Building codes in the U.S. require to install fire-resistant materials in the framework of a structure that can decelerate the spread of a fire and prevent extensive structural damage. Those materials are usually sprayed on beams and columns and may protect the buildings from collapsing for hours. Nevertheless, active measures to put out a fire (e.g. sprinklers) are not required by the regulations.
The current codes are effective in ensuring people's health and protecting buildings from total failure in the vast majority of cases. Nevertheless, there are certain cases in which the installed systems along with the fire department assistance have not proved to be adequate. Researchers from NIST assembled concrete floors supported by steel beams, a structural pattern that is frequently found in high-rise office buildings, and set them on fire. The span of the concrete slabs was 12.8 meters in length, typical for such buildings. Moreover, the floors were connected to the steel beams either by shear tab or double angle connections. In addition, to simulate the weight of people and objects on the floor, they incorporated a force produced by a hydraulic system. Finally, the typical fire-resistance equipment suggested by the current regulations was added to the structural parts.
Scientists found out that those buildings are not totally safe. In particular, the problem arises when structural parts experience temperature rise and dilate. Steel beams that are connected to support columns are elongated resulting in a connection failure that causes the structural collapse.
The results of the tests showed that no floor was able to completely withstand the extreme heat, however, some performed better than others. The shear tab connections fractured and collapsed within the first hour of testing but, the double angle connections remained undamaged. However, after the fire was put off and the steel beams cooled, they also failed due to contraction.
A solution that researchers suggest is to take into consideration those forces applied by a fire incident in the design of a building. However, they point out that the current knowledge on the actual forces is inadequate and more experimental data needs to be derived. “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. Then the programs can be expanded to different building configurations and used for design,” Lisa Choe, lead author of the study and a NIST structural engineer, stated.