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Photo courtesy of Structurlam

Building construction is constantly evolving as architects, engineers and scientists develop new innovations and construction methods. Buildings utilizing mass timber as part of their structure have been growing in number worldwide, as developers strive for sustainable and efficient construction. 

In the United States, buildings with a mass timber structure are being planned and constructed, driven by building owners, managers and designers who see timber as a positive solution given the sustainability credentials and efficiencies in construction. The mass timber solutions include products such as glulam, cross-laminated timber (CLT), nail laminated timber (NLT), mass plywood panels (MPP), laminated veneer lumber (LVL) and dowel laminated timber (DLT). High-rise mass timber buildings are being planned and constructed in the U.S. today. 

The two most common forms of timber construction are light timber framing and mass timber. Mass timber differs in that the products are “engineered,” as they are factory made from smaller sawn wood members, and the sizes are typically 12 inches (300 millimeters) or more. Mass timber is becoming popular because it offers an economically favorable construction method for mid-rise and high-rise building types. 

When compared to other materials, mass timber provides lighter construction that can result in savings in foundation works. Another advantage is the amount of offsite prefabrication that allows for highly accurate production, leading to faster overall construction times. Quicker construction further reduces costs and increases return on investment. Mass timber is also quieter in construction, involves a smaller work crew and is safer overall. Owners and developers looking for a highly sustainable building solution can achieve their goals through mass timber.

Mass Timber and Codes

Most states adopt one or more model building codes, with the majority adopting the International Code Council International Building Code; others also adopt the National Fire Protection Association Life Safety Code, NFPA 101. Many states further amend the model codes to provide the basis for construction compliance. Other codes and standards impact construction, fire protection systems, maintenance and firefighting operations, including the International Fire Code.

Within the IBC, timber can be utilized in Types III, IV and V construction. Types III and IV can be constructed to a maximum building height of 85 feet, typically to seven floors. The American Wood Council and WoodWorks provide resources and technical guidance on how timber construction can be utilized to maximize height and area.

For buildings with an occupied floor above 75 feet (defined as “high-rise”), the IBC requires an increased level of fire protection and structural performance. Represented by construction Types IA and IB only, non-combustible construction is required for the primary structural elements (i.e., columns, beams, floors and load-bearing walls). Fire protection and structural protection are also increased for high-rise buildings, with a minimum two-hour fire resistance rating (FRR). The primary structure is required to have sufficient resistance to survive full burnout of a fire, in the highly unlikely scenario where the sprinklers fail and the fire department has limited intervention. Compared to a mid-rise building, there’s a significant increase in expected structural performance for all high-rise buildings in fire. 

Mass Timber Construction – What About Fire?

For fire performance, light-timber construction is encapsulated within non-combustible gypsum drywall to achieve protection from fire and an FRR. Mass timber is different in that when it is exposed to fire it achieves an FRR through the insulating benefits of charring, a process of inherent protection based on the section size. Mass timber members can be designed to carry applied forces when exposed to fire, given that the rate of charring and insulation to the strong wood is highly predictable. Guidance on engineering design for mass timber fire resistance is provided within the National Design Specification for Wood Construction (AWC, 2018).

To assist architects, engineers and contractors, mass timber suppliers provide code-compliant fire tested solutions for building permit approval. The IBC has several methods for fire resistance compliance through fire testing or calculation. Fire testing must meet either ASTM E119 or UL 263, which apply a standardized heating regime to a loaded building element to determine the fire rating. All North American CLT suppliers have had their CLT panels fire tested to meet ASTM E119, demonstrating a two-hour FRR. These test reports are available from each supplier to support the use of their products as load-bearing floors or walls. Another code-compliant method for proving an FRR is using the calculation methodology from the CLT Handbook or the NDS.

Cross Laminated Timber Performance in Fire

CLT performance in fire has been very well studied, but the performance is not always well understood given the complexities related to the char rate being dependent on ply thickness, number of “plys” and the CLT adhesive used. There has been significant fire testing in Europe, Canada and more recently, in the U.S. The outcomes from fire testing have been consistent, showing that CLT will char in a predictable manner and fire resistance can be calculated based on the number and thickness of “plys” that each panel is made up of and assumptions on adhesive type. This has led to CLT guides such as the NDS and CLT Handbook providing an FRR calculation method to support building permit applications for CLT being used for floors and walls.

An area of recent research is the differing adhesives being used by manufacturers. There have been two types of CLT adhesive used – with one adhesive system being more resistant to the heat. To improve CLT performance in fire, the North American manufacturing standard (ANSI-APA PRG-320) has recently been updated. From 2021, all CLT panels will be required to use adhesives that provide a greater resistance to the heat of a fire and improve the fire performance of the CLT panels.

CLT Construction Details

Details to provide compliant construction are particularly important for mass timber buildings. Some key areas are:

  • Concealed Spaces. Mass timber buildings that utilize CLT as the primary wall and floor elements have no concealed spaces within the primary structure due to the solid timber construction of CLT. Spaces for building utilities such as pipes, cables and ducts can be pre-formed with gypsum drywall solutions to provide fire separation similar to other forms of construction. Therefore, concealed spaces, which are an issue for potential fire spread, are mostly eliminated with mass timber construction. 
  • Connections. Connections in mass timber construction aren’t as uniformly specified. CLT panel-to-panel connections are specified by each supplier, with the connections having been fire tested as part of a wall or floor assembly to meet ASTM E119. For the beams that support the CLT floor, fire testing for three differing glulam beam-to-column connections has recently been undertaken. The fire test reports are available on the Think Wood website.
  • Penetrations. “Through penetrations” occur in all buildings for plumbing, electrical cables, telecommunications, heating and cooling. For penetrations in CLT walls and floors, the installation of a collar, fire mastic or fire damper that has been previously tested for use in a CLT wall or floor penetration is specified. Currently, there are few of these products available in the United States, and more fire testing is underway to increase the catalog of products available to designers.
Fire Risk During Construction

Fires during construction are a significant issue with light-frame timber buildings given that the fire protection of gypsum drywall and sprinkler protection aren’t installed until the building is nearly completed thereby placing the incomplete building at risk. Since most mass timber construction has no need for welding or grinding, one of the major ignition sources is eliminated. A major advantage of mass timber construction is that structural members, such as glulam and CLT, do not rely on additional protection measures, such as drywall, to resist fire. The required structural fire ratings are provided by each member as soon as it is installed. 

Preventing construction fires from starting is always important, and the methods used include eliminating cooking onsite, improved nighttime security and hot work supervision (where required). The use of NFPA 241 “Standard for Safeguarding Construction, Alteration, and Demolition Operations” provides a means by which to reduce the fire risk and consequences of construction fires.

What is Coming?

Currently, prescriptive code legislation limits mass timber buildings to heights of up to 85 feet, and in many jurisdictions, there has been a reluctance to approve mass timber buildings. This is primarily due to a lack of familiarity with the material and the extent of fire testing available to the design community. Continued education by organizations such as WoodWorks, Think Wood and AWC are key to increasing the number of skilled architects, engineers and contractors with mass timber experience.

Importantly, the ICC has convened an Ad-Hoc Committee on Tall Wood Buildings to research and address future code changes to allow mass timber buildings to be built above 75 feet. The initial voting has been very positive to the code change proposals.

As knowledge and understanding of mass timber’s fire performance continues to develop, there will be more opportunities for mass timber construction to be designed and approved. Through increased education, the industry should expect to see more mass timber buildings being constructed.


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