NOTE This standard covers the manufacture, fabrication, quality assurance, fire-resistance design, delivery, and installation of engineered mass timber structural elements. Mass timber is a class of large-format engineered wood members built up from smaller laminations, plies, or veneers, valued for prefabricated speed of erection, exposed-wood architecture, carbon sequestration, and an inherent fire-resistance mechanism based on a predictable surface char layer. Elements covered include cross-laminated timber (CLT) panels, structural glued-laminated timber (glulam) beams and columns, nail-laminated timber (NLT) panels, dowel-laminated timber (DLT) panels, and mass plywood panels (MPP). (1.1.1)

NOTE Steel connectors furnished as part of the mass timber connection system are included in this standard. It does not cover the following, which are specified elsewhere: (1.1.3)

NOTE The following standards are referenced in this section. The edition adopted by the authority having jurisdiction (AHJ) governs where it differs from the edition cited here. (2.1.1)

NOTE IBC Type IV construction was subdivided in the 2021 IBC into three subtypes - IV-A, IV-B, and IV-C - that differ in required fire-resistance rating (FRR), allowable height, and the percentage of timber that may be left exposed. The applicable IBC edition must be confirmed with the AHJ, because the 2018 IBC does not contain these subtypes: (3.1.1)

NOTE The char-calculation method relies on a predictable rate at which the exposed wood surface converts to a protective char layer; below the char and a heat-affected zone, the residual cross-section retains structural capacity. (3.1.5)

NOTE A nominal char rate of 1.5 in/hr applies to glulam and to CLT manufactured to ANSI/APA PRG 320; the residual section is reduced by the char depth plus an additional zero-strength layer per NDS Chapter 16 before checking structural adequacy at the fire limit state. (3.1.7)

NOTE Mass timber products manufactured to ANSI A190.1 (glulam) and ANSI/APA PRG 320 (CLT) require third-party inspection and a quality mark; without a named accepted certification body the owner has no mechanism to verify compliance. (5.1.1)

NOTE Mass timber moves dimensionally with moisture: it shrinks perpendicular-to-grain as it dries toward its in-service equilibrium moisture content (EMC), and it checks or warps if delivered or stored too wet. Controlling moisture is the single most important condition for a durable installation, both at delivery and during construction. Concrete topping or acoustic mat installed over wood above 16% moisture content traps moisture against the wood and risks long-term decay. (6.1.1)

NOTE Standard interior-use adhesive is not rated for sustained wet-service exposure; exposed exterior or semi-exposed members require an adhesive qualified to ASTM D2559 for exterior conditions. (6.1.5)

NOTE CLT is a panel built from an odd number of layers (plies) of dimension lumber stacked at right angles and bonded into a single solid panel; the cross-orientation gives the panel two-way capacity and dimensional stability. ANSI/APA PRG 320 defines performance classes: E-rated classes (E1, E2, E3) are governed by the modulus of elasticity and bending strength of the boundary laminations and are selected for structural bending about the major strength axis, while V-rated classes (V1, V2, V3) are visually graded and govern appearance as well as strength. Specifying a performance class without also specifying the layup and ply orientation is a common ordering error; E-rated and V-rated classes are not interchangeable, and a wrong call generates RFIs late in fabrication. (7.1.1)

NOTE Glulam is an engineered member made from dimension-lumber laminations bonded face-to-face with the grain running parallel to the member length; it is used for beams, girders, columns, and arches at spans and loads beyond sawn timber. It is specified by a combination symbol that fixes its structural grade, and separately by an appearance grade that fixes its surface quality. The two are independent: 24F-V4 uses visually graded laminations while 24F-E8 uses E-rated laminations, and they carry different allowable stresses and costs. Specifying only an appearance grade (for example, "Architectural") without a combination symbol leaves the structural grade ambiguous and is a frequent source of bid-stage RFIs. (8.1.1)

NOTE Where CLT and glulam do not suit the application, three alternative mass timber panel systems are available: (9.1.1)

NOTE Mass timber connections range from proprietary concealed steel connectors and self-tapping screws to exposed bearing or notched connections; the choice drives both erection method and the fire-protection requirements of the connection. Concealed steel connectors are protected from fire by the surrounding wood, whereas exposed steel connectors may require supplemental fire protection to meet the connection's required FRR. (10.1.1)

NOTE Self-tapping-screw withdrawal and lateral values vary by manufacturer; specifying a generic "self-tapping screw" without citing an ICC ESR report number and edition leaves the Contractor unable to demonstrate code compliance. (10.1.3)

NOTE Mass timber buildings resist wind and seismic loads through diaphragms and shear walls; CLT diaphragms and CLT shear walls have design values and detailing rules in AWC SDPWS. (11.1.1)

NOTE Bare mass timber floors transmit airborne and impact sound readily and often fail the vibration frequency thresholds expected of office and residential occupancies; a resilient mat and concrete topping are the usual remedy and must be specified deliberately, not "optional." A floor-ceiling assembly built up with a resilient underlayment mat and a concrete topping over the timber deck raises both STC and IIC ratings and damps floor vibration. (12.1.1)

NOTE At each floor line, the joint between the mass timber floor and the exterior wall or facade must be firestopped to maintain the floor's fire rating and to resist vertical fire spread up the building perimeter; this is frequently missed in early curtainwall coordination. (13.1.1)

NOTE Mass timber is prefabricated to tight tolerances so that connectors and panel joints align in the field; the manufacturing tolerances of ANSI A190.1 and ANSI/APA PRG 320 must align with the connection shop drawings. (14.1.1)

NOTE Mass timber is most vulnerable to moisture and mechanical damage between delivery and enclosure; a deliberate storage and cover plan is required, not optional housekeeping. (17.1.1)