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CallisonRTKL recently conducted research to explore the feasibility of using mass timber technology for high rise construction. The study envisioned a 40-story (420-foot-tall) residential tower in downtown Seattle using engineered woods such as cross laminated timber (CLT), nail laminated timber (NLT), and glulam panels as the primary structural material.

Mass timber products are the only renewable structural building materials of scale; they also have the lowest energy consumption across their life cycle. They are sourced from sustainably managed forests or from cheap, sustainable softwood and possess excellent structural stability.

Our research showed a hybrid and composite format of construction was the optimal solution for a structure of this magnitude. The tower core, two extending outrigger walls, and the foundation are the only components that are constructed in-situ with reinforced concrete. Lower floor columns are fabricated in steel and two other outrigger walls in carbon fiber, using the structural performance advantages of lightweight carbon fiber strands in a hexagonal net configuration for wind-load resistance. The material-specific advantages of mass timber construction (MTC), reinforced concrete, and carbon fiber are strategically put to work to form an innovative high-performance hybrid structural system.

Structural design

CallisonRTKL’s research envisioned a 40-story residential tower in downtown Seattle using engineered woods. Rendering: CallisonRTKL Inc.

To better understand the performance of a high rise timber tower, our team conducted a comparison of a typical concrete building and a mass timber-framed building with the same height. For both, the concrete core is continuous from roof level to base level. Outrigger walls were assumed to run to the third level and sit on columns that extend to base level. In the lower levels, the concrete core is modeled with 15-inch-thick walls to the perimeter of the core and 8-inch-thick walls internally. The outer core wall thickness has been modeled with a reduction from 15 inches to 12 inches at level 15, and 9 inches thick at level 30.

Concrete strength in the core and shear walls varies from 65 MPa at level 10 to 50 Mpa at level 20 and 40 MPa above. If the core walls were made out of timber, their thickness could be comparable to typical concrete core walls for support of gravity loads, but for lateral loading, the timber planks require significant shear connectors across joints to provide the stiffness required. These connectors require careful design to ensure that shear can be transferred between timber panels without panel failure and that the connection method is buildable. As such, timber core walls have not been developed as part of this project.

Also, when comparing the standard concrete example to a timber floor with a concrete topping system, there is a 38 percent reduction in the floor plate mass. Without changing the wall design, this results in a 28 percent reduction in the total building mass.

Wind resistance

The performance of a tall building exposed to wind is a function of the dynamic properties of the building. The taller the building, the lower the frequency becomes and the more significant the dynamic component is in estimating wind loads. A frequency of less than 1.0 Hz is the standard for determining if the structure has a significant resonant response. Both base and timber building solutions have a fundamental frequency of less than 0.5 Hz.

Keeping the building core and shear wall system the same for both examples and using timber floors with a reduced floor mass as opposed to concrete floors results in a positive increase in the building’s natural frequency. This reduces the resonant response of the building and results in lower design wind forces.

Therefore, building mass reduction indicates a notionally stiffer building; however, as the main concrete core structure used to resist the lateral loads is unchanged, lateral drift under static lateral loads also does not change. The increase in frequency is due to the reduced mass, and the benefit — the building’s response under wind loads — is important in assessing the building’s dynamic performance under both wind and earthquake loads.

Fire safety

In addition to structural wind load resistance requirements, the tower’s core walls are designed to be in-situ reinforced concrete for better compliance with fire-safety regulations. Tower core walls encapsulate all vertical circulation, including a designated fire service access elevator and two pressurized escape staircases. The tower is outfitted with an automatic sprinkler system throughout.     

Mass timber products resist fire well due to their make-up, scale, and thickness. When exposed to flames, they will develop a protective charring layer at their surface instead of catching fire and burning in a way that is associated with regular timber frame products and construction. Char rate of 1.5 inches to 1.75 inches per hour is the required char thickness, which should be added to the structurally required dimension of a respective assembly.

Seismic resistance

Seismic loads are a function of the building mass. Around the globe, there are wide variety of systems used that idealize the impact of earthquake loads on a system of lateral loads. This is a representation of the inertial loads generated in the building as the ground beneath moves. The building mass is the common element when calculating seismic loads: The loads are directly proportionate to the building mass, so a reduction in mass will result in an equivalent reduction in seismic loads.

A means to dissipate the energy generated through ground movements without compromising the structure or risking collapse is of utmost importance. This is achieved through reinforcement detailing in concrete structures, which enables ductile behavior under seismic forces. Energy is dissipated through the cracking of concrete, while the reinforcement ensures that the structural integrity is maintained and concrete confined. 

The Trees and Timber Research Institute of Italy (IVALSA) carried out testing on a seven-story CLT building in Japan (Ceccotti, A., 2010, Cross Laminated Timber Introduction to Seismic Performance, Trees and Timber Institute IVALSA-CNR National Research Council, Italy). The building was placed on a shake table and demonstrated ductile behavior and good energy dissipations. The most influential factor in the performance was the nature of the mechanical connections used.

The tower core, two extending outrigger walls, and the foundation are constructed in-situ with reinforced concrete. Lower floor columns are fabricated in steel and two other outrigger walls in carbon fiber. Image: CallisonRTKL Inc.

Construction technology

Construction of tall buildings in mass timber relies heavily on a tower crane for all hoisting needs since all mass timber components are prefabricated. Modularized, factory-built, and site-delivered kitchen and powder room pods will put additional strain on the tower crane schedule. Careful assessment should be made relative to “hook time” requirements and availability. Depending on site logistics and the schedule for construction, an additional crane could be considered for a more efficient construction process.

CLT and NLT floor panels are typically set one at a time using a tower crane or some other lifting device. While more conventional construction such as steel erection uses a “Christmas tree” method (several steel beams are hoisted simultaneously and placed in the same pick), mass timber floor panels will allow the overall completion of the floor assembly in a more efficient manner.

Depending on size, most timber columns can be lifted into place with a two-person crew once the columns have been landed on the destination deck. This is advantageous compared with other structural systems because it does not rely on the use of cranes other than hoisting the materials onto the level they will be installed. A comprehensive erection plan linked with fabrication, delivery, and manpower schedules is a prerequisite for smooth onsite operations and successful installation. 

Conclusion

Overall, the Seattle mass timber tower design defines a vision to build tall, dense, resilient, sustainable, and low-carbon developments in the coming decades. Such an endeavor will obviously require substantial political leadership, regulatory approvals, and financial commitment and resources. This study suggests that using mass timber in a creative and responsible way reduces carbon footprints and brings us closer to the goal of a more sustainable future.


Amir Lotfi, LEED AP BD+C, is an associate at the global architecture, design, and planning firm CallisonRTKL in the Seattle office. Amir specializes in the mixed-use and tall building markets and has worked on projects around the world. He can be reached at amir.lotfi@callisonrtkl.com.

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