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TABLE 4: Comparison of Construction Types III.IV and V Exterior wall materials FRTW FRTW FRTW or CLT Any wood including Any wood including mass timber mass timber Exterior bearing wall FRR 2-hour 2-hour 2-hour 1-hour 0-hour Interior framing materials Any wood including Any wood including Heavy timber Any wood including Any wood including mass timber mass timber including mass timber mass timber mass timber Sources-IBC Section 602,Table 601 and Section 2304.11 Selecting a Construction Type For example, if a building's size permitted the use of Type Selection of construction type for mass timber projects is V-B construction, it could still be completely framed with one of the more significant design considerations.While it's noncombustible materials while being classified as V-B. common to choose construction type based on structural Similarly,a Type III or V building could be framed with a material—i.e., to assume that steel and concrete structures combination of combustible and noncombustible materials. should be Type II, light-frame wood should be Type V,and As noted,from a cost efficiency perspective, it is usually exposed heavy timber should be Type IV—this approach can best to start a building analysis with Type V-B construction lead to additional costs.While Type IV construction can be as this provides the most flexibility in terms of allowable used for exposed mass timber projects,a full understanding use of materials throughout the building while minimizing of the allowable use of materials in all five construction types, requirements for fire resistance-rated assemblies and as well as the unique allowances and limitations associated structural elements. However,Type V-B is also the most with each,will help to inform the most efficient design. restrictive in terms of allowable building size. All three of these factors—allowable building size,allowable use of To optimize a building design from a construction type perspective,it is best to start with Type V-B construction structural materials,and required fire-resistance levels— and work up toward higher construction types.This avoids are interconnected. unnecessary defaults or assumptions—and unnecessary Construction type,which is a direct function of the types of costs.The fact that certain materials are being used doesn't materials used in a building(and vice versa) looks at limiting mean there is only one option for construction type.Similarly, the size of the building based on fire and life safety risks. In a mix of occupancy groups doesn't dictate that certain other words, how much of a building's contents and structure materials, construction types or building configurations are are combustible has an impact on presumed risk,which in required. For example,a mass timber building may have turn affects the code's allowances for building size. Fire- isolated steel,concrete or masonry structural elements, but resistant measures required of the building's structure dictate this doesn't mean that Type I or II Construction is necessary, the length of passive fire endurance(i.e., 1 hour, 2 hours). nor does it mean that some or all of the building can't be framed with mass timber. Likewise, a building with mass If Type V-B construction doesn't allow as large a building timber elements has options other than Type IV construction. as desired,the next step is to check Type V-A.The main Note IBC Section 602.1.1) differences between V-B and V-A are fire-resistance rating requirements and allowable building size. If Type V-A doesn't 602.1.1 Minimum requirements.A building or portion allow the desired size,Type III-B is the next choice,with Type thereof shall not be required to conform to the details of a III-A following.Type IV construction has similar allowable type of construction higher than that type which meets the building size limits as Type III-A;however,there are nuances minimum requirements based on occupancy even though to the selection of one or the other as discussed below. certain features of such a building actually conform to a higher type of construction. Table 4 summarizes the main differences between construction Types III, IV and V, as well as the different types This section permits the use of elements commonly used in of wood systems permitted in each. These allowances are a higher construction type without requiring that the entire shown in IBC Section 602,Table 601 and Section 2304.11. building meet all of the provisions of that construction type. 5