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

A 7/16-inch wood structural panel does not satisfy the prescribed minimum thickness for fireblocking. Therefore, option C is the exception.

IBC Section 718.2.1 permits one thickness of 23/32-inch wood structural panel where the joints are properly backed. A 7/16-inch panel is substantially thinner and cannot be substituted without an approved alternative-material determination. The code also recognizes one-half-inch gypsum board and properly installed batts or blankets of mineral wool or other approved materials.

Mass timber complying with Section 2304.11 is also permitted as fireblocking. Its large cross section provides resistance to the movement of flame, hot gases, and smoke through a concealed space that is at least equivalent to the nominal lumber traditionally accepted for this purpose.

The inspector must distinguish fireblocking from a tested firestop system. Fireblocking divides or interrupts concealed combustible cavities; it does not automatically protect a penetration through a fire-resistance-rated wall or floor. Installation must completely fill or close the required space, remain securely supported, and comply with requirements for joints, edges, penetrations, and mineral-fiber compression.

References/Topics: 2021 IBC Section 718.2.1; Section 2304.11; approved fireblocking materials in mass timber construction.

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IBC Section 703.7 requires sealant or adhesive to resist air passage at the abutting edges and intersections of mass timber building elements where those elements are required to be fire-resistance rated. Section 1705.20 then requires periodic special inspection where those prescribed sealants or adhesives are installed. Option C repeats that first required location accurately.

Option D is defective because the code requires sealing at an intersection between mass timber and another material only where both intersecting building elements are required to be fire-resistance rated. The phrase “one or the other” materially weakens the condition. Option A is also too broad. A code-defined joint is not automatically subject to Section 703.7; fire-resistant joint systems are separately regulated by Section 715 and, where applicable, Section 1705.18.

Option B describes the exception in reverse. Sealant or adhesive need not be added under Section 703.7 where it is not a required component of the tested fire-resistance-rated assembly. When it is part of the listed or tested assembly, it must be installed accordingly, but that statement does not identify the primary location requested.

References/Topics: 2021 IBC Sections 703.7 and 1705.20; sealing of adjacent rated mass timber elements; periodic inspection.

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Option A is the incorrect statement and therefore the required answer. Under 2021 IBC Table 601, an exterior bearing wall in Type IV-C construction requires a two-hour fire-resistance rating , not one hour.

The tall mass timber classifications progressively vary the degree of noncombustible protection and allowable exposure, but the structural fire-resistance requirements remain substantial. Type IV-A exterior bearing walls require three hours. Type IV-B exterior bearing walls require two hours. Type IV-C exterior bearing walls also require two hours. Thus, options B, C, and D correctly reproduce the Table 601 values.

The special inspector must also recognize that Table 601 is not necessarily the only rating criterion affecting an exterior wall. Exterior walls are additionally subject to the requirements associated with fire separation distance, particularly Table 705.5. Where another IBC provision requires a higher rating, that more restrictive condition controls.

For mass timber construction, compliance also depends on the prescribed protective system, member dimensions, connections, joints, intersections, and exterior-side protection. A wall cannot be accepted as compliant solely because the timber cross section appears substantial; it must conform to the approved rated design.

References/Topics: 2021 IBC Section 601; Table 601; Section 602.4; Table 705.5.

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A conventional 2 × 6 solid-sawn stud is a light-frame member and does not satisfy the minimum cross-sectional dimensions associated with mass timber or heavy-timber construction. Therefore, option A is the exception.

The IBC defines mass timber as Type IV structural elements primarily composed of solid, built-up, panelized or engineered wood products that meet the minimum cross-sectional dimensions required for Type IV construction. A nominal 2 × 6 stud is substantially smaller than those dimensions and is ordinarily classified as conventional light-frame construction. Its use might be permitted in a particular nonstructural partition or another specifically authorized application, but it is not itself a qualifying mass timber structural element.

CLT is expressly recognized as a mass timber panel product. Parallel-strand lumber is a form of structural composite lumber and can qualify where it satisfies the applicable minimum dimensions, product-standard requirements and approved structural design. Dowel-laminated timber is a built-up panelized timber product and may likewise serve as a mass timber element when its dimensions, engineering and approval satisfy the construction documents and code.

The classification depends on the completed element’s dimensions and approved structural function—not merely on the fact that it is manufactured from wood.

References/Topics: 2021 IBC Chapter 2 definitions of “Mass Timber” and “Conventional Light-Frame Construction”; Sections 602.4 and 2304.11.

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ASTM E84 cannot be used to establish the hourly fire-resistance rating of a structural member or assembly. Therefore, option B is correct.

ASTM E84 evaluates surface-burning characteristics , principally flame-spread and smoke-developed indices. Those measurements characterize how a material ' s exposed surface behaves when subjected to the standard tunnel test. They do not demonstrate that a wall, floor, roof, beam, column, or structural assembly will maintain integrity and structural performance for one, two, or three hours.

Fire-resistance ratings are instead established through the methods recognized by the IBC, including standard fire testing—principally ASTM E119 or UL 263—prescriptive rated construction provided by Section 721, and calculated fire-resistance procedures under Section 722. The code also recognizes appropriately documented approved designs and other approved methods within the Section 703 framework.

This distinction is particularly important in mass timber work because ASTM E84 may appear elsewhere in the code for flame-spread limitations on materials, but an acceptable E84 result does not establish structural fire endurance.

The special inspector must determine which approved fire-resistance methodology applies and verify that the installed assembly precisely reflects that design.

References/Topics: 2021 IBC Sections 703, 721 and 722; ASTM E119; UL 263; distinction between fire resistance and ASTM E84 surface-burning testing.

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ASTM C920 is the required standard for sealants used under the IBC mass timber air-sealing provision. Option C is therefore correct. Section 703.7 applies to Type IV-A construction and requires compliant sealant or adhesive at specified interfaces involving fire-resistance-rated mass timber elements.

The sealing requirement reduces the movement of air, heat, and combustion gases into gaps that could expose additional timber surfaces and undermine the rated performance assumed by the approved design. The sealant must not merely appear to close the opening; it must be the specified product and installed within its approved joint dimensions and substrate conditions.

ASTM A36 is a structural steel material specification. ASTM E119 establishes procedures for fire testing building construction and assemblies but does not qualify elastomeric sealant products. ASTM F1667 covers driven fasteners such as nails, spikes, and staples and has no application to this sealing requirement.

Under Section 1705.20, installation is subject to periodic special inspection where the Section 703.7 sealant is applied. Inspection should occur before concealment and confirm continuity, adhesion, backing, joint depth, surface cleanliness, and compliance with the approved documents.

References/Topics: 2021 IBC Sections 703.7 and 1705.20; Type IV-A construction; ASTM C920.

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Option A reproduces one of the two permitted exterior gypsum-board fastening methods in 2021 IBC Section 722.7.2.2. Type X gypsum board used as noncombustible protection on the exterior side of mass timber walls may be attached using galvanized nails of minimum 12 gage with a 7/16-inch head and sufficient length to penetrate the mass timber at least 1 inch .

The code also permits ASTM C1002 Type S, W, or G screws, but those screws must likewise penetrate the mass timber a minimum of 1 inch . Options C and D are therefore incorrect because they specify only 3/4-inch and 7/8-inch penetration, respectively. Option B is incorrect because it substitutes 16-gage nails for the minimum 12-gage requirement.

Section 722.7.2.2 additionally requires exterior Type X gypsum-board layers to be fastened 12 inches on center each way and 6 inches on center at joints or ends. Panel-edge fasteners must be located at least 1 inch and not more than 2 inches from the edge.

These fastening details are part of the conditions necessary to claim the protection contribution in Table 722.7.1(2).

References/Topics: 2021 IBC Sections 722.7.1 and 722.7.2.2; exterior Type X gypsum-board protection.

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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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The intended answer is ANSI/AWC NDS Chapter 16. The video option uses “AWS,” but the correct standards organization is the American Wood Council—AWC . Option D is therefore correct based on its intended reference.

IBC Section 722.1 expressly permits exposed wood members and wood decking to have their calculated fire resistance determined in accordance with Chapter 16 of the ANSI/AWC National Design Specification for Wood Construction. The method evaluates charring, the effective char depth, the reduced residual cross section, elevated-temperature effects, loading, and the remaining structural capacity for the required exposure period.

ACI 216.1 applies to concrete and masonry fire resistance. TMS 0216 addresses masonry assemblies. ASCE 29 Chapter 5 provides procedures for calculating the fire resistance of structural steel assemblies. Those references are code-recognized methods, but they apply to different construction materials.

For mass timber inspection, the approved calculations should identify the exposed surfaces, required rating, timber species or product, member dimensions, design loads, protective contribution, and connection treatment. Field reductions in member dimensions can invalidate the calculated fire resistance.

References/Topics: 2021 IBC Section 722.1; ANSI/AWC NDS Chapter 16; calculated fire resistance of exposed wood members and decking.

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The calculation begins with 2021 IBC Table 506.2. Because the building is a Group B occupancy, has more than one story, and is protected throughout by an automatic sprinkler system complying with Section 903.3.1.1, the applicable tabular classification is “SM”—a multistory, fully sprinklered building.

For Group B and Type IV-A construction, Table 506.2 assigns an SM allowable area factor of 324,000 square feet. However, this is not the final total allowable building area. Section 506.2.1 requires a single-occupancy building with more than three stories above grade plane to use Equation 5-2. Where no frontage increase is stated, the calculation is:

324,000 square feet × 3 = 972,000 square feet.

The story-area multiplier is limited to three even though the building contains 15 stories. The code does not permit multiplying 324,000 square feet by all 15 stories. Additionally, the actual area of an individual story cannot exceed its calculated allowable area. Option A represents a different construction-type area factor, while option B does not result from the required IBC multistory calculation. The building is not automatically unlimited merely because it is Type IV-A and sprinklered.

References/Topics: 2021 IBC Sections 506.2 and 506.2.1; Equation 5-2; Table 506.2, Group B, Type IV-A, SM area factor.

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