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Cross-Laminated Timber: Coming Soon to a Project Near You

Every design project is a learning opportunity.

Bergmeyer is currently designing a mixed-use multi-family building at 74 Main Street in Durham, New Hampshire. Early in schematic design, we were exploring innovative and sustainable construction methods. At the same time, we prepared this report through a separate research study on these methods in collaboration with H+O Structural Engineering. We were excited to learn that the 2021 International Building Code (IBC) will “rebrand” heavy timber construction into four new construction types, raising the allowable height limits for many types of engineered wood structures.

These new construction types will be adopted statewide in Massachusetts in 2021:

  • Type IV-A – Mass timber up to 18 stories with 2- and 3-hour non-combustible protection enclosing all timber elements.
  • Type IV-B – Mass timber up to maximum 12 stories, limited exposed timber permitted with a 2-hour fire resistance.
  • Type IV-C – Mass timber up to 8 stories (residential), or 9 stories (business), no protections needed if timbers are intrinsically designed for 2-hour fire resistance.
  • Type IV-HT – New name for Type IV in the previous code

Although Type IV-A and Type IV-B allow for heavy timber construction of similar scale to conventional high-rise construction types, both require some sort of non-combustible fireproofing over all or part of the structural framing system – meaning the beauty of wood beams and floor slabs would need to be concealed. Type IV-C, however, will not require covering of the structural frame. This makes it ideal for mid-rise multi-family construction seeking to take advantage of the beauty of an exposed cross-laminated timber (CLT) structure.

While 74 Main Street is going full steam ahead with an equally innovative design and sort of hybrid structural approach (read more about the project through the “On the Boards” profile on our website), our interest in studying CLT technology with Type IV-C construction was piqued. We saw a world of new possibilities for sustainably designed multi-family buildings and jumped at the chance to explore this with like-minded partners like H+O Structural Engineering.

CLT: The Basics

Cross-laminated timber is a prefabricated engineered wood panel. Each CLT panel consists of several layers of dimensional lumber stacked in alternating directions, bonded with structural adhesives, and pressed to form a solid, rectangular panel. The panel is then sized and shaped with a CNC machine, allowing finished panel dimensions to achieve a high level of accuracy. By alternating each wood layer, structural rigidity is obtained in both directions, like a reinforced concrete slab.

Building with CLT is fast, efficient, and has environmental benefits that can result in significant cost savings. Because CLT is produced off-site in a factory, on-site construction time, and, therefore, on-site labor is reduced. Additionally, mass timber structures are 30% lighter than the steel equivalent; 60% lighter than an equivalent post-tensioned concrete building allowing for a smaller foundation and lower seismic loads.

The prefabrication process, as well as the fabrication of the CLT panels themselves, allow for reduced construction waste. Manufacturers use Building Information Modeling (BIM) to send panel designs directly to the CNC machines often used in the prefabrication process. This coordination enables greater control of the construction process, reduced material waste, and quicker fabrication times.

Given that Bergmeyer and our research design partners, H+O Structural Engineering, were already adept with the use of BIM and productively sharing our design models with consultants, builders, and fabricators, we were eager to see what we could create.

Our Mixed-Use Prototype

“Panelized” wood framing is the most economical system in low-rise multifamily residential, hotel and assisted living structures, but conventional panelized wood construction has been historically limited to 5 stories for both residential and business occupancies. Podium structures, where 4 or 5 stories of panelized wood construction is supported by Type IA construction, are often utilized in urban developments to increase building height and maximize the cost-advantages of wood construction while introducing mixed-use or parking areas on the bottom levels. Taking advantage of the new opportunities provided by the 2021 IBC and the design and cost advantages afforded by CLT construction, we set out to design an ultra-efficient mid-rise multi-family prototype with our partners, H+O.

Our prototype building uses CLT floors, laminated “glulam” heavy timber columns, and a double beam system along with a modular steel stair and cast-in-place concrete elevator shafts. The double beam system allows us to minimize beam depth, also minimizing floor to floor heights to allow for 7-stories of mass timber over slab on grade construction. This configuration keeps the building below the height designation of Massachusetts “high-rise” – thereby avoiding additional requirements and costs. By eliminating the Type IA concrete podium required to develop a 5-on-2 podium structure, the 7-story mass timber structure is cost competitive and yields higher ceiling heights, schedule advantages and market-differentiation. Eliminating concrete and steel as much as possible results in significant costs savings and a reduced carbon footprint.

The rectangular shape and repetitive structural bays allowed us to maximize the residential unit count and gave us great flexibility for the units’ sizes. The twenty-seven-foot bay size we chose gave us an efficient floor plan that was easily arranged into studio, one and two-bedroom units – depending on the desired unit mix.

The centrally located stair and elevator cores are ideally located to provide lateral bracing, reducing the need for additional structure, and creating the greatest floor to floor height possible in relation to the overall building height. Isolating the structural system on either side of the centrally located corridor meant the glulam beams could stop on either side of the corridor walls – allowing MEP ducts, pipes, and services to run the corridor's length unimpeded by the beams.

The CLT and glulam structure were highly compatible with a panelized façade system on the building's exterior – another opportunity for cost savings. The structural system's rigidity and strength allow façade panels to be hung from the system – potentially without additional reinforcement (depending on the finish materials used). Using a system of only 5 different façade panel "types" but repeating them in a varied and interesting way allows for endless façade expression possibilities.

Finally, from a user-experience point of view, the potential to create dramatic and attractive interior spaces for these residential units is compelling. The flexibility of the façade system allowed the exterior glazing and balconies to be maximized. The clean lines of the exposed wood structure and floors bring exceptional warmth to the spaces while eliminating the need for redundant interior finishes. The overall look and feel combine the romantic tactility of historic New England mill buildings with the dynamic transparency of the most contemporary high-rise construction.

The Wrap-Up

Building mid-rise structures with cross-laminated timber construction has been an interesting and promising idea for some time. When the 2021 IBCC is adopted in Massachusetts, the multifamily market will be ready to leverage the potential for this construction type. With the inherent cost and time savings that come with timber construction along with numerous environmental benefits – primarily being a lower embodied carbon than any comparable structural systems – CLT should take off. For more information, or to see if CLT is right for your next project, reach out to our team of experts at hello@bergmeyer.com.

Download our CLT White Paper, a research collaboration with H+O Structural Engineering:
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