Cost-effective, code-compliant and sustainable, mid-rise wood construction is gaining the attention of design professionals nationwide, who see it as a way to achieve higher density housing at lower cost—while reducing the carbon footprint of their projects. Yet, many familiar with wood construction for two- to four-story residential structures are not aware that the International Building Code (IBC) allows wood-frame construction for five stories and more in building occupancies that range from business and mercantile to multi-family, military, senior, student and affordable housing.
“Once designers know that wood offers all the required safety and structural performance capabilities and meets code requirements for mid-rise, the most appealing feature of wood tends to be its price,” says Michelle Kam-Biron, P.E., S.E., a senior technical director with WoodWorks in California. “Multi-family housing was one of the first market segments to rebound from the recession—because it’s more affordable than single-family housing while offering advantages such as less upkeep and, in most cases, closer proximity to amenities. Wood construction is attractive for multi-family projects because it offers a high percentage of rentable square footage at a relatively low cost, but its benefits are equally applicable to other occupancy types.”
In addition to lower material costs, wood buildings typically offer faster construction and reduced installation costs. For example, after completing the first phase of a developer-funded five-story student housing project using steel construction, OKW Architects in Chicago switched to wood. “The 12-gauge steel panels were expensive, very heavy and difficult to install; and welding and screwing the shear strap bracing was very time consuming,” says project architect Eileen Schoeb. “Using wood was far more economical for the second phase.” Farooq Maniar, president of California-based Farooq Maniar, Inc. and structural engineer for a number of senior apartment projects also finds wood construction much less costly. “The lead time and construction time are far less than they are for steel.”
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| Built to LEED® Platinum certification, New Genesis Apartments in Los Angeles, CA, designed by Killefer Flammang Architects, is a mixed-use and mixed- income wood-frame building with commercial retail space.
Photo courtesy of KC Kim, GB Construction |
Wood construction also offers advantages for project teams seeking green building certification or simply to reduce the environmental impact of their buildings. Wood is the only major building material that grows naturally and is renewable, and life cycle assessment studies show that wood is better for the environment than steel or concrete in terms of embodied energy, air and water pollution, and other impact indicators.1
From a carbon footprint perspective, wood continues to store carbon absorbed during the tree’s growing cycle, keeping it out of the atmosphere for the lifetime of the building. The manufacturing of wood products also results in far less greenhouse gas emissions than other materials. For example, according to a new carbon calculator,2 one California apartment complex with 251 apartments housed within five stories of wood-frame construction stores nearly 4,000 metric tons of carbon dioxide equivalent (CO2e) in its lumber and sheathing, while an estimated 8,000 metric tons of emissions were avoided by using wood instead of steel or concrete.3 According to the U.S. Environmental Protection Agency, this equates to the annual emissions from more than 2,300 cars.4
“Wood is also versatile and adaptable,” says Kam-Biron. “Modifications on the jobsite tend to be straightforward and are easily made.” A survey of 227 buildings demolished in the last decade in Minneapolis/St. Paul found that buildings are often torn down within 50 years, regardless of material, because of changing needs and increasing land values as opposed to performance issues.5 Overall, wood buildings in the study had the longest life spans, showing that wood structural systems are fully capable of meeting a building’s longevity expectations. However, when you consider the embodied energy in demolished buildings and the implications of material disposal, the fact that wood is adaptable—either through renovation or deconstruction and reuse (with minimal additional processing)—is a significant advantage.
MASTERING WOOD CONSTRUCTION DESIGN
As with any other type of construction, mastering the technical details of wood-frame construction is critical to creating cost-effective buildings that are safe and durable. Building codes require all building systems to perform to the same level of safety, regardless of material used, and wood-frame structures can be designed to meet or exceed standards for (among other things) fire protection, seismic performance and resistance to high winds.
The following pages will provide an overview of technical considerations related to the design, safety and structural performance of mid-rise wood buildings, as well as two trends that are expanding the opportunities for wood use in multi-story design.
Building Code Requirements
The IBC is the predominant U.S. model building code, having been adopted by most states with or without amendments. Chapters 16, 17 and 23 cover structural wood design and construction. (Non-structural provisions such as heights and areas are covered elsewhere.) The IBC classifies buildings into five types of construction. Construction Types I and II are generally limited to non-combustible materials such as concrete and steel, although wood can be used in all types of construction to varying degrees. Type III allows both non-combustible and combustible materials, while construction Types IV and V can have combustible building materials.
Multi-story wood construction generally falls under Types III and V. (Less used is Type IV construction, also known as Heavy Timber, in which interior building elements have no concealed spaces.) Each building type is further subdivided into A and B, which have different fire-resistance-rating requirements, with A being the more rigorous. (See section online: Fire Protection Requirements.) “From a cost perspective it makes no sense to use Type I for five stories,” says Tim Smith, AIA, founding principal of Togawa Smith Martin, Inc., Los Angeles, and a pioneer of five-story wood framing in California. “Type I is more realistic for taller buildings. Type III for wood construction helps fill the gap between low-rise and taller buildings.”
| Mixed-Use Podium Building | ||
Project: Parkside mixed-use project, a combination of the Parkside affordable housing project (77 apartment units) and East Village Community Church (11,000-sq-ft sanctuary and related facilities) Parkside, a $20-million LEED® Platinum project, is an example of using code provisions to increase height and area. Installing sprinklers (according to NFPA 13) in the wood-framed portion permitted a height increase of one story to a total height of 85 ft with five stories, explains building code and life-safety consultant James E. Churchill, P.E., president of CHURCHILL ENGINEERING, INC. An additional 11,040 sq ft in the wood portion of the building was permitted through the frontage provision (IBC 506.2), which allows an increase in building area if the building fronts on a public way or open space. (Unlike the IBC, the California Building Code [CBC] permits an increase in building area based on frontage, but not on the use of sprinklers if that provision has already been employed to gain an additional story.) The concrete podium provides a three-hour fire-resistant horizontal assembly which separates the wood stories above from the stories below, allowing the wood building and the podium to be treated as separate buildings for the purposes of determining area limitations, continuity of fire walls, number of stories and type of construction. Two vertical three-hour fire walls extend from the first floor level to the roof level, separating the residential portion of the building into three separate buildings. Without the special provisions of IBC Section 509.2, which permit portions of structures to be treated as separate buildings, the fire walls would have had to run from the foundation level to the roof level, dividing the parking garages, thus making the building program impossible to deliver. |
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| Library Square in British Columbia includes five stories of wood-frame housing over a library and retail space.
Photo courtesy of naturallywood.com |
Permissible Increases in Area and Height
While Table 503 of the IBC lists allowable building heights and floor areas for different construction types, there are provisions for increases. For Type IIIA, for instance, an allowable floor area of 24,000 sq ft as stipulated in Table 503 for Group R-2 occupancies could be increased to 90,000 sq ft per story. Such provisions include:
Open front areas
IBC Section 506.2 permits area increases up to 75 percent for buildings with open spaces around their perimeters such as yards, courts, parking areas and streets, which provide fire-fighting access.
Sprinkler systems
For most occupancy groups, increases to the allowable height (and number of stories) and floor area are permitted according to IBC Section 504.2 with the use of an approved automatic sprinkler system in accordance with the National Fire Protection Association (NFPA) 13 standard.
“Type IIIA construction for Residential Group R is allowed to be four stories and 65 ft high while a Type VA building is permitted to be three stories and 50 ft,” explains Kevin Cheung, Ph.D., P.E., chief engineer for the Western Wood Products Association. “However, when protected by automatic sprinklers, Type IIIA and Type VA buildings are allowed to be five and four stories, respectively. Type IIIA is permitted an increase in height to 85 ft and Type VA an increase to 70 ft.” (See Table of IBC Allowable Heights and Areas for Residential Construction.)
In the Pacific Northwest (Washington, Oregon and Idaho), the model code has been amended to allow Type V residential buildings to have up to five stories of wood-frame construction. In Canada, the British Columbia building code was revised in 2009 to permit residential wood construction up to six stories.
Use of fire walls to “separate” buildings
While the code does not explicitly require fire walls, they may be utilized in many cases to expand the prescribed size of a building. (See Parkside sidebar above.) IBC Section 706.4 requires a three-hour rating for R-2 (long-term residential) occupancy of Type IIIA construction.
| Five Stories of Wood Over Concrete Slab | ||
Project: Advanced Individual Training Barracks with Company Operations LS3P ASSOCIATES LTD. and Clark Builders Group designed and constructed the design/build Advanced Individual Training Barracks with Company Operations in Fort Lee, VA, as the seed project under a Multiple Award Task Order Contract (MATOC) for the U.S. Army Corps of Engineers. The $68,169,000 LEED® Silver project consists of two five-story barracks situated on an 11-acre site. Each 180,000-sq-ft building has 600 beds and meets IBC 2006, NFPA 101 egress and life safety provisions, plus Anti-Terrorism/Force Protection Criteria. Set on a reinforced concrete mat slab, the Type IIIA five-story wood-framed buildings are clad primarily in brick. “The team provided three structural options: cast-in-place concrete, light-gauge steel framing and panelized wood stud framing,” recalls Chris Ions, AIA, LEED AP, vice president/principal of LS3P ASSOCIATES LTD. “The steel came in $2 million cheaper than the cast-in-place concrete and the wood came in $2 million cheaper than the steel, a significant difference.” The advantage of working as a design/build team was being able to explore a number of framing choices before deciding, which was the most cost efficient. To minimize the impact of shrinkage, the walls are balloon framed with flooring rim beams on the inside, rather than on top of the top plates (platform framing) at each floor of vertical framing. The first floor has the heaviest framing with triple studs of southern yellow pine, selected for its high compression strength. The first three stories of brick veneer were supported on the foundation; steel relieving angles through-bolted to the wood framing were used to support brick at the fourth and fifth floor levels. “The greatest challenge of wood construction is looking at every assembly, transition and penetration to ensure that fire-rated assemblies are truly continuous,” says Ions. |
IBC Section 706 permits portions of a building separated by one or more fire walls to be considered as separate, side-by-side buildings. In this way, wood-frame buildings can be designed as separate but connected buildings for code-compliance purposes.
Mezzanines
IBC Section 505 excludes mezzanines from the determination of number of stories or building area. Mezzanines may be one-third of the floor area of the room or space beneath. A loft or penthouse plus a mezzanine on the first concrete podium floor could add two “stories” to the building. (See Union Square sidebar below.)
| Seven Residential Levels | ||||
Project: Union Square Union Square made use of code provisions to increase the height of the project by adding two additional floors for residential use. First, utilizing IBC Section 505, designers added a mezzanine, increasing the number of wood-framed levels to six. Second, since the project was not located in a retail neighborhood, the concrete level at grade was designed to incorporate residential stoop units, each with access to the street. The building was thus able to achieve seven levels of residential use with a density of 143 units per acre.
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Podium design
In order to gain an additional story for multi-level wood-framed buildings, architects and engineers are increasingly turning to podium or pedestal design rather than building directly on a concrete slab on grade. IBC Section 509.2 allows five- or six-story wood-frame structures over one level of typically concrete Type IA construction. These “five-over-one” and “six-over-one” buildings are treated in the code as two structures separated by a three-hour fire-resistance-rated horizontal assembly. The podium is considered as a separate and distinct building for the purpose of determining height and area limitations and continuity of fire walls. For the purposes of IBC Table 503, the height of the two buildings is measured from the grade plane.
FIRE PROTECTION REQUIREMENTS
Fire-resistance rating requirements for building elements are provided in IBC Table 601. Chapter 7 of the IBC covers materials and assemblies used for fire resistance-rated construction and separation of adjacent spaces.
Wood-frame building elements that require fire-resistance are required to have the fire-resistance rating be determined in accordance with the test procedures set forth in ASTM E119, Standard Test Methods for Fire Tests of Building Construction Material. The code also provides alternative methods for determining fire resistance per IBC Section 703.3, which includes prescription tables in 720 or calculations for fire-resistance in 721.
Fire-Resistance Assemblies
One-hour and two-hour fire-resistance requirements are generally provided by fire-rated assemblies that include gypsum sheathing.
Lists of accepted rated wood floor and wall assemblies are available from the American Wood Council, DCA 3: Fire-Rated Wood-Frame Wall and Floor/Ceiling Assemblies at www.awc.org, and from the APA (W305) atwww.apawood.org. Other sources are (UL) Fire-resistance-rated Systems and Products, the UL Fire Resistance Directory, and the Gypsum Association’s Fire Resistance Design Manual.
Fire-Retardant-Treated Wood (FRT)
The building code (IBC Section 2303.2) permits exterior two-hour rated bearing walls in Type IIIA construction to be constructed of fire-retardant-treated (FRT) wood, an acceptable substitute for a non-combustible material.
Designing for Different Fire Ratings
One framing consideration for Type IIIA is that floors and walls may have different fire-ratings. Since load bearing exterior walls are required to have a two-hour rating, the intersection of a one-hour floor requires detailing. This can be accomplished by allowing the wall studs with one layer of gypsum to extend to the underside of the floor sheathing, and supporting the floor framing off the top plates with the use of a hanger designed to span over a layer of gypsum sheathing. Another framing option is to hang the floor framing off a 2x ledger, which provides as much if not more fire resistance as the layer of gypsum. The one-hour wall rating plus the one-hour floor system yields the required two-hour rating.
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| Typical vertical fire wall with three-hour rating. Note: building departments in high seismic areas have allowed floor sheathing to be continuous. |
Image courtesy of CGC Inc.
Fire Walls
Fire walls are not required by the building code; however, they may be utilized to increase the square footage of a building. For R1 (short term residential) and R-2 Type IIIA structures, the IBC requires a non-combustible three-hour fire-resistant wall between buildings. However, for Type V construction, walls are permitted to have a combustible two-hour fire-resistance-rating. Under the NFPA 221 Standard for High Challenge Fire Walls, Fire Walls and Fire Barrier Walls, designers may build a two-hour rated fire wall using two contiguous one-hour fire-resistance-rated assemblies.
HOW TO FRAME WITH WOOD
In addition to selecting the appropriate framing technique, designers of wood buildings must consider factors such as shrinkage, differential movement, and seismic requirements.
Platform Framing and Balloon Framing
There are two common types of framing for wood construction. For Type VA buildings, where exterior walls require a one-hour fire-resistance rating, traditional platform framing is usually used, where the joists sit on top of the double top plates of the wall. Balloon framing is where the joists hang off the ledger that is attached to the structural studs that frame the building. In modified or semi-balloon framing, the floor framing hangs off the double top plates; it is often used as an alternate to platform-framed structures for both Type VA and Type IIIA construction.
Shrinkage
Regardless of the framing type, building designs must compensate for the fact that wood shrinks as it dries. Shrinkage continues until wood reaches its Equilibrium Moisture Content (EMC), which averages 8-12 percent of moisture content for most structures in the U.S. The Western Wood Products Association (www2.wwpa.org) offers a technical guide that includes formulae for calculating shrinkage for different wood species across the country as well as a downloadable shrinkage estimator.
“Shrinkage calculations aren’t complex,” says Kam-Biron, “but it’s an area designers aren’t always familiar with, and it can be challenging to detail for differential movements between two different materials and overall shrinkage.” The shrinkage effects must be considered for horizontal framing members (width or thickness) in the wall (top/sill plates) and floor (joists) design. Wood is anisotropic, meaning the dimensional change in wood is unequal in different directions. In most softwoods, radial shrinkage (across growth rings) is approximately 4 percent and tangential shrinkage (parallel to growth rings) is approximately 8 percent from green (unseasoned) to typical EMC for structures in the U.S. Longitudinal shrinkage (parallel-to-grain) for vertical framing members is generally negligible and does not affect building performance. Therefore, the majority of shrinkage will occur in the top plates, sill plate and sole plates, and possibly the floor joists—depending on how the floor framing members are framed to the wall. If the framing is balloon framed or modified balloon framed, then sawn lumber joists won’t play a huge role in overall movement from shrinkage because balloon framing, unlike platform framing, does not accumulate shrinkage over all floors. “Unseasoned (green) sawn lumber will shrink more compared with seasoned (dried) lumber,” says Cheung “Shrinkage should be considered for wood-frame buildings over three stories. The good thing about wood is that it will dry naturally.” Among the many ways to minimize shrinkage is letting the wood dry during construction, or using products and systems such as pre-engineered metal-plate connected wood trusses for floor and/or roof framing, and manufactured wood products (laminated veneer lumber, I-joists, etc.). I-joist floor systems are dimensionally stable and offer minimal inter-floor shrinkage.
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| Accumulated shrinkage for a five-story platform framed building can amount to between 1-¼ in. and 1-½ in.
Image courtesy of WoodWorks |
Additionally, there can be some overall settlement of the building that may occur due to gaps in the building construction that can contribute to the overall vertical movement. Some contractors will distribute the dead load throughout the height of the building and allow the building to acclimate to the environment and/or air dry prior to installation of the drywall, thus allowing the building to naturally settle.
Differential Movement
Allowing for differential movement between wood and other structural elements and building finishes is critical. Steel, concrete and brick continue to expand and contract due to temperature changes, while wood generally maintains its dimensions having reached its EMC. Differential movement occurs when, for example, floor joists are supported by a wood-frame wall at one end and by the masonry block of an elevator shaft at the other end. Areas such as stairwells, shafts and vaulted ceilings require attention for differential movements as do plumbing, electrical and mechanical systems. Using flexible joints such as flexible pipe, conduit, couplings, elbows, and tees for electrical, mechanical and plumbing between floors can prevent potential problems. The design of the joints between building envelope components, such as windows and doors, must also allow for differential shrinkage.
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| Options for managing shrinkage: (A) supporting joists with hangers in balloon framing can reduce the effect of ½-in. shrinkage; (B) engineered floor I-joists. Platform framing option: (C) open-web trusses and lumber joists may be used in traditional platform systems.
Image courtesy of Togawa Smith Martin |
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Semi-balloon framing option for Type IIIA construction. When floor trusses hang off the top plate, shrinkage is contained in one joint.
Image courtesy of Togawa Smith Martin |
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An example of a 1-in. sealant joint that allows the window to move down. If the joint were ½-in. the window frame may bind.
Image courtesy of Canadian Wood Council |
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| Plumbing joints with pipes nested in joint to allow for vertical movement of structure.
Photo courtesy of WoodWorks |
Plumbing sleeve connected to structure with foam wrap to allow vertical movement of structure.
Photo courtesy of WoodWorks |
Seismic Requirements
Earthquakes and rigorous seismic requirements are a well-known aspect of building on the west coast, but other parts of the country, especially in the east, are not immune to earthquake activity and the need to comply with seismic codes. Wood structures offer a high strength-to-weight ratio over those built with steel and concrete (concrete walls are about seven times heavier than typical wood-frame walls) resulting in a low inertia force during earthquakes. In wood-framed buildings, the large number of walls and floors often used in a project transfer lateral loads induced by winds and seismic forces.
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| After being lifted to a shake table and subjected to four progressively intense earthquakes, this six-story light-frame building was found to have only minimal damage to the gypsum wallboard and nail connections. (View a video of the test on YouTube.6)
Photo courtesy John Van De Lindt, Univ. of Alabama |
Wood-frame construction also provides numerous load paths through shear walls and diaphragms, which typically have hundreds of structural elements and thousands of nail connections, adding ductility and redundancy to the system. This means that overloading can be transferred to alternate load paths should some connections fail. In contrast, structures supported by heavy non-wood frames have relatively few structural members and connections, which can lead to overloading. Moreover, the large number of walls reduces the loads shared by each wall. Tests and observations from past earthquakes show that wood buildings have performed well. For example, a six-story light-frame wood building tested on the world’s largest shake table in Japan resisted a major 2,500-year earthquake with minimal damage.
Current building code requirements for wood diaphragms, shear walls and holdown devices work effectively in creating earthquake-resistive structures. Horizontal diaphragms in roofs and floors transfer the horizontal forces to the shear walls. Shear walls with holdowns such as an anchor tie-down rod system resist the overturning forces in an earthquake, while wood studs or columns absorb the compression forces.
In high seismic areas, mid-ply shear walls (i.e., walls sheathed with structural wood panels with studs nailed on both sides) are often used on exterior, hallway and demising walls. These have been recognized for providing superior performance against strong forces resulting from wind storms and earthquakes.
| Five Stories of Wood over a Concrete Podium | ||
Project: Americana at Brand—Excelsior Condominiums The Americana in Glendale, CA consists of five stories of luxury condominiums and retail space. The residential part of the building is Type IIIA wood framing, while the retail structure is Type IA raised concrete podium with two levels of subterranean parking. Utilizing the special provisions of IBC Section 509.2, the Type I retail use was considered a separate building for purposes of determining area, number of stories, and construction type. Since the project is in a high-traffic section of Glendale, the bottom floor was used for stores and replicates the European planning concept of complementing retail with residential occupancy above. |
INNOVATIVE WOOD PRODUCTS AND BUILDING SYSTEMS
There are a number of innovative design technologies such as panelized systems and cross laminated timber (CLT) that are increasing the opportunities for mid-rise wood buildings.
Panelized Systems
Specifying prefabricated or factory manufactured wall and roof panels can provide better quality wall construction since the panels are created off-site in a moisture controlled environment. Wall panelizing is particularly useful for jobsites that don’t have adequate space to construct the walls and can speed up the erection time considerably. Wall panels can optimize stud design and increase sound proofing and energy efficiency. Panels may range from 4 to 30 ft long, and are lifted into place by crane.
Cross Laminated Timber (CLT)
Cross laminated timber is a multi-layer wood panel in which each layer is oriented crosswise to its adjacent layer for increased dimensional stability and structural performance. Widely used in Europe and now available in North America, it is considered viable for buildings up to 12 stories and even higher.
In the U.K., CLT has been used to construct an eight-story all-wood building, while North American applications include a two-story office building in Montana, and a five-story heavy timber/CLT hybrid building at the University of British Columbia.
Among CLT’s attractive structural characteristics are high dimensional stability, high axial load capacity, high shear strength, rigidity around openings and negligible settlement effects. CLT assemblies also offer inherent fire resistance due to thick cross-sections that, when exposed to fire, char at a slow and predictable rate, and the ability to resist lateral loads caused by earthquakes or high winds. Other benefits include speed of construction, thermal performance, and the environmental advantages offered by all wood products—including a low carbon footprint.
CLT is also price competitive. Costs for using CLT in Stadthaus, a residential building in London with eight stories of wood over a concrete podium, were 15 percent less than for a comparable concrete design.
Although CLT is not yet referred to in the IBC, specifiers can propose it as an alternative solution. For more information, APA (www.apawood.org) has developed an American National Standard, ANSI/APA PRG 320 2011: Standard for Performance Rated Cross-Laminated Timber.
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| Wood I-joists are lightweight and designed to carry heavy loads over long spans. Resiliency channels dramatically reduce the amount of sound conducted though the structure.
Image courtesy of Maxxon Corporation |
CONCLUSION
The last few years have seen a trend toward taller wood buildings, driven by their acceptance in building codes and the value they provide. Design professionals are capitalizing on wood’s ability to offer high density at less cost than other materials. They also appreciate wood’s versatility, adaptability and light carbon footprint. However, while today’s building codes recognize wood’s safety and performance capabilities in buildings that are five and six stories—and these are becoming increasingly common—innovative technologies such as CLT can be expected to propel designers of mid-rise wood buildings to even greater heights
