Building and Infrastructure Construction

Modified Fire Resistive (ISO Class 5, IBC Type IB)

Modified Fire Resistive construction is ISO Class 5. ISO Class 5 encompasses IBC Type IB.

  1. Learn the elements of Modified Fire Resistive Construction. Modified Fire Resistive Buildings are buildings where the exterior bearing walls and load-bearing portions of exterior walls must be of noncombustible materials or of masonry, but exterior nonbearing walls and wall panels may be slow-burning, combustible, or with no fire- resistance rating.
    • Buildings with exterior walls, floors, and roofs of masonry materials described in the definition of fire resistive (Construction Class 6) — less thick than required for fire-resistive structures but not less than four inches thick, or
    • Fire-resistive materials with a fire-resistance rating less than two hours but not less than one hour

Structural Steel Protection

  1. Note that modified fire-resistive buildings also include structural steel protection techniques — fire-protection material applied to steel. Materials include:
    • concrete
    • plaster
    • clay tile
    • brick or other masonry units
    • gypsum block
    • gypsum wallboard
    • mastic coatings
    • mineral and fiberboard
    • mineral wool

Ceilings Protecting Steel Beams or Joists

What happens when there is no fire-protection material applied to steel beams or joists that support floors or roofs? ISO still considers a building modified fire resistive if it has a suitable ceiling. Ceilings can be plaster or gypsum wallboard or suspended mineral tile. The entire floor-ceiling (a fire-resistive ceiling protecting a floor) or roof-ceiling (a fire-resistive ceiling protecting roof supports) should conform to construction details in a UL-listed or Factory Mutual (FM)-approved design. ISO individually evaluates each approved design.

  1. Note the advantages of the modified fire resistive construction:
    • uses noncombustible materials
    • allows greater height and area than other construction classes
    • uses load-bearing members or assemblies that resist damage from fire
  2. Note that modified fire-resistive construction has these disadvantages:
    • expensive to construct and repair
    • provides a false sense of security