International Code Council 93-Tall-Mass-Timber-Buildings - 93-Tall-Mass-Timber-Buildings

Option A correctly reflects the required noncombustible protection for protected Type IV-A mass timber. Under IBC Section 602.4.1.2.1, the noncombustible protection applied to interior faces of Type IV-A mass timber must contribute the protection time determined by the applicable provisions of Section 722.7, but not less than 80 minutes .

The broader Type IV provisions are particularly important for interior exit and elevator hoistway enclosures. In qualifying tall mass timber buildings with an occupied floor more than 75 feet above the lowest level of fire department access, mass timber interior exit and elevator hoistway enclosures are permitted up to the specified height limits only where protected in accordance with Section 602.4.1.2. For a two-hour, 120-minute-rated enclosure, at least 80 minutes must therefore come from the prescribed noncombustible protection.

Type IV-B likewise uses an 80-minute minimum for protected interior mass timber, so option B ' s 40-minute value is incorrect. Type IV-C has different exposure allowances and shaft-protection provisions, but option C does not state the controlling tall-building enclosure requirement tested here. Type IV-HT is not governed by the Type IV-A tall mass timber enclosure protection rule.

References/Topics: 2021 IBC Sections 602.4 and 602.4.1.2 through 602.4.1.2.1; Section 722.7.

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The controlling provision is 2021 IBC Section 504.4 and Table 504.4, which establish the allowable number of stories above grade plane according to occupancy classification, construction type, and sprinkler protection. The question identifies three controlling variables: Group E occupancy, Type IV-A construction, and an NFPA 13 automatic sprinkler system installed in accordance with Section 903.3.1.1.

In the Group E row, the sprinklered “S” value for Type IV-A construction is nine stories above grade plane. Therefore, option D is correct. The three-story value applies to the nonsprinklered Type IV-A condition and must not be used because the question expressly states that the building is sprinklered. The six-story value corresponds to Type IV-B construction under the same sprinklered Group E condition, while the four-story value—although not listed as an answer—would apply to Type IV-C.

The special inspector should distinguish the story limitation in Table 504.4 from the separate building-height-in-feet limitation in Table 504.3. Both requirements apply concurrently; compliance with one does not permit the other to be exceeded.

References/Topics: 2021 IBC Section 504.4; Table 504.4, Group E, sprinklered Type IV-A construction; Section 903.3.1.1.

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The code provides a specific combustible-material exception for qualifying water-resistive barriers , making option D correct. The exterior-protection provisions for Types IV-A, IV-B, and IV-C require the exterior side of mass timber walls to receive noncombustible protection with at least a 40-minute assigned protection time, and components of the exterior wall covering are generally required to be noncombustible.

A water-resistive barrier may be combustible only where it satisfies stringent fire-performance criteria. Among those criteria, the 2021 IBC limits peak heat-release rate and total heat release under ASTM E1354 and establishes flame-spread and smoke-developed limits under ASTM E84 or UL 723. The provision therefore does not create a blanket exemption for arbitrary combustible membranes.

Insulation, exterior cladding, and architectural components forming part of the regulated exterior wall covering must comply with the applicable noncombustibility requirements unless some other express code allowance applies. The WRB is the specifically identified exception tested by this question.

During inspection, product identification and fire-test documentation should be checked rather than assuming that a membrane qualifies merely because it is marketed as a water-resistive barrier.

References/Topics: 2021 IBC Sections 602.4.1.1, 602.4.2.1 and 602.4.3.1; exterior wall protection and qualifying water-resistive barriers.

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The minimum required fire flow is 500 gallons per minute. Therefore, option C is correct.

Under 2021 IFC Section 3313.2, an approved water supply must be available when combustible building materials for the building under construction are delivered to the project site. The hydrant supplying the required flow must be located within the prescribed distance of the combustible materials as measured along an approved fire apparatus access route. Where one hydrant cannot provide coverage to all combustible material locations, additional hydrants are required.

The 500-gpm requirement is the initial minimum associated with the delivery of combustible building materials. It should not be confused with the potentially greater fire-flow requirements applicable after vertical construction of a Type III, IV, or V building begins. Depending on fire separation distance and the building’s calculated final fire-flow demand, a larger supply may be required under subsequent provisions.

Options A and B provide inadequate suppression capacity. Option D is not the baseline amount prescribed for the condition stated in the question. Inspectors and site safety personnel should verify that the water source is operational and accessible before substantial quantities of mass timber arrive on-site.

References/Topics: 2021 IFC Sections 3313.1 and 3313.2; construction-site water supply and combustible building materials.

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For the illustrated two-layer protection system, the 5/8-inch Type X gypsum-board layer directly adjacent to the connection is assigned 20 minutes, while the second layer is assigned 40 minutes. Therefore, option C is correct.

The standard assigned protection time for one layer of 5/8-inch Type X gypsum board is 40 minutes. However, the thermal-separation methodology applies a 0.50 reduction factor to the final or base Type X layer located directly against the protected connection. That layer therefore contributes 20 minutes. The outer layer retains its full 40-minute assigned value, producing a combined assigned protection time of 60 minutes for the two-layer configuration.

The layers are not interchangeable for calculation purposes because the reduced contribution applies specifically to the layer adjacent to the protected connection. Options A and B incorrectly assign the same value to both layers, while option D reverses the required allocation.

During inspection, the special inspector should verify layer sequencing, board thickness and type, fastening, joint staggering, edge support, protection at corners, and continuity around the connection. Missing or incorrectly sequenced layers can invalidate the calculated thermal-separation time.

References/Topics: AWC Technical Report 10; AWC 2021 Fire Design Specification Section 3.5.2.1; assigned protection time for Type X gypsum board.

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Two layers of 5/8-inch Type X gypsum wallboard are required for the condition shown, so option D is correct. The relevant AWC TR10 connection-protection methodology distinguishes between outer gypsum layers and the base layer immediately adjacent to the connection .

For 5/8-inch Type X gypsum board, an outer layer contributes 40 minutes. However, when gypsum board is being relied upon as the complete thermal-separation protection for a connection, TR10 reduces the contribution of the final/base layer to 50 percent. Therefore, the base layer contributes 20 minutes , while the second, exterior layer contributes 40 minutes . The total is:

20 minutes + 40 minutes = 60 minutes .

That satisfies the one-hour required protection period.

This is why simply looking at the normal 40-minute contribution of one 5/8-inch Type X layer would be incorrect for this connection condition. TR10 expressly uses the reduced base-layer contribution when gypsum constitutes the connection protection. The same methodology shows that four 5/8-inch Type X layers are needed for 120 minutes: 20 + 40 + 40 + 40 = 140 minutes, satisfying two hours.

References/Topics: AWC Technical Report No. 10, Sections 4.4.2 and connection-protection examples. TR10 states that outer 5/8-inch Type X layers provide 40 minutes each while the base layer provides 20 minutes.

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The permitted reduction is one hour. Table 601 assigns a two-hour base fire-resistance rating to exterior and interior bearing walls in Type IV-C construction. Footnote “a” to the table permits the ratings of the primary structural frame and bearing walls to be reduced by one hour where those elements support a roof only.

Accordingly, a Type IV-C bearing wall that supports only roof construction may be assigned a one-hour rating after application of the permitted reduction. The allowance recognizes that the element is not carrying occupied floor levels above and therefore presents a lower consequence of progressive structural failure than a wall supporting one or more floors.

The reduction is not automatic merely because a bearing wall is located at the upper story. The approved structural documents must establish that the wall supports only the roof and does not carry floor loads, transfer loads, elevated mechanical platforms, occupied rooftop structures, or other construction that invalidates the condition.

Options A and B are not reductions specified by Table 601. Option D would eliminate the entire two-hour rating and is not permitted. The inspector should verify the applicable rating and the precise load path before accepting reduced fire protection at the top story.

References/Topics: 2021 IBC Table 601, Footnote “a”; Type IV-C bearing walls and roof-support rating reduction.

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Minimum prescriptive dimensions are not themselves a component added into a calculated fire-resistance time. Therefore, option A is the exception.

IBC Section 602.4 states that a mass timber element’s required fire resistance can be based on the fire-resistance contribution of the mass timber, the noncombustible protection, or a combination of those two components. Options B, C, and D reflect those permitted approaches.

Minimum dimensions under Section 2304.11 serve a different function. They establish the minimum cross-sectional size and construction characteristics required for qualifying heavy timber or mass timber elements. A member can satisfy the minimum dimensional requirement without automatically achieving every required one-, two-, or three-hour fire-resistance rating. The structural fire calculation must evaluate the remaining effective section after charring, elevated-temperature effects, applied loading, and any credited protective materials.

The inspector must therefore verify both requirements independently: the installed member must meet the minimum code dimensions and must match the approved fire-resistance design. Increasing a member to the minimum prescriptive size does not by itself replace the required calculation, tested assembly, or approved protection system.

References/Topics: 2021 IBC Sections 602.4, 703.2, 722.7 and 2304.11; calculated mass timber fire resistance.

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The required separation is 15 feet , making option B correct. Section 602.4.2.2.4 of the 2021 IBC specifically regulates the separation distance between unprotected mass timber elements in Type IV-B construction.

Within each dwelling unit or fire area, unprotected portions of mass timber walls and ceilings must be separated from other unprotected wall and ceiling portions by not less than 15 feet . For ceiling-related surfaces, the separation is measured horizontally along the ceiling. For wall-to-wall exposure, the applicable separation is measured horizontally along the floor.

This requirement operates in addition to the maximum exposed-area restrictions. A designer therefore cannot merely demonstrate that the exposed ceiling is below 20 percent or that exposed walls are below 40 percent of floor area; the exposed regions must also satisfy the required spatial separation.

The purpose is to prevent multiple exposed mass timber regions from acting as closely spaced fuel packages during a compartment fire. During inspection, dimensions should be checked against the approved documents before required gypsum or other noncombustible protection is completed.

References/Topics: 2021 IBC Section 602.4.2.2.4; separation distance between unprotected Type IV-B mass timber elements.

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