WOOD VERSUS ALTERNATIVE MATERIALS

Numerous studies have been conducted comparing the environmental impacts of using wood versus other materials. In 2005, the Consortium for Research on Renewable Industrial Materials (CORRIM) conducted two landmark studies on the life cycle environmental impacts of wood compared with steel and concrete. Phase I of the research examined the impacts of comparable wood versus concrete homes in Atlanta and Minneapolis; these results found wood to be better for the environment in terms of embodied energy, air and water pollution, and global warming potential. Phase II of the research looked more closely at carbon footprint, and found that the carbon stored in wood products offsets many of the emissions from other products.

Environmental Benefits

Although cost is the main reason cited for the use of wood in five- and even six-story buildings, climate change has been the biggest motivator for those developing systems that allow taller wood structures. Wood is the only major building material that grows naturally and is renewable. Life cycle assessment studies consistently show that wood outperforms steel and concrete in terms of embodied energy, air pollution, and water pollution. It also has a lighter carbon footprint—because wood products continue to store carbon absorbed by the trees while growing, and wood manufacturing requires less energy and results in less greenhouse gas emissions.

In fact, wood buildings lock in carbon for the lifetime of a structure, while the manufacture of steel and concrete produces large amounts of CO2. The International Energy Agency (IEA) estimates that for every 22 pounds of cement created, 13 to 20 pounds of CO2 are produced.⁷

Cost Competitive

Because mid-rise wood structures have proven to be cost competitive, many in the industry expect that tall wood structures built with mass timber building techniques will be cost competitive with concrete and steel structures up to 30 stories. The fact that architects have their choice of different configurations provides flexibility, giving them the ability to use various mass timber building products depending on market availability. As additional design development occurs, and as additional manufacturing capacities come online, most experts expect costs to become even more competitive.

Besides offering a cost-effective construction option, owners and developers will also save money because mass timber structures can be installed much more quickly than steel or concrete. For example, development company Lend Lease estimates that the $11-million Forté apartment building in Melbourne, Australia, was built 30 percent faster because the materials were prefabricated. Off-site panelization saves money by speeding erection, which leads to quicker occupancy. Wood structures are also lighter weight compared to concrete, which can reduce foundation costs. This feature is particularly important in areas with poor soil where foundation costs are already high, as was the case with the Forté project.

Cree Buildings' Tahan says they expect costs to decrease as the company makes further improvements in technology and system design. During the research phases, Cree GmbH did a cost analysis, comparing a mass timber structure built using the LifeCycle Tower design and a wooden core with a concrete building with a concrete core. “Overall, our analysis showed that costs were not much different, and the larger the volume, the more affordable wood becomes,” Tahan notes. “In addition, the CO2 differences are huge.”

When PE International, a firm specializing in sustainability, compared the CO2 equivalents of the LifeCycle Tower mass timber building technique with a similar building of reinforced concrete, they determined that using wood resulted in a 92 percent reduction in CO2. In comparison, costs of the wood and concrete systems were relatively close.⁸

Tall Wood Fire Safety

While fire safety is often considered one of the primary barriers to building tall structures with wood, research shows otherwise. Mass timber buildings behave very well in fire, primarily because the wood's thick cross-section chars slowly. Once formed, char protects the structural integrity of the wood inside and prevents further degradation. In addition, mass timber assemblies also have fewer combustible concealed spaces. The solid wood panels themselves essentially form the fire-rated assemblies between building compartments, reducing a fire's ability to spread undetected.

There are two design approaches to accessing the acceptable structural passive fire protection measures in a mass timber building. Encapsulation is used to provide fire-resistance rating to timber structures, but charring is increasingly accepted around the world as a valid means of achieving reliable and safe structural performance in fire.

Encapsulation. Designers can apply one or two layers (depending on the fire assembly required by code) of fire-rated gypsum board to the underside of floors and throughout the building to reach the desired protection level. This method is similar to standard construction techniques used to construct fire-rated floor, roof, and wall assemblies in both combustible and noncombustible building types.

CLT FIRE TESTING RESULTS

In October 2012, the American Wood Council (AWC) conducted a successful fire resistance test on a load-bearing CLT wall at NGC Testing Services in Buffalo, New York. The test, conducted in accordance with ASTM E-119-11a (Standard Test Methods for Fire Tests of Building Construction and Materials), evaluated CLT’s fire resistance properties. The five-ply CLT wall (approximately 6-7/8 inches thick) was covered on each side with a single layer of 5/8-inch Type X gypsum wallboard and then loaded to 87,000 lbs., the maximum load attainable by the NGC Testing Service equipment. The 10 x 10 foot test specimen lasted three hours, five minutes, and 57 seconds (03:05:57)—well beyond the two-hour goal.

Charring. The solid wood members used in mass timber construction allow a char layer to form. This, in turn, helps insulate the remaining wood from heat penetration. The fire-resistance rating of large-sized members can be calculated based on minimum structural thicknesses and the remaining sacrificial thickness available for charring. By combining modern fire suppression systems and compartmentalization, structures can be detailed to safely resist fire without encapsulation, using charring calculation methods. This eliminates the need for the gypsum board, reducing building weight and cost while showcasing the natural beauty of the exposed wood.⁹

Structures built using hybrid panels similar to those used in the LCT ONE project in Austria can take advantage of the additional fire protection properties provided by the concrete in the slabs. Cree GmbH has developed fire-rated assemblies for their wood hybrid panels offering 90-minute and two-hour ratings.

Performance

Wood also has a number of other properties that make it well-suited for tall structures.

• Structural/seismic: On a strength-to-weight ratio, engineered wood products generally match, and in some cases exceed the performance of reinforced concrete.¹⁰ In addition, timber's weight is just 25 percent of reinforced concrete, placing less gravity and seismic loads on the structure and foundation.¹¹ Mass timber building components are dimensionally stable and rigid, creating an effective lateral load resisting system. They also offer good ductile behavior and energy dissipation. Extensive seismic testing has found that CLT panels perform exceptionally well in multi-story applications, with no residual deformation. In fact, Japanese researchers tested a seven-story CLT building on the world's largest shake table. The structure survived 14 consecutive seismic events with almost no damage.¹²

• Thermal: The environmental benefits of wood extend through the life of the structure because of its excellent thermal performance. Wood's thermal properties are determined by U-value, or coefficient of heat transfer, which relates to panel thickness. Thicker panels have lower U-values; they are better insulators and therefore require less insulation. Because the components are solid, there is little potential for airflow through the system. The way in which mass timber buildings are constructed also improves its thermal performance. CLT, LVL, and LSL can be manufactured using CNC equipment to precise tolerances, so panel joints fit tightly, which results in excellent energy efficiency. One study found that the interior temperatures of a finished CLT structure could be maintained with just one-third the normally required heating or cooling energy.¹³ LCT ONE, Cree GmbH's eight-story structure in Austria, was designed to meet Passivhaus (Passive House) standards, intended to reduce carbon emissions by 90 percent when compared to a structure built according to current energy codes.

• Acoustics: Tall buildings are often used for apartment or condominiums, where noise control is critical. Test results¹⁴ show that, because the mass of the wall contributes to its acoustic performance, mass timber building systems provide appropriate noise control for both airborne and impact sound transmission, often without the need to add additional acoustical layers. Projects gain additional acoustic benefits because builders use sealant and other types of membranes to provide air tightness and improve sound insulation at the interfaces between the floor and wall plates.