IIIB Construction

[Penguin87]

And as a follow-up, PEMB's don't have load bearing walls. Period, end of discussion.
They use giant steel metal building frames and possibly columns on the grid lines. ALL of the building loads are carried by these metal frames. Lateral is provided by cross-bracing of some sort.
The "walls" and roof are attached to the metal frames, and neither are load bearing in any definition of the term.

PEMBs are, of course, entirely designed by the PEMB company.

You have two options to add a fire rated wall to a PEMB:
-IMP wall or roof panels, either foam or mineral wool filled
-Infill the walls between the PEMB framed with CMU block to get your 3+ hour rating. You will have to deal with the columns, however, which you can wrap w/ gyp board.

I just finished up a Type 2B PEMB and it was a PITA. The contractor kept asking to use plywood in the walls for sheathing!

 

[Penguin87]

A typical metal building with rigid frames, purlins, and metal siding is a type IIB building. Trying to call it a type III building and leaving the rigid frames unprotected because the purlins are attached to the outside edge of the rigid frames is stretching the code beyond the intent.

The code allows you to downgrade the construction type as far as you want to go. You can, for instance, label a fully cast-in-place concrete structure as a Type VB, if you want. This means that you can use combustible materials in the construction if you want. But, even if you don't, there is nothing stopping a building to be permitted as a Type VB building if it meets the area, egress, # stories, height limits and Table 601/705.5 requirements.

In many jurisdictions, going with the lower construction type requirements results in cheaper permitting fees. However, of course it limits what the owner can do with the building in the future.

 

[Yankee Chronicler]

Edit - I believe you are thinking of Type IV / Heavy Timber construction. Type IV are min. 2-hour masonry load bearing exterior walls w/ timber framing on the interior.
Type IIIB is considered "standard construction."

I had forgotten that the ICC International Building Code introduced an entirely new category of construction typoes. Under BOCA, there were only four major types, with sub-categories. The IBC moved what had been Type 3 under BOCA to Type IV, and introduced an entirely new Type III -- Combustible/Noncombustible.

 

[steveray]

At work during luch today I dug out the old copy of our state building code that was based on the 1970 BOCA code. I had forgotten that, back then, type 3 construction had three sub-types. All were masonry bearing wall, bu the 3A was heavy timber construction; type 3B was ordinary, protected; and 3C was ordinary, unprotected.

Apologies for the poor quality of the photos. The old pages are starting to discolor, and my desk doesn't have the best lighting in the universe.

Looks like we "downgraded" our A&B to what was B&C almost....Still nothing great on the importance of the exterior walls...

 

[classicT]

If you are confused as to what a "Primary Structure" is, its part of the structural frame. Check under Chapter 2 - Definitions:

[BS] BEARING WALL STRUCTURE: A building or other structure in which vertical loads from floors and roofs are primarily supported by walls.
[BG] PRIMARY STRUCTURAL FRAME. The primary structural frame shall include all of the following structural members:

1. The columns.
2. Structural members having direct connections to the columns, including girders, beams, trusses and spandrels.
3. Members of the floor construction and roof construction having direct connections to the columns.
4. Members that are essential to the vertical stability of the primary structural frame under gravity loading.

Edit - I believe you are thinking of Type IV / Heavy Timber construction. Type IV are min. 2-hour masonry load bearing exterior walls w/ timber framing on the interior.
Type IIIB is considered "standard construction."

Not sure what code you are relying upon for construction type classification, but you are not aligned with the IBC.

2021 IBC ​

602.3 Type III​

Type III construction is that type of construction in which the exterior walls are of noncombustible materials and the interior building elements are of any material permitted by this code. Fire-retardant-treated wood framing and sheathing complying with Section 2303.2 shall be permitted within exterior wall assemblies of a 2-hour rating or less.

602.4 Type IV​

Type IV construction is that type of construction in which the building elements are mass timber or noncombustible materials and have fire-resistance ratings in accordance with Table 601. Mass timber elements shall meet the fire-resistance-rating requirements of this section based on either the fire-resistance rating of the noncombustible protection, the mass timber, or a combination of both and shall be determined in accordance with Section 703.2. The minimum dimensions and permitted materials for building elements shall comply with the provisions of this section and Section 2304.11. Mass timber elements of Types IV-A, IV-B and IV-C construction shall be protected with noncombustible protection applied directly to the mass timber in accordance with Sections 602.4.1 through 602.4.3. The time assigned to the noncombustible protection shall be determined in accordance with Section 703.6 and comply with Section 722.7.
{continues...}

 

[Henchalwoog]

Just a note to end the thread - I received an opinion from ICC regarding type IIB PREMB buildings and it indicated the frame should be protected...completely opposite of what I would have thought:
In accordance with Table 601 for a building of Type IIIB construction, primary structural frame members are not required to have a rating (Item 1) and exterior bearing walls are required to have a 2-hour rating (Item 2). As such, the highest fire-resistance rating is 2 hours. While Section 704.10 addresses load-bearing structural members “within” exterior walls, this section has also historically included load-bearing structural members located “along the outer lines” of a building or structure. As such, in my opinion, if the steel columns located at the exterior wall are load-bearing, then they would be required to have a 2-hour rating regardless of whether the wall itself is load-bearing.

 

[Henchalwoog]

oops - should be IIIB PEMB

 

[Yankee Chronicler]

Just a note to end the thread - I received an opinion from ICC regarding type IIB PREMB buildings and it indicated the frame should be protected...completely opposite of what I would have thought:
In accordance with Table 601 for a building of Type IIIB construction, primary structural frame members are not required to have a rating (Item 1) and exterior bearing walls are required to have a 2-hour rating (Item 2). As such, the highest fire-resistance rating is 2 hours. While Section 704.10 addresses load-bearing structural members “within” exterior walls, this section has also historically included load-bearing structural members located “along the outer lines” of a building or structure. As such, in my opinion, if the steel columns located at the exterior wall are load-bearing, then they would be required to have a 2-hour rating regardless of whether the wall itself is load-bearing.

Wow! That's convoluted, and it shows why when looking at the tables we have to look at the footnotes.

Aside from the fact that I've never encountered a PEMB that wasn't classified by the designer as type II-B construction, if someone wants to declare it type III-B then things get interesting. Looking at Table 601 for fire resistance ratings, in type III-B construction the only element requiring a 2-hour (or any) rating is exterior bearing walls -- which a PEMB doesn't have. A PEMB does have a primary structural frame, and the table says that requires a rating of 0 hours -- no rating.

But wait -- the plot thickens. Where it says "Primary structural frame" it says to see Section 202. That's definitions.



Okay. So the main framing members of a PEMB are pretty clearly part of the primary structural frame, by definition. So what? But there's also a footnote 'f' in there. And footnote 'f' to Table 601 says, "Not less than the fire-resistance rating as referenced in Section 704.10." So now we have to look at Section 704.10.



This is where I return to the fact that opinions from ICC staff are not binding, they are advisory. They are sometimes helpful, and they are sometimes wrong. Section 704.10 addresses load-bearing structural members located "within" the exterior walls or on the outside of the building. The editor in me says that "within the walls" doesn't mean "inside the building," it means contained between the inner and outer surfaces of the wall construction. The studs of a house are "within" the exterior walls because they are between the exterior sheathing and the sheetrock interior finish. The primary frame of a PEMB is "inside of" the exterior walls, separated from the exterior walls by the depth of the purlins, it is not "within" the exterior walls.

I see where the ICC staffer who offered that opinion seems to have been coming from, but from an editorial and grammatical perspective my response is "You can't get there from here" -- the code doesn't say what he/she says it says. Very simply, a primary structural frame member that is near the inside face of the exterior walls is not an "exterior load-bearing structural member." It is what it is -- a load-bearing structural member that is entirely inside of the building and which happens to be near (but not "within") the exterior wall.

 

[Paul Sweet]

This interpretation follows the intent of Type III construction. It used to be referred to as "Ordinary Construction" which required 2-hour rated (originally masonry) exterior walls but allowed wood, steel, and/or concrete for the interior structure. A PEMB would have been classified as Non-Combustilbe (Type IIB) if the interior structure and partitions were noncombustible, or Frame (Type V) if wood was used for interior structure or partitions.

 

[walker.t]

it shows why when looking at the tables we have to look at the footnotes.

Very nice outline.

They are sometimes helpful, and they are sometimes wrong.

And they can also be in disagreement with other ICC publications:

2018 IBC Illustrated Handbook commentary on 704.10 Exterior Structural Members (partial quote, emphasis added]
The code provides that structural frame elements in the exterior wall…

Using just “in” cannot give the sense that it means “within the space defined by the exterior walls.”

 

[redeyedfly]

Using just “in” cannot give the sense that it means “within the space defined by the exterior walls.”

Good thing the code uses the word "within" to remove all doubt.

 

[Tim Mailloux]

Wow! That's convoluted, and it shows why when looking at the tables we have to look at the footnotes.

Aside from the fact that I've never encountered a PEMB that wasn't classified by the designer as type II-B construction, if someone wants to declare it type III-B then things get interesting. Looking at Table 601 for fire resistance ratings, in type III-B construction the only element requiring a 2-hour (or any) rating is exterior bearing walls -- which a PEMB doesn't have. A PEMB does have a primary structural frame, and the table says that requires a rating of 0 hours -- no rating.

But wait -- the plot thickens. Where it says "Primary structural frame" it says to see Section 202. That's definitions.

View attachment 16780

Okay. So the main framing members of a PEMB are pretty clearly part of the primary structural frame, by definition. So what? But there's also a footnote 'f' in there. And footnote 'f' to Table 601 says, "Not less than the fire-resistance rating as referenced in Section 704.10." So now we have to look at Section 704.10.

View attachment 16781

This is where I return to the fact that opinions from ICC staff are not binding, they are advisory. They are sometimes helpful, and they are sometimes wrong. Section 704.10 addresses load-bearing structural members located "within" the exterior walls or on the outside of the building. The editor in me says that "within the walls" doesn't mean "inside the building," it means contained between the inner and outer surfaces of the wall construction. The studs of a house are "within" the exterior walls because they are between the exterior sheathing and the sheetrock interior finish. The primary frame of a PEMB is "inside of" the exterior walls, separated from the exterior walls by the depth of the purlins, it is not "within" the exterior walls.

I see where the ICC staffer who offered that opinion seems to have been coming from, but from an editorial and grammatical perspective my response is "You can't get there from here" -- the code doesn't say what he/she says it says. Very simply, a primary structural frame member that is near the inside face of the exterior walls is not an "exterior load-bearing structural member." It is what it is -- a load-bearing structural member that is entirely inside of the building and which happens to be near (but not "within") the exterior wall.

Foot note f only applies to bearing walls. The exterior walls of a PEMB are not bearing walls and 704.10 does not apply. What does apply is any fire rating to those non load bearing exterior walls based on fire separation distance per table 602.