A new way to build tall

April 2014 » Feature Articles » Materials
SOM’s Timber Tower Research Project investigates timber as sustainable materials for cities of the future.
Benton Johnson, P.E., S.E., and David Horos, P.E., S.E., LEED AP

Skidmore Owings & Merrill LLP (SOM) undertook the Timber Tower Research Project with the aim of developing a structural system for tall buildings using mass timber as the main structural material in order to minimize its embodied carbon footprint. A prototypical concrete building — the Dewitt-Chestnut Apartments in Chicago — was chosen as a benchmark for the investigation. The 395-foot-tall, 42-story building designed by SOM was built in 1966.

The proposed system, first revealed in a June 2013 report, is called the “Concrete Jointed Timber Frame.” The main structural elements are mass timber, with supplementary reinforced concrete utilized at the highly stressed locations of the structure — the connecting joints. This system relies on the inherent strengths of both materials while applying sound tall building engineering fundamentals to the problem. The result is an efficient structure that SOM believes could compete with reinforced conventional structural systems while reducing the embodied carbon footprint of the structure by 60 to 75 percent.


Detailed view of a typical floor. Photo: Skidmore, Owings & Merrill.

Architectural detail of the wood structure proposed in the Timber Tower Research Project report. Photo: Skidmore, Owings & Merrill.

The use of mass timber is vital in these structures – providing the strength and stiffness necessary for multi-story gravity and lateral loads. Photo: credit Skidmore, Owings & Merrill. ​

Why tall buildings matter

SOM’s interest in developing a timber tower is rooted in our pursuit of sustainable urban development. Projections estimate the current world population of 7 billion people will increase to 11 billion people by 2050. More significantly for the building industry, the number of people living in cities is predicted to double from 3.5 billion to 7 billion people during these years. Tall buildings are one solution for growing cities — provided they are developed in sustainable ways that limit environmental impacts.

Current tall building technology and materials pose a challenge. Positive effects include reducing urban sprawl, promoting alternative transportation, and efficiently using energy in their operation. But these benefits come with a higher embodied carbon footprint when accounting for the additional carbon dioxide emissions necessary to produce the materials necessary to construct the building. On a square-foot basis, a tall building’s embodied carbon footprint is significantly higher than a low-rise building. This is because the structure is generally responsible for the majority of the building’s embodied carbon footprint and tall buildings require considerably more structure to support additional height. The choice of a structural system for a tall building can have a significant impact on its overall embodied carbon footprint.

Choices

Steel, concrete, masonry, and wood are the four primary materials structural engineers use to design buildings. Tall buildings are almost exclusively limited to steel or concrete for two reasons. First, non-combustible materials are generally required by building codes for buildings taller than four stories. Second, steel and concrete have higher material strengths than masonry and wood, which makes them

natural choices to support the large loads of tall buildings. Wood has generally been relegated to supporting low-rise buildings, although recent developments in mass timber technology can overcome these challenges. Mass timber products such as cross-laminated timber (CLT) can be built up using small pieces of dimensional lumber and structural adhesives to create panels up to 1 foot thick, 10 feet wide, and 40 feet long. These panels can provide the basis for floors and shear walls with the structural sizes necessary to support a tall wooden building. Additionally, they behave like heavy timbers in a fire and form an insulating char layer, which protects underlying material. This charring behavior is predictable and preserves a portion of the member’s structural strength, making performance-based fire design of mass timber structures possible. These new developments have made mass timber a viable choice for multi-story structures.


Detailed views of framing. Photo: credit Skidmore, Owings & Merrill.

Detailed views of framing. Photo: credit Skidmore, Owings & Merrill.

Detailed views of framing. Photo: credit Skidmore, Owings & Merrill.

The sustainability of wood is also a factor in the growing interest in multistory timber buildings. Wood requires less energy to produce compared with structural steel and reinforced concrete and, more importantly, wood is approximately 50 percent carbon by weight, making it a carbon sink — the natural result of photosynthesis. These traits make mass timber an attractive material from which to construct the sustainable cities of the future.

SOM has applied its decades-long expertise in tall building design to further this concept through the Timber Tower Research Project. The report identifies key design and construction issues related to tall mass timber buildings and proposes the “Concrete Jointed Timber Frame” structural system. This specific system is optimized for tall buildings and could be competitive with existing tall building structural systems for the size represented by the 42-story benchmark structure. Reflecting SOM’s integrated approach to design, the solution balances the requirements of building marketability, economy, and sustainability.

Optimizing materials

In general, the most marketable building layout is an open floor plan that allows a variety of room layouts and maximum flexibility for future changes. These specific features helped SOM choose the DeWitt-Chestnut Apartments as an appropriate benchmark structure. This leasable area distance in Dewitt-Chestnut is 28 feet, 6 inches, with a clear span of 26 feet, 3 inches. It is most advantageous to span this distance with a flat mass timber panel, which minimizes floor-to-floor height of the building. Early investigations revealed that the required panel thickness to span this distance was 13.5 inches. This was thought to be too thick compared with reinforced concrete to be economically viable — necessitating alternative methods to span the required distance.

Vibration due to occupant activity was found to be the controlling design consideration for the mass timber floors. SOM analyzed the floors according to American Institute of Steel Construction design guide 11, utilizing the velocity-based methodology. Increasing floor stiffness is the most effective way to control vibrations. The floor stiffening effect of end rotation restraint (fixed end condition) is an efficient way to reduce vibrations. SOM discovered that an 8-inch-thick mass timber floor panel could be used if end restraint was provided. This requires moment connections at the intersection of mass timber floor panels with vertical elements, including mass timber shear walls and structural glued laminated timber perimeter columns. Several connection schemes were considered to provide the necessary moment connections. The most reasonable solution: steel reinforcing epoxy connected to the mass timber and cast in reinforced concrete joints because of the ability of reinforced concrete to resist complex load paths. These reinforced concrete joints are able to resist floor-to-floor compression, shear, bending moments, and torsion, thus creating an efficient composite-timber system.

Reinforced concrete joints proved useful meeting other criteria. The concrete jointing between timber floors and timber shear walls provides a link beam between individual wall panels. This creates the stiff lateral load resisting system required for a tall building. Demands on the link beams were found to be beyond the capacity of a structural glued laminated wooden link beam, which required the use of a material other than wood. Concrete joints and link beams were also useful in the design of the lateral system to resist net uplift due to lateral loads. The prototypical building has approximately 40 percent of the dead load of the benchmark building. This creates net uplift forces at the extremities of the lateral load resisting system. This net uplift would have been exacerbated without the concrete joints, which provide more than 50 percent of the entire structure dead load, but only 20 percent of the structural material volume for a typical floor.

Comparing the structural material required to construct the benchmark and prototypical buildings shows that the proposed system is very efficient in material consumption and could be competitive with reinforced concrete. Minimizing the mass timber structural materials used will help reduce costs and minimize new demands on forest resources.


The 400-feet-tall, 42-story building documented in the 72-page report and 33 supporting drawings demonstrate the technical feasibility of meeting architectural, structural, interior, and building service requirements. Photo: credit Skidmore, Owings & Merrill.

Next Steps

SOM believes that the “Concrete Jointed Timber Frame” is technically feasible from the standpoint of structural engineering, architecture, interior layouts, and building services. Verifying the actual performance of the structural system relative to the theoretical behavior will require additional research and physical testing. The system has been developed with considerations for constructability, cost, and fire protection, but further reviews from experts in these fields and physical testing related to fire will be necessary before the system can be fully implemented in the market. Lastly, the design community needs to work creatively with forward-thinking municipalities and code officials to make timber buildings a viable alternative for more sustainable tall buildings by using the latest in fire engineering and performance-based design.

For more information, the full Timber Tower Research Project is available for download at SOM’s website, www.som.com.

Benton Johnson, P.E., S.E., associate at Skidmore Owings & Merrill LLP, Chicago, is the project engineer on the Timber Tower Research Project. David Horos, P.E., S.E., LEED AP, a director at SOM, is the senior engineer on the Timber Tower Research Project.


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