Steel Bar Joist Trusses And Steel C-Beams - Part 2

Many renovated buildings replace wood and masonry structural elements with steel. The replacement of heavy wooden roof and floor joists with the lighter steel open-web bar-truss joists is a common renovation change. A lightweight steel open-web...


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Fire size. The size of the fire is the final factor that can affect steel failure. If a small-area fire comes in contact with a portion of a large steel beam, the steel will absorb heat and transfer it away from the flaming area to cooler parts of the structural element. A fire could burn for some time before it heats the entire steel beam to its failure temperature. On the other hand, a large-area fire in which flames involve much of the steel beam in a short time will heat the steel beam to its critical temperature more quickly. A "flash fire," suddenly involving a large area with flame, can heat steel to its failure temperature rapidly.

Lessons to Be Learned

There is a difference between the terms "non-combustible" and "fire resistance." Steel is non-combustible, not fire resistive. Non-combustible steel will not add fuel to a fire, but steel cannot resist fire. It will collapse from the effects of fire. The heat of a fire destroys the load-bearing qualities of steel. If you want to make steel fire resistive, you must protect it. You can make steel fire resistive by covering it with insulation. Heat-resistant materials such as concrete or spray-on insulation can give steel fire resistance for one, two or three hours, depending on the type of insulation, or you separate it from a potential fire area with a membrane ceiling.

Unprotected lightweight-steel bar-joist trusses and steel C-beams are not fire resistive. They can collapse after five to 10 minutes of fire exposure. Lightweight steel beams are designed for low-hazard occupancies with contents that do not experience severe fires. Unfortunately, the design of a non-combustible type II building with floors or a roof built with steel bar joist trusses or C-beams does not anticipate a possible occupancy change from low-hazard content to a high-hazard content or flammable-liquid use. When an occupancy hazard increases from low hazard to a moderate or high hazard, additional fire protection should be mandated.

During the renovation or a building where there is a change in roof construction from wood to unprotected lightweight steel bar joist or steel C-beam, there may be an advantage of reducing the fuel in the roof or floor construction that would contribute to a fire. There will, however, be an increased potential for firefighter death and injury from building collapse.