Separation required between Type 3B and Type 2B?

[Humberbm]

I have an existing vehicle repair service garage (S-1 occupancy) that is single story, non-sprinklered Type 3B construction. The client wants to do an addition and we're looking at constructing a metal building, non-sprinklered Type 2B construction- also S-1 occupancy. The allowable building height, stories and area are all compliant with either S-1 for Type 2B or Type 3B. Path of logic has me thinking that if I'm allowed to build a Type 3B building addition without any separation, then a Type 2B building addition (PEMB- entirely non-combustible) is a better option and thus shouldn't require separation. I can't find a direct answer either way in the code- so what I am missing?

This feels like the logic of mixed-use, non-separated where if the height and area for the most strict occupancy can be applied to the entire building and still be compliant, then separation within is not required.

 

[Codeguy49]

I have an existing vehicle repair service garage (S-1 occupancy) that is single story, non-sprinklered Type 3B construction. The client wants to do an addition and we're looking at constructing a metal building, non-sprinklered Type 2B construction- also S-1 occupancy. The allowable building height, stories and area are all compliant with either S-1 for Type 2B or Type 3B. Path of logic has me thinking that if I'm allowed to build a Type 3B building addition without any separation, then a Type 2B building addition (PEMB- entirely non-combustible) is a better option and thus shouldn't require separation. I can't find a direct answer either way in the code- so what I am missing?

This feels like the logic of mixed-use, non-separated where if the height and area for the most strict occupancy can be applied to the entire building and still be compliant, then separation within is not required.

From the model 2021 IBC and Commentary - "Buildings and structures erected or to be erected, altered or extended in height or area shall be classified in one of the five construction types defined in Sections 602.2 through 602.5."

This means the Type of Construction must be the same for "a building", including additions. This is supported by the commentary, which explains:

"The use of multiple construction classifications in a single building is very limited and can only be done when specifically called out in the code. An example of combining types of construction is an apartment building of Type VA construction located above an open parking structure of Type IA construction, as described in Section 510.2. Several other special provisions found in Section 510 also permit a building of multiple construction classifications.

A more common example is where a single structure is divided into two parts by using a fire wall, resulting in two separate buildings or structures each of which may be of a different type of construction. Where a structure contains more than one building (for example, separation by a fire wall), each building is to be individually assigned a type of construction."

The idea to use II-B Construction might be tempting but this may come with added requirements so I'd try to stick with III-B.

 

[Tim Mailloux]

as long as your fire separation distance is greater than 30 feet from adjoining property lines (table 705.5) or other buildings on the site, there is no reason why you could not classify your PEMB addition as type 3B constriction.

 

[steveray]

as long as your fire separation distance is greater than 30 feet from adjoining property lines (table 705.5) or other buildings on the site, there is no reason why you could not classify your PEMB addition as type 3B constriction.

Which in my heart I disagree with but technically by code is sort of correct...

 

[Tim Mailloux]

sort of correct?

 

[RLGA]

A building is only required to be classified according to the type of construction for which it complies. Section 602.1.1 does not require compliance with a construction type higher than that to which it meets, even if the building or portions of the building conform to a higher construction type. You can add an addition that, on its own, would qualify as a Type IIB building as a part of a Type IIIB, as long as the addition also complies with all of the requirements for a Type IIIB building. This includes providing the 2-hour, noncombustible, exterior load-bearing walls; however, with PEMBs, the exterior walls are typically not load-bearing, so that requirement would likely not apply.

 

[Tim Mailloux]

A building is only required to be classified according to the type of construction for which it complies. Section 602.1.1 does not require compliance with a construction type higher than that to which it meets, even if the building or portions of the building conform to a higher construction type. You can add an addition that, on its own, would qualify as a Type IIB building as a part of a Type IIIB, as long as the addition also complies with all of the requirements for a Type IIIB building. This includes providing the 2-hour, noncombustible, exterior load-bearing walls; however, with PEMBs, the exterior walls are typically not load-bearing, so that requirement would likely not apply.

and per table 601, the rating for non load bearing exterior walls in IIIB construction is governed by table 705.5 per fire separation distance.

 

[Humberbm]

as long as your fire separation distance is greater than 30 feet from adjoining property lines (table 705.5) or other buildings on the site, there is no reason why you could not classify your PEMB addition as type 3B constriction.

I wondered about this too! Interesting thought. PEMB walls are not load bearing so this should work by logic...

 

[Yankee Chronicler]

A building can be classified only as one type of construction, and that's determined by the classification of the lowest/least portion of the building. You can start with a building of type I-A construction and if you tack on an addition of type V-B construction the entire building is then classified as type V-B construction.

The only way to do an addition of a lower construction type without reclassifying the existing building is to erect a fire wall between the existing building and the addition.

 

[RLGA]

and per table 601, the rating for non load bearing exterior walls in IIIB construction is governed by table 705.5 per fire separation distance.

True, but based on Type IIIB, which would require a 2-hour rating at 0 to <5 feet and a 1-hour rating from 5 to <30, even though a Type IIB building does not require the 1-hour rating at 10 feet or greater.

 

[Humberbm]

A building can be classified only as one type of construction, and that's determined by the classification of the lowest/least portion of the building. You can start with a building of type I-A construction and if you tack on an addition of type V-B construction the entire building is then classified as type V-B construction.

The only way to do an addition of a lower construction type without reclassifying the existing building is to erect a fire wall between the existing building and the addition.

Right, but I'm going the other way. My existing building is Type IIIB and my addition is IIB. It just seems silly to think that I can build the addition out of masonry and give it a combustible wood truss roof, but a fully non-combustible building is simply not allowed

 

[Yankee Chronicler]

Right, but I'm going the other way. My existing building is Type IIIB and my addition is IIB. It just seems silly to think that I can build the addition out of masonry and give it a combustible wood truss roof, but a fully non-combustible building is simply not allowed

Yes, you can go the other way. But you have to be clear that the result will be a building ALL OF WHICH is classified as type III-B, not partially II-B and partially III-B. If you want to maintain the individual construction type classifications, you will need a fire wall.

 

[steveray]

sort of correct?

I don't believe that was the intent of allowing IIIB to be basically the same as IIB for H&A purposes, even though that is what it says now...I believe the 2 hour exterior walls were keeping fire in or out thereby preserving the building and adjacent ones from collapse from fire on one side or the other...But I haven't had the time to research and hopefully prove that...

 

[Tim Mailloux]

I don't believe that was the intent of allowing IIIB to be basically the same as IIB for H&A purposes, even though that is what it says now...

its been that way since the 2018 IBC

 

[steveray]

its been that way since the 2018 IBC

Way longer than that....

 

[Yankee Chronicler]

The construction type descriptions got bastardized back in the 1980s or 1990s, well before the ICC even existed.

When I started playing at being an architect in the 1970s, we were under an amended version of the BOCA Basic Building Code. I think it was what we now call type III that was named "Masonry Bearing--Ordinary Joisted," and it was specifically intended to cover buildings with load-bearing masonry exterior walls and wood floor and roof framing. Then, somewhere along the way, BOCA kept the basic classification and description but changed the name, opening up what had been an easily-understood construction type to other configuration. [Hmmm ... or that may have been type IV. Type III may have been Heavy Timber.]

In the SSBC, there were



and



And now we have the IBC, in which type III is

 

[steveray]

[Hmmm ... or that may have been type IV. Type III may have been Heavy Timber.]

Looks that way...

 

[Paul Sweet]

"its been that way since the 2018 IBC"
"Way longer than that...."

1922 Building Code Recommended by the National Board of Fire Underwriters:
Section 10, par. 3(a)
"Ordinary Construction. A building having masonry walls, with floors and partitions of wooden joist and stud construction. The supportint posts and girders may be of wood, or of metal protected as required in Section 114."

Sections 26 & 31 required masonry bearing walls to be at least 12" thick. This is probably where the 2 hour exterior wall rating came from when Ordinary Construction became Type III.

 

[steveray]

"its been that way since the 2018 IBC"
"Way longer than that...."

1922 Building Code Recommended by the National Board of Fire Underwriters:
Section 10, par. 3(a)
"Ordinary Construction. A building having masonry walls, with floors and partitions of wooden joist and stud construction. The supportint posts and girders may be of wood, or of metal protected as required in Section 114."

Sections 26 & 31 required masonry bearing walls to be at least 12" thick. This is probably where the 2 hour exterior wall rating came from when Ordinary Construction became Type III.

And notice nothing on bearing or not....