How to Interpret 2-Hour "Building" in NFPA 72?

5/20/2021

NFPA 72, 2016 Edition uses the phrase "2-hour fire-rated construction" in a few places; what does this refer to?

It's confusing to me since a variety of elements within the building could be 2-hour rated. For example, NFPA 72-2016 Section 24.3.13.7 states:

"Two-way in-building wired emergency communication systems shall have a pathway survivability of Level 2 or Level 3. Exception: Level 1 shall be permitted where the building is less than 2-hour fire-rated construction."

Buildings are not designated with a fire-rating, elements are. How do you interpret this?

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6 Comments

CJ Bonczyk

5/20/2021 08:42:54 am

SFPE has an article that talks about fire ratings/enclosures and pathway circuit survivability requirements. Not sure if this helps provide a little bit of clarification regarding a specific construction types.

https://www.sfpe.org/publications/magazine/fpeextra/fpeextra2017/fpeextraissue14

Todd E Wyatt

5/20/2021 08:53:50 am

SHORT ANSWER
A building which is "less than 2-hour fire-rated construction" would be of a Type of Construction of Type III, IV or V.

LONG ANSWER
Fire-resistance ratings (FRR) of buildings can be divided into (2) categories : Type of Construction and separation of adjacent spaces.

All buildings are assigned a Type of Construction (see IBC Ch 6) which asssigns a FRR of the "building elements" : primary structural frame (columns & beams), bearing walls (exterior and interior),floor constrcution, and roof construction. There are (5) Types : Type I (most stringent) to Type V. Type I & II are those in which the building elements are "noncombustible". Type III may have both noncombustible (e.g. masonry exterior walls) and combustible materials (e.g. wood framing). Type IV are mass timber or noncombustible building elements. Type V is all others. Each type has subsets (e.g. IA & IB) which have different FRR. The Type of Construction is determined by the Design Team based on the "allowables" (IBC Ch 5, max bldg height, max number of stories, and max area per story) based on the building's Occupancy Classification (IBC Ch 3).

The other FRR categories are assigning specific areas, rooms, and spaces protection by FRR assemblies (IBC Ch 7) such as fire walls, fire barriers, fire partitions, smoke barriers, smoke partitions, shaft enclosures, and horizontal assemblies based on hourly ratings of 0.5, 1, 2, 3, and 4 hours. Elevator hoistways may be protected by 1-hour shaft enclosures or by 2-hour shaft enclosure based on how many stories they connect (<4 or 4 or more) and by the FRR of the floor construction (Type of Const) they penetrate.

James

5/20/2021 08:57:53 am

Under that premise of a "building" being less than 2-hour, would a Type II-B building, without any rated partitions or barriers, be considered having 2-hour fire-resistance-rated construction?

Not trying to be argumentative, I'm just trying to understand. I haven't heard it presented that way, I've only known the later point for FRR assemblies.

Todd E Wyatt

5/21/2021 08:27:33 am

I neglected to include Type IIA (1-Hour) and Type IIB (0-Hour) with the above Type III, IV or V Types of Construction that would be less than 2-hours.

The FRR of individual rooms/spaces, and areas are not directly related to the building's Type of Construction. Depending on the building's Occupancy Classification(s) (e.g. Business, Storage, Mercantile), a Type IIA (1-hour) or Type IIB (0-hours) building could be 4 stories that would require its elevator hoistways, mechanical shafts, and interior exit stairways to all be protected by 2-hour FRR assemblies. These 2-hour FRR assemblies, if they were NOT load-bearing, would be required to be supported by 2-hour (floor) construction. These 2-hr floor constrcution would also be required to be supported by 2-hour columns and beams.

If instead, the elevator hoistways, mechanical shafts, and interior exit stairways are all independenlty load bearing, they indivdually would have a 2-hr FRR but all the surrounding building elements (primary structure, floor const, foof const) could be Type IIB and 0-hr rated. It seems counterintuitive to require 2-hour FRR elevator hoistways, mechanical shafts, and interior exit stairways with 0-Hr FRR building elements that theoretically would burn, fail, and collapse around these vertical elements.

sean

5/21/2021 07:46:21 pm

easiest way it that it is contained and isolated from the rest of the building, think shaft, stairways, mechanical / electrical rooms that are stacked

fire rated access doors and panels link

8/3/2021 08:52:35 am

The facts that have been discussed here are really important. Thank you so much for sharing a great post.

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How to Interpret 2-Hour "Building" in NFPA 72?

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