Buildings can potentially experience structural failures hours after a fire has been extinguished, according to a new study led by Thomas Gernay, assistant professor of civil and systems engineering and online master’s program instructor at Johns Hopkins Whiting School of Engineering.
The findings, which focus on concrete edifices, address a crucial and overlooked aspect of buildings’ resistance to flame.
“What we learned is that even after the temperature of the fire had peaked, the temperature inside the building’s structural elements can continue to rise, stressing and changing them in complex ways. This essentially means the building’s frames and walls’ load-bearing capacity progressively degrades even as things cool,” said Gernay, a leading expert on fires and the built environment.
Gernay noted that these processes occur in all types of structures and materials, so understanding and quantifying this behavior in concrete edifices enabled the team to propose new methods to improve structures’ fire resilience in general. The team’s results appear in Engineering Structures: “Numerical analysis of the effects of fire with cooling phase on reinforced concrete members.”
Gernay and researchers at Politecnico di Milano (Polytechnic University of Milan) used Gernay’s SAFIR software to model what happens when reinforced concrete columns, beams, and walls are subjected to simulated but realistic fires that flare up before gradually cooling down. Standard fire resistance tests heat concrete structures until they fail, but the researchers sought to observe the impact of shorter fires over a longer period.
The researchers found that, as a fire subsides, the outer layers of a concrete building cool first, while the inner cores remain hot, redistributing stress on internal structures and their components.
These stressors, when added to the damage and degradation of materials caused by the fire itself, may cause entire buildings to become unstable long after the fire is out.
In addition, the team found that how fast a fire cools affects a structure’s risk of collapse. Buildings that cool down more slowly and gradually have a higher likelihood of collapse than do those that cool more quickly.
“The findings provide engineers with new methods to design concrete buildings that can withstand this cycle of exposure to a full fire and then the cooling off period,” Gernay said. “The ultimate goal is preventing collapses not only while a fire is occurring, but also in the hours and days after.”
This research was supported by the American Concrete Institute (ACI) Foundation.