One of the most interesting and exciting projects I ever worked on was the evaluation of a large industrial building constructed in the mid-1920s that had a fire in the early part of this current century. The building had a steel structure, covered with brick, and the tallest portion was the most severely damaged. The area had to be accessed with an 80-foot boom lift, which was the reason for the excitement! Fortunately, my then employee had a lot of experience with mobile equipment. He had to dodge around the trusses to get us to the areas of concern. We were way up in the air!
The purpose of our evaluation was to provide a second opinion about the damage to the steel beams and trusses. The bricks were clearly spalled and had to be replaced. This had been determined by the first engineering company to inspect the scene. They said that the steel beams were also damaged, however, and needed to be replaced. The building owners were not enthusiastic about replacing the steel structure for business-disruption reasons. The steel did not “look damaged” to the building owners.
When I first understood the specific reason for the desired second opinion, I was thinking that the first engineering company was correct. If the bricks were damaged, the steel must have been damaged because day one of my freshman materials science and engineering class emphasized that ceramics generally have higher heat resistance than metals.
Of course there are always exceptions to these general rules. Polymers in general have the lowest strength of the three main groups of materials, but mercury (a metal) is liquid at room temperature, while most polymers (human-made and natural, such as spider webs, silk, cotton and wool) are solid at room temperature and have quite good strength, especially when the ratio of the load-bearing capability to the weight of the component is considered. During this fire investigation, I found that there is a type of brick that has very poor resistance to thermal shock. Apparently, these bricks were of that type.
As the building owner pointed out, the steel structure did not appear damaged in any way by the fire. There was no sagging or buckling.
There is a very interesting article written by Ray Tide, “Integrity of Structural Steel After Exposure to Fire.” Tide points out that most members of the general public do not understand how hot steel gets in its normal processing. Hot-rolled steel is done at temperatures in the red heat range, which is well over 1500°F. Most building fires do not get that hot.
When I was in “metallurgy school,” we learned that modern high-strength, low-alloy steels were “invented” during the World War II effort. That was some 20 years after this building was constructed. So, I assumed that the steel beams that we were going to examine would be typical low-carbon, hot-rolled steel. Since the beams showed no sagging or twisting, I assumed that it was going to be fairly simple to demonstrate that the beams were still sound.
Check back to see the conclusion of our investigation.