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30 csengineermag.com MAY 2019 A "sum of the ratios" type equation allows for a combination of the exposed ceilings and walls within a dwelling unit or fire area—e.g., area of exposed ceilings equal to 10% of the floor area plus area of exposed walls equal to 20% of the floor area. Unprotected areas of ceilings and walls also need be located with at least 15 feet horizontal separation. This requirement is to avoid re-radiation of heat between adjacent exposed mass timber elements which could extend the duration or severity of an uncontrolled fire. As an aside, the most frequent application of exposed mass timber in buildings to date is a portion of the ceiling alone. Common Requirements: Additional requirements applicable to Type IV-A, IV-B and IV-C were included in the G108-18 proposal and include: No exposed mass timber in concealed spaces; concealed space permitted only with noncombustible protection as required for the interior mass timber • Exterior side of exterior walls protected by a non-combustible material—e.g., 5/8" Type X gypsum sheathing • No combustible exterior wall coverings except for certain water-resistant barriers • No exposed mass timber on the inside and outside surfaces of exit enclo- sures and elevator hoistways in high-rise buildings (occupied floor > 75 feet from lowest fire department access) • Noncombustible construc- tion only for exit enclosures and elevator hoistways greater than 12 stories or 180 feet. Allowable Building Sizes As the ICC Tall Wood Building Ad Hoc Committee developed and tested requirements for the new construction types, the allowable heights and areas were developed with the following performance objectives as the basis: •No collapse under reasonable scenarios of complete burnout of fuel without automatic sprinkler protection being considered •No unusually high radiation exposure from the subject building to adjoining properties to present a risk of ignition under reasonably severe fire scenarios •No unusual response from typical radiation exposure from adjacent properties to present a risk of ignition of the subject building under reasonably severe fire scenarios •No unusual fire department access issues •Egress systems designed to protect building occupants during the design escape time, plus a factor of safety •Highly reliable fire suppression systems to reduce the risk of failure during reasonably expected fire scenarios; the degree of reli ability should be proportional to evacuation time (building height) and risk of collapse The comprehensive package of approved changes met these objectives, and allowable heights and areas for select occupancies in the 2021 IBC are shown in the table below. I-A I-B IV-A IV-B IV-C IV-HT Occupancies Value Allowable Building Height above Grade Plane, Feet (IBC Table 504.3) A, B, R S Unlimited 180 270 180 85 85 Allowable Number of Stories above Grade Plane (IBC Table 505.4) A-2, A-3, A-4 S Unlimited 12 18 12 6 4 B S Unlimited 12 18 12 9 6 R-2 S Unlimited 12 18 12 8 5 Allowable Area Factor (At), Feet 2 (IBC Table 506.2) A-2, A-3, A-4 SM Unlimited Unlimited 135,000 90,000 56,250 45,000 B SM Unlimited Unlimited 324,000 216,000 135,000 108,000 R-2 SM Unlimited Unlimited 184,500 123,000 76,875 61,500 S is sprinklered with NFPA 13 sprinklers. SM is the multi-story allowable area factor. Underlined entries are the new additions. Select Height and Area Limits by Construction Type FRA-742_TALL-WOOD_Charts_March2019.indd

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