Are wood-frame high rises a fire risk?
A new way to make wood panels lets builders erect 14-story structures, made mostly of wood; here's a look at what they are and the risks they present
By Robert Avsec
In May 2016, Australia became the latest country to change its building codes to allow the construction of timber-framed structures up to eight stories. In doing so, Australia joined a growing number of countries including the U.S., Canada, Sweden and the United Kingdom that have ventured into this new age of building construction — tall timber construction.
Interest in tall timber construction in the U.S. began in Portland, Ore. in the mid-1990s when architects, developers and fire protection engineers first began designing and building structures greater than four stories using a combination of concrete and wood products.
One of the first tall-timber construction projects completed in the U.S. was the Cornerstone condominium building in Portland. Completed in August 2000, the Cornerstone consisting of retail spaces on the ground story with 46 residential units occupying the five upper stories.
The rectangular structure measures 50 feet by 100 feet with a total area of 32,500 square feet and stands 65 feet tall.
Construction was slated to begin in October 2016 on an even taller structure in Portland. The Framework Building is planned to be a 12-story mixed-use building made primarily from timber.
What’s enabling this revolution in high-rise building construction is cross-laminated timber. CLT is a multi-layer wooden panel made of lumber. Each layer of boards is placed cross-wise to the adjacent layers to increase the panel’s rigidity and stability.
Each panel can have between three to seven layers, or more, normally in odd numbers, constructed symmetrically around the middle layer. These ready-to-use components are then assembled into complete frameworks.
Understanding the construction
Cross-laminated timber is not new, per se. In the past few decades, European builders, engineers and architects have quietly developed the material. In the last five years, 17 buildings ranging from seven to 14 stories tall have been constructed using mass timber around the world, the majority in Europe.
CLT is not light-weight construction. Rather, it’s a new take on the heavy timber construction of days gone by. The global timber industry has developed structural elements using CLT that exceed the load-bearing performance of heavy timber with less mass and weight. Those structural elements are said to be comparable to the performance of steel and concrete, again, with less mass and weight.
CLT makes it possible to use lower-grade lumber for the interior layers and higher-grade lumber for the exterior of each panel.
A combination of factors is at work behind the growing popularity of wooden high-rises. One is the desire to reverse declines in rural lumber manufacturing in the timber-rich western United States, particularly since the 2008 recession.
Architects and developers also see the use of CLT as having several distinct benefits over steel and concrete components.
For one, there’s less noise and congestion on the building site. Whereas the clear majority of construction activity using concrete and steel occurs on the construction site, CLT structural components are constructed at the factory, usually near the timber source, and then transported to the site for assembly.
Second, there are fewer work crews and a shorter construction timeline. Structural elements such as walls, floors and roof assemblies arrive at the construction site ready for assembly as door and window openings, utility routes, and connection points have all been pre-fabricated at the factory.
Then there’s the environmental factor. Buildings made primarily of wood are said to have significantly lower carbon emissions and use less energy than those made from traditional materials such as concrete.
Building code review
The 1997 Uniform Building Code provided for construction of Type V-1 Hour Protected structures up to four stories, provided that fire protection systems were installed. Use of the UBC was prevalent in the western U.S. until it was replaced by the International Building Code in 2000.
In Portland, the city government approved a statute, Title 24.85, that allowed an additional Type V-1 Hour Protected story to be added with the following provisions:
- Type V-1 Hour Protected (up to five stories) over Type I Construction with a three-hour separation, i.e., concrete slab.
- The top four stories must be R-1 Occupancy.
- There must be an installed fire protection system throughout the structure.
- The maximum building height cannot exceed 65 feet.
- Firefighting equipment access requirements must be complied with.
In addition, supplemental inspections were required for the wood-framed portion of the structure, primarily of the lateral force resisting elements. The additional required inspections included the nailing of floor and roof diaphragms and shear walls, installations of metal strapping, hold downs, collectors and stud wall sill and top plate connections.
One challenge that tall timber construction faces in the U.S. is that getting new codes approved by the International Code Council, the nonprofit organization that develops building standards, is a three-year cycle.
A proposal was submitted for a nine-story mass timber code, which was denied. The next opportunity to submit a new proposal will be in 2018, for the 2021 edition codes.
Code officials may be opening the lines of communication. In January 2016, the ICC board of directors voted to create a tall wood ad hoc committee, which will study tall timber construction and may develop code changes to be submitted for the 2021 International Building Code.
How fire safe is CLT?
In 2000, one of the first research studies, Timber Frame 2000, was conducted on the use of CLT for a structure of six-stories. The study was a partnership between the British Research Establishment and Chiltern International Fire, and jointly sponsored by the U.K.'s timber frame industry and the U.K.’s Department of the Environment, Transport and the Regions.
The U.K. timber industry saw a desire to build structures up to five or six stories using light timber, but building codes limited timber buildings to three stories. This prompted a series of tests on a six-story light timber framed building.
Results of those tests were used to demonstrate that light timber frame buildings could meet the functional safety requirements that would be required for non-combustible steel or concrete buildings. The test had two key objectives.
The first was to evaluate the fire resistance of a multi-story timber frame building subjected to severe natural fire exposure, particularly structural integrity or loadbearing capacity and compartmentation or the prevention of fire spread from the apartment of fire origin. The second was to provide data to help develop fire engineering design principles for medium-rise timber frame buildings above four stories.
The six-story test building contained 24 apartments (four per story). The designated fire test compartment was one of the two bedroom apartments on the third story.
The test fire was ignited in the living area of the apartment and progressed to flashover after about 24 minutes. Peak temperatures in the apartment reached 1,000 C and remained at or close to this level until the test was stopped after 64 minutes, having exceeded the planned termination criteria. Researchers used an array of instrumentation throughout the test building to collected fire performance data. Analysis of that data led researchers to these five conclusions.
- The performance of the complete timber frame building subject to fire is at least equivalent to that obtained from standard fire tests on individual elements.
- Fire conditions in the living room represented an exposure approximately 10 percent more severe than a standard 60-minute fire resistance test.
- The standard of workmanship (particularly in the installation of plasterboard or sheetrock) is a critical component to the necessary fire resistance.
- Correct location of cavity barriers and fire stopping is important in maintaining structural integrity.
- Vertical flame spread from floor to floor via windows needed to be addressed.
The results of the study prompted changes to the prescriptive code in the U.K. to increase the height limit to six stories for buildings using CLT structural elements.
In Canada, FPInnovations tested CLT panels, as walls and floors, to demonstrate that the panels had a certain level of fire performance comparable to non-combustible building elements like concrete.
The tests showed the walls and floors could be designed for up to three hours of fire resistance, in many cases exceeding the code requirement for structural element fire resistance ratings.
The researchers presented the study’s results to the Canadian code council and requested changes in the building codes based on the demonstrated performance through fire testing. Thus, the use of CLT has been adopted into the prescriptive building codes in Canada.
Future of tall timber construction in the U.S.
To date, the building codes in Europe tend to have taller height allowances compared with the U.S. This makes the approvals process quite a bit easier across the pond, but the prescriptive limit isn’t a deal-breaker.
For example, the height limit in Australia is capped at three stories for wood buildings, yet Australia recently built the tallest modern timber building in the world.
How? In an interview for Arch Daily in March 2014, Robert Gerard, a licensed fire protection engineer in California, said, “The primary difference in terms of approvals, I’d say, is there’s a greater understanding of timber fire safety risks in those regions, resulting from research and testing and educating building authorities. This is something we’re currently working on in the U.S.”
The building codes in the U.K. and Australia are a bit different than those in the U.S. The codes in those countries make greater use of performance requirements and rely less on prescriptive regulations. Instead, building codes in those countries rely more on the use of functional objectives.
Functional objectives are fire safety principles that apply to all buildings regardless of building material. When code enforcement officials, architects and developers agree that the functional objectives have been satisfied, this allows greater flexibility in the building material and design.
It is this difference between building codes in the U.S. and countries such as Australia that has generated such interest for tall timber construction and a greater degree of acceptance in other countries.
As evidenced by the experiences early adopters in Portland, Ore. and Seattle, architects and developers in the U.S. are going to continue to follow the examples of their counterparts in other parts of the world in designing and building taller structures that make extensive use of CLT.
Because of that, fire service leaders in the U.S. must understand this construction trend and become more involved in the revisions to building and fire codes as they relate to tall timber construction.