Cost-effectiveness, material use efficiency and the ready availability of labor and materials make light-frame construction the most common type of wood construction in North America.
Approaches include platform, balloon or semi-balloon framing, which distinguish the wall-to-floor connection.
- Platform – The floor bears directly on the wall below. Ease of constructability makes this the most common approach.
- Balloon – The wall extends two or more stories and the floor is hung off a ledger connected to the wall. Balloon framing is often used in industrial and retail applications where a parapet is needed.
- Semi-balloon – The floor hangs from the top plate of the wall below and the wall above is stacked directly on the wall below. This approach is popular in multi-story applications because it helps create fire resistance continuity and minimizes shrinkage.
- Chapter 23 of the 2012 International Building Code (IBC) covers general guidelines for structural wood products and the associated minimum design requirements. This chapter offers an overview of wood design and references these American Wood Council publications:
- The National Design Specification® (NDS®) for Wood Construction includes design equations and properties for most wood products, systems and connections. It is supplemented by design examples and commentary.
- Special Design Provisions for Wind and Seismic contains information on lateral-force-resisting system design. It includes design methods, equations and system capacities.
- The 2012 Wood Frame Construction Manual (WFCM) for One- and Two-Family Dwellings includes tabulated engineered and prescriptive design and construction provisions for connections, wall systems, floor systems and roof systems based on the specific loads from ASCE 7-10 Minimum Design Loads for Buildings and Other Structures.
- Light-frame systems are typical in Type V and Type III construction and are described in IBC sections 602.3 and 602.5 respectively.
- Roofs/floors – Typical light-frame roof and floor systems consist of repetitive framing members such as rafters or trusses with wood structural panel decking. Framing components include solid sawn dimension lumber, I-joists, structural composite lumber (LVL, LSL, PSL), and parallel chord and pitched trusses. Oriented strand board (OSB) and plywood are used interchangeably as decking material.
- As part of the lateral resisting system, roofs and floors are designed as horizontal diaphragms and may require special considerations for high loads or irregular shaped structures. Diaphragm design can also affect the structure’s load distribution. In making an assumption as to whether a diaphragm is flexible or rigid, it is important to analyze the lateral deflections.
- Roofs shed water and act as part of the thermal building envelope. In many non-residential applications, low-slope roofs are common and may need special attention to ensure building longevity. How the roof is insulated can often affect the building’s thermal performance. Prescriptive paths within the energy codes appear to prefer continuous insulation, but code commentary and performance path offer opportunities for other insulation techniques.
- Walls – Typical light-frame walls consist of studs spaced at 12 to 24 inches on center (o.c.) with a double top plate and single bottom plate, and are often sheathed with wood structural panels. Construction depends on the amount of load bearing on the wall, its height and whether it’s part of the lateral-force-resisting system.
- Walls higher than 12 feet are often designed as tall walls and may require additional engineering.
- Shear walls refer to wall components that participate in the lateral resistance of the structure and can be designed a number of ways. Methods include segmented, perforated and force transfer around opening.
- Advanced framing is a combination of techniques used in light framing but is most typically recognized as using stud spacings larger than 16 inches o.c. A goal of advanced framing is to increase the insulation cavities of the exterior building envelope, thus yielding a higher system R-value and better thermal performance.
Mass timber is a category of framing styles typically characterized by the use of large solid wood panels for wall, floor and roof construction. Products in the mass timber family include cross-laminated timber (CLT), nail-laminated timber (NLT), dowel-laminated timber (DLT), glue-laminated timber (glulam, or GLT when used as panels), and structural composites such as laminated veneer lumber (LVL) and laminated strand lumber (LSL).
These products, combined with a heightened awareness of wood’s carbon benefits, have focused attention on the possibility of “tall wood” buildings, either made entirely from wood products or a combination of wood and other materials. Around the world there are now dozens of timber buildings constructed above eight stories tall.
In the United States, such buildings have been constrained by a strong reliance on prescriptive building code limits and less willingness to use performance-based fire protection engineering. That said, mass timber construction has grown significantly; more than a hundred projects have been constructed using mass timber over the past couple years and hundreds more are in design. Most of these projects are within the size limits of the building code, such as T3 Minneapolis, a 6-over-1 office building developed by Hines. A few, such as the eight-story Carbon 12 project in Portland, Oregon, have successfully used an alternative means process to go beyond the prescriptive code limits. Many projects teams are in discussion with their jurisdictions to explore other tall timber buildings.
In January 2019, the International Code Council (ICC) approved a set of proposals to allow tall wood buildings as part of the 2021 IBC. Based on these proposals, the 2021code will include three new construction types—Type IV-A, IV-B, and IV-C—allowing the use of mass timber or noncombustible materials. These new types are based on the previous Heavy Timber construction type (renamed Type IV-HT), but with additional fire-resistance ratings and levels of required non-combustible protection. The code will include provisions for up to 18 stories of Type IV-A construction for Business and Residential Occupancies.
The 2015 IBC streamlined the acceptance of CLT buildings by recognizing CLT products manufactured to the ANSI/APA PRG 320-2011: Standard for Performance-Rated Cross Laminated Timber. Under the 2015 IBC, CLT at the required size is specifically stated for prescribed use in Type IV buildings. However, CLT can be used in all types of combustible construction—i.e., wherever combustible framing or heavy timber materials are allowed.
The National Design Specification® (NDS®) for Wood Construction is referenced throughout the IBC as the standard for all wood design and is applicable to CLT as it is for all timber products.
Cost-effective materials, fast erection and improved worker safety make panelized wood roof systems a good choice for commercial and industrial buildings, particularly low-slope roof structures such as big box stores and warehouses. Using certified roof erectors can help to ensure the quality and speed of erection.
Types of Panelized Roofs:
The two most common types of panelized wood roof systems in North America are all-wood and hybrid systems. (See examples here.)
- An all-wood system consists of glued-laminated (glulam) beam girders with wood purlins (glulam, I-joists or open web wood trusses), wood sub-purlins and a wood structural panel deck. Commonly seen in buildings with spans up to 40 feet, this system is particularly well suited for applications where conveyer equipment is hung from the roof structure or in food processing facilities that need to minimize dust from overhead joists. It is also a good choice for developers and designers who want to take advantage of wood’s aesthetic for an exposed roof structure.
- The hybrid system uses steel purlin and girder trusses together with wood sub-purlins and a wood structural panel deck. Wood decking allows better economy than steel, both in terms of material and installation cost. This is often the system of choice for large warehouse and industrial structures.
- The American Wood Council’s (AWC’s) Special Design Provisions for Wind and Seismic includes information on high-load diaphragm design useful in panelized roofs. High load wood diaphragm design (accommodating ASD capacities up to 1,500 plf for seismic and 2,000 plf for wind) is possible in configurations using tall and/or heavy wall systems, especially in combination with very large diaphragms.
- AWC’s National Design Specification® (NDS®) for Wood Construction provides important design factors for members and connections that may have more impact in panelized roofs where value engineering is standard practice for the economy of the system.
- Unlimited areas are often achieved under 2015 International Building Code Section 507. However, other low-rise commercial and industrial applications use these large roofs under standard provisions of Chapter 5 using sprinkler increases and the subnotes included with Table 601.
Timber-frame designs are characterized by the use of timbers (typically larger than 4×6) arranged in two-dimensional frames at a consistent interval throughout a building. These two-dimensional frames typically consist of posts connected with beams or trusses, which resist the building’s gravity loads, while lateral forces are resisted by infill walls or bracing. Foundations are typically pads and/or piers as opposed to a continuous strip footing.
The term timber-frame describes the structural configuration of a broad category of systems (see below for examples) and should not be confused with heavy timber in the building code. The term heavy timber relates to fire resistance and is specifically defined as a construction type within the code.
The members used in timber framing are typically large enough for the structure to be classified under the International Building Code (IBC) as Type IV (heavy timber) construction. However, Type IV structures must meet other prescriptive requirements in addition to the size of framing (e.g., no concealed spaces), and designers may also choose to use a timber-frame system in Type V construction.
- General requirements are covered in Sections 602.4 and 2304.11 of the 2015 IBC or the American Wood Council publication, WCD5 – Heavy Timber Construction.
- Lumber decking requirements are referenced in Section 2304.9 of the 2015 IBC.
- Information on fire resistance can be found in the American Wood Council publication TR10 – Calculating the Fire Resistance of Exposed Wood Members or in Chapter 16 of the National Design Specification® (NDS®) for Wood Construction.
- Lumber decking diaphragm requirements can be found in the American Wood Council’s Special Design Provisions for Wind and Seismic, Sections 126.96.36.199, 188.8.131.52 and 184.108.40.206.
- Embedded post foundation design requirements are covered in 2015 IBC Section 1807.3.
Typical Timber-Frame Systems
- Post and Beam – Vertical load bearing system often accompanied by a masonry shear wall, though wood-frame shear walls or structural insulated panel systems are also feasible. Roof and floor assemblies often consist of tongue-and-groove decking overlaid by wood structural panels to provide horizontal diaphragm capacity.
- Post Frame – Economical alternative to prefabricated steel buildings for low-rise commercial buildings. In addition to gravity loads, lateral forces are also resisted by this timber frame system via embedded posts in the soil, or combined embedded posts and shear walls. Although this system was initially used mostly in agricultural applications, advances in material and foundation technology have led to a wide range of commercial and residential opportunities.
- Heavy Timber Braced Frames – With special connection detailing, a post and beam system can be turned into a lateral-resisting system. Although not recognized as a seismic-resisting lateral system in ASCE 7-10 Chapter 12, Table 12.2-1 Seismic Design Coefficients, designers in high seismic areas have been able to establish a response and over-strength factor through the alternate methods procedure of IBC Section 104.11. These systems can be used in low seismic areas without issue.
- Timber Bridges – Glued-laminated (glulam) members are often used in simple-span and multi-span bridges with composite concrete or all-wood slabs supported by arched, trussed, cable-stayed and other structural timber configurations.