FBC 5TH EDITION TABLE 508.4

[mjabell]

Per table 508.4 a Business 'B' occupancy is not required to be separated from an S-1 storage occupancy of moderate hazard (contains flammable items such as clothes, aersols, etc). But a 'B' occupancy is require to have a 1 or 2 hour separation (sprinkler/non-sprinkler) from an S-2 storage occupancy of low hazard (non-combustible items). This seems to be backwards - is there a logic I am missing? I have encountered this on 3 projects over the last year. What is the reasons behind this separation discrepancy.

[Jerry Peck - Codeman]

Part of the answer may be in your statement that “S-2 storage occupancy of low hazard (non-combustible items)” ... some S-2 items listed are “non-combustible”, however, some items listed under S-2 are combustible.

Another part of the answer may be that S-2 includes open or enclosed parking garages and Table 508.4, note b., references private or pleasure vehicles may have the separation reduced by 1 hour but to not less than 1 hour, this would be to separate a parking garage from the B occupancy.

However, let's start from the previous section - 508.3 Nonseparated occupancies.

- 508.3 Nonseparated occupancies.
- - Buildings or portions of buildings that comply with the provisions of this section shall be considered as nonseparated occupancies.

- - 508.3.1 Occupancy Classification.
- - - Nonseparated occupancies shall be individually classified in accordance with Section 302.1. The requirements of this code shall apply to each portion of the building based on the occupancy classification of that space. In addition, the most restrictive provisions of Chapter 9 which apply to the nonseparated occupancies shall apply to the total nonseparated occupancy area. Where nonseparated occupancies occur in a high-rise building, the most restrictive requirements of Section 403 which apply to the nonseparated occupancies shall apply throughout the high-rise building.

One choice is to use nonseparated occupancies, however, doing so creates greater limitations on the designer and the structure as the most restrictive requirements of any of the nonseparated occupancies applies to all other nonseparated occupancies.

Thus the usual choice is to go with separated occupancies, which leads us to the apparent contradiction of S-1 (higher hazard) not being separated from B whereas S-2 (lower hazard) is required to be separated, which gives the designer the ability to do different things with each separated occupancy.

- 508.4.4 Separation.
- - Individual occupancies shall be separated from adjacent occupancies in accordance with Table 508.4.

The following is from the 2010 FBC-Building commentary, however, some section and table numbers have changed so I changed those in the commentary below (i.e., Table 508.4 in the 2014 edition was Table 508.3.3 in the 2010 edition, thus I changed "Table 508.3.3" in the commentary below to read "Table 508.4".)

- Table 508.4 provides the required separation between the fire areas containing the separated uses. For example, a completely sprinklered building of Type VB construction contains areas devoted to business and assembly occupancies. The designer has chosen the separated uses option and has completely separated the areas containing the two different occupancies by fire barrier walls and horizontal assemblies having a minimum 1 hour fire resistance rating in accordance with Table 508.4. This is found by consulting the box that intersects with “A,E,D” in the first column with “B,F-1,M,S-1” in the first row and, because the entire building is sprinklered, the fire resistance rating of the fire barrier walls and horizontal assemblies is required to be 1 hour, as indicated in the column designated “S.” Had the building not been sprinklered, the required rating would have been 2 hours, as indicated in the column designated "NS”.
- - Note that Table 508.34 contains groupings of some of the occupancies, including:
- - • A, E, D
- - • I
- - • R
- - • F-2, F-3, S-2, U
- - • B, F-1, M, S-1
- - • H-3, H-4, H-5
- - With the remaining occupancies, Group H-1 and H-2 residing in their own group.

- - These groupings are groups of occupancies that share the same level of hazard, mainly with respect to fire safety. It is possible, therefore, to have two occupancies that comply with these separated use provisions that require no separation between them. For instance, a mixed occupancy of Groups B and M would not be required to have separation between them, but the provisions for calculation of the sum of the ratios of actual areas could still be applied to this circumstance. See commentary to Section 508.4.

(underlining is mine)
- TABLE 508.4
- - The purpose of Table 508.4 is to set forth the fire resistance rating required for fire barrier walls and horizontal assemblies used to separate occupancies, The fire -resistance rating of these parathion between different occupancies is based on the relative anticipated fire severity of the occupancies.
- - Note a: See the commentary for Section 903.2.4.2. (*)
- - Note b: Many business and mercantile occupancies are very similar in operation and to the types of storage that may occur. Storage rooms that are greater than 100 square feet (9 m2) in area and satisfy any of the three options listed are not required to be separated from the business or mercantile occupancy.
- - - It should be noted that Group S 1 occupancies would not require a separation in accordance with Table 508.4 but Group S 2 would be required to be separated.
- - Note c: The fire resistance rating of spaces used solely for private or pleasure vehicles may be reduced by 1 hour.
- - Note d: See the commentary for Section 406.1.4. (*)
- - Note e: Kitchens and the restaurants they serve need not be separated from each other by fire barrier wall

(*) I did not change these underlined section numbers nor did I verify that these underlined sections numbers are correct as they are not applicable to this discussion.

In a nutshell ...

The above does not provide a good or clear explanation as to why a lesser hazard (S-2) is required to be separated from a B while a higher hazard (S-1) does not, however, S-2 includes parking garages ... my logic puts that as indicating that some of the S-2 uses require separation (with parking garages being the indicator of this logic) whereas S-1, which does not include parking garages, does not. Private parking garages even require a "separation" (albeit a nonrated separation) between a private dwelling and its own attached private garage.

Illogical? Seems to be that way at first, but if we use the stated presumption that S-1 and B are presumed to have the same level of hazard, then there is no need to separate them.

And if we go with S-2 as including parking garages, and we acknowledge that parking garages require separation, then requiring separation of S-2 from a B makes sense.

That is all I have been able to find out so far.

[mjabell]

Thanks for thorough research and answer. The more I looked at it the more the "parking garage" became the driving factor. I'll continue to go by the code but as always err to the side of safety.