Steel is used in heavy construction because it is relatively inexpensive, easy to use and provides great strength under normal conditions. However, heat is its weakness. Exposed steel expands and may fail under fire conditions.
Physical damage can cause the concrete to separate from the steel compromising the structure's fire-resistive integrity. Exposed steel can fail when its average temperature reaches approximately 1,000 F. The high temperature will weaken the steel resulting in sagging that might result in catastrophic structural failure.
According to the National Fire Protection Association (NFPA) and the International Code Council (ICC), steel structural elements must be protected to a minimum one-hour fire-resistance rating. The steel's fire-resistance rating is achieved through a concrete, or intumescent (swells when heated) paint, coating. The current term for spray fireproofing is “spray-applied fire resistive material.”
The most common fire-proofing materials are low-density fiber or cement coatings that are sprayed directly onto the steel. These require the oily coating found on new steel to be removed prior to spray application or the material will not stick. However, a thin film of rust will help adhere these materials to bare metal.
Fire code requirements
Fire inspectors should be familiar with the many elements that make up fire-resistive construction, and if they see damaged components, they should order corrective action.
The model fire codes authorize the fire inspector to require repairs to damaged fire-resistive assemblies. For additional information, refer to NFPA 1, Fire Code, Chapter 12; International Building Code, Chapter 7; and International Fire Code, Chapter 7.