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Do You Know Your Construction Classes? Construction Class 6: Fire-Resistive

August 19, 2013

We’ve reached the end of the construction class road! Fire-resistive construction is perhaps the most expensive and is best and most commonly used in high-rise buildings. As the name would imply, it resists fire the best of all of the construction classes, but resisting fire doesn’t necessarily mean it will remain structurally sound at high enough sustained heat. Construction Class 6 (CC 6) buildings are designed to resist the heat of a fire for at least 2 hours (R.2 hours).

“Buildings where the exterior walls and the floors and roof are constructed of masonry or fire-resistive materials having a fire resistance rating of not less than 2 hours.” Commercial Lines Manual, Rule 15.B.6

Exterior Walls or Exterior Structural Frame:

–          Structural, horizontal, and vertical load-bearing protected metal supports, including concrete units

–          Hollow masonry not less than 8 inches thick

–          Solid masonry, including reinforced concrete not less than 6 inches thick

–          Brick not less than 6 inches thick

–          Assemblies with a fire-resistance rating of not less than R.2 hours

–          NOTE: Panel or curtain walls of masonry may be of any thickness

The following is an excellent picture of a CC 6 building undergoing a remodel:

 CC6 building construction

The top arrow, in green, is pointing to what a protected steel structural element would generally look like. Next time you’re in the parking garage of a large high-rise building, look around and you may see columns that look like this.

The bottom arrow, in blue, is pointing to a portion of the beam that is missing fire protectant.

The right arrow, orange, is pointing to an unprotected, non-load-bearing steel beam. Because it’s non-load-bearing, it doesn’t have to be protected. However, if this beam were made of wood or another less fire-resistive material, the building would not be CC 6.

Floor and Roof:

–          Reinforced concrete with slabs not less than 4 inches thick.

–          Floor and roof assemblies with a fire-resistance rating of not less than R.2 hours.

This is as simple as it sounds. Floors and roofs of a CC6 building must be 4 inches thick or more, or they must have a fire-resistance rating of R.2 hours or more. In reality, there are multiple ways this can appear:

fire resistive building construction

The ceiling assembly in the photo above shows the same sprayed-on protection as on the steel floor and ceiling members.

Construction Class 6 building

The photo above shows a building with concrete columns and concrete floors for each level, with a concrete roof.  Assuming the concrete floors are 4 inches thick or more, this qualifies as a CC 6 building.

The previous articles have outlined the six different construction classes WSRB uses: Frame, Joisted Masonry, Non-Combustible, Masonry Non-Combustible, Modified Fire-Resistive, and Fire-Resistive.  But what happens when a building is not the same material all the way through?  Perhaps an addition was put on the building at some point over time, or maybe the upper floors are of a different construction than the lower floors.

Next week, we’ll round out the Know Your Construction Class series with a brief discussion of mixed construction classes. Stay tuned!

Learn about other Construction Classes here!

Article by: Kristen Skinner

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