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Do You Know Your Construction Class 5: Modified Fire-Resistive, Where R-Ratings Aren’t Just for Movies

August 12, 2013

When dealing with Construction Class 5 (CC 5), it is first helpful to know about R-Ratings.  Fire-Resistive Ratings (or R-Ratings, as we call them) are how building codes, including the International Building Code (IBC), the National Fire Protection Association (NFPA), WSRB, and other agencies, determine how resistive a building’s construction will be in a fire. Construction type definitions can vary slightly depending on the agency implementing the code and their application. WSRB uses the Underwriters Laboratories (UL) Directory for Roofing Materials and Systems as our guidelines for the fire-resistive capabilities of any particular construction. While this directory is too large to summarize (over 950 pages) it’s the basis for the following—and good light reading for the weekend!

–          When the phrase “R.1 hour” is used below, it means the structure should remain structurally sound for more than 1 hour.

–          “Less than R.2 hour” means the structure will probably not remain structurally sound for more than 2 hours.

“Buildings where the exterior walls and the floors and roof are constructed of masonry or fire-resistive rating of one hour or more but less than two hours.”

Exterior Walls or Exterior Structural Frame:

–          Structural, horizontal and vertical load-bearing protected metal supports, including horizontal pre-stressed concrete units with a fire resistance rating of R.1 hour, but less than R.2 hour.

–          Hollow masonry less than 8 inches thick

–          Solid masonry, including reinforced concrete, less than 6 inches thick

–          Brick less than 6 inches thick

–          Assemblies with a fire resistance rating of at least R.1 hour, but less than R.2 hours

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

What does this mean? For metal buildings it means if the walls and load-bearing supports are protected to withstand the heat of a fire for at least one hour, but not more than 2 hours, it would be classed as CC 5 (assuming conforming walls and floors). For concrete buildings, the walls would be less than 6-inches thick—5 ½ inches is the most common we see in this category. Keep in mind that if the load-bearing supports or roof are of wood construction or unprotected metal, it could change the Construction Class.

Floors and Roof:

–          Reinforced concrete with slabs less than 4 inches thick

–          Floor and roof assemblies with a fire resistance rating of R.1 hour, but less than R.2 hours.

Assuming the walls conform to the rules above, the floors must also conform. Concrete floor must be no more than 4-inches thick and any other floor and roof assemblies (especially metal) must be at least R.1 hour, but less than R.2 hours.

Most buildings that fall under this construction class are multiple-story office or habitational-type risks, though they can house just about anything.  This construction is one of the hardest to recognize without extensive training, and even with, building construction plans can be crucial in the determination.

Fire resistive building construction

The building pictured above is a glass and concrete office building, rated as CC 5 due to the thickness of the flooring at each story.

construction class 5

This building is also rated CC 5 due to the fire-resistiveness of the construction used.

Click here to read about more construction classes!

Article by: Kristen Skinner

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