building size limit for that construction type. In some instances, alterations and additions, either horizontally or vertically, can still be accomplished with wood.
Determining maximum building height and area
Notable features provided in the 2015 CCWD, updated from the 2012 and 2009 versions, are the convenient tables to help determine maximum building size for eight common occupancy groups using newly formatted height and area tables of the 2015 IBC. Users will recognize allowable areas for sprinklered and unsprinklered are both provided in the building area tables in the back of the CCWD. Occupancies discussed include:
- Group A−Assembly;
- Group B−Business;
- Group E−Educational;
- Group F−Factory/Industrial;
- Group I−Institutional;
- Group M−Mercantile;
- Group R−Residential; and
- Group S−Storage.
One should note Group H (Hazardous Occupancies) and Group U (Utility and Miscellaneous Occupancies) may be built using wood construction, but are beyond CCWD’s scope. Section 510 of the code also contains special provisions for buildings sometimes referred to as ‘podium’ construction.
Using a special horizontal fire-resistance rated separation, two different construction types can be used in a single building. This is typically used to place wood-frame construction over a concrete parking structure, with the required three-hour-rated horizontal separation between the occupancies. Code changes in recent years have added more flexibility by widening the number of use groups in either the upper or lower building, and by permitting multiple stories of concrete construction below a wood frame structure above. These occupancy sections also contain special provisions for protection of exits.