March 1, 2018
by Rich Mitchell, Josh McDowell, and Dietrich Wieland
The Pacific Northwest is the site for a renaissance in heavy timber construction that is now beginning to spread across the country. Wood, instead of steel, is being used to construct modern, multistory, and creative office buildings. This article explores the context for this trend, the motivations behind adopting a century-old construction approach, and what constitutes modern heavy timber.
First though, a few definitions—the term ‘heavy timber’ has been around for centuries, and commonly refers to a building made up of large wood beams and columns. Mass timber refers to a system using one or more of a few structural materials—glued-laminated timber (glulam), cross-laminated timber (CLT), or nail-laminated timber (NLT). Glulam has been used in the United States for more than 80 years and is very common. CLT is a large, prefabricated member made up of layers of framing-lumber laminated perpendicular to one another on their wide faces. This technique has been around for decades in Europe, and is now gaining traction in the United States. An alternate to CLT, NLT is fabricated in-place using framing-lumber and nails. A common floor structure in historic wood-framed warehouses, this system can support relatively large loads.
There is a set of interlocking reasons behind the rise in heavy timber construction. With advances in wood technology, and an innovative approach leveraging the structural system for uses beyond its basic function, heavy timber results in structurally resilient, sustainable, and budget-friendly buildings. It also supports a healthy work environment, which is connected to nature. On top of this, heavy timber buildings represent an authentic continuation of historic building practices—a qualitative but very valuable element for many modern tenants.
Once the decision has been made to use a heavy timber system, the architect, engineer, and contractor must then work together to address a series of design considerations, including fire rating, seismic code, and logistics.
To explore the growth of heavy timber construction, one must first understand the motivations for using the system, and then the technical considerations required for a successful project.
Several complementary themes influence the rising trend in heavy timber construction. In the authors’ experience, there exist three major motivating factors—response to context, sustainability and work environment, and the economics of a project that pencils out. These factors are interrelated, with aspects in each affecting the delivery of the rest.
Response to context
Heavy timber buildings can provide a compelling contextual response to a project’s environment, particularly where local building traditions historically employed a wood vernacular. Two contrasting approaches include the Clay Creative building in Portland, Oregon, and the Hudson building in Vancouver, Washington.
Clay Creative, a six-story, 8361-m2 (90,000-sf) office building, is located in the heart of Portland’s Central Eastside Industrial District. The creation of a contemporary reflection of the district’s historical building style—with exterior masonry walls, wood column, floor, and roof structures—was one of the project’s primary design drivers. As a nod to these historical traditions, Clay Creative opted to incorporate 50 x 152-mm (2 x 6-in.) on-edge NLT decking as the primary floor structure over glulam columns and beams. The perimeter structure shifts to structural steel to accommodate the loads of masonry veneer, and to handle seismic loads without sacrificing window openings around the entire exterior. This approach merged the historical traditions of floor construction with the modern technology of laminated timber and steel systems.
In contrast, the Hudson is located within Vancouver’s downtown historic district where new buildings are encouraged to respond to historical context. Although similar to Clay Creative, this three-story, 4181-m2 (45,000-sf) commercial office building takes things a step further by emulating traditional building techniques with a historically authentic design. It uses on-edge NLT decking and glulam columns and beams. The masonry veneer and structural steel are replaced entirely with structural clay masonry in the exterior walls, which act as a lateral force-resisting system. This combination of heavy timber and structural brick is both aesthetically and functionally reflective of Vancouver’s built-environment history.
Sustainability and work environment
Environmental concerns can also drive the use of wood in today’s structures and, in the process, serve as an organizing strategy for a project’s sustainability objectives. Wood is a renewable resource with clear advantages over steel and concrete in terms of its total embodied energy. For example, Clay Creative’s carbon emissions is 60 percent lower than an average building of its size—a reduction caused partly by its heavy timber structural system. (For more information, consult the 2016 AIA Oregon Architecture Awards Carbon Calculator for Clay Creative.) A lower manufacturing footprint and wood’s availability as a regionally sourced material reduced its embodied energy.
As an exposed structural system, heavy timber can also positively impact the health and wellness of building occupants. Wood is a natural material, helping people connect to the natural environment as part of a biophilic design response. The premise of biophilic design is that a stronger human-nature connection can reduce stress, improve cognitive and creative functions, increase productivity, and support the overall health and well-being of inhabitants.
The authors’ recent experience with heavy timber projects shows the cost to be comparable to other conventional structural systems, particularly if the heavy timber is left exposed (eliminating other finishes as well as reducing floor-to-floor height requirements). Exposed timber, when used as a finish, sustainably reduces extraneous material and systems, and reinforces biophilia. The impact of fire-rating requirements must also be considered. There are many such variations on the configuration of heavy timber buildings (e.g. stories, fire rating, ceilings, underfloor air distribution, etc.) that affect the overall cost, but on recent projects, the authors have seen effective design decisions bring the cost of heavy timber buildings on par with more conventional construction. As designers (and new tenants) for these types of buildings, the writers understand building owners are attracting a high-paying, stable tenant base, which seeks authentic and creative work environments. Higher rents and rapid lease-up help to offset minimal differences in building costs.
As heavy timber grows in popularity, its nuances are becoming more familiar, and design, detailing, fabrication, and construction costs are going down. CLT prices are dropping as more mills come on board and increase output. Recently, the Framework project, a new 12-story, 45-m tall (148-ft) office building in Portland, was granted a permit to start construction. Currently, the International Building Code (IBC) only permits at most a six-story building using Type IV construction. This would be the first CLT building taller than 26 m (85 ft) in the United States. It required rigorous fire, seismic, and other safety tests to prove an acceptable durability to the City of Portland Building Department in comparison to typical steel and concrete construction. Grant money is available in certain cases to encourage innovation in this area. For Framework, testing and research at Portland State University and Oregon State University were funded in part by a $1.5 million U.S. Tall Wood Building Prize Competition award.
These motivating factors—response to context, sustainability and workplace environment, and economics—have convinced multiple owners and developers to adopt heavy timber for new multistory buildings. Of course, the process is not over once a decision is made. Major design considerations are involved in implementing a heavy timber structural system. Like any material choice, heavy timber brings with it a distinct set of opportunities and challenges.
There are a few notable design considerations in using a heavy timber system, including fire rating, structural integrity, and constructability logistics like material lead time, skills and craftsmanship required, and protecting the material during construction. As with any system that is slightly removed from the status quo, a broadening of technical knowledge is necessary. For example, some of the techniques required to maintain the wood’s finish may be unfamiliar to contractors accustomed to steel framing. However, none of these considerations present a great challenge—in fact, there exists an opportunity in each.
A thorough grasp of a heavy timber building system’s fire rating is an integral part of the design process. This starts with an understanding of how fire interacts with wooden structural elements. Heavy timber is flammable, but depending on the situation (e.g. type of fire or fuel in space), charred wood can protect a structural element’s core for an acceptable code-prescribed amount of time. The American Institute of Timber Construction (AITC) has developed methods for determining the necessary size of structural members to achieve a specific fire rating (one- or two-hour) while still meeting the structural demand. A large amount of research is being done on this topic as heavy timber is increasingly proposed for bigger buildings.
Protection of connections is a critical structural issue. While the wood can be designed to develop a protective char layer, the steel hangers and bolts typically used in non-rated wood construction lose structural integrity well before the wood elements they connect. In response, creative designers have developed column and beam connections concealed within the wood members, so the wood surrounding the connections provide a char layer of protection. As CLT is increasingly used, a wider array of improved concealed connections is being developed. Coordination of the connection design with the fabricator and other subcontractors is central to the success of mass timber buildings with fire-rated construction.
Wood structures are not necessarily any more flammable than other common building materials. Earlier this year, London, England’s Grenfell Tower fire was exacerbated by highly flammable insulation and skin materials. Steel structural elements lose most of their structural integrity when heated above 288 C (550 F) and concrete starts to crack and spall at around 427 C (800 F). When properly sized, the char layer protects the structural core of a wood member, and can continue to perform as intended around 315 C (600 F) to 426 C (800 F). (For more information, read the 10th edition of Calculating the Fire Resistance of Exposed Wood Members by the American Forest and Paper Association [AF&PA], and the American Wood Council [AWC].) Wood begins to lose its structural integrity when the char depth reduces the cross-sectional area of a wood member to a point where it cannot support the imposed load.
As with any project, the unique circumstances or project vision may pose challenges requiring creative solutions. We have found heavy timber systems can accommodate the challenges posed. For the Hudson project, a two-story atrium required a different fire rating than the rest of the building. This difference
in rating meant there was a potential problem in the aesthetics of the atrium versus timber in the rest of the building: The owner’s vision included a cohesive
and seamless system of exposed wood elements. To address the challenge, the thickness of the NLT deck was increased, and members in and around the atrium were upsized so they could remain exposed and meet the owner’s aesthetic goals. The changes were achieved with fairly simple detailing at transition points, which camouflaged the interior connections.
Another important consideration is the seismic performance of the heavy timber system, particularly in active areas like the Pacific Northwest. In high seismic regions, use of mass timber structural elements is made more attractive because the material is lighter than steel and concrete, which reduces the building’s seismic mass. While this system was codified years ago in Europe, the seismic demands in the United States are significantly higher. Research and new products are developing higher-capacity elements, achieving strength and ductility properties similar to more conventional systems.
Design firm Skidmore, Owings and Merrill is overseeing testing with Oregon State University of a CLT/concrete composite system, which increases span length and load capacity of floor elements. Combined building systems—concrete cores or steel frames as the vertical lateral system and mass timber for the floor—are a solution for those concerned about a mass timber system for an entire structure.
Meanwhile, the Hudson building was designed using non-bearing structural masonry exterior walls as shearwalls, which support a glulam beam and column system with 50 x 100-mm (2 x 4-in.) NLT floors and 50-mm (2-in.) concrete topping slab. The structural system was studied extensively at the start of the design process, with many systems evaluated. In the end, the hybrid system was chosen as a cost-effective way to achieve a robust structural system, achieving the look and feel the building owner desired, while being able to quickly go through the permit review process.
Early adopters of this technology have laid the groundwork, but more testing is currently underway to improve the entire project delivery process. Based on existing research, the 2015 IBC explicitly allows use of CLT construction, spelling out structural and fire-rating considerations. As more states adopt this new code, owners will begin to ask about CLT for their projects.
The State of Oregon recently developed Statewide Alternate Method, No. 15-01, Cross-Laminated Timber Provisions, which brought forward several CLT provisions in the 2015 IBC, and included conservative seismic design parameters for CLT as seismic force-resisting elements. In some cases, plywood sheathing can be eliminated to allow CLT to act alone as the horizontal diaphragm, reducing costs.
In other states, use of CLT as a lateral force-resisting system can be achieved through the alternate material and method provisions in IBC, which allows for a performance-based design of these elements. This requires additional research and analysis on the structural engineer of record’s part, and the path to approval should be coordinated with the building official at the start of the design process.
The final major consideration is constructability, with a few factors—material lead time, contractor’s craft skill, and protecting the wood during the construction process—requiring special attention. With proper planning, none of these factors pose a significant barrier to a project’s success, but they all should be carefully addressed to ensure efficient, high-quality project delivery.
Engineered wood products (e.g. glulam) tend to be commonly available in most locations. CLT requires more planning, as production is limited in the country—Canada has more availability. As this product becomes codified and interest increases, local production will likely ramp up. Materials for the NLT systems are very simple to get, but take some onsite time to fabricate and install.
Building with wood requires a certain skillset and craftsmanship that is different from other types of commercial construction. The resurgence of wood
as a viable commercial construction material offers builders an opportunity—which many relish—to display advanced woodworking ability. Since the heavy timber structural system is often left exposed as an aesthetic element in the building’s finished condition, it is critical the wood is skillfully crafted. Fire ratings requiring connection protection is another aspect, which needs skill and precision. Fabricators who use computer numerical control (CNC) tools provide a heightened level of precision, helping with complicated connections and shapes, and reducing material waste.
Protection of material during construction
How wood is stored onsite, either in stockpiles or in the partially constructed building, has always been a concern. Again, with the heavy timber structural system often also acting as a finish element, wood protection can become increasingly critical. Members can come to the site wrapped and left that way until the last minute. Placing concrete over the top of a wood deck (NLT or CLT) can be really problematic, and preventing bleed water from staining the
wood is important. At the Hudson, a membrane was installed between the wood and concrete to prevent staining. The weather also needs to be considered—it is smart to schedule construction during the dry months.
Heavy timber is gaining popularity as it simultaneously addresses many common goals in modern multistory developments. The material is sustainable, can often be locally sourced, and, as a finish element, elegantly expresses an immediate connection to both nature and historic context. This benefit lends heavy timber buildings an atmosphere of authenticity. When left exposed as a design feature, timber tells a story that is immediately and viscerally understood by building occupants.
The use of timber in the structural system can support an overarching design centered on well-being and biophilia, a common goal in contemporary workplaces. Adopting a heavy timber system also allows designers, builders, and owners to illustrate a connection to the past and often to the surrounding neighborhood while constructing forward-thinking buildings in response to contemporary needs. Moreover, heavy timber is cost-effective. When the structural system is exposed—a basic aesthetic driver in these projects—other materials used in the ceiling become extraneous. Approached properly, there is minimal difference in cost between a heavy timber or hybrid building and one with a steel structure.
With these motivations in mind—sustainability, context, and the bottom line—heavy timber expertise is growing in demand. Once the decision to use a heavy timber system has been made, a thorough understanding of fire rating, structural integrity, and constructability logistics is necessary. Each of these factors has its own set of nuances unique to heavy timber, but none are deal-breakers. In fact, as more buildings get designed and built with heavy timber systems, the process and available options are improving rapidly. Innovative thinking, coupled with a respect for traditional practices, is revolutionizing the bones of modern buildings.
Rich Mitchell, AIA, LEED AP, GGA, is the managing principal at Mackenzie, an architectural/engineering consultancy firm. Mitchell has an acute understanding of the resurgence in heavy timber construction. A longtime advocate of sustainable design, he is the Green Building Initiative’s 2017 chair of the board of directors. Mitchell can be reached at email@example.com.
Josh McDowell, SE, PE, LEED AP, is a current firm principal at Mackenzie and leads the company’s structural engineering department. He has more than two decades of experience in the field, an extensive background in seismic strengthening code compliance, and regularly serves as project manager for seismic analysis and retrofit projects throughout the Northwest. McDowell oversaw the structural design of The Hudson heavy timber project. His e-mail is firstname.lastname@example.org.
Dietrich Wieland, RA, LEED AP, is the director of sustainability at Mackenzie, and he oversees the firm’s mixed-use team. Specializing in adaptive reuse and urban infill projects, his professional experience includes mixed-use, commercial, and urban redevelopment projects. He led the design effort for both the Hudson and Clay Creative heavy timber projects. Wieland can be reached via e-mail at email@example.com.
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