Materials In Action  

When an architect specifies a building material, that choice casts a long shadow. While most of the environmental effects from materials occur during the extraction and production phases, the building material influences a structure’s environmental footprint well after, throughout the operations phase and beyond. What are the life cycle costs of the material? How durable is it? Is the material thermally efficient? Is it susceptible to moisture damage? Can it withstand seismic activity? What are the code considerations? Can it be recycled or reused, and at what cost to the environment? These are the kinds of questions that should be considered in the earliest project phases. The answers will determine, in part, a structure’s sustainability quotient. This article will address, through research and facts, the differences between wood, concrete, and steel in terms of basic material properties as well as their performance during the building operations phase. Topics will also include end-of-life issues, including the impacts of recycling and re-use, and code changes that have allowed the increasing use of wood in construction.

Design Considerations

Prior to specifying a material, certain issues should be thoroughly investigated.

Durability

Good design and quality construction are important factors in a building’s longevity, as is maintenance. “Any building of wood, concrete or steel could last an indefinite period of time, provided there is proper maintenance," says Scott Lockyear, Senior National Director for U.S. WoodWorks, an initiative of the Wood Products Council established to provide free technical support and education related to non-residential and multi-family wood buildings. “The critical thing is moisture control. Without it, concrete will spall, wood will decay, and steel will rust."

Exterior of Arena Stage, Mead Center for American Theater, Washington, D.C. Hybrid wood, concrete, and steel structures are often good solutions in sustainable building. Photo by Nic Lehoux courtesy of Bing Thom Architects

Wood buildings have lasted for centuries.

Building materials tend to deteriorate and fail via well-known mechanisms. Fungi are the major cause of wood deterioration when wood is exposed to constant wetting without preservative treatment or the ability to dry. However, wood is relatively resistant to high humidity and many of the conditions and chemicals that adversely affect steel and concrete, such as corrosive salts, dilute acids and sea air. Provided its surface is protected from rust, steel can maintain its strength indefinitely. For construction steels, corrosion is the most common and expensive form of material degradation.

The most effective and common procedure for preventing or slowing corrosion is to eliminate contact with water, either by coatings or by protection within a building envelope. Steel studs and many other components are protected from water electrochemically by galvanizing, which does not eliminate contact with water. Although concrete itself does not corrode or decay, it almost inevitably cracks, and concrete cannot be used structurally without steel reinforcement. Cracking of concrete exposes concealed steel reinforcement to more moisture and corrosive chemicals which, in turn, further erodes the steel components and leads to further cracking and spalling of concrete.

Steps to Decades of Reliable Service

To enable wood to have a long service life, the following four factors are critical: Moisture control Architects should fully understand moisture loading, including the source of water, how the water is transported, and how to control and remove it. Wood construction maintained at a moisture content of 19 percent or less will not decay. In fact, decay doesn't generally occur until the moisture content reaches 26 percent or more. Protected from water or vapor condensation, exposed to normal atmospheric conditions, interior wood has a moisture content that rarely exceeds 14 percent. Termite control As termites thrive in wet environments, controlling moisture goes a long way toward controlling termites. Soil and foundation barriers and bait systems can also help prevent infestation by insects. Use of durable materials Wood that comes into contact with the ground or certain climates may need greater protection; naturally durable wood species, such as yellow-cedar or Douglas-fir, or preservative-treated wood may be necessary. Quality assurance Where moisture and/or insects are an issue, quality control is critical in constructing building assemblies to resist negative effects, as is proper maintenance to keep the structure dry.

Building materials can be durable but good design and consideration for future use are equally important. A study by FPInnovations¹ examined service lives of buildings in Minneapolis/St. Paul. The author investigated building demolition in 227 residential and non-residential buildings. Some 66 percent of non-residential wood structures were over 50 years old, while a similar percentage of concrete buildings were under 50 years old, and nearly 90 percent of steel buildings were under 50. However, the most common reasons for demolition were not related to material degradation, but to changing land values, lack of suitability for current use, and lack of maintenance for non-structural components. The relative ease of expanding and modifying wood-frame structures may have contributed to their longer life.

Because of the unpredictability of future building needs, a design that lends itself to renovation or adaptation can extend a building’s life span and reduce waste.

Wood is particularly versatile and flexible, which makes it an easy construction material for renovations. For example, Ardencraig House in Vancouver, British Columbia, comprises four townhomes designed within the framework of an existing heritage home and garage. Over 90 percent of the wood in the original structure was retained in adapting the house. Salvaged materials from deconstruction of the garage were used to construct a coach house behind the main structure. Salvaged framing members were used to strengthen roof trusses and increase the space available for insulation.

Strength

The strength of a building material refers to its ability to withstand an applied load without failure. Several types of load can be applied—tension, compression, torsion, bending, and shearing.

Steel is one of the strongest materials for tensile strength, the amount of stretching a material can take before breaking or failing. It is also one of the few materials that is equally strong in tension and compression. There are many different steel alloys, but they all have similar stress versus strain ratios. All steel alloys have the same modulus of elasticity, which refers to the material’s stiffness, or the ratio of the material’s allowable stress versus strain. Steel’s modulus of elasticity is 29 million pounds per square inch (psi), compared to concrete’s 5 million and wood’s 2 million. However, every steel alloy represents a different yield strength, which is the highest force a material can take before it deforms. The most common alloy, carbon steel, or ASTM A36, has a yield strength of 36,000 psi; ASTM A441 has a yield strength of 40,000 to 50,000 psi; ASTM A572 has a yield strength of 42,000 to 65,000 psi. Building codes provide an allowable stress between 33 percent and 75 percent of steel alloy’s yield strength. The steel industry is creating new and stronger alloys. Common carbon steel, ASTM A36, for example, is slowly being replaced by ASTM A572 Grade 50, which is 77 percent stronger.

Flexure test machine for testing full size panels for bending movement and stiffness Photo courtesy of CertiWood

Concrete is one of the strongest materials for compressive strength; tremendous loads can be put on concrete without crushing it. Most concretes can handle 2,000 to 3,000 psi. The American Concrete Institute defines high-strength concrete as having a compressive strength greater than 6,000 psi, but the advent of high-strength concrete has pushed concrete’s compressive strength up to 19,000 psi. Making high-strength concrete involves optimizing the use of basic concrete ingredients. Fly ash and silica fume, commonly used admixtures, provide additional strength, as do superplasticizers which, when combined with a water-reducing retarder, provide workability at low water-cement ratios, resulting in stronger concrete. Most often used in high-rise structures, high-strength concrete is specified where reduced weight is important, and because it carries loads more efficiently than conventional concrete, it can reduce the total amount of material needed, lowering overall building costs. On the other hand, high strength concrete may be more expensive than conventional concrete. According to the Portland Cement Association, one of the tallest concrete buildings in the United States is Chicago’s 311 South Wacker Drive, which at 969 feet, uses concrete with compressive strengths up to 12,000 psi.

Earth Systems Science Building, University of British Columbia Location: Vancouver, British Columbia (Canada) Architect: Perkins+Will Canada This five-story building was built using a combination of massive timber systems including cross-laminated timber (CLT), composite laminated strand lumber/concrete floors, and glued laminated timber (glulam) heavy timber braced frames. CLT was used for the roof and exterior canopies. Photo: KK Law, courtesy naturallywood.com

The Crossroads Location: Madison, WI Architects: Uihlein/Wilson Architects, Inc.; EwingCole For Promega, a leading biotechnology firm headquartered in Wisconsin, CLT and glulam met all of the architectural and engineering goals for its new 52,000-square-foot client and staff reception area known as The Crossroads. Architecturally, using CLT for the roof allowed exposed interior surfaces, which enhanced the glulam beam and column superstructure. From an engineering point of view, CLT allowed the team to increase deck spans while still supporting heavy snow loads. Photo: Uihlein/Wilson Architects, Inc.

However, concrete doesn’t have the same advantage when it comes to tensile strength. In building construction, rebar, or reinforcing steel bars, provides the tensile strength lacking in concrete. Concrete has a very low coefficient of thermal expansion, and as it matures, concrete shrinks. All concrete structures will crack to some extent, due to shrinkage and tension.

Wood’s strength is dependent on loading direction—it is strongest in tension along the fibers and weakest in radial and tangential directions. When loaded longitudinally along the grain, wood can have a strength-to-weight ratio advantage relative to steel of 2:1. However, when wood is loaded in other directions, including radial and tangential to the grain, this advantage disappears. Wood’s psi varies among species: western red cedar may have a psi of 7,500, Douglas-fir a psi of 12,400, and mahogany a psi of 25,400.

For decades, the wood industry has been evolving high-strength products in the form of engineered wood—plywood, oriented strand board, glulam beams, I-joists, and laminated veneer lumber, to name a few examples. Generally stronger than traditional lumber, engineered wood is often made from (among other things) chips, particles, fibers and wood from small-diameter trees not suitable for lumber—which is part of the reason the wood industry is able to utilize more than 99 percent of every tree harvested and brought to a mill.²

One innovative engineered wood product is CLT, a material widely used in Europe that is poised to significantly increase the possibilities for North American wood buildings. CLT is comprised of boards stacked together at right angles and glued over their entire surface, creating a product that retains its static strength and shape, and allows the transfer of loads on all sides. Besides being dimensionally stable, it can span long distances and be erected rapidly.

Internationally, CLT has propelled wood construction to new heights, the most recent example of which is the Forté, a 10-story apartment building in Australia. It offers the structural simplicity needed for cost-effective projects, as well as benefits such as rapid installation, reduced waste, energy efficiency and exceptional design versatility.

In North America, CLT is relatively new but quickly gaining momentum. In 2012, the American National Standards Association approved ANSI/APA PRG 320-2012 Standard for Performance-Rated Cross-Laminated Timber, a product standard that details manufacturing and performance requirements for qualification and quality assurance. Due to recently approved code changes, CLT is scheduled to be included in the 2015 International Building Code. In the meantime, a handful of innovative designers have already built CLT structures in the U.S. and Canada, having had them approved under the relevant code through an alternative or innovative solutions path.

Moisture Resistance

“All materials have challenges when it comes to moisture; however, when moisture is managed properly, wood exceeds expectations," says Cheryl Ciecko, ALA, AIA, LEED AP, noting that wood acts as a moisture sink and a thermal break.

Lumber grading rules and many building codes require wood be dried to 19 percent moisture content or below—still substantially below the fiber saturation point of 28 percent, the level at which mold or decay can begin to thrive. Decay fungi feed on wood and require oxygen and moisture to thrive. Because damaging fungi affect wood primarily when the moisture content exceeds the fiber saturation point for a prolonged period of time, adverse effects can be prevented by avoiding direct contact between untreated wood and the ground or other moisture sources. Treating wood with preservatives will also protect it from undesirable fungi and insects.

All materials, including steel and concrete are susceptible to mold, since dirt or dust can be the food source, along with moisture. Bulk water, air infiltration and condensation can be a source of moisture in all types of buildings. While wood acts as a thermal break due to inherent insulating properties, steel, concrete and masonry are thermal bridges which can provide a cold surface on which warm, moist air can condensate, increasing the potential for deterioration or mold. “This moisture due to condensation can be a huge problem," says Ciecko. “Wood can hold some moisture for short periods of time, acting as a moisture sink, without harm. However, steel cannot, making it potentially susceptible to corrosion with even small amounts of water contact."

A study by FPInnovations showed that interior wood paneling can reduce peak moisture loads in a typical Canadian house by 10 to 25 percent—a scenario that leads to both improved user comfort and reduced need for air conditioning.³

Fire Resistance

Cross-section taken from 3-ply CLT panel protected by two layers of gypsum board and exposed to the standard fire exposure (CAN/ULC S101) for 1 hour and 15 minutes. Photo courtesy of FPInnovations

According to the National Fire Protection Association, property loss from fire was estimated at $11.7 billion in 2011.⁴ While no building is completely fireproof, construction materials and systems can improve a building’s fire safety. Concrete, and especially Insulating Concrete Form (ICF), is a good fire-resistant material. Unlike wood, concrete cannot burn; and unlike steel, it won’t soften or bend. Since it doesn’t burn, concrete doesn’t ignite, nor does it release toxic fumes or smoke, nor melt when it is exposed. Concrete will only break down at temperatures of thousands of degrees Fahrenheit. Concrete’s thermal mass properties—slow absorption and release of heat—help to mitigate fire risk, and it is able to achieve fire-resistance ratings without additional fireproofing. However, concrete can be subject to severe spalling, particularly if it has an elevated moisture content. Fireproofing is available for concrete but this is typically not used in buildings. Instead, it is used in traffic tunnels and locations where a hydrocarbon fire is likely to break out.

Structural steel requires fireproofing to prevent the steel from weakening in the event of a fire. When heated, steel expands and softens, eventually losing its structural integrity and, under extreme conditions, melting. According to the National Institute of Standards and Technology, when exposed to fire, steel loses its strength and stiffness much faster than high-strength concrete. With a lower thermal conductivity, high-strength concrete will maintain its structural integrity for a longer period of time in a fire situation.

Although seemingly counter-intuitive, wood can be an excellent performer under fire conditions. According to the Southern Forest Products Association, wood outperforms non-combustible materials in direct comparison fire tests. A 2x4 timber tie maintained more of its original strength under higher temperatures and for a longer period than did aluminum alloy or mild steel. This is because of wood’s unique charring properties. When wood burns, a layer of char is created which helps to maintain the strength and structural integrity of the wood beneath—a scenario that enables an exposed heavy timber system to achieve a fire-resistance rating of up to 90 minutes.⁵ Properly designed wood-frame walls, floors and roofs using conventional wood framing, wood trusses and wood I-joists, can also provide fire resistance ratings for up to two hours.

Another test comparing the performance of a glulam beam to a steel beam conducted at the Southwest Research Institute demonstrated the fire performance superiority of the glulam beam when both members were directly exposed in an ASTM E119 fire test.⁶

Seismic Considerations

Wood-frame house from the earthquake stricken Sichuan Dujiangyan area in China still stands. Photo courtesy of Forestry Innovation Investment China

While seismic design is a complex undertaking subject to many variables, certain general material considerations can be stated. To withstand earthquakes, buildings are designed to be flexible and move without breaking. This ability to yield and deform without fracturing is called ductility. According to Timber Engineering Europe, the type of construction that causes the most fatal injuries in earthquakes is unreinforced brick, stone, or concrete buildings that tend not to be flexible and to collapse when shaken. Metal can be formed to flex and bend without breaking, allowing the building to sway and reducing the stress on the building.

Both the elastic limit and ultimate strength of wood are higher under short periods than under longer times, permitting higher working stresses under short-term live loads, such as heavy winds and seismic loads. In fact, one of the most earthquake-resistant building types is considered to be a low wooden structure anchored to its foundation and sheathed with plywood. Such low-rise wood-frame structures with correct wall bracing, connectivity and anchorage provide safety during seismic activity. The many nailed joints in wood-frame structures make them inherently more ductile, which enables them to dissipate energy from the sudden shock of an earthquake. Numerous load paths, such as those found in wood buildings, help avoid collapse in the event that some connections fail. Because structures constructed of other materials have relatively few structural members and connections, the failure of one load can lead to overloading of adjacent members or joints. Also, since forces in an earthquake are proportional to the weight of a structure, lightweight structures fare better. Concrete walls are seven times heavier than typical wood-frame walls.

Seismic simulation testing in 2009 on a high-capacity mid-rise structure registered a major earthquake with minimal damage.

In California’s 1994 Northridge earthquake, where peak ground accelerations nearly broke records and were considerably higher than codes specified, many large structures collapsed. At a hearing before the U.S. House of Representatives by the Committee on Space, Science and Technology, a reason given for relatively limited death and damage toll was “The earthquake occurred at 4:31 a.m., when the majority of people were sleeping in their wood-frame, single-family dwellings, generally considered to be the safest type of building in an earthquake."⁷

In that same earthquake, many welded steel moment frame buildings, which appeared to be earthquake-proof, were damaged, prompting the Federal Emergency Management Agency to initiate new design approaches that would minimize future seismic damage. As a consequence, all pre-qualified connection details and design methods contained in the building codes of that time have been rescinded, with new provisions stipulating that new designs be substantiated by testing.

In 2008, an earthquake struck Wenchuan, China, claiming the lives of hundreds of thousands of people. Most of the buildings that collapsed or sustained structural damage were masonry or concrete structures. In comparison, the few wood-frame houses within the earthquake region performed well, with some not even sustaining superficial damage. The seismic performance of the wood-frame buildings prompted the building of multi-story wood-frame buildings within the earthquake area as demonstration projects.⁸

Materials in the Construction Phase

Because of its high strength-to-weight ratio, steel framing is easy to handle on site and can take less framing material than wood for an equal size structure. Steel can be manufactured to specific lengths minimizing jobsite scrap and, because it does not warp or split, the use of steel eliminates the need to sort out poor quality product. Builders are increasingly using panelization—either building panelized components themselves or purchasing pre-assembled wall, floor and truss components—which reduces construction time and costs. Prefabricated steel structures can be lifted into place with cranes and bolted together, for relatively quick building erection.

CLT panels were used on site to build Norwich Academy in the UK. Architect: Sheppard Robson Photo courtesy of Kier Contractors

According to the Reinforced Concrete Construction Committee, cast-in-place concrete can reduce project startup time and start-to-finish time compared with steel. Materials can be ordered for just-in-time delivery from local suppliers as opposed to steel, where pre-ordering and long lead times are common. Concrete industry data claim that the ready availability of concrete can save up to 20 weeks or more from notice-to-proceed to the start date and that, once construction begins, 13 percent fewer delays are reported during framing compared to steel.⁹

Wood’s advantages during construction stem from its flexibility in enabling a contractor to make modifications on site—including everything from a building’s orientation to its floor plan, number of rooms, interior design, and overall appearance. In addition, timber’s thermal efficiency means walls can be slimmer, creating more usable floor space.¹⁰

In terms of sustainability documentation, wood, and only certified wood, can come with a chain of custody that verifies it is sourced from sustainably managed forests and other responsible sources (e.g., recycled content). Forest products are tracked throughout the entire supply chain from harvesting, processing and transportation to the end consumer.

Another pressing issue during construction is the availability of materials and the impacts of getting the materials to the site. While the materials that go into cement—lime, iron, silica, alumina, and fly ash—are not often local, cement makes up only 15 percent of concrete. Steel and wood typically require a greater transport effort. A recent study on the matter was conducted in accordance with the UK’s PAS 2050 (Publicly Available Specification), a measurement tool/protocol for companies to make credible decisions based on a product’s greenhouse gas emissions. The study, by The Athena Sustainable Materials Institute and FPInnovations on the carbon footprint of transporting wood products from British Columbia, Canada to the UK, showed that, despite being transported more than 9,900 miles, the four types of wood under study—softwood lumber, softwood plywood, western red cedar lumber and western red cedar siding—represent a net carbon sink upon delivery; that is, each product stores more carbon than is emitted during its respective harvest, manufacture and transport. Unlike wood, steel has no such carbon storage capacity to offset the environmental costs of transportation.

Materials in the Use Phase

Buildings represent the highest form of energy usage in North America. According to the U.S. Department of Energy, buildings account for 38 percent of total U.S. energy consumption, more than industrial and transportation usage combined, and more than 70 percent of United States electricity consumption.

Energy Terms

Certain terms related to the use phase should be understood, and distinctions made between embodied and operating energy and their interrelation.

There are two forms of embodied energy in buildings: initial embodied energy and recurring embodied energy. For building materials, initial embodied energy is the non-renewable energy consumed in raw material acquisition, processing, manufacturing, transportation to site, and construction. Recurring embodied energy is the non-renewable energy consumed to maintain, repair, restore, refurbish or replace materials, components or systems during the life of the building. Recurring embodied energy is related to the durability of the building materials, the building’s components and systems, how well these are maintained, and the life of the building.

Operating energy refers to the energy buildings consume for heating, cooling, ventilation, lighting, equipment and appliances. Increased insulation in foundations, walls, doors, and windows may improve the operating efficiency of a structure, but the insulation products themselves may increase the structure’s embodied energy. Appreciating—and balancing—the need to reduce operating energy in buildings and its effect on embodied energy is critical to true sustainability.

Energy performance of buildings depends more on insulation, air sealing and other factors than the choice of structural material. All buildings are typically insulated well, so they tend to have essentially comparable energy performance. However, a building's embodied energy is very much affected by its structural material. The chart above is based on life cycle assessment (LCA) with calculations by The Athena Sustainable Materials Institute for identical 2,400-square-foot homes designed according to standard practice in the locales for which they were intended. Over 60 years, the wood house had less embodied energy than the concrete or steel-framed structures. Concrete structures had the highest embodied energy. For further information on the study and its assumptions, see the Canadian Wood Council’s Energy and the Environment in Residential Construction.

According to the NAHB Research Center, insulated concrete forms (ICF), rigid plastic foam forms that hold concrete during curing and remain afterwards to serve as thermal insulation for concrete walls, are superior insulators to standard concrete. Typical insulation values for ICFs range from R-17 to R-26, compared to between R-13 and R-19 for most wood-framed walls. The higher the R-value, the more thermal resistance.

But materials with a heavy carbon footprint like concrete can really skew the balance between embodied and operating energy. As shown in the accompanying graphic, a typical concrete house has nearly as much energy embodied in the materials as it takes to run the house for 20 years. If the embodied energy is increased by adding much more insulation, it is important to make sure that the savings in heating energy will be greater than the embodied energy in the insulation. As building operations become more and more energy efficient, the embodied energy may even be eventually larger than the operating energy.

Thermal Properties

The thermal conductivities of building materials have direct bearing on their performance as insulators. A low thermal conductivity indicates that a material is a poor conductor of heat and therefore a good insulator. Any material with open pockets of air will insulate better than a material that is solid. Wood is made up of thousands of tiny open cells that limit its ability to conduct heat. The thermal properties of wood are 400 times better than steel and 10 times better than concrete.¹¹ Their higher conductivity means steel and concrete must overcome lower R-values associated with thermal bridging; consequently they require more insulation to provide the same level of energy efficiency.

In the graph above illustrating energy performance in two similar houses near Chicago, the steel house has significantly more insulation than the wood house and still can’t perform as well. In addition, the steel house has a lot more embodied energy, which is not reflected in the graph.

The two identical houses, unoccupied but with heating and cooling systems operating, were built to standard practice for wood frame and steel frame. Data was measured for one year and also simulated with software in order to normalize and validate results. Both houses have fiberglass insulation between the studs. The steel house has more insulation volume, because its studs are spaced at 24 inches vs. 16 inches for the wood house. In addition, the steel house has a 3/4-inch layer of rigid polystyrene board mounted to the outside of the framing sheathing—standard practice for steel framing—to reduce conduction of heat at the studs. Yet even with these benefits—more fiberglass, the additional layer of polystyrene foam, and fewer thermal bridges because the studs are spaced further apart the steel house still cannot perform as well as the wood house. Steel framing requires a layer of exterior polystyrene foam, which adds cost and embodied environmental effects. If the wood studs were spaced at 24 inches, like the steel (which is sometimes the case), the wood performance would be even better.¹²

End of Life—A Closer Look at Recycling and Reuse

The U.S. EPA estimates that the U.S. generated more than 160 million tons of building-related construction and demolition (C&D) waste materials in 2003—nearly 53% of which was the result of demolition activities, 38% from renovations, and nine percent from new construction. Only 40% of this waste was estimated to be reused, recycled, or sent to waste-to-energy facilities, leaving 60% going to landfills.¹³

The concrete and steel industries have made great strides in recycling their materials. Recycled concrete aggregate has found ready markets as road base, asphalt pavement, soil stabilization, pipe bedding and landscape materials; occasionally it is used as aggregate for new concrete. Steel is widely recycled as well. In a year, the American Iron and Steel Institute maintains, the North American steel industry saves the equivalent energy, from recycling alone, to power about 18 million households for a year.

As stated earlier, wood product companies in the U.S. and Canada utilize nearly 99 percent of their manufacturing inputs. The challenge for the future expansion of wood recover is to increase utilization of wood classified as "post-consumer," including municipal solid waste and C&D waste. For some wood products, such as pallets and railway ties, this is already common practice. For others there are barriers. According to the report, Wood Reuse and Recycling in North America, successful and/or recycling of wood products often depends on condition and appearance issues. While "timbers" are either used "as is" or remanufactured into products like flooring, "lumber" and other structural materials can be challenging or costly because of a variety of issues. According to the report, there are opportunities for increased recycling, but barriers to be overcome.

While recycling is considered beneficial, a closer look is warranted. Although reusing concrete aggregate is referred to as recycling, some maintain it’s actually downcycling, as the greatest economic value is in the cement, which can’t be reused. “Design for Deconstruction," a report prepared for California’s Chartwell School partially funded by the EPA, states, “Crushed aggregate even tends to require higher cement mix designs, offsetting some of the benefit and reducing virgin aggregate use." While recycling saves energy, it also expends energy and sometimes in large amounts, particularly in the case of steel. The Chartwell School study says that recycling steel takes 50 percent of the energy required to refine steel from ore.

In some instances, a material with no recycled content can actually be a more sustainable choice than a recycled material. Steel, for example, typically has at least 25 percent recycled content, while most structural wood products have none. But to achieve a higher recycled content percentage, manufacturers change the content or processing of a product in a way that has other environmental implications. Life cycle assessment illustrates these impacts. The accompanying bar chart uses LCA to compare the environmental profile of two standard structural post-and-beam systems (one with wood and one with steel), and a third option using a theoretical steel with 100 percent recycled content. The wood system is glulam beams on wood columns with no recycled content. The steel system is standard wide flange beam on hollow structural section columns, recycled content is 25 percent, typical for the industry. The wood performance is set as the benchmark at 100 percent, and the other sets of bars are shown by percentage better or worse than the wood. Figures were calculated using The Athena Impact Estimator for Buildings.

An alternate to recycling is designing for deconstruction, which requires less use of resources—in other words, design to facilitate salvaging components during demolition for reuse in their original high-value format. The U.S. Green Building Council's Leadership in Energy and Environmental Design (LEED®) rating system encourages the use of both recycled and salvaged materials.

The Recycled Content credit (MRc4) rewards materials with recycled content such that the sum of post-consumer recycled content plus 1/2 of the pre-consumer content constitutes at least 10 percent (1 point) or 20 percent (2 points), based on cost, of the total value of the materials in the project.

The Materials Reuse credit (MRc3) rewards projects that “use salvaged, refurbished, or reused materials, the sum of which constitutes at least 5 percent (1 point) or 10 percent (2 points), based on cost, of the total value of materials on the project."

Another rating system that’s gaining traction is the Living Building Challenge (LBC), a program of the Cascadia Region Green Building Council, which encourages increasing improvements, rather than reducing impacts. In contrast with LEED®, LBC projects are not certified until they have proven their intended design and requirements. Among the projects to achieve certification so far: the Living Learning Center at Tyson Research Institute, near St. Louis, and the Omega Institute in Rhinebeck, New York, both of which make high use of salvaged materials, particularly wood.

At this time, using salvaged materials is not without its obstacles. The salvaged wood market is established in some areas, with large timbers and old growth lumber especially sought after. Salvaged wood requires very little additional energy to process, and is generally confined to transportation. As with wood, steel salvage yards exist in some regions. Yet one of the greatest disincentives to using salvaged material of any type is that it is not always on hand in the right dimension, amount, or timeframe—a situation that could be remedied by basing a design on the availability of the salvage.

Evolving Building Codes

Coupled with advances in wood science and building technology, increasing recognition of wood's structural and performance capabilities has expanded the options for wood use in construction. Earlier in this article, reference was made to the fact that CLT is scheduled to be included in the 2015 IBC. This particular code change is getting a lot of attention because of the groundbreaking nature of CLT; nine- and ten-story CLT buildings exist and exciting concepts have been developed for going higher still. However, the possibilities for wood use have actually been expanding since the IBC was introduced in 2000.

The University of Washington West Campus Student Housing project includes five buildings, each with five stories of wood-frame construction over two stories of concrete. This 2-story podium configuration, which is common in Seattle and will also be permitted under the 2015 IBC, allowed the project team to meet ambitious design goals within the University's tight budget. Photo courtesy of W.G. Clark Construction and Mahlum Architects

The IBC consolidated three regional model building codes into one uniform code that has since been adopted by most jurisdictions. It created more opportunities for wood use by (among other things) recognizing additional fire protection techniques, and combining the maximum allowable heights and areas from the three legacy codes into one (thus increasing what's allowable in some jurisdictions).

In a Type III building, for example, it is possible to achieve an eight-level wood building that's approximately 85 feet high. Like all construction types, Type III has base limitations with regard to height, number of stories and square footage. However, the IBC allows increases to these tabular amounts per other code sections. For example, when a building has an NFPA 13-compliant automatic sprinkler system, the floor area can be increased by 300 percent for a one-story building and 200 percent for a multi-story building. In addition to the area increase, IBC Section 504.2 allows a 20-foot increase to the tabular building height and an additional story above the grade plane. The exception to this is Group I-2 occupancies, which include hospitals and nursing homes and are not allowed the extra story.

Adding an automatic sprinkler system not only means that a five-story wood building is allowed, it means a maximum height of 85 feet instead of 65 feet. The question then becomes, how can the vertical envelope be maximized to take advantage of the extra height when a typical five-story building is only about 55 feet? The first step is to add another level. Under the 2009 IBC, a wood-frame mezzanine can be added on top of a multi-story wood building. The area of the mezzanine can’t be more than 1/3 of the floor below and isn’t defined as a 'floor' or 'story.' Starting with the base height and then adding a sprinkler system and now a mezzanine brings the building to six levels of wood-frame construction and about 65 feet in height.

A Type I podium increases the height of the building even further. The wood-frame and concrete portions of a podium building are designed as two separate structures with a 3-hour fire separation in between. A podium designed for retail is typically about 15 feet high. So, adding the sprinkler system, mezzanine and now a podium takes the building to seven above-ground levels and about 70 to 75 feet in height.

The final step, to achieve 85 feet, is to use a sloping site to advantage. The IBC recognizes that the world isn’t flat. It allows semi-basements or daylight basements providing they don’t extend from grade more than 12 feet at any one point and don’t extend more than 6 feet from the average grade. As with mezzanines, this 'basement' level is not considered a 'floor' or 'story,' but it helps to achieve an eight-level building that’s in the range of 85 feet high.

Whether a designer is considering wood because of cost, aesthetics, low carbon footprint or any of its other benefits, it's useful to recognize that building codes recognize wood's structural performance capabilities in a broad range of applications, from the light-duty repetitive framing common in small structures to the larger and heavier framing systems used to build arenas, schools and other large buildings. Engineered wood products such as CLT also offer exceptional stability and strength and have made wood a viable alternative to steel or concrete in many applications.

Materials In Action Matter

In building operation and end of life, wood continues to prove its value as a sustainable building material. It is a strong, durable, natural insulator that can be designed to perform well under fire conditions and to withstand seismic forces. It has less embodied energy than steel or concrete. It lends itself to recycling and reuse without significant energy input. It continues to store carbon absorbed by the trees during their growing cycle—keeping it out of the atmosphere for the life of the building, or longer if reclaimed and used elsewhere. Wood waste made into viable products can help spur the economy in rural areas.

This is the second of a three-part series documenting the environmental footprint of wood, concrete, and steel. The third and final article, A Natural Choice, will cover how these materials factor into green design and high-performance buildings as well as how green design projects are currently defined.

Endnotes
1.)	Survey on Actual Services Lives for North American Buildings, J. O'Connor, Forintek (now FPInnovations)
2.)	Utilization of Harvested Wood by the North American Forest Products Industry, Dovetail Partners, Inc., 2012
3.)	Wood Can Help Control Indoor Relative Humidity, FPInnovations (then Forintek), 2004.
4.)	According to the National Fire Protection Association, property loss from fire was estimated at $11.7 billion in 2011. http://www.nfpa.org/research/fire-statistics/the-us-fire-problem
5.)	Southern Pine Use Guide, Southern Forest Products Association, http://www.raisedfloorlivingpro.com/pdfs/publications/Southern-Pine-Use-Guide.pdf
6.)	Superior Fire Resistance, American Institute of Timber Construction, http://www.aitc-glulam.org/shopcart/Pdf/superior%20fire%20resistance.pdf
7.)	The January 17, 1994 Northridge Earthquake, an EQE Summary Report, http://www.lafire.com/famous_fires/1994-0117_NorthridgeEarthquake/quake/00_EQE_contents.htm
8.)	Forestry Innovation Investment, China
9.)	Wisconsin Ready Made Concrete Association, http://www.wrmca.com/mid-high-rise-structural-concrete-framing.html
10.)	NBS, http://www.thenbs.com/topics/constructionproducts/articles/ achievingThermalComfortInTimberFrameBuildings.asp
11.)	Thermal Performance of Light-Frame Assemblies, Canadian Wood Council, http://cwc.ca/documents/IBS/IBS5_Thermal_SMC_v2.pdf
12.)	Steel vs. Wood Long-Term Thermal Performance Comparison: Valparaiso Demonstration Home, NAHB Research Center, http://www.huduser.org/portal/publications/destech/steelval.html
13.)	US Environmental Protection Agency, http://www.epa.gov/wastes/conserve/imr/cdm/index.htm

The reThink Wood initiative is a coalition of interests representing North America’s wood products industry and related stakeholders. The coalition shares a passion for wood and the forests it comes from. Innovative new technologies and building systems have enabled longer wood spans, taller walls, and higher buildings, and continue to expand the possibilities for wood use in construction. www.rethinkwood.com