|FPE Extra Issue 7, July 2016|
Issue 7, July 2016
Tall Mass Wood Buildings in the US—What’s Happening?
By David Barber
Multi-story mass wood buildings are being planned and constructed globally due to the need for green and sustainable architecture and the availability of innovative materials such as cross-laminated timber (CLT) and glulam. The construction industry has also recognized that mass wood offers a number of benefits for economically favorable construction: a reduction in overall building weight, accurate off-site prefabrication, faster construction times and improved on-site safety for workers, all increasing the return on investment.1
Tall mass wood buildings are being constructed taller in different parts of the world, with Norway’s 14-story “Treet” building recently being completed2 and an 18-story University of British Columbia student housing building under construction.3 Within the U.S., mass wood buildings are becoming more widely considered. The recent U.S. Department of Agriculture (USDA) Tall Wood Building Competition4 awarded two winners currently in design: a 10-floor residential building in New York City (475 W. 18th St.)5 and a 12-floor mixed office and residential building in Portland, Oregon (Framework).6 Both buildings will be using glulam as the primary structural gravity frame and CLT for the floors and lateral load-resisting walls. The USDA competition has encouraged other developers and architects to plan mass wood buildings, and more are expected in the U.S. in the near future.
Each U.S. state adopts one or more model building codes. All 50 states adopt the International Code Council’s International Building Code (IBC),7 with some states also adopting the National Fire Protection Association’s NFPA 101: Life Safety Code®.8 Each state adapts and amends the model codes to provide the basis for construction compliance.
Figure 1—University of Massachusetts Amherst Design Building
Tall Mass Wood Buildings—Alternative Approach to Code
For a building in which the primary structural frame is mass wood and the height (or area) exceeds that stated within chapter 5 of the IBC, an approval process following the methodology of “alternative materials, design and methods of construction and equipment,” is required. This approach requires the authority that has jurisdiction to determine compliance, based on technical documentation, analysis and testing provided by the project team.
For interior finishes, exposed wood needs to be tested to meet ASTM E 8414 or UL 72315 to show that it can meet the requirements of the IBC. These tests provide a classification of how quickly flame will spread on a material and the smoke that develops, which will result in a classification of “A,” “B” or “C” material, with “A” being the less combustive.
Exposed wood will meet a class “B” or “C” classification and hence can be used within the interior spaces of most buildings. Evidential test information is available from AWC.16
Wood as the Primary Structural Frame
Wood products can be engineered and designed for fire safety. The properties of wood are known and have been studied for well over 50 years with numerous fire tests undertaken to ASTM E11917 or UL 26318 to prove load-bearing fire resistance ratings (FRR). When wood is exposed to fire, the outer layer burns and turns to char. The pyrolysis occurs at a temperature of approximately 572 F (300 C) and creates a protective char layer that acts as insulation and delays the onset of heating for the unheated, or cold, layer below. This process of charring allows wood elements to achieve an inherent fire resistance. Research and testing have shown that the fire performance of exposed wood is predictable so that it can be used as an engineering material.19 The charring rate, section size and required fire duration can be used to calculate the fire resistance time for a wood element. A summary of fire test data for engineered wood assemblies exposed to fire can be found in White20 and Buchanan.21
Figure 2—Before and after a 1.5 hr ASTM E119
fire test for a glulam beam (image: APA)
Figure 3—Mass wood primary structural frame for Framework,
Portland, Oregon (image: Lever Architecture)
Light wood frame assemblies are constructed with concealed spaces in walls, floors and ceilings, whereby the assembly of fire-rated gypsum board on studs provides the structural stability and assists to prevent the spread of noise and allows the passage of building utilities. These concealed spaces can present a location for fire spread if they are not correctly fire stopped (blocked).
Connections for mass wood buildings are not as uniformly manufactured as they are for steel or concrete construction, as larger and taller mass wood buildings are still relatively new in the U.S. market. Connections is an area that requires further research, testing and verification.
Penetrations that occur in a building for utilities, such as plumbing, electrical cables, telecommunications, heating and cooling, need a fire-tested compliant method of sealing to achieve the FRR of the wall or floor of the penetration.
Fire Risk during Construction
Fire safety during construction is a relevant risk for mass wood buildings. An advantage of mass wood construction compared to other forms of construction is the lower risk of fire ignition due to welding, cutting and other “hot works,” which are significantly reduced for the construction process. Mass wood floor and wall elements can be prefabricated off-site to include plumbing, electrical and HVAC utilities in place. The prefabricated elements are also screw-fixed in place, with welding and spark producing cutting eliminated from the site.
David Barber is with Arup.
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