Research by engineers at the National Institute of Standards and Technology (NIST) suggests that structures built to code are not always equipped to survive the forces induced by extreme shifts in temperature when a fire breaks out.

The data gained by building and then destroying concrete slabs with fire could help researchers develop and validate new design tools and building codes that bolster fire safety.

Study results were reported in the Journal of Structural Engineering. (Learn more about building code standards at Engineering360.)

Inside a fireproof compartment, NIST researchers subjected full-scale replicas of office building floors to fires produced by three gas-fueled burners. Source: NISTConventional fireproofing

In the United States, fireproofing materials are sprayed or painted onto weight-bearing beams or columns to slow their temperature rise in case of a fire. These materials, which are typically the only fire-resistance measures integrated into building skeletons, are required by building codes to be thick enough to delay structural deterioration for a certain number of hours.

The responsibility of putting fires out or preventing them from spreading typically falls on measures outside of the structural design, such as sprinkler systems and local fire departments.

NIST researchers said the current approach to fire safety is typically sufficient to protect most buildings from collapse. However, rare situations occur in which fire protection systems and firefighting efforts are not enough. In those cases, a fire rages uncontrolled and flames burn so hot that they overwhelm the defense of the fireproofing and lead to structural failure and even collapse.

Structural limitations

In engineering terms, components of a building undergo thermal elongation at elevated temperatures. Steel beams are typically bound at their ends to support columns, which typically remain cool and maintain their shape for longer. This is because of additional fireproofing and the reinforcement of the surrounding structure. With little room for thermal expansion, beams that heat up during fires could press up their uncompromising boundaries, potentially breaking their connections and causing floors to collapse.

The NIST researchers said that to better prepare buildings for worst-case scenarios, structural designs may need to account for the forces introduced by fires. But because the behavior of a burning building is complex, structural engineers need help predicting how their designs would hold up in an actual fire. Computer models that simulate building fires could provide invaluable guidance.

"The main purpose of this experiment is to develop data from realistic structure and fire conditions that can be used for developing or validating computational programs," said Lisa Choe, NIST structural engineer and lead author of the study.

The shear tab connection (left) is a single flat plate welded to the support column and bolted onto one side of the steel beam. The double angle connection (right) is composed of two L-shaped plates, or angle legs, welded to the support column and bolted onto both sides of the steel beam. In this study, the double angle-connected beams endured greater deformations and temperatures before failing. Source: B. Hayes/NISTStructures are seldom fire-tested at a realistic scale. Standard tests make use of laboratory furnaces that typically only accommodate individual components or small assemblies. These often lack the kinds of end connections that are used in buildings. Within the National Fire Research Laboratory (NFRL), engineers are able to build and safely burn structures as tall as two stories.

Fire-testing components

Mimicking the design of floors from high-rise office buildings, Choe and her colleagues at the NFRL formed concrete slabs atop steel beams spanning 12.8 m (42 ft), a typical length in office buildings. The floors were suspended in the air, fastened at their ends to support columns either by double angle or shear tab connections.

The engineers also used a hydraulic system to pull down on the floors, simulating the weight of occupants and moveable objects like furniture. The beams also coated in fireproofing material with a two-hour fire-resistance rating to meet building code requirements.

Inside a fireproof compartment, three natural-gas fueled burners torched the floors from below, releasing heat as rapidly as a real building fire. While the compartment warmed up, various instruments measured the forces felt by the beams along with their deformation and temperature.

As temperatures within the compartment surpassed 1,000° C, the expanding beams, having been constrained between two support columns, began to buckle near their ends.

After around one hour of heating, the shear tab connections of one beam — having dropped more than 2 ft — fractured, leading to collapse. The beams with double angle connections, however, remained intact. They ultimately failed hours after the furnaces were shut off, as the beams cooled and contracted back upwards, breaking the double angle connections.

While the study's small sample size means conclusions about buildings in general could not be drawn, the NIST team found that the beams with double angle connections endured greater forces and deformations from the temperature changes than those with shear tab connections.

"The influence of the thermal elongation and contraction is something that we shouldn't ignore for the design of steel structures exposed to fires," Choe said.