Modified Fire Resistive

Construction types

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

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

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
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.

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

Note that modified fire-resistive construction has these disadvantages:
expensive to construct and repair
provides a false sense of security

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