Fire behavior of building construction (2020s)

• The National Fire Research Laboratory (NFRL) is a facility at NIST that enables combined structural and large-scale fire tests.
• Using the NFRL, NIST researchers have gathered data on important new building materials and structures exposed to real fires.
• These tests have helped us understand how fire weakens structures and causes them to fail, leading to recommendations on how to make buildings that are safer during severe fires.
• Results from these tests have supported changes to building codes in North America.

A fire can weaken a building enough to cause it to collapse, endangering anyone still inside. Structural fire protection engineers look for ways to prevent that from happening.

The most effective way to tell whether part of a building will collapse is to test it using a real fire in a laboratory. These tests help building designers and regulators figure out what’s safe. Starting in the early 1910s, NIST developed tests for individual construction elements like steel columns and wall sections to rate them based on how much heat they can safely withstand for various amounts of time. These ratings are central to building codes.

Structural engineers believed that the right combination of structural elements would help buildings withstand fire more effectively than individual components alone. If larger combinations of these elements, along with their connections, could be tested and rated, it would allow for the design of stable structures at lower cost. This approach could also foster innovation in structural design and the use of new construction materials.

The solution was for NIST to expand its experimental facility for making such measurements from the modest single compartment of the 1910s to the current multifloor, multiroom National Fire Research Laboratory (NFRL).

Completed in 2015, the NFRL allows researchers to conduct large fire experiments with well-characterized measurement of heat release rates.

The NFRL

The weight of a building plays a crucial role in determining whether a fire will cause to it collapse. However, constructing a full-scale building for experiments is often impractical. At the NFRL, a hydraulic loading system is used to apply force to a portion of a structure, simulating the downward pressure from multiple floors above. This allows engineers to test how structures of different heights perform under realistic fire conditions, pushing them to the point of collapse. The insights gained help engineers better understand structural limits, leading to more rational, resilient and economical designs.  

The following three examples show how NFRL research is already contributing to the advancement of building codes and our understanding of structural performance in fire.  

Cross-laminated timber

Cross-laminated timber (CLT) is a relatively new construction material with a lot of potential. CLT is layers of lumber glued together with the grain of each layer at a right angle to its neighbors. It’s like a more robust version of plywood. The material is strong enough to support multistory buildings. It has the potential to enable faster, more sustainable construction than steel and concrete.  

Since wood can burn, one of the major barriers for adoption of CLT construction was the question of how well it would hold up during and following a fire. In 2017, NIST staff, in collaboration with the National Research Council Canada and the Fire Protection Research Foundation, conducted large-scale experiments to study the impact of fires on CLT. These experiments helped establish exposure limits under various conditions, leading to an addition to the ANSI/APA PRG 320 standard and facilitating changes to the International Building Code to expand CLT use in North America.

Steel walls during earthquakes

During an earthquake, gas lines or electrical systems can break, increasing the chances of a fire. Fire in a building already damaged by an earthquake can increase the chances of collapse, especially because there may be aftershocks that shake the building after it has already been weakened by a fire.

Using the NFRL in 2019, NIST examined the resistance to horizontal force of steel wall construction under combined simulated earthquake and realistic fire conditions.  

The results provide guidance for how building designers should construct walls to resist the side-to-side forces of an earthquake aftershock following a fire.

Steel-concrete floors in steel-framed buildings

Steel-framed buildings commonly have floors made of concrete reinforced with steel. In 2022, NIST researchers examined how these floors react to fire and what causes them to fail. In the NFRL, they built a full-scale, two-story steel frame with realistic connections and reinforced concrete slabs.

The researchers found that the prescribed minimum amount of steel reinforcement required by United States standards for everyday conditions may not be enough to keep the floor from collapsing during a fire. The experiments focus on achieving specific structural fire safety goals, rather than just following a set of rules.

These experiments also provided crucial data needed to make accurate computer models of steel-framed buildings subjected to fire. Both the data and the models will help building designers ensure that their designs will resist collapse during a fire.

Additional Reading:

Journal article: Structural Fire Experimental Capabilities at the NIST National Fire Research Laboratory. Fire Technology, 2016.

NFPA report: Fire Safety Challenges of Tall Wood Buildings – Phase 2: Task 2 & 3 – Cross Laminated Timber Compartment Fire Tests, 2018.

Standard: ANSI/APA PRG 320-2018. Standard for Performance-Rated Cross-Laminated Timber, 2018.

NIST report: Influence of Fire on the Lateral Resistance of Cold-Formed Steel Shear Walls – Phase 2: Oriented Strand Board, Strap Braced, and Gypsum-Sheet Steel Composite, 2019.  

NIST news: How Fire Causes Office-Building Floors to Collapse, 2020. 

NIST feature story: Built to House an Inferno, 2017.