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.
Within the IBC, wood building elements are referred to as combustible construction, while concrete and steel elements are referred to as non-combustible construction. Non-combustible construction can be used in construction Types I and II. Wood construction can be used in Types III, IV and V. Guidance on using wood for buildings is freely available on the American Wood Council (AWC) website.9
Current examples of code compliant mass wood buildings include the six-floor Hines T310 in Minneapolis, Minnesota; the four-floor University of Massachusetts Amherst Design Building;11 the four-floor Redstone Arsenal hotel in Alabama;12 and the four-floor Albina Yard13 in Oregon. Albina Yard is using U.S.-manufactured CLT.
With the resurgence of mass wood as a construction material in the U.S., there is growing interest in building structures taller than the current IBC limit of six floors.
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.
As with any new and innovative forms of construction, a number of design and construction issues are required to be addressed for a tall wood building to gain approval. These issues include interior finishes, fire ratings, concealed spaces, connections, penetrations and construction fire safety.
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
The IBC references the AWC’s “National Design Specification for Wood Construction”22 as the method for calculating the fire resistance for an exposed wood member, providing engineers with a well-proven methodology for FRR analysis. The larger section sizes of glulam and CLT, typically in excess of 6 inches (150 mm) that are used for mass wood buildings, result in members achieving an FRR of 1 hr and can be designed to achieve 2 hr or more.
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).
Mass wood buildings that use CLT as the primary wall and floor elements have no concealed spaces due to the solid wood construction. Spaces for building utilities have to be formed once the structure is in place, normally with gypsum board shaft solutions. In mass wood buildings that use CLT, concealed spaces are limited in number and fire protected.
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.
Connection design requires input from the structural engineer, architect and fire protection engineer to develop a design that provides sufficient structural capacity in fire and to provide the aesthetics required by the architect. Connections are addressed through a number of design approaches to prove an FRR, such as calculation methods and, where required, through fire testing.
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.
As with other buildings, the method for achieving compliant through penetrations in CLT walls and floors are based on using an approved fire-tested product (e.g., collar, fire mastic, fire damper, etc.) that has been fire tested for use in a CLT wall or floor. The number of products available in the U.S. for mass wood building elements is limited but is growing through new fire testing.
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.
With the causes of construction fires well-researched and known,23 mass wood construction managers are proactive at enforcing safety requirements to reduce the risk of fires. The smaller workforce required at a mass wood site is also an advantage for training and implementation of site safety requirements.
The desire to build sustainable buildings has led many architects and developers to consider mass wood. The substantial fire research and testing in North America has shown that mass wood as a structural material is predictable in its fire performance and hence can be used as part of the primary structural frame. However, mass wood buildings are currently limited by code in height and area. An alternative engineering approach is the required method for approval for high-rise buildings.
As knowledge and understanding of the fire performance of mass wood products such as CLT develop, the potential for changes in building codes becomes increasingly possible. Through increased education to improve the understanding of how mass wood performs in fire and targeted research to assist with the implementation of innovative mass wood elements, we should expect to see more tall wood buildings being designed, safely constructed and used.
David Barber is with Arup.
1USDA Forest Service, Integrated Technology in Architecture Center, AIA, FPInnovations, University of Utah, 2015, “Solid Timber Construction—Process, Practice, Performance,"http://itac.utah.edu/ST_Perform.html
4USDA Tall Wood Building Competition, https://tallwoodbuildingcompetition.org
5475 W. 18th St., New York City, http://www.shoparc.com/projects/475-west-18th
6Framework, Portland, Oregon, http://leverarchitecture.com/work/framework-2
7International Code Council, 2015, “International Building Code”
8National Fire Protection Association, 2015, “NFPA 101—Life Safety Code”
9American Wood Council http://www.awc.org
14ASTM E84-15b, “Standard Test Method for Surface Burning Characteristics of Building Materials,” ASTM International, West Conshohocken, Pennsylvania, 2015
15UL 723-2008, “Standard for Test for Surface Burning Characteristics of Building Materials”
16American Wood Council, 2010, “Flame Spread Performance of Wood Products Used for Interior Finish,” DCA 1
17ASTM E119-16 “Standard Test Methods for Fire Tests of Building Construction and Materials,” ASTM International, West Conshohocken, Pennsylvania, 2015
18UL 263-2011, “Standard for Fire Tests of Building Construction and Materials”
19American Wood Council, 2003, “Calculating the Fire Resistance of Exposed Wood Members,” TR-10, http://www.awc.org/publications/download.php
20White R., 2016, “Analytical Methods for Determining Fire Resistance of Wood Members,” SFPE Handbook of Fire Protection Engineering, 5th Edition
21Buchanan, A., 2001, “Structural Design for Fire Safety,” John Wiley and Sons
22American Wood Council, 2015, “National Design Specification for Wood Construction”
23Campbell, R., 2014, “Fires in Residential Properties Under Construction or Undergoing Major Renovation,” National Fire Protection Association