R2 over S2 parking garage in wood frame type VA

[Scott555]

Building design using 2006 IBC:
A Mixed-use 4-story R2 apartments over S2 below grade plane basement for private enclosed parking.
Type VA construction, fully sprinklered to increase the 3 stories to 4 (as per table 503 and section 504.2).

Question: can the horizontal separation between the basement/first floor be built in wood frame floor trusses assuming the proper fire rating between the R2 and S2 uses is met? I seem to recall this was allowed (wood floor truss) in previous BOCA code versions, but am unsure in 2006 IBC (section 509.2 and 509.4).

Thanks in advance,
Scott

[Jerry Peck - Codeman]

Question: can the horizontal separation between the basement/first floor be built in wood frame floor trusses assuming the proper fire rating between the R2 and S2 uses is met?

Yes, as long a all fire-resistance ratings (ceiling/floor, walls, etc.) are met - you are not allowed to use the sprinkler system to reduce any of the required fire-resistance ratings as you have already used the sprinkler system to increase the height/area.

[Scott555]

Thanks....although I am getting a lot of resistance because of section 509.2. Doesn't this section require type 1 construction making the ceiling/floor construction require steel or concrete only? Or am I missing something?

[Jerry Peck - Codeman]

Thanks....although I am getting a lot of resistance because of section 509.2. Doesn't this section require type 1 construction making the ceiling/floor construction require steel or concrete only? Or am I missing something?

Or unless I am missing something (like everyone, I miss things too), so let's go through 509.2 and seem then continue with other sections to find out what, if anything, I am missing. (bold and underlining are mine)
- 509.2 Group S-2 enclosed or open parking garage with Group A, B, M, R or S above. A basement and/or the first story above grade plane of a building shall be considered as a separate and distinct building for the purpose of determining area limitations, continuity of fire walls, limitation of number of stories and type of construction when all of the following conditions are met: (Jerry's note: Unless I am missing something, you are not considering this a separate and distinct building, right? Or did I miss that in the first information you provided? If I did not miss that in the first information you provided, this entire section does not apply.)
- - 1. The basement and/or the first story above grade plane is of Type IA construction and is separated from the building above with a horizontal assembly having a minimum 3-hour fire-resistance rating.
- - 2. Shaft, stairway, ramp or escalator enclosures through the horizontal assembly shall have not less than a 2-hour fire-resistance rating with opening protectives in accordance with Table 715.4.
- - - Exception: Where the enclosure walls belowthe horizontal assembly have not less than a 3-hour fire-resistance rating with opening protectives in accordance with Table 715.4, the enclosure walls extending above the horizontal assembly shall be permitted to have a 1-hour fire-resistance rating, provided:
- - - - 1. The building above the horizontal assembly is not required to be of Type I construction;
- - - - 2. The enclosure connects less than four stories;
- - - - and
- - - - 3. The enclosure opening protectives above the horizontal assembly have a minimum 1-hour fire protection rating.
- - - 3. The building above the horizontal assembly shall be permitted to have multiple GroupAuses, each with an occupant load of less than 300, or Group B, M, R or S uses.
- - - 4. The building below the horizontal assembly is a Group S-2 enclosed or open parking garage, used for the parking and storage of private motor vehicles.
- - - - Exceptions:
- - - - - 1. Entry lobbies, mechanical rooms and similar uses incidental to the operation of the building shall be permitted.
- - - - - 2. Multiple Group A uses, each with an occupant load of less than 300, or Group B or M uses shall be permitted, in addition to those uses incidental to the operation of the building (including storage areas), provided that the entire structure below the horizontal assembly is protected throughout by an approved automatic sprinkler system.
- - - 5. The maximum building height in feet shall not exceed the limits set forth in Section 503 for the building having the smaller allowable height as measured from the grade plane.

The above section is used when a choice is made to consider the area (basement or first story above grade) as a separate and distinct structure, did I miss that in your first information?

Table 503, for Type VA type of construction to be used for Group R2 use, allows 3 stories and 12,000 square feet per story.

Section 504.2, for Group R2 which is fully sprinklered with an automatic sprinkler system which meets 903.3.1.2, allows an increase in height of 20 feet and the maximum number of stories is increased by 1 story, i.e., Table 503 allows 3 stories, 1 additional story is allowed for the sprinkler system, which makes 4 stories.

- TABLE 503
- - ALLOWABLE HEIGHT AND BUILDING AREASa
- - - Height limitations shown as stories and feet above grade plane.
- - - Area limitations as determined by the definition of “Area, building,” per story

- STORY ABOVE GRADE PLANE. Any story having its finished floor surface entirely above grade plane, except that a basement shall be considered as a story above grade plane where the finished surface of the floor above the basement is:
- - 1. More than 6 feet (1829 mm) above grade plane; or
- - 2. More than 12 feet (3658 mm) above the finished ground level at any point

As I understand your description, you plan on having 4 stories "above grade plane", right? Table 503, with the increase allowed by 504.2, allows 3+1=4 stories "above grade plane" for Type VA and R2 use. As such, at this point there is no need to consider the S2 occupancy as a "separate and distinct structure" and thus no reason, to this point, to apply section 509.2.

Section 506 addresses area modifications, with 506.3 addressing the automatic sprinkler system increase. The automatic sprinkler system allows the area per story to be increased by and additional 200 percent with more than one story above grade, i.e., 12,000 sf + 24,000 sf 200% additional = 36,000 sf per story. Again, no fire-resistance rating substitution in accordance with Table 601, Note e is allowed.

Section 506.4 has a limiting affect on the maximum area of a building with more than one story above grade: two stories above grade and the maximum area per story is multiplied by 2, i.e., a two story above grade building would have 36,000 sf x 2 = 72,000 sf maximum for the building; three or more stories about grade and the maximum area per story is multiplied by 3, i.e., a 4 four story above grade building would have 36,000 sf x 3 = 108,000 sf (not the 144,000 sf one might expect). There are some exceptions to this section, such for unlimited area building and those with the appropriate automatic sprinkler system, which means such a sprinklered building would be allowed the 144,000 sf (36,000 sf x 4 stories = 144,000 sf).

Section 506.4.1 addresses mixed occupancies, this further modifies and limits the above in accordance with sections 508.3.2 and 508.3.3.

Section 508 addresses mixed use and occupancy, which you have, the most restrictive of the occupancies control, and the R2 numbers used above are more restrictive than the S2 numbers would have been, thus no need to recalculate the numbers using the S2 occupancy.

Unless I missed something going through the above, I see no need to consider the two occupancies as distinct and separate buildings, thus 509.2 would not be applicable.

Where you able to follow through the above, if so, what was your outcome?

[Scott555]

Jerry, sorry for the long delayed reply...

My footprint is approximately 15,000 sf. So you are suggesting I do not need to separate the S2 from the R2 which would make distinct separate buildings per section 509.2 and just apply restrictions for R2 to whole structure as non-separated uses?

[Jerry Peck - Codeman]

Jerry, sorry for the long delayed reply...

My footprint is approximately 15,000 sf. So you are suggesting I do not need to separate the S2 from the R2 which would make distinct separate buildings per section 509.2 and just apply restrictions for R2 to whole structure as non-separated uses?

Scott,

Following the code through, that is what it says. From my post above:
- "Section 506.4.1 addresses mixed occupancies, this further modifies and limits the above in accordance with sections 508.3.2 and 508.3.3.

Section 508 addresses mixed use and occupancy, which you have, the most restrictive of the occupancies control, ... "

You would need the separation required in those two sections and in re-directed sections (the code often takes you back and forth between sections), I recommend going with the most restrictive, in Table 508.3.3, R (see note d, which says see section 406.1.4) and S-2 (see notes c and d), the 'S' column ('sprinklered') shows a separation of 1 hour, and note c allows a reduction in separation by 1 hour, but I would recommend going with a 2 hour separation as that is not much more expensive and offers a much higher degree of protection for the occupants, i.e., 'reduces the chances of accidental injuries and the litigation which will follow'.

What is that old saying? An ounce of prevention is worth a pound of cure.

Besides, following through the referenced sections may end up with a 1 or 2 hour separation based on what the other sections allow, don't allow.

[Scott555]

Ok, although I am a bit confused on why there is a choice between separated (Type IA construction) and non-separated (using only one or two hour with wood combustible construction). Why is it a "choice" and not a requirement one way or another? Very misleading and subjective for such a huge issue...

[Jerry Peck - Codeman]

You have a single building with mixed use and occupancy, and in going over the code sections and taking the various paths available for various options, I have not found a section which I believe precisely addresses your description, not even section 509 Special Provisions.

None of the specific sections in 509 address your description, and the sections do not state that they are required, those sub-section appear to only be allowances, i.e., if-this-is-done, this-is-allowed. In fact 509.1 says:
- 509.1 General. The provisions in this section shall permit the use of special conditions that are exempt from, or modify, the specific requirements of this chapter regarding the allowable heights and areas of buildings based on the occupancy classification and type of construction, provided the special condition complies with the provisions specified in this section for such condition and other applicable requirements of this code.

Your description does not fit the combinations listed in 509, therefore 509 does not apply. I suspect that the AHJ is trying to apply 509.2 as your requirement as it closely fits what you described them as saying. However, one must keep in mind that 509.1 may be used to "permit" the use of the special conditions which follow, this does not state that the section is a requirement instead of other applicable code sections.

The code does not, cannot, specifically address every conceivable configuration, that is where the AHJ can apply this:
- 104.11 Alternative materials, design and methods of construction and equipment. The provisions of this code are not intended to prevent the installation of any material or to prohibit any design or method of construction not specifically prescribed by this code, provided that any such alternative has been approved. An alternative material, design or method of construction shall be approved where the building official finds that the proposed design is satisfactory and complies with the intent of the provisions of this code, and that the material, method or work offered is, for the purpose intended, at least the equivalent of that prescribed in this code in quality, strength, effectiveness, fire resistance, durability and safety.

Meaning it comes down to the AHJ being able to allow what you are proposing. Likewise, the AHJ also has the authority to not approve what you are proposing. An often repeated answer to a code question is: "It depends.", and with questions like this one, "it depends" to a large degree on what the AHJ will or will not approve.

Scott, I see you are reading this as I am correcting some of my wording, please make sure to re-read it again for the final working. Jerry

[Scott555]

So the short answer is I can do type VA over basement garage in all wood frame and 1 hour separation between garage and first floor using sprinklers.

The only reason one would consider doing a separated mixed use and applying 509.2 and using type 1A construction would be for square footage of footprint limitations. In other words, if the building was so large it couldn't qualify for Type VA area requirements (with all the increases added) THEN 509.2 would apply and type 1A would be required. Correct?

[Jerry Peck - Codeman]

As correct as I can determine by the code.

Your AHJ may state otherwise, and if they apply 509.1, you should point out that 509 says that it "shall permit the use of" the special conditions and that you are not applying any of those special conditions.