What the Fire Service Needs to Know about Tall Wood Buildings

Firefighters and fire chiefs: Forget everything you know about wood construction. Tall wood buildings (TWB)—or more accurately tall mass timber buildings—are not the conventional, lightweight wood, stick-built construction with which the fire service is familiar. There are no individual 2 x 4s, I-joists or open web trusses. In fact, there is no dimensional lumber or conventional lightweight wood products of any kind in tall mass timber buildings.

There are 14 proposals before the International Code Council (ICC) to incorporate TWB code restrictions into the International Building Code (IBC). These code provisions introduce three new construction types—Type IV-A, -B and -C. They also outline standards for the manufacture of mass timber, height and area restrictions, enhanced fire protection (both active and passive), fire safety during construction, water supply requirements, standards on sealants and adhesives, non-combustible protection, and fire-resistance ratings of tall mass timber.

What is tall mass timber?

The mass timber in tall mass timber buildings are large pre-manufactured panels for beams and columns, floors, walls and roof assemblies. The products used in tall mass timber buildings are cross-laminated timber (CLT), glued-laminated timber (GluLam) and structural composite lumber (SCL). Mass timber can also include traditional sawn members, but no members are smaller than what would be traditional heavy timber (HT) under Type IV-HT provisions already in the code. What they all have in common is that they are multiple layers (laminations) of wood to form massive panels, beams and girders. For example, a CLT floor system could have as many as seven to nine laminations of wood. The CLT panels are typically utilized in an odd number of laminations. The individual laminations are laid perpendicular to the preceding lamination. This results in a floor system of solid timber that is typically 7 to 12 inches thick. There is an inherent fire resistance to heavy or mass timber because of the layer of char that occurs during a fire that protects the inner structure of the beam or panel. Testing has shown that heavy mass timber chars at a predictable rate that is comparable to dimension lumber. 

If you combine the inherent fire resistance of heavy or mass timber with protection from non-combustible insulators, such as Type X gypsum board (testing has shown each layer of five-eighths-inch Type X gypsum provides approximately 40 minutes of fire resistance), the fire-resistance rating of CLT meets or exceeds the fire-resistance ratings of Type I-A or I-B non-combustible construction materials, such as concrete and protected steel.

Tall mass timber buildings are not a new technology. The first patent issued for CLT use was in 1994, and the first CLT building was in 1995. CLT construction dates to the early 1990s in Germany and Austria. These structures have been built in this country, in Portland and Minneapolis, and abroad, in Canada, Norway, the United Kingdom, Finland, Italy, Australia and Austria.

First materials ignited

Most fires that occur in structures are room-and-contents fires, with the first material ignited being the contents of the room. As long as combustible building contents exist (e.g., furniture, computers, personal belongings, files), there will never be a truly non-combustible building. The building materials utilized in construction will never prevent a room-and-contents fire, but they must be able to resist the fire for an acceptable period of time. The automatic sprinkler system keeps the fire in check, preventing flashover, and the fire alarm notifies the occupants. Eventually, any building material will fail if the fire burns long enough. Steel will lose strength and fail, concrete will spall and fail, and wood will char and fail. That is why fire testing is essential for all building materials. (The testing of mass timber will be discussed later.) 

A key requirement of Type IV-A and IV-B construction is non-combustible protection. The amount and arrangement of non-combustible protection allows the contents fire to decay without substantially involving the mass timber. 

Why these codes were proposed

There are tall mass timber buildings that have already been constructed in this country, and dozens that are being proposed. Because the IBC does not specifically provide for this type of construction, the projects are being approved under IBC Section 104.11: Alternative materials, design and methods of construction and equipment.

Most fire departments have little to no input on what land-use projects/bills get approved. In some parts of the country, the fire departments get little input on plan review. Building departments are approving tall mass timber buildings under the “alternative means and materials” section of Chapter 1 of the IBC, and when approved in this manner, the building official has total discretion. There has been no consistent standard in the code to regulate these structures.

TWB ICC Ad Hoc Committee

In December 2015, the ICC board of directors established an ad hoc committee of designers, code officials and members of the fire service, including firefighters, fire chiefs and fire protection engineers. Their task was “to research and design fire testing of mass timber and to draft code changes that ensure that tall mass timber buildings have redundant and rigorous fire safety systems that will protect the public that occupy tall mass timber buildings and the first responders that respond to them in emergencies.” 

The committee’s proposals seek to codify standards for these structures so that there will be consistency across the nation. To that end, the fire service will know what to expect when responding to these structures and how they will perform under fire conditions.

The committee created a set of performance objectives to inform the creation of its proposals:

1. No collapse under reasonable scenarios of complete burn-out of fuel without automatic sprinkler protection.
2. No unusually high radiation exposure from the subject building to adjoining properties to present a risk of ignition under reasonably severe fire scenarios.
3. No unusual response from typical radiation exposure from adjacent properties to present a risk of ignition of the subject building under reasonably severe fire scenarios.
4. No unusual fire department access issues.
5. Egress systems designed to protect building occupants during the design escape time, plus a factor of safety.
6. Highly reliable fire suppression systems to reduce the risk of failure during reasonably expected fire scenarios.

The committee was formed into four working groups: Standards & Definitions, Fire Safety, Code, and Structural. The committee’s proposals identified a rigorous set of fire protection requirements that ensured that during reasonable fire events, no structural collapse will occur despite a complete burn-out of the room and contents. This performance is expected to occur even in the rare event of a sprinkler system failure.

Fire testing

The TWB ad hoc committee found that fire testing was required to validate the performance of tall mass timber buildings. Five full-scale, multi-story tests were developed to simulate the three new construction types: Types IV-A, -B and -C. The tests were created to evaluate the following:

1. Contribution of mass timber to a fire
2. Integrity of the structural members
3. Performance of connections
4. Performance of through-penetrations
5. Conditions for responding personnel 

The test was of one-bedroom apartments, fully furnished on two levels. Outside both apartments was a one-hour corridor leading to a stair enclosure. The test was a realistic non-standard furnishing-fueled compartment fire with over 2,200 degrees F early peak temperatures, followed by decay unlike ASTM E119, which is the standard for testing building construction techniques and materials.

Several tests were conducted that assumed the unlikely event of a sprinkler system failure, while others confirmed the effectiveness of sprinkler systems in mass timber fire events:

1. A mass timber structure fully protected by two layers of five-eighths-inch Type X gypsum wallboard. The fire burned itself out after 3 hours with no significant charring on the protected mass timber surfaces.
2. A mass timber structure with 20 percent of the CLT ceiling exposed. Test concluded after 4 hours, with apartment contents burned out and CLT self-extinguished due to char that protected the underlying mass timber.
3. A mass timber structure with two CLT walls fully exposed. Apartment furnishings and contents consumed after 4 hours and CLT self-extinguished after formation of protective char surface.
4. A fully exposed mass timber structure with sprinkler system. A single sprinkler quickly contained the fire.
5. A fully exposed mass timber structure with sprinkler system. The fire was allowed to grow to flashover—23 minutes—before being quickly extinguished by sprinklers.

The fire test in which there was no sprinkler activation registered as a 23-megawattfire, one of the most intense fires ever at the ATF laboratory. Only two labs in North America can handle a fire of that intensity. To put the fire in context, an intense camp fire generates approximately 300–500 kilowatts. It would be the equivalent of having 45 roaring camp fires in a small apartment. The fire loading alone (the contents of the apartment) was the equivalent of setting 300 gallons of gasoline on fire in a small apartment.

During a fire resistance test at the ATF laboratory, a five-ply cross-laminated panel wall was subjected to temperatures in excess of 1,800 degrees F and lasted for 3 hours and 6 minutes. The code required a fire resistance rating of 2 hours. To review a summary of the fire tests, visit http://bit.ly/ATF-firetestreport. To watch the accelerated video of the fire tests, visit http://bit.ly/ATF-firetestvideos. 

Fire resistance

The fire-resistance rating of mass timber consists of the inherent fire resistance of the mass timber itself combined with additional fire resistance provided by non-combustible protection of the mass timber structural members with five-eighths-inch Type X gypsum board. Each layer of five-eighths-inch Type X gypsum has been shown to provide 40 minutes of fire resistance in addition to the mass timber itself in multiple E119 tests. 

Type IV (mass timber) fire-resistance requirements are identical to that of Type I (non-combustible) construction. The table below shows the comparison between Type I-A and IV-A, Type I-B and Type IV-B, Type II-A and Type IV-C. The table illustrates that, across the board, mass timber’s fire-resistance rating meets or exceed that of its Type I or II counterparts. 

Height and stories limitations

Height and stories limitations have been a source of concern for the fire service. The full allowable height, area and stories tables can be found in proposals G75-18: Table 504.3; G80-18: Table 504.4; and G84-18: Table 506.2. This article addresses the height and stories proposals for the two most common occupancy types that the fire service encounters in tall buildings: Office buildings or “B”-use group, and Residential buildings or “R”-use groups, which include R-1 (hotels and dormitories) and R-2 (apartment buildings and condo buildings).           

A sprinkler-equipped office building of Type I-A (concrete and fire protected steel) can have an unlimited number of stories above grade plane. If limited to 420 feet in height above grade plane, a Type I-A high-rise office or residential tower can have the fire-resistance rating of its basic building elements (except columns) reduced to 2 hours or the same fire-resistance rating of a Type I-B building. The only extra requirement is for the NFPA 13 sprinkler system to sprinkler control valves equipped with supervisory initiating devices and water flow initiating devices for each floor (mandatory in the 2018 NFPA 13).

By contrast, a Type IV-A (protected mass timber) office or apartment building that has the same unreduced fire-resistance rating as a Type I-A building is limited to 18 stories in height.

The “super high-rise” water supply provisions for all other types of construction are triggered at 420 feet above grade plane. For tall mass timber, those provisions are triggered at 120 feet above grade plane, which would include two water mains from different streets to supply the fire pump(s).

The ad hoc committee carefully considered the proposed type of construction, the non-combustible protection, fire resistance and sprinkler systems before recommending the height, area and stories proposals.

Fire safety during construction

Fires occurring during construction are always a concern of the fire service. Before fire protection systems are in place and functional, construction sites are always vulnerable to the scourge of fire. Most of these fires occur on light-frame wood projects, not mass timber buildings. Mass timber is installed as it is delivered. The committee understood the hazard, and put forward proposals that deal with fire safety during construction that are more stringent than the current code.                 

The proposal requires that the mass timber must be protected (encapsulated with gypsum board) before the building extends beyond a specified number of levels. The builder/developer must confer with the local fire department to determine the water flow and pressure needed to meet the fire department’s demand for a specific site, and the water supply must be established when construction begins.

An example of the protection required while “going vertical” is that before placing the eighth-level floor panels, while there is active construction on the seventh floor, passive fire protection must be in place for building components on level three and below. The passive fire protection in place would include, but not be limited to, draft-stopping, fire-blocking and encapsulation of structural members with a non-combustible insulation. As with all buildings, standpipes must go up with the building and be functional. The FDC must be readily accessible.

Enhanced fire protection

Tall mass timber buildings feature enhanced fire protection that are more conservative than anything currently in the code. The following list includes some of the enhanced features proposed by the TWB Ad Hoc Committee:

• All mass timber buildings are required to provide at least 2 hours fire resistance for structural members, and Type IV-A is required to provide 3 hours of fire resistance for the primary structural frame and bearing walls.
• Dual water supplies are required for mass timber buildings exceeding 120 feet, whereas Type I-A (non-combustible) buildings must exceed 420 feet to trigger a dual water supply.
• Non-combustible exterior cladding is required on all mass timber buildings, whereas combustible exterior cladding is permitted on Type I-A or -B buildings.
• Type IV-A has no exposed mass timber; IV-B allows limited exposure, but separated, interior mass timber.
• Type IV-C is limited to the same height (85 feet) as existing heavy timber buildings, but provides additional stories in A, B and M occupancies, and one additional story in group M occupancy buildings.
• All new mass timber building types require all concealed spaces to be protected with non-combustible fire-resistive material.
• Joints and penetrations are to be protected, tested and inspected as per current building code.
• Annual inspections are required to verify that required passive fire protection remains in place.
• Special inspections are required to ensure that the CLT is being properly installed. 
• The proposals from the TWB Ad Hoc Committee are at a minimum, as stringent as the present code, and in many aspects, they are more stringent.

In conclusion

After consultation with its membership, the ICC board of directors saw a need to create an ad hoc committee to study the fire safety implications of tall combustible buildings. After two years of study, discussion, testing and analysis, the committee concluded that the proposals recommended would provide life safety protections to the public and first responders that are equal to or greater than tall buildings that are made of steel or concrete.