What Happens to Steel Beams in a Structural Fire?

Steel's strength relies on its crystalline structure, which begins to change around 1,000 degrees Fahrenheit. At this temperature, steel loses about half its yield strength, the point where it permanently bends. In a typical house fire, temperatures can exceed 1,500 degrees, causing beams to soften and deform under their own weight. This weakening happens long before the steel melts, which occurs above 2,500 degrees. The rate of strength loss accelerates as the temperature climbs, making time a critical factor in fire scenarios.

Only a qualified structural engineer can determine a steel beam's safety after a fire. They perform calculations and may order tests, such as hardness measurements or ultrasonic testing, to measure the remaining strength. Their assessment decides whether a beam can be left in place, needs reinforcement, or must be completely replaced. Never assume a beam is safe based on a visual inspection alone, as internal stresses and metallurgical changes can remain unseen but critically weaken the component.

Your first priority is safety: do not re-enter the structure until fire officials deem it safe. Contact your insurance company and document all damage with photos. A general contractor or restoration pro can help secure the site, but a structural engineer's report is needed for repairs. Policies vary, and your insurer decides what is covered. The repair process often involves installing temporary supports before permanent beam work begins, which stabilizes the area for further assessment and construction.