While wood-frame construction is nothing new, forward-thinking engineers, architects and builders are turning heads by using wood increasingly in five-story-and-higher multi-family structures.
From the outside, these wood-frame structures look like typical condominiums and other housing complexes. Inside, however, savvy building professionals are using both traditional framing and new products, such as cross laminated timber (CLT), to take advantage of wood's environmental and performance benefits while remaining cost-effective.
Despite the growing number of wood-frame buildings, some people in the industry are still surprised to learn that wood can be used to build anything higher than a four-story residential structure.
The International Building Code (IBC) allows wood-frame construction for five stories and more by meeting Type III-A construction requirements; Type V-A construction can be used in the Pacific Northwest. Type IV construction, also known as heavy timber construction, can also be used, but no concealed spaces are allowed.
According to IBC Table 503, Type III-A residential construction allows a building of up to four stories and 65 feet in height, while Type V-A construction allows three stories and 50 feet. Per Section 504.2, when the structure is fully sprinklered to meet the requirements of National Fire Protection Association (NFPA) 13, "Standard for the Installation of Sprinkler Systems,' designers may add up to 20 feet in height and another story. A wood building categorized as Type III-A construction is very similar to one of Type V-A construction. However, among other things, the code requires the use of fire-retardant treated (FRT) wood for exterior walls and exterior bearing walls need to be 2-hour fire resistance rated.
There are other provisions for increases. For example, Section 505 of the 2009 IBC allows a Type III building to have six or seven "levels' of wood-frame construction if the level qualifies as a mezzanine instead of a story. Another level above grade may be added if the building includes a concrete podium, per Section 509. This allows the wood structure to be considered as a separate and distinct building for the purpose of determining area limitations, continuity of firewalls, limitation of number of stories and type of construction.
Also, because Section 502 of the IBC defines grade plane to be "the average of finished ground level adjoining the building at exterior walls,' the overall height of the building may be measured from the lowest point between the building and the lot line, instead of from the podium, allowing additional height.
Many developers are using these allowances to their advantage.
The Marselle condominium complex in Seattle added a mezzanine level on top of the allowed five-story structure to extend the building envelope to the 85-foot maximum zoning height allowed. Doing so gave the top level condo units water views of Seattle's Lake Union, increasing the value of the complex by $1 million.
Once designers learn how wood meets code requirements for mid-rise buildings, the most appealing feature of wood-frame construction tends to be cost. Wood works well for multi-story projects because it offers a high percentage of rentable square footage at a relatively low cost. If the Marselle complex had been built using concrete, the engineer estimates that the cost would have been about 30 percent higher.
Contractors used panelized walls for Marselle, which also made a big economic impact. Crews were able to install the walls in less than two weeks per floor, which lowered construction costs significantly. Panelization also simplified construction logistics because space was limited on the job site (as is common in most urban infill projects).
Pre-panelized walls add value in other ways. Because they are assembled in a controlled environment, finished panels typically meet tighter tolerances, which can help reduce shrinkage due to gaps. Cost of on-site labor and construction time can be reduced by 50 percent, allowing wood to be even more competitive.
Meet the multi-story challenge
Wood-frame construction can be effectively used in multi-story residential buildings when proper considerations are made.
Because wood is a natural material, consider shrinkage for any multi-story application.
Shrinkage calculations aren't complex, but they can be challenging to detail. In general, longitudinal shrinkage for vertical framing members is usually small and does not typically impact building design. However, shrinkage effects must be considered for horizontal framing members in the wall (top/sill plates) and floor (joists) design. To minimize the likelihood of accumulated shrinkage throughout the structure, use balloon framing or modified balloon framing with sawn lumber instead of platform framing, or use I-joists, which have very little shrinkage.
Mechanical devices also offer options. Marselle's contractors used a continuous rod tie-down system with a shrinkage compensation device to limit deflection and avoid wall separation due to movement under wind and seismic forces.
They also included an expansion joint in the cladding and were careful to specify proper panel spacing in the exterior wall sheathing. They also included a provision in the mechanical systems to allow for movement.
When a building is constructed with different materials, it experiences differential movement when these materials expand and contract under varying temperatures.
Considerations can easily be made to accommodate various building material combinations. For example, there's a misconception that when brick veneer is used as an exterior finish on a wood structure, its maximum height is limited to 30 feet (plate height) or 38 feet (gable height). However, if the brick is supported on the foundation and designed to carry its own weight, then the brick veneer may be allowed to exceed these heights. The brick veneer must be supported laterally by the wood structure, and wood studs must be designed to resist these loads.
Research shows that, with a proper moisture control system in place, the deflection criteria for the wood studs can be limited to L/240. Mid-rise building designers must also consider that the wood structure may shrink while the brick veneer may expand, and provide detailing to allow for this differential movement.
Multi-story wood-frame construction can be configured to meet or exceed the most demanding earthquake design requirements. Wood has several characteristics that enhance its performance during a seismic event, including ductility, strength and stiffness, light weight, load path redundancy and others.
Mid-rise wood-frame podium structures consist of two parts – a flexible wood-frame upper portion and a rigid lower portion, generally a concrete slab. Many building designers use two-stage analysis for lateral design, which enables them to treat the upper and lower portions as separate buildings. Two-stage analysis provides an alternative to having to design the entire structure for the same seismic response coefficient R, which would mean (among other things) designing the upper portion for a minimum of 30 percent higher forces.
Widely recognized for its environmental attributes, wood is a renewable, sustainable building material that can help the industry meet its green building goals. (For supporting information on wood's environmental attributes, visit www.woodworks.org/why-wood/sustainable-design)
For example, lifecycle assessment studies show that wood products require less energy across their lifecycle, are responsible for less air and water pollution, and have a lighter carbon footprint. As trees grow, they absorb carbon dioxide from the atmosphere, release the oxygen and incorporate the carbon. When trees are harvested and manufactured into products, they continue to store much of this carbon indefinitely. Building professionals also choose wood instead of fossil fuel-intensive products such as concrete or steel to avoid greenhouse gas emissions.
Innovation at work
New products continue to enhance wood's ability to add value in multi-story applications. CLT is widely used in Europe and gaining ground in North America. The Stadthaus building in London, framed with CLT, is nine stories high (eight stories of CLT over one story of concrete). When Bridport House was constructed in 2011, also in London, the eight-story CLT structure formed the largest timber-built apartment block in the world. A 10-story CLT project is also under construction in Melbourne, Australia.
CLT offers high strength and the structural simplicity needed for cost-effective multistory buildings, as well as a lighter environmental footprint than concrete or steel. It also provides numerous other benefits, including quicker installation, reduced waste, improved thermal performance with a tighter building envelope and design versatility.
By using wood-frame construction to build five-story and higher structures, designers can take advantage of wood's environmental and performance benefits to add value, while providing a cost-effective solution for their clients.
Michelle Kam-Biron, P.E., S.E., is senior technical director with WoodWorks (www.woodworks.org). A collaborative effort of North American wood associations, WoodWorks provides free technical support, education and resources regarding the use of wood in non-residential and multi-family buildings.