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MAY 2019 csengineermag.com 29 Fire-Resistance Ratings: The table below contains the minimum FRR requirements for the new construction types found in IBC Table 601. Included for comparison are the requirements for Type I-A and I-B. The new Type IV-A has the same base FRR requirements as Type I-A. Type IV-B and IV-C have the same base FRR as Type I-B. Noncombustible Protection: A requirement unique to the new construction types is noncombustible protection (for mass timber). This noncombustible material applied to the mass timber helps determine fire behavior by delaying the contribution of the mass timber structure in a fire and has an added benefit of increasing the fire-resistance rating of the mass timber element. The minimum contribution of noncom- bustible protection was set by the ICC Tall Wood Building Ad Hoc Committee to be no less than 2/3 of the Table 601 required FRR for mass timber elements. All mass timber in Type IV-A construction requires noncombustible protection. Most of the mass timber in Type IV-B requires noncombustible protection with limited exposed mass timber elements. Select mass timber elements in Type IV-C require noncombustible protection. A summary of the noncombustible protection required for building components in the different construction types is shown in the table below. Three new types: Conceptually, Type IV-A is similar to type I-A with equal or greater FRR requirements and no exposed mass timber. Type IV-C is similar to Type IV-HT with almost all of the interior mass timber permitted to be exposed; however, most structural building components have a 2-hour FRR in addition to minimum heavy timber sizes. In surveying the existing and desired applications of mass timber in the U.S. and around the world, the committee determined that an intermediate construction type between IV-A and IV-C was needed, resulting in Type IV-B. Type IV-B has similar FRR requirements as IV-C; however, all the mass timber requires noncombustible protection except the following: • Area of ceilings not greater than 20% of the floor area • Area of walls not greater than 40% of the floor area Building Element I-A I-B IV-A IV-B IV-C IV-HT Primary Structural Frame 3* 2* 3 2 2 HT Ext. Bearing Walls 3* 2* 3 2 2 2 Int. Bearing Walls 3* 2* 3 2 2 1/HT Floor Construction 2 2* 2 2 2 HT Roof Construction 1½* 1* 1½ 1 1 HT *These values can be reduced based on certain conditions in IBC 403.2.1, which do not apply to Type IV buildings. Required Fire-Resistance Ratings by Construction Type (IBC Table 601) FRA-742_TALL-WOOD_Charts_March2019.indd 1 IV-A IV-B IV-C IV-HT Interior Surface of Building Elements Always required. 2/3 of FRR, 80 minutes minimum Required with exceptions. 2/3 of FRR, 80 minutes minimum Not required* Not required* Exterior Side of Exterior Walls 40 minutes 40 minutes 40 minutes Not required* Top of Floor (above Mass Timber) 1" minimum 1" minimum Not required* Not required* Ceiling (below Mass Timber) Per interior protection Per interior protection Not required* Not required* Shafts 2/3 of FRR, 80 minutes minimum, inside and outside 2/3 of FRR, 80 minutes minimum, inside and outside 40 minutes minimum, inside and outside Not required* *Not required by construction type. Other code requirements may apply. 5/8 " Type X gypsum = 40 minutes. Required Noncombustible Protection on Mass Timber Elements by Construction Type

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