Pitch
Modify the building codes to allow wood construction for tall buildings and more use types to decrease Somerville’s carbon footprint.
Description
Summary
Urban Wood
Somerville’s built environment is dominated by the wood construction of its residential urban fabric. Wood construction is key for obtaining carbon neutrality on an urban scale due to the material’s inherently low carbon footprint and its high carbon capture. Somerville’s legacy of wood construction is already a model for a low carbon-footprint urbanism, and expanding this legacy would build Somerville’s carbon-neutral future. The current building code limits the height and use of wood buildings. Somerville should revise the building code to allow for taller wood buildings with a greater range of possible use types.
Carbon Footprint
The carbon footprint of a wood building is low, especially when compared to steel and concrete. Wood can be harvested from sustainable forests whereas steel and concrete are environmentally destructive to extract and process their raw materials. Wood is lightweight relative to its strength so it uses far less energy to transport materials and reduces the necessary size of foundations to support buildings.
Carbon Capture
Wood is an ideal carbon-capture material. Photosynthesis transforms light energy, carbon dioxide and water into wood and free oxygen. Somerville’s wood buildings sequester carbon on an urban scale for the life of the building and longer if materials are recycled. Somerville’s built environment is an urban-scaled carbon-capture system par excellence.
Building Codes
Nationally and internationally, municipalities and state agencies have adopted performance-based building codes that allow for taller wood buildings with a greater range of possible use types. Somerville’s current building code is prescriptive: it mandates that buildings over four stories be constructed of non-combustible materials, such as steel and concrete. A performance-based building code will accept any construction type as long as it can be proven to perform as well as the prescriptive code mandates for structural and fire ratings.
What actions do you propose?
Urban Wood
Somerville’s built environment is dominated by the wood construction of its residential urban fabric. Wood construction is key for obtaining carbon neutrality on an urban scale due to the material’s inherently low carbon footprint and its high carbon capture. Somerville’s legacy of wood construction is already a model for a low carbon-footprint urbanism, and expanding this legacy would build Somerville’s carbon-neutral future. The current building code limits the height and use of wood buildings. Somerville should revise its building code to allow for taller wood buildings with a greater range of possible use types.
Carbon Footprint
The carbon footprint of a wood building is low, especially when compared to steel and concrete. Wood can be harvested from sustainable forests whereas steel and concrete are environmentally destructive to extract and process their raw materials. Wood is lightweight relative to its strength so it uses far less energy to transport materials and reduces the necessary size of foundations to support buildings.
To help contextualize the potential carbon impact of Urban Wood, we can use the Assembly Row development as an example. Constructing the 6.6 million square feet in Assembly Row using wood rather than concrete and steel could have avoided an estimated 440,000 metric tons of CO2 equivalent greenhouse gas emissions, while sequestering an additional 20,000 metric tons of carbon in the wood structures. This total carbon benefit is equal to taking nearly 12,500 cars off the road, and is greater than the green house gas (GHG) emissions from operating the development for a year and a half.
There are many other possible large scale developments to unfold in Somerville, particularly around the Union Square area. As the city grows, Urban Wood could play a significant role in the overall carbon picture for the city, and could serve as an excellent example for other communities to adopt similar carbon saving building codes.
Carbon Capture
Wood is an ideal carbon-capture material. Photosynthesis transforms light energy, carbon dioxide and water into wood and free oxygen. Somerville’s wood buildings sequester carbon on an urban scale for the life of the building and longer if materials are recycled. Somerville’s built environment is an urban-scaled carbon-capture system par excellence.
Building Codes
Nationally and internationally, municipalities and state agencies have adopted performance-based building codes that allow for taller wood buildings with a greater range of possible use types. Somerville’s current building code is prescriptive: it mandates that buildings over four stories be constructed of non-combustible materials, such as steel and concrete. A performance-based building code will accept any construction type as long as it can be proven to perform as well as the prescriptive code mandates for structural and fire ratings.
Actions
The actions for this proposal are quite simple (if not easy): create a performative-based option in the Massachusetts State Building Code (780 CMR) that allows for taller wood buildings and a greater range of possible use types for wood buildings. This would entail the City of Somerville Office of Strategic Planning and Community Development working with the Commonwealth of Massachusetts to amend the building code. Many national and international precedents exist for building code revisions incorporating performance-based approaches that allow for tall wood buildings of diverse use types. For example, the Province of British Columbia passed the “Wood First Act” in 2009 to “facilitate a culture of wood by requiring the use of wood as the primary building material in all new provincially funded buildings, in a manner consistent with the British Columbia Building Code.”
Performance-based Building Code
Performance-based building codes are also known as objective-based building codes, or alternative solutions-methods, because they achieve the same performative measures of a conventional building code but through alternative construction types. Performance-based code provisions are not limited by the conventional relationships between construction types and use types described in the current Massachusetts State Building Code, such as only using non-combustible construction types for tall buildings. Instead, performance-based building codes allow building designers to demonstrate that the intent of the building code to protect the health, safety and welfare of the public can be satisfied with alternative building assemblies. For example, when considering the fire safety, the building designers can demonstrate that a tall building constructed of mass timber can achieve the same fire safety requirements as that same building constructed from non-combustible materials by incorporating known and rigorously-tested fire-rated construction assemblies.
A revised building code should also specifically require that all wood used for new building projects be certified as being harvested from sustainable forests. This will ensure that the wood used in future development will not in turn damage any other natural environments.
Mid-Rise Buildings
Somerville is not a city of super tall buildings, but it is a city of low and mid-rise buildings. Mid-rise buildings from 5 to 20 stories are in an ideal height range to realize the advantages of tall wood construction. Very tall buildings must be designed to withstand extreme lateral loads from the wind and seismic movement that create significant tensile forces in a buildings structure. Tensile forces in super tall buildings are better resisted using steel or steel-reinforced concrete construction due to the tensile strength of steel. Mid-rise buildings, however, require much less structure devoted to lateral loads than super tall buildings. Instead, the primary structural loads that a mid-rise building must resist are vertical loads from gravity. Gravitational loads create compression forces, and wood performs particularly well in compression. “Mass timber” construction systems are an optimal choice both structurally and financially for buildings up to 20 stories tall, and form a perfect compliment to wood light frame residential buildings of 1 to 4 stories.
Mass Timber
Mass timber construction systems include Cross Laminated Timber (CLT: an engineered wood panel consisting of layers of dimensional lumber glued at right angles to one another to form structural panels), Glue Laminated Timber (Glulam: dimension lumber laminated together to create beams of any shape and size), and Structural Composite Lumber (SCL: a family of solid engineered wood products created by heating wood veneers, strands or flakes and adding adhesives under pressure).[Rethink Wood] SCL products include Laminated Veneer Lumber (LVL), Parallel Strand Lumber (PSL), Laminated Strand Lumber (LSL) and Oriented Strand Lumber (OSL). Unlike traditional heavy timber structures, these engineered lumber types can use fast-growth wood from sustainably managed forests. Mass timber systems also speed the construction assembly and greatly reduces the amount of construction waste, further contributing to its overall carbon neutrality.
Combustible Construction
The Massachusetts State Building Code looks at most wood construction as “combustible construction,” but different wood construction types represent different characteristics in the case of a fire. For example, the components of heavy timber and mass timber systems have a much greater mass than a typical wood light frame construction types such as balloon or platform frame. During a fire, the greater the mass, the longer it takes the wood to reach its critical ignition temperature. Furthermore, this slow-to-burn quality means large members can be deeply charred without losing their structural strength. Therefore, mass timber structures can resist the deleterious effects of fire for much longer than wood light frame structures due to the greater thermal lag and larger size.
Mass timber structures actually enjoy some advantages over steel in a fire. Long before a fire is hot enough to melt the metal, the steel in a steel frame building reaches a critical temperature called a “yield point” at which time it loses its stiffness and fails catastrophically. Mass timber, however, can burn for a long period of time and still maintain a core of structurally viable material. Additionally, during a fire, the outer layers of large timber structural elements develop an outer layer of char that naturally acts as a fire resistive protection for the inner layers of wood.[Michael Green] This is why some species of trees remain standing even though they have experienced a forest fire. The extra time it takes for a mass timber to burn to a point of failure provides extra time to both evacuate a building and to safely suppress the fire.
Most important to fire safety are the regulated fire safety standards in the Massachusetts State Building Code that provide integrated fire suppression systems such a sprinklers, fire rated wall and floor assemblies, and fire separation of habitable spaces from emergency exits. Under this proposal, such fire safety systems would be identical for tall wood buildings and tall steel or concrete buildings, only the construction material would be different.
Incentives
The City of Somerville could institute incentives for building tall wood buildings such as tax incentives for building with carbon-neutral or carbon-sequestering materials like mass timber. The city could also increase the percentage of lot coverage, provide certain zoning relief, or increase the allowable floor area for buildings that use carbon-responsible construction systems.
Building owners, designers and contractors would also be attracted to mass timber construction for mid-rise building because the construction costs are generally lower than for steel and concrete. For example, Waugh Thistleton Architects, designers of a nine-story residential tower in London, determined that their building was 15% less expensive than an equivalent concrete building.[Mass Timber and Wood Framing] Why? Mass timber buildings weigh less so transportation weight and its associated energy expenditure are reduced. Foundations for timber buildings support less weight, and so they require less material thereby reducing both cost and embodied energy. This is particularly powerful because most foundations use concrete, which has a very high carbon footprint. Also, mass timber components are fabricated in a factory, which increases quality and reduces waste. Structural connections are simplified with wood because it does not need to be welded or cured. And construction time is reduced because large prefabricated components are rapidly lifted into place by a crane.
Urban Wood
A performance-based building code could allow for the construction of mid-rise residential, commercial, institutional and research buildings in Somerville. This would combine Somerville’s legacy of low-rise wood frame construction with a future of mid-rise mass timber construction making it a leader as an “Urban Wood” city with a significant reduction of carbon output for material production and building construction while greatly increasing the city’s overall carbon sequestration. Somerville’s neighborhoods have been sequestering carbon in its wood frame construction for over 150 years, thereby compounding its carbon footprint over a long duration. The city can build on this legacy by extending wood construction at an even greater scale of mid-rise buildings into the future. Imagine the carbon advantage if all of the new buildings in the unfolding Assembly Row were built with mass timber! Carbon sequestration at this scale could lead the drive toward the Mayor’s goal of carbon neutrality for the City of Somerville.
Who will take these actions?
The primary catalysts for amending the Massachusetts State Building Code would be the Somerville Office of Strategic Planning and Community Development (OSPCD). This group would need to collaborate with the Commonwealth of Massachusetts Board of Building Regulations and Standards (BBRS) and the Massachusetts State Fire Marshal to affect change in the building regulations.
OSPCD should collaborate with the plethora of institutions of higher education in the vicinity that are conducting research on alternative construction systems. Additionally, the OSPCD could engage critical professional organizations that represent the array of building design professionals, such as the Boston Society of Architects (BSA), the eastern Massachusetts chapter of the American Institute of Architects (AIA); the Boston Association of Structural Engineers (BASE), the eastern Massachusetts chapter of the Structural Engineers Association (SEA); representatives from the development community, such as the Greater Boston Real Estate Board (GBREB); and representatives of the wood industry, such as the American Wood Council [AWC].
The OSPCD could identify specific academic institutions, and architectural, engineering, construction, and real estate development firms that could have a focused role in developing the parameters for a performance-based building code. By combining the available expertise in the area, Somerville could become a national leader in sustainable and carbon-neutral design for the built environment.
What are the key challenges?
Carbon Sequestration
This proposal addresses an urban-scale opportunity for carbon neutrality in Somerville by capitalizing on its existing Urban Wood architecture and paving the way for even greater carbon sequestration for the future. Architect Michael Green reports the follow statistics regarding the carbon sequestration properties of wood:
“Wood stores somewhere between 1 to 1.6 tonnes of carbon dioxide per cubic meter of wood depending on species, harvesting methods and secondary manufacturing methods. A typical North American timber-frame home captures about 28 tonnes of carbon dioxide, the equivalent of seven years of driving a mid-size car or about 12,500 liters of gasoline."[Michael Green]
Aggregating the carbon sequestration described above for the whole City of Somerville represents a massive savings of carbon generated that would otherwise pollute our atmosphere and oceans.
“Every minute, on average, new growth adds more than 50,000 cubic feet of wood to U.S. forests, sequestering approximately 800 million metric tons of CO2. In that same average minute, more than 24,000 cubic feet of wood are removed from U.S. forests to build homes, make furniture, and create other products."[Evaluating the Carbon Footprint of Wood Buildings]
Although it is difficult to predict the future rate of development in Somerville, and therefore difficult to calculate with precision the amount of carbon that would be sequestered over time, what we do know is that addressing carbon production and sequestration of building at the urban scale will have a greater impact on carbon neutrality than any other urban system.[R. Sarthre and J. O’Connor]
What are the key benefits?
Outcomes
The outcomes for instituting performance-based building code options and incentives for producing buildings using mass timber construction systems are potentially significant:
- Somerville could measure the carbon neutrality of its existing building stock and understand that it already represents a carbon-responsible Urban Wood built environment
- Somerville could extend the existing legacy of Urban Wood into the future by allowing for, and incentivizing, mass timber construction
Benefits
The benefits of deploying performance-based building code options and incentives for producing building using mass timber construction systems are both immediate and long term:
- Somerville would become a national leader for sustainable carbon-neutral building design, construction and development
- Somerville would reduce carbon at the massive scale of its entire built environment for multiple generations to come
More tall buildings in Somerville would create greater urban density, and greater urban density means greater use of mass transit systems and less development in greenfield sites at the periphery of the metropolitan region.
What are the proposal’s costs?
Costs
The beauty of this proposal is that it does not cost the City of Somerville anymore than the salaries already being paid to its employees now. If Somerville did decide to provide tax incentives for building with mass timber for mid rise buildings, then this cost should be calculated to be offset by the addition tax revenues generated by the new construction.
Also, it does not cost the building owners, developers and contractors any more to build with wood than its non-combustible counterparts, steel and concrete: in fact, it costs less!
Time line
Changing the Massachusetts State Building Code would take a number of years. However, the building code is revised continuously so the Massachusetts Board of Building Regulations and Standards (BBRS) is well practiced in the evolution and improvement of the code:
Year 1:
- Outreach to the Massachusetts Board of Building Regulations and Standards (BBRS) and the State Fire Marshal to describe the goals of the Mayor of Somerville to achieve carbon neutrality
- Outreach to institutions of higher education, building design, construction and development industry, and code officials to contribute knowledge for a performance-based building code
Year 2:
- Demonstrate how building codes in other jurisdictions globally have been revised to include performance-based building codes.
- Create a draft of a revised building code that includes performance-based options
- Disseminate a draft of a revised building code for public feedback and feedback from academic and profession communities
Year 3:
- Revise the draft of a revised building code that includes performance-based options
- Submit the final version of a building code that includes performance-based options to the BBRS
Year 4:
- Enact the revised building code that includes performance-based options
- Institute incentives to facilitate the design and construction of mass timber mid-rise buildings
- Conduct information sessions to the building design, construction and development industry to explain the code and its incentives in Somerville
Years 5-10:
- Review and approve viable performance-based construction options for mass timber construction in Somerville
- Revisit and revise the performance-base code options to best serve the city and the building industry
- Revisit and revise the incentives for building with mass timber construction systems
Thereafter:
- Build a city of wood to reduce the carbon footprint of material production and construction processes
- Build a city of wood to sequester carbon for generations to come
Become the premier Urban Wood City in the nation
Related proposals
Value not set.References
General Bibliography
“Structural Composite Lumber (SCL),” Rethink Wood:
http://www.rethinkwood.com/masstimber/structural-composite-lumber-scl
“Mass Timber and Wood Framing: New and Traditional Approaches Reduce Cost and Meet Code for Mid-Rise Construction” Architectural Record Magazine, published by McGraw Hill Construction, November (2012):
http://continuingeducation.construction.com/article_print.php?L=324&C=946
Michael Green, “A Case for Tall Wood Buildings: How Mass Timber Offers a Safe, Economical, and Environmentally Friendly Alternative for Tall Building Structures,” sponsored by the Canadian Wood Council (CWC), February (2012):
http://cwc.ca/wp-content/uploads/publications-Tall-Wood.pdf
“Evaluating the Carbon Footprint of Wood Buildings: Reducing greenhouse gases with high-performance structures,” Rethink Wood:
http://www.rethinkwood.com/sites/default/files/Evaluating-Carbon-Footprint-CEU.pdf
Sarthre, R. and J. O’Connor, A Synthesis of Research on Wood Products and Greenhouse Gas Impacts,” FP Solutions, October 2010:
http://www.woodworks.org/wp-content/uploads/FPI-Greenhouse-Gas.pdf
“Wood Shine in Sustainable ‘Show and Tell:’ Bullitt Center’s heavy timber frame teaches environmental and structural lessons,” Wood Products Council:
http://www.woodworks.org/wp-content/uploads/CS-Bullitt.pdf
Benton Johnson and David Horos, “A New Way to Build Tall,” Civil + Structural Engineering Magazine, published by ZweigWhite LLC, April (2014): 58-60:
http://cenews.com/article/9658/a-new-way-to-build-tall
Skidmore, Owings and Merrill, Architects, “Timber Tower Research Project,” Skidmore, Owings and Merrill (SOM) website:
http://www.som.com/ideas/research/timber_tower_research_project
The University of British Columbia, Center for Interactive Research on Sustainability (CIRS) website:
http://cirs.ubc.ca/building/building-manual/structural-systems
By:
Team Urban Wood
