Timber Tower Research Project
The Timber Tower Research Project by Skidmore,
Owings & Merrill, LLP (SOM) was publically released in June of 2013,
and is available for download at SOM’s website. The goal of the
research project was to develop a structural system for tall buildings
that uses mass timber as the main structural material and minimizes the
embodied carbon footprint of the building. The structural system
research was applied to a prototypical building based on an existing
concrete benchmark for comparison. The concrete benchmark building is
the Dewitt-Chestnut Apartments, a 395-foot tall, 42-story building in
Chicago designed by SOM and built in 1966.
SOM’s proposed system is the "Concrete Jointed
Timber Frame". This system relies primarily on mass timber for the main
structural elements, with supplementary reinforced concrete at the
highly stressed locations of the structure: the connecting joints. This
system plays to the strengths of both materials and allows the
structural engineer to apply sound tall building engineering
fundamentals. The result is believed to be an efficient structure that
could compete with reinforced concrete and structural steel systems,
while reducing the embodied carbon footprint of the structure by 60 to
75%.
Project Basis
The basis of the research project was rooted in
sustainable urban development. Recent population projections have
estimated the current world population of 7.0 billion people to increase
to 11.0 billion people by the year 2050. More importantly, the number
of people that will be living in cities has been estimated to double
from 3.5 billion people to 7.0 billion people in the same time frame.
Tall buildings will likely be needed in order to house that many
additional people in growing cities. Tall buildings constructed to meet
population demands need to be developed in sustainable ways to limit
environmental impacts.
Tall buildings built using current technology and
materials pose a challenge to sustainable city development because they
offer both positive and negative environmental impacts. Positive
impacts include reducing urban sprawl, promoting alternative
transportation, and efficient energy use. These benefits come at the
cost of emitting more carbon dioxide to produce the materials and to
construct the building. These carbon emissions are referred to as the
embodied carbon footprint of a building. A tall building’s embodied
carbon footprint is significantly higher relative to low-rise buildings
on a per square foot basis. This is because the structure is usually
responsible for the majority of the building’s embodied carbon
footprint, and tall buildings require far more structure to support
their height. The structural system chosen for a tall building can have a
significant impact on the overall embodied carbon footprint of the
building.
Architectural detail of the wood structure proposed in the Timber Tower Research Report.
Design and Sustainability Issues
Structural engineers currently have four primary
materials in which to design buildings: steel, concrete, masonry, and
wood. Tall buildings currently use steel or concrete almost exclusively,
for two reasons. First, with some limited exceptions, non-combustible
materials are required by most building codes for buildings greater than
four stories tall. Second, steel and concrete have higher material
strengths than masonry and wood, making them a natural choice for tall
buildings which require support of very large loads. These factors have
generally limited wood use to low-rise buildings. Recently, developments
in mass timber technology are overcoming 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 achieve
panels as large as 1foot thick and 40 feet long. These panels can be
used as floors and shear walls with structural sizes necessary to
support a tall wooden building. Wood members of this size have an
equally important characteristic; they behave like heavy timbers in a
fire and form an insulating char layer which protects underlying
material. The charring behavior is predictable and preserves a portion
of the member’s structural strength, making performance based fire
design of mass timber structures possible. Mass timber has made wood a
viable choice for multi-story buildings as evidenced by completed
projects in Europe and Australia, and many other proposed projects
around the globe.
The structural and fire engineering advancements
of mass timber have made recent multi-story wood buildings possible.
However, the sustainability of wood seems to be an equally important
consideration in the resurgence of multi-story timber buildings. Wood
has been shown to be more sustainable than other materials because it
generally requires less energy to produce compared to structural steel
and reinforced concrete. More importantly, wood is approximately 50%
carbon by weight, a carbon sink that is the natural result of
photosynthesis. These sustainable aspects of wood make mass timber an
attractive material from which to construct the sustainable cities of
the future. The intersection of increasing urban populations, need for
tall buildings, and the sustainability of wood has led to the
increasingly popular concept of tall wood buildings. SOM has committed
decades of tall building design expertise to furthering this concept,
through the Timber Tower Research Project, by identifying key design and
construction issues related to tall wood buildings and proposing the
"Concrete Jointed Timber Frame" structural system. This system is
optimized for tall buildings and could be competitive with existing tall
building structural systems. The proposed system balances the
requirements of building marketability, economy, and sustainability.
SOM’s Timber Tower Research Project proposes a
"Concrete Jointed Timber Frame" system that relies primarily on mass
timber for the main structural elements, with supplementary reinforced
concrete at the connecting points.
Material Optimization
The primary goal of any structural system is to
provide a marketable and valuable building to the owner and occupants. A
marketable building must have adequate and flexible floor area to
layout useful space for the occupants. The most marketable building
layout is an open floor plan which allows a variety of room layouts and
maximum flexibility for future changes. An open floor layout requires
that the floor structure span the entire distance of the leasable area.
This distance in the Benchmark Dewitt-Chestnut building was 28 feet 6
inches, with a clear span of 26 feet 3 inches. The most advantageous
system to span this distance is a flat mass timber panel which minimizes
floor-to-floor height of the building. The required panel thickness to
span the required distance was determined to be 13½ inches. This
thickness was thought to be too great compared to the material required
for the Reinforced Concrete Benchmark to be economically viable.
Therefore, alternative methods to span the required distance were
investigated in order to reduce the amount of structural materials
used.
The controlling design consideration for the mass
timber floors was determined to be vibration due to occupant activity.
The floors were analyzed according to American Institute of Steel
Construction Design Guide 11, utilizing the velocity-based methodology,
which was found to be more useful for flat slab-type floors. Evaluation
of the criteria shows that increasing floor stiffness is the most
effective way to control vibrations. The floor stiffening effect of end
rotation restraint (fixed end condition) was quickly realized as an
efficient way to reduce vibrations. It was determined 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 such as mass timber shear
walls and structural glued laminated timber perimeter columns. Several
connection schemes were investigated to provide the required moment
connections. Steel reinforcing epoxy connected to the mass timber and
cast-in reinforced concrete joints were determined to be the most
reasonable solutions due to 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.
The reinforced concrete joints also proved to be
useful in other tall building aspects. The concrete jointing between
timber floors and timber shear walls provides a link beam between
individual wall panels. This creates a stiff lateral load resisting
system which is required for a tall building. It was also determined
that the demands on the link beams were beyond the capacity of a
structural glued laminated wooden link beam, requiring the use of a
material other than wood. The 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% of the
dead load of the Benchmark Building. This led to net uplift forces at
the extremities of the lateral load resisting system. This net uplift
would have been exacerbated without the concrete joints which account
for over 50% of the entire structure dead load, yet only 20% of the
structural material volume for a typical floor.
A comparison of the structural materials required
to construct the Benchmark and Prototypical building shows that the
proposed system is very efficient in material consumption and could be
competitive with reinforced concrete. The goal of minimizing the
structural materials used, namely mass timber, will help reduce costs
and minimize new demands on forest resources which may become strained
due to increasing populations and demands.
The non-structural effects of the proposed system
were evaluated and the most notable effect was the acoustic treatment
required on top of the mass timber floors in order to achieve a
marketable acoustic rating. The most effective treatment was determined
to be a 2-inch-thick gypsum concrete topping. This treatment thickness,
in addition to potential ceiling finishes, required 3 inches of
additional floor-to-floor height in order to maintain the same
floor-to-ceiling height as the Benchmark building. This has impacts on
wind loads on the building, and non-structural costs such as the
exterior wall system.
Conclusion
SOM believes that the proposed system is
technically feasible from the standpoint of structural engineering,
architecture, interior layouts, and building services. Additional
research and physical testing is necessary to verify the actual
performance of the structural system relative to the theoretical
behavior. SOM has also developed the system with consideration for
constructability, cost, and fire protection. Reviews from experts in
these fields, and physical testing related to fire, is also required
before this system can be fully implemented in the market. Lastly, the
design community must continue to work creatively with forward thinking
municipalities and code officials using the latest in fire engineering
and performance based design to make timber buildings a viable
alternative for more sustainable tall buildings.▪
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Timber Tower Research Project
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