Building and Infrastructure Construction

Joisted masonry (ISO Class 2, IBC Type III, IBC Type IV)

Joisted Masonry construction is ISO Class 2. ISO Class 2 encompasses IBC Type IIIA and IBC Type IIIB. Regardless of whether the IBC classification is A (protected) or B (unprotected) the ISO Class is 2. IBC Type IV is Heavy Timber construction and is considered ISO Class 2. The reason is that the heavy timbers perform well and do not fail early in a fire.

  1. Learn the elements of Joisted Masonry construction. Joisted Masonry buildings are buildings with exterior walls of masonry or fire-resistive construction rated for not less than one hour and with combustible floors and roofs. There are several types of masonry used in the exterior bearing walls of joisted masonry buildings:
    • brick
    • concrete — either reinforced or non reinforced
    • hollow concrete masonry units
    • tile
    • stone
  2. Note that exterior bearing walls may also be any noncombustible materials with fire-resistance ratings of not less than one hour.
  3. Be familiar with the variations of Joisted Masonry Construction. There’s one variation on joisted masonry construction that doesn’t change the construction class — heavy timber or mill construction. Heavy timber construction uses wood members much larger than those found in frame (Construction Class 1) or other joisted masonry construction. If the building uses steel columns or beams for walls, the beams must be protected so they have a fire-resistance rating of not less than one hour. Heavy Timber Construction (IBC Type IV); ISO classifies the building as heavy timber construction if it meets these requirements:
    • walls of masonry construction
    • floors of 3 inch wood plank or 4 inch laminated plank, both surfaced with 1 inch flooring
    • roof of 2 inch wood plank, 3 inch laminated plank, or 1-1/8 inch tongue-and-groove plywood deck
    • wood column supports not less than 8 inch x 8 inch, wood beams or girders not less than 6 inch x 6 inch, or protected metal
  4. Understand the advantages of jointed masonry:
    • harder to ignite
    • consumed more slowly by fire
    • more structural stability
    • greater salvage value
    • lack of concealed spaces (Heavy Timber)
  5. Be aware that jointed masonry construction has these disadvantages:
    • floors and roofs of combustible materials subject to damage by fire
    • presence of concealed spaces