Designing for Fire Safety
Cross-Laminated Timber Meets 2015 International Building Code
During the week of October 21, 2012, the International Code Council (ICC) approved language for the 2015 International Building Code (IBC) that declared cross-laminated timber (CLT) was approved as a building element for non-residential structures (2015 IBC 602.4).
This breakthrough codification of CLT quickly inspired a wide variety of mass timber development—from schools and offices to mid-rise/multifamily, civic, institutional, industrial and retail—and the code enforcement that goes with it. Today a new class of CLT-based mid-rise structures are in active development in Minneapolis, New York City, and Portland, Oregon.
Tested & Approved
The 2015 IBC requires all buildings to perform to the same level of safety regardless of the materials specified. As expected, CLT has been subjected to strenuous time-to-failure testing. Prior to the October 2012 hearings, an independent fire endurance test was performed on CLT at NGC Testing Services in Buffalo, N.Y. The fully-accredited NGC testing complex is one of North America’s most sophisticated, featuring a 40,000 square foot fire-testing laboratory.
The NGC test was performed in accordance with ASTM E119 fire testing standards, using a 5-layer CLT wall (about 7 inches thick) that was covered on each side with a single layer of 5/8-inch Type X gypsum wallboard.
3 Hours, 6 Minutes
NGC testing experts then loaded the wall to the maximum their stress-testing equipment allowed, though well below the engineered strength of the CLT wall in question. The CLT wall was then exposed to a standard fire that reached over 1,800 degrees Fahrenheit within 90 minutes of exposure.
IBC code calls for a 2-hour fire-performance rating. The load-stressed specimen exceeded the 2-hour standard, demonstrating structural integrity for 3 hours, 6 minutes.
This edition of Count Counts looks at what the 2015 IBC now allows in CLT-based construction. Why is CLT becoming a building material of choice for a growing group of architects and owners? What types of CLT structures are now recognized by the 2015 code?
A Difference That Matters
When the question is fire safety, wood is typically held to a higher standard than other building materials. It may be understandable, but is it justified? “Probably not,” says David Barber, a principal and fire-protection engineer for Arup, a global engineering company. “Of course there’s a fire safety risk. But there’s a fire safety risk with all buildings.”
One reason for the misconception may be the confusion between traditional light-frame lumber and CLT. CLT is a densely-compressed designed panel composed of multiple layers of wood laminations oriented at right angles to one another. For example, an 8 foot by 50 foot and 7-inch-thick CLT panel weighs about 8,000 pounds. A CLT panel arrives at the jobsite precut, shaped, and routed to accommodate utilities, then erected in a carefully engineered sequence.
Widening IBC code recognition underscores CLT’s safe, predictable fire safety performance.
Today the 2015 code classifies structures by construction type to account for the expected response building elements will have in a fire: Types I, II, III, IV, and V. Types I and II must only have noncombustible building elements, with Section 603 exemptions as noted. Building types III, IV, and V are allowed to incorporate CLT elements, with Type IV notable for its broad allowance for CLT in exterior and interior construction.
Table 601 of the 2015 IBC may be of special interest to many code enforcement officials. The table shows the required fire resistance of building elements (structural frame, walls, floors, and roofs) for each construction type. Ratings are presented in hours.
Type IV wood structural elements are assumed to have inherent fire resistance due to their required minimum dimensions (no fire resistance rating is necessary except for exterior walls). Fire resistance is defined as the rate a building material or assembly degrades due to fire. In effect, Type IV recognizes one of primary public safety attributes of CLT in a fire: char formation.
Because CLT beams, columns, and panels are exceptionally thick and solid, they char on the outside at a slow and predictable rate while retaining strength. This protective seal of char slows combustion and allows time for building occupants to evacuate. The char shields the wood from further degradation, helps sustain structural integrity, reduces combustible material, and reduces heat and flame propagation.
The gathering momentum of CLT-based design and construction reflects the rapid recognition that this important new building material offers in terms of cost, versatility, and sustainability within code.
Building codes are paying less attention to the combustibility of building components and more attention to design and construction techniques intended to improve fire resistance and protect public safety. Now more than ever, it’s important for code officials to understand the structural and fire performance of CLT.
The views and opinions expressed in this article are those of the sponsor and do not necessarily reflect those of the International Code Council, or Hanley Wood.