Bullitt Center the first Mass Timber Building in Seattle in 80 years
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The Bullitt Center, arguably the “greenest office building in the world” was the first mass timber commercial building constructed in Seattle in over 80 years. It has a structural frame of glulam columns and beams, floor decks of 2x6” lumber set on edge and “nail-laminated” together, and plywood used for structural diaphragm and shear wall panels – all conforming to prescriptive code requirements. 100% of the wood used in the project was FSC certified from local sources to meet the rigorous standards of the “Living Building Challenge.”
The Bullitt Center, designed to last 250 years, was the first mass timber commercial building constructed in Seattle since the 1920s, and is helping spark a renaissance of mass timber construction for taller wood buildings just beginning to emerge in 2013 when the building was completed. Although allowed in national building codes for over a century, Type IV Heavy Timber construction was all but abandoned as a commercial framing system for most of the 20th Century - in favor of steel, iron and reinforced concrete construction. The Bullitt Center is a six-story podium structure (four floors of wood over two stories of reinforced concrete) that was approved through prescriptive compliance with the 2009 Seattle Building Code Chapter 6; the code specifies exterior walls made of non-combustible materials with interior structural members of solid or laminated wood without concealed spaces. Bullitt Center is a groundbreaking demonstration of the “Living Building Challenge” a rigorous green building standard that requires all wood in the building to be from 100% sustainably-certified wood sources within 600 miles of the site.
|Code Requirement||Compliance Path|
|Section 602.4 Type IV construction (Heavy Timber) defines buildings with interior building elements of solid or laminated wood||Prescriptive compliance, yet the first such building in over 80 years!|
Type IV construction requires wood columns and beams to be oversized so that in the case of a fire, the building will retain its structural strength long enough for occupants to escape and fire crews to arrive. Bullitt Center structural wood exceeded those requirements, with wood sizes ranging from 5-1/8” x 15”, up to 12-1/4” x 21 inches. In an intense fire, steel will weaken and may collapse before the required 1-hr fire-tested time has elapsed, if the steel structural member is not encapsulated in a fire-protective covering like gypsum wall board. Wood on the other hand will char and burn at a slow and predictable rate even when exposed, retaining its strength for longer than the required 1 hour. Wood actually performs better than other common materials for fire safety when appropriately sized.
Wood columns and beams must also be assembled so that steel connectors are either protected from heat-related failure, or constructed so they are not vulnerable to such failure. Bullitt designers opted for the latter, using a familiar bucket connector system, installed so that each of the main girders and beams has at least 3” of material bearing directly on the timber columns that support them from below. So In the event of a fire, even if the steel connectors are weakened by heat, the girders and beams still will not fail.
Another consideration for wood buildings is slow shrinkage over time. Especially if loads are applied perpendicular to the grain, compression of the wood can result in shrinkage of up to a ¼ inch per floor. The cumulative effect of this can create real problems for a high-performance building. To avoid this problem, the team used a glulam wood column system with vertical grain orientation for the entire height of the wood structure (4 floors). The top of one column is connected to the bottom of the next with a steel tube inset at each joint, creating in effect a “continuous” column system that won’t shrink over time.
The floor and roof structures are constructed with “Nail-laminated” solid wood decks, conforming to prescriptive codes. Floors were hand-assembled by nailing 2x6 #2 fir lumber set on edge and nailed together one board at a time in a staggered pattern, “so that no continuous line of joints will occur except at points of support.” The floor decks so constructed, were then topped by ½-inch plywood to create a structural diaphragm to resist lateral forces, then covered with an insulation mat as a noise barrier, followed by a three inch slab of concrete as thermal mass in support of the Center’s innovative natural ventilation strategy.
As required for Type IV, the exterior “curtain wall” is made of non-combustible glass, steel and aluminum with operable windows and automated shading system that controls daylighting, thermal gain and the aforementioned natural ventilation strategy.
|“This is not your grandfather’s heavy timber structure” by Brad Kahn Daily Journal of Commerce, Green Building Blog, Aug. 17, 2012. (Jun. 24, 2013, 11:20 PDT)||WoodWorks Case Study WW-011 • Bullitt Center • ©2013 WoodWorks|
|Bullitt Center Financial Case Study|
Wood was chosen for the building’s structural frame because it is a renewable material, and when sourced from a responsibly managed forest, it is an environmentally superior resource when compared to the high embodied energy and carbon footprint of steel and concrete. Instead of causing greenhouse gas emissions, the frame will actually sequester (store) carbon for the 250 year life span of the building and beyond, offering a carbon-positive solution. Wood was also preferred by the building owners and designers because when left exposed as an aesthetic interior element it expresses the identity of the Pacific Northwest region’s history and architectural style in which timber figures prominently.
The building frame is constructed of approximately 119,000 board feet of Douglas-fir glulam beams and columns, sourced from 100% Forest Stewardship Council (FSC) Certified wood from mills within 600 miles of Seattle. In addition to requiring local and sustainable wood, to achieve the rigorous “Living Building Challenge” (LBC) the beams were prohibited from containing any of the 362 toxic substances on the LBC’s ‘Red List’ such as off-gassing lamination adhesives and finishes commonly found in building components. Although not required by code, these standards were applied to all the building’s wood products including structural members, plywood and other lumber used for interior trim, wall framing and other uses.
Court said “A lot of people were surprised to learn that a wood structural system was possible here, but this project helped them learn about wood’s environmental advantages in terms of embodied energy and life cycle assessment. Wood really outperforms steel and concrete in the right applications. With this project, we think perceptions about wood are going to change.”
This project is part of a renaissance of mass timber construction for taller wood buildings happening around the world. The International Building Code allows for buildings up to 6 stories, however, several buildings 12-18 stories are currently being designed in the US and Canada taking advantage of the greater structural strength of cross-laminated timber (CLT). CLT are mass timber panels 3, 5 or 7 layers thick (up to 9” thick) in dimensions up to 8 feet wide by 40 feet long that are manufactured in a plant and can be quickly assembled on site. In such buildings, CLT panels are used in numerous ways to support vertical loads, provide lateral (shear) resistance, and create floor and roof decks. Vancouver, BC Architect Michael Green has developed a system he believes can safely and cost effectively support construction of CLT buildings up to 30 stories, even in high seismic zones.
Design / Build Process
LBC requirements stipulated that the Bullitt Center’s design and construction team meet a number of criteria, including responsible site selection, 100 percent on-site renewable energy generation, 100 percent of water needs provided by harvested rainwater, and on-site waste management.
Cost / Benefit
Total project cost for the Bullitt Center was estimated at $32.5 million, or $577 per square foot—about 23% more than the cost of a comparable building. Designers knew going in that a cutting edge “living building” with a planned life expectancy of 250 years would cost more than a typical commercial office building designed to last just 40-50 years. Yet construction costs alone were only about $50 per square foot more than a comparable building and in line with other more demanding institutional projects (e.g. hospital or lab). Much of the premium costs were actually soft costs such as the building’s painstaking integrated design process, negotiating with regulatory agencies, and other hurdles often faced by exemplary projects. Architect Brian Court said “People have a strong reaction to the $577 figure, but when you’re trying to change laws about how rainwater can be used and how graywater can be treated and used in commercial structures it, you’re breaking new ground, dealing with a lot of issues which won’t factor into future projects once the precedent has been proved. Plus, we’re going way beyond code minimums here. It’s an investment in the future: initial construction costs may be higher, but being net-zero energy and water, this building has essentially prepaid utility bills. Over the lifespan of the typical building, which averages about 50 years, it’s going to be more profitable than most if not all commercial office buildings. And as more living buildings come on line in Seattle and elsewhere, we are confident that costs will also come down.”
As part of the LBC certification, a post-occupancy survey of the building occupants revealed that their single favorite thing about the building was the exposed timber structure and ceiling decks.
|Owner: Denis Hayes, President The Bullitt Foundation (206) 343-0807||Designer: Brian Court, AIA The Miller Hull Partnership 206-254-2016||Developer: Joseph David Point 32|
|Owner: Nouri Samiee, Structural Plans Seattle Dept. of Construction and Inspections 206-733-9057|