Evaluating high-rise wood construction

Advances in technology have allowed designers to challenge the status quo, think outside the box, and design high-rises comprised mostly of wood. It’s critical to understand the history associated with high-rise construction, current trends in the industry, and insights on the code-development process associated with high-rise wood construction.

07/12/2018


Learning objectives 

  • Learn about the difference between noncombustible and combustible construction materials.
  • Understand why wood construction is becoming more common for high-rise buildings.
  • Review the codes and standards as they relate to timber construction.

There has been a worldwide movement to use wood materials within high-rise buildings over the course of the past 20-plus years, and it has gained more exposure in the past 5 years. With stagnation in design comes innovation; and with most innovations, it takes time to reach the masses. The thought of constructing high-rise buildings out of wood, a combustible material, has design professionals' interests piqued, code professionals dubious of the viability, and fire chiefs skeptical. How can a high-rise building be constructed of wood? It's already a challenge to design, build, and protect in the event of fire, but there are codes, tests, and additional challenges to be aware of during the process. 

Noncombustible vs. combustible construction

Many architects and engineers that design high-rise buildings are accustomed to noncombustible construction. The International Building Code (IBC) provides the design criteria for all buildings, and in general, the larger the building, the greater the likelihood that it's going to be built of noncombustible, fire-resistive construction. The IBC requires a 50-story high-rise hotel tower to be constructed of Type IA construction (noncombustible construction), with structural members having a minimum fire-resistance rating and additional life safety features incorporated into the design.

Concrete and steel are industry standards for high-rise construction, but it wasn't always that way. Part of the reason the industry moved away from wood was the Great Chicago Fire of 1871, which destroyed more than 17,000 structures over the course of 2 days. The fire was able to spread so easily due to the close proximity of wood structures. The aftermath spurred economic development, and architects were designing high-rises in place of the structures that fell due to the fire. Based on the recent past, the use of noncombustible materials was seen as favorable. Now in the 21st century, as steel prices continue to climb and with a greater emphasis being placed on sustainability, wood is making a comeback.

The wood in question is referred to as mass timber, not to be confused with stick-frame construction that is typically seen in residential construction and low- to mid-rise buildings. Mass timber is essentially composite wood systems combining multiple pieces of wood to increase their compressive and tensioned strength; large solid-wood panels for floor, wall, and roof construction. There are several different categories of mass timber including cross-laminated timber (CLT) and glued-laminated timber (glulam), which will be the focus since they appear to be the most widely used components of mass timber for these building types.

CLT is comprised of structural-grade lengths of timber, which are anywhere from 5/8 to 2 in. thick and 2.5 to 9.5 in. wide, glued together in an alternating pattern with formaldehyde- or polyurethane-based adhesive. CLT panels can be made into nearly any size, up to 10 ft wide, 60 ft long, and 20 in. thick. These are ideal for floor and roof construction as well as shear walls in buildings. CLT panels are usually comprised of three, five, or seven layers (sometimes more); the layer count is always in odd numbers as the layers are constructed in symmetry around the middle panel. Glulam is comprised of individual wood laminates that are joined in parallel along their length to create members of virtually any size. These are typically used for beams and column construction. Adhesives used for glulam are moisture-resistant.

The move toward wood construction

There are several reasons for the movement toward wood construction. It is a sustainable building material, as wood can be regrown. In addition, the materials used for construction are typically sourced locally, creating local jobs. Since the recession began a decade ago, the foresting industry in the Western United States has taken a hit, and it's believed that wood construction will help get it back on its feet. There are fewer carbon emissions as a result of this type of building. It's estimated that one-third of global greenhouse gases are produced by buildings, of which about 10% is related to the manufacturing of concrete and steel. Trees are considered a "carbon sink," which means that they absorb and hold carbon until such a time that they decompose or are burned.

Figure 1: A glulam beam to column connection after a post-fire test. Courtesy: David Barber, ArupAccording to a study in the Journal of Sustainable Forestry, substituting wood for other building materials could prevent between 14% to 31% of global carbon emissions. It's assumed that the forests will regrow, and the wood is either reused or disposed of responsibly after the building lifespan. Currently, reducing carbon emissions by that amount is certainly going to get some attention among the construction community, especially those that are environmentally conscious. In addition, CLT panels and glulam can be brought onsite and assembled easily, with members being manufactured down to a fraction of an inch. Members also can be put in place with a high degree of accuracy. Since the timber is prefabricated offsite and then brought onto the construction site, there is less construction traffic and congestion and the time to complete construction is slashed significantly.

Wood buildings around the world

CLT construction has been used in Europe and elsewhere around the world for the past 20-plus years. There are buildings located in Finland, Sweden, Norway, the U.K., Canada, Australia, and the United States. The world is ahead of the United States in this endeavor based on the ability to use performance-based designs, which emphasize the goals and functional objectives, which are fire safety principles that apply to all buildings regardless of building material. When stakeholders agree that the functional objectives have been met, it allows for greater flexibility in the design process. The United States has historically used prescriptive-based codes, such as the IBC, which details what types of materials may or may not be used based on the use of the building, height, and area.

Below is a partial list of projects using wood building materials that have been previously built, are in construction, or are in the design phase.

  • The Forté building is a 10-story apartment building in Australia. The building is comprised of 759 CLT panels of European spruce. At its time of completion, it was the tallest timber-framed building in the world.
  • The Treet building is a 14-story luxury apartment building in Norway. The structure consists of glulam load-bearing structure and prefabricated modular units. The wood is sourced from Norway and all of the main load-bearing structure is wooden.
  • Brock Commons is an 18-story residence hall at the University of British Columbia. To highlight the speed of construction, the primary structure was completed less than 70 days after the prefabricated components were first delivered to the site, which took more than 4 months off the construction schedule if it was built using traditional noncombustible materials.
  • Framework is a 12-story building under construction in Portland, Ore. It will be the United States' tallest human-occupied wood structure when it's completed. It recently won a $1.5 million grant by the U.S. Department of Agriculture for research into using these building materials. It's currently slated for completion in 2019.
  • The HoHo Tower is a 24-story mixed-use building in Vienna, Austria. The project will include a hotel, apartments, a restaurant, a wellness center, and offices. Seventy-six percent of the structure will be constructed from wood, saving more than 2,800 tons of carbon dioxide emissions.
  • The W350 is a 70-story, 1,148-ft-tall building under development in Tokyo. It's proposed to open in 2041 and will cost about $5.6 billion. The W350 is a nod to the Sumitomo Forestry company, as 2041 will be their 350th year in business and the building will be 350 meters high. It will be comprised of 90% wooden material with steel vibration-control framework. 

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