Type IV-C generally permits interior mass timber surfaces to remain exposed, but shaft and elevator hoistway enclosures are a specific exception to that broad exposure allowance. Section 602.4.3.6 requires shafts, elevator hoistways, and interior exit stairway enclosures constructed with mass timber to receive noncombustible protection having a minimum assigned protection time of 40 minutes . Option A is therefore correct.

Critically, protection is required on both the inside and outside of the shaft enclosure . The 40-minute value is an assigned contribution from the noncombustible protection; the complete enclosure must still satisfy whatever overall fire-resistance rating is required by Sections 713, 713.4, Table 601, or other applicable provisions.

Option B incorrectly applies the general Type IV-C exposed-interior allowance to a specifically regulated shaft condition. Option C is incomplete because 1/2-inch Type X gypsum board provides only 25 minutes under Table 722.7.1(2), not the required 40 minutes. A 5/8-inch Type X board provides a recognized 40-minute contribution when installed in accordance with Section 722.7.2. Option D does not state the prescribed Type IV-C shaft requirement.

References/Topics: 2021 IBC Sections 602.4.3.2 and 602.4.3.6; Tables 722.7.1(1) and 722.7.1(2).

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Type IV-B construction is fundamentally protected mass timber construction, but the 2021 IBC permits specifically limited areas of interior mass timber to remain exposed. Section 602.4.2.2.2 establishes two independent exposure limits. Unprotected portions of mass timber ceilings, including attached beams, are limited to 20 percent of the floor area of the dwelling unit or fire area. Unprotected portions of mass timber walls, including attached columns, are limited to 40 percent of that floor area . Accordingly, option C correctly states the permitted percentages.

These percentages do not operate in isolation. Exposed areas must also comply with Section 602.4.2.2.4, which establishes separation requirements between unprotected mass timber surfaces. Where both walls and ceilings are exposed within the same dwelling unit or fire area, Section 602.4.2.2.3 requires use of the mixed-unprotected-area equation rather than independently maximizing both percentages.

Option A describes neither Type IV-B nor its controlled-exposure concept. Option B incorrectly characterizes Type IV-B as requiring complete encapsulation. Option D reverses the ceiling and wall percentages.

References/Topics: 2021 IBC Sections 602.4.2.2.2, 602.4.2.2.3 and 602.4.2.2.4; Type IV-B permitted exposed mass timber.

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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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Type IV-HT construction permits concealed spaces where those spaces comply with the specific protection alternatives prescribed for traditional heavy-timber construction. One permitted method is to completely fill the concealed space with noncombustible insulation. Therefore, option D is correct.

This provision should not be confused with the concealed-space requirements applicable to the tall mass timber classifications—Types IV-A, IV-B, and IV-C. Those classifications employ dedicated noncombustible protection and assigned-time requirements intended to control the contribution of concealed combustible construction during a fire. Type IV-HT follows a different concealed-space framework and allows protection through approved alternatives, including sprinkler protection, gypsum-board enclosure, or complete filling with noncombustible insulation, as applicable.

The insulation must completely occupy the concealed volume. Merely installing partial batt insulation, leaving unprotected voids, or relying on nominally fire-resistant thermal insulation does not satisfy the provision. The inspector should verify the material’s noncombustibility, continuity, placement around obstructions, and conformance with the approved construction documents before the space becomes inaccessible.

References/Topics: 2021 IBC Section 602.4.4; concealed spaces in Type IV-HT construction; noncombustible insulation protection method.

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A separate inspection by the building official is required where the fire resistance of a structural connection in Type IV-A, IV-B, or IV-C construction is provided by wood cover calculated in accordance with Section 2304.10.1. Therefore, option C is correct.

The required inspection occurs after the calculated wood cover has been installed but before additional coverings or finishes conceal the connection. This sequencing allows the building official to verify the actual cover thickness, geometry, continuity, fit, fastening, and correspondence with the approved fire-resistance design.

The requirement is distinct from routine Chapter 17 special inspection. Although a qualified special inspector may observe connection installation and mass timber erection, Section 110.3.5 expressly identifies a required building-department inspection for this calculated wood-cover condition. The inspection cannot be bypassed merely because the connection was observed during structural installation.

A rated joint system under Section 715 is governed by its tested or listed assembly requirements but is not the circumstance identified by this specific inspection provision. The other options do not state the code trigger. Inspectors should therefore coordinate notification before the connection is enclosed because subsequent destructive exposure may otherwise be necessary.

References/Topics: 2021 IBC Sections 110.3.5 and 2304.10.1; inspection of calculated wood cover protecting mass timber connections.

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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 required minimum wood protection thickness is 1.75 inches , making option B correct. AWC Technical Report No. 10 establishes a thermal-separation methodology for protecting steel connectors and fasteners with sacrificial wood cover. Because the question specifies that the wood cover itself must provide the entire 60-minute fire-protection period, the protection thickness must be calculated using the thermal-separation provisions rather than relying on residual fire resistance from the connector or steel plate.

TR10 applies a reduction to the protection contribution of the base wood layer. Its connection examples use the relationship in Section 4.4.1.3, with the required thermal protection time divided by 0.85. For a 60-minute requirement, back-calculating the prescribed nonlinear wood charring equation gives approximately 1.71 inches of required wood thickness. The practical minimum corresponding to the available choices is therefore 1.75 inches.

The principle is critical for special inspection: steel connection components have high thermal conductivity and must remain below the temperature limits assumed by the connection fire design. The inspector must verify actual wood-cover thickness, fastening, continuity, and any gaps before concealment.

References/Topics: AWC Technical Report No. 10, Sections 4.4.1.3 and 4.5; connection thermal separation; wood protection of steel connectors. TR10 demonstrates the same calculation methodology for wood-covered steel connections.

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The connection detail specifies the CLT-to-glulam screws at 24 inches on center , so option D is correct. This is a construction-document interpretation question rather than a universal prescriptive screw-spacing requirement imposed by the IBC.

The approved structural detail controls the required spacing for this particular connection because the registered design professional has established the connection geometry and fastener schedule based on the applicable structural loads, CLT panel configuration, glulam member, fastener properties, and required load transfer. The 24-inch dimension must therefore be maintained in the field unless an approved revision changes the connection design.

IBC Table 1705.5.3 is directly relevant to the special inspector’s responsibilities. For screws used in Type IV-A, IV-B, or IV-C mass timber connections, periodic special inspection includes verification of diameter, length, head type, spacing, installation angle, and installation depth . The special inspector is therefore expected to compare actual field installation against precisely this type of approved connection detail.

Substituting 12-, 16-, or 20-inch spacing without design authorization would constitute a deviation from the approved documents, even if the alternate spacing appeared more conservative.

References/Topics: Approved structural connection detail shown in the exhibit; 2021 IBC Section 1705.5.3 and Table 1705.5.3, inspection of screw spacing and installation.

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Option B is correct because the required protection time placed over a mass timber structural connection may be established through a code-recognized fire-resistance evaluation of that protective construction under standard fire exposure.

IBC Section 2304.10.1 regulates connections in Types IV-A, IV-B, and IV-C. The code allows connection fire performance to be demonstrated through testing where the connection is incorporated into the fire-resistance test or through an approved engineering analysis. AWC further clarifies that the protection applied over the connection must provide the necessary protection time and that its performance may be established using the methods recognized by IBC Section 703.3, including testing and engineering analysis based on ASTM E119 or UL 263 performance.

Option A is too narrow: testing an isolated fastener does not establish the fire protection of the entire connection. Options C and D also use the wrong temperature location. Where an engineering-analysis route is used, the temperature criterion applies to the connection itself , including connectors, fasteners, and portions of wood incorporated into the structural connection—not simply to the protected structural element or supported member.

References/Topics: 2021 IBC Section 2304.10.1; Section 703.3; AWC guidance on fire resistance of mass timber connections.

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A proprietary field-glued mass timber beam splice requires special inspection because the connection relies on a field-applied adhesive process that is not covered as a routine prescriptive mass timber connection in Table 1705.5.3. The controlling mechanism is IBC Section 1705.1.1, which authorizes the building official to require special inspection for unusual materials, systems, or installation procedures, particularly where compliance depends on additional manufacturer installation instructions.

The ICC code-development rationale for the tall mass timber provisions specifically identifies field-glued mass timber beam or panel splices as an example of work for which special inspection can be required. Inspection must address the approved evaluation report and manufacturer’s instructions, including substrate preparation, moisture and temperature limits, adhesive storage and expiration, mixing or proportioning, application quantity, open time, member alignment, clamping or pressure, and curing conditions.

Approval by the permit applicant has no regulatory significance. Verification by the registered design professional may supplement the inspection program but does not replace the approved special inspector. A building official may observe the work, but routine field verification by that official is not the prescribed quality-control procedure.

References/Topics: 2021 IBC Sections 1704.2, 1704.3 and 1705.1.1; special cases; proprietary field-glued mass timber splices.

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