IIIB Construction

[steveray]

Because of the other thread and me being one of the best (or worst) thread drifters here.....A lot of parts on this and looking to make the clarifying change where it makes sense...Being from a "mill town", I was always under the understanding that the 2hr exterior wall (bearing or not) was to contain the fire and keep it from spreading to adjacent buildings and maybe that is what we lost....

The question being: IF the primary frame "supports" the exterior wall is it part of the exterior wall and requires the same rating and where do we need to tweak that language....? I think the language in 704.10 has gotten it closer, but at the same time it hasn't nailed it, so the confusion is about equal...



In a PEMB the exterior wall is clearly not a load bearing wall:

[BS]WALL, LOAD-BEARING. Any wall meeting either of the following classifications:

1. 1.Any metal or wood stud wall that supports more than 100 pounds per linear foot (1459 N/m) of vertical load in addition to its own weight.
2. 2.Any masonry, concrete or mass timber wall that supports more than 200 pounds per linear foot (2919 N/m) of vertical load in addition to its own weight.

[BS]WALL, NONLOAD-BEARING. Any wall that is not a load-bearing wall.

They might be "within" if you consider supporting within.....

704.10 Exterior Structural Members​

Load-bearing structural members located within the exterior walls or on the outside of a building or structure shall be provided with the highest fire-resistance rating as determined in accordance with the following:

1. As required by Table 601 for the type of building element based on the type of construction of the building.
2. As required by Table 601 for exterior bearing walls based on the type of construction.
3. As required by Table 705.5 for exterior walls based on the fire separation distance.

Group B code changes are right around the corner...

 

[classicT]

I was always under the understanding that the 2hr exterior wall (bearing or not) was to contain the fire and keep it from spreading to adjacent buildings

That is not the intent of Table 601.

Table 601 does not relate to fire compartmentalization. It is only focusing on the ability of the structure to remain standing during fire conditions. Based upon that ability, the allowable height, stories, and area are established based upon the level of risk.

 

[classicT]

Given my preceding post, understanding that the intent of Table 601 is to establish the ability of a structure to withstand fire conditions, I asses that the application of 704.10 becomes clear.

Type III-B are intended to have exterior walls that can withstand fire conditions for 2-hours. For a PEMB, I do not see any way that you can separate the columns from the exterior wall assembly. I will acknowledge that depending on the configuration of the exterior wall, the columns may be located interior to the building, with purlins and cladding being the primary components of the exterior wall. Columns could be located within the interior floor space of the building. I would however contend, given the intent of fire resiliency, the columns fall subject to Condition #2 of 704.10.

Column protection is further defined in 704.2.

2021 IBC 704.2 Column Protection​

Where columns are required to have protection to achieve a fire-resistance rating, the entire column shall be provided individual encasement protection by protecting it on all sides for the full column height, including connections to other structural members, with materials having the required fire-resistance rating. Where the column extends through a ceiling, the encasement protection shall be continuous from the top of the foundation or floor/ceiling assembly below through the ceiling space to the top of the column.
Exception: Columns that meet the limitations of Section 704.4.1.

 

[steveray]

That is not the intent of Table 601.

Table 601 does not relate to fire compartmentalization. It is only focusing on the ability of the structure to remain standing during fire conditions. Based upon that ability, the allowable height, stories, and area are established based upon the level of risk.

Note the use of my understanding was....But there is nothing inherent on the gravity loaded exterior walls that will keep them standing for the 2 hours and that is part of my struggle...You can and do have floor diaphragms and other structural member with a zero rating that can result in a collapse of the exterior load bearing walls in a much shorter time than 2 hours....

 

[steveray]

Given my preceding post, understanding that the intent of Table 601 is to establish the ability of a structure to withstand fire conditions, I asses that the application of 704.10 becomes clear.

Type III-B are intended to have exterior walls that can withstand fire conditions for 2-hours. For a PEMB, I do not see any way that you can separate the columns from the exterior wall assembly. I will acknowledge that depending on the configuration of the exterior wall, the columns may be located interior to the building, with purlins and cladding being the primary components of the exterior wall. Columns could be located within the interior floor space of the building. I would however contend, given the intent of fire resiliency, the columns fall subject to Condition #2 of 704.10.

Column protection is further defined in 704.2.

2021 IBC 704.2 Column Protection​

Where columns are required to have protection to achieve a fire-resistance rating, the entire column shall be provided individual encasement protection by protecting it on all sides for the full column height, including connections to other structural members, with materials having the required fire-resistance rating. Where the column extends through a ceiling, the encasement protection shall be continuous from the top of the foundation or floor/ceiling assembly below through the ceiling space to the top of the column.
Exception: Columns that meet the limitations of Section 704.4.1.

Purlins are not load bearing by definition and there is no reference to "supporting" construction here like there is for fire barriers....I agree we can push and call it part of the exterior wall, but that is what I want to fix,....

BS]WALL, LOAD-BEARING. Any wall meeting either of the following classifications:

1. 1.Any metal or wood stud wall that supports more than 100 pounds per linear foot (1459 N/m) of vertical load in addition to its own weight.

 

[classicT]

Purlins are not load bearing by definition and there is no reference to "supporting" construction here like there is for fire barriers....I agree we can push and call it part of the exterior wall, but that is what I want to fix,....

BS]WALL, LOAD-BEARING. Any wall meeting either of the following classifications:

1. 1.Any metal or wood stud wall that supports more than 100 pounds per linear foot (1459 N/m) of vertical load in addition to its own weight.

Purlins are not load bearing walls, I agree.

They are however load bearing structural members. Take note of the language in 704.10.

Load-bearing structural members located within the exterior walls

 

[Yankee Chronicler]

Something about forgetting the lessons of history ...

As codes have "evolved" and attempted to make their language more modern, they have lost sight of how somethings got into the codes and were classified. I'd have to dredge up the old 1970s-era BOCA books to find the exact terminology (and I'm not in the office at the moment) but, from memory, under the old BOCA codes in the 1970s what we know call III-B ("combustible-noncombustible) was originally called "masonry bearing-ordinary joisted." So the provisions applicable to type III-B construction were really codifying traditional, solid masonry, mill-type construction, with thick masonry walls and the floor framing supported directly on the masonry walls at the exterior perimeter. This construction type also encompassed the tradition Philadelphia/Baltimore row house, in which the party walls are solid brick masonry and the floor joists run transverse, from party wall to party wall with beveled ("fire-cut") ends so the joists could fall out if burning in a fire without damaging the masonry pockets.

By changing the name for type III-B construction, we seem to have forgotten what it was intended to describe. The exterior walls were not rated to protect adjacent structures, they were rated because they supported the floors.

 

[Yankee Chronicler]

1. They might be "within" if you consider supporting within.....
   

   704.10 Exterior Structural Members​

   Load-bearing structural members located within the exterior walls or on the outside of a building or structure shall be provided with the highest fire-resistance rating as determined in accordance with the following:
   1. As required by Table 601 for the type of building element based on the type of construction of the building.
   2. As required by Table 601 for exterior bearing walls based on the type of construction.
   3. As required by Table 705.5 for exterior walls based on the fire separation distance.
   4. Group B code changes are right around the corner...

The frames of a pre-engineered metal building are "inside" of the exterior walls, but they are certainly not "within" (contained inside of) the exterior walls. The exterior walls of a typical PEMB are a single, corrugated sheet of metal siding with some insulation stretched across the interior face. Unless you put something between the exterior skin and the interior facing of the insulation, there's nothing "within" the exterior walls.

 

[steveray]

By changing the name for type III-B construction, we seem to have forgotten what it was intended to describe. The exterior walls were not rated to protect adjacent structures, they were rated because they supported the floors.

Why would we care about the support of an unrated floor made out of fuel?

 

[steveray]

Purlins are not load bearing walls, I agree.

They are however load bearing structural members. Take note of the language in 704.10.

Good point...I'm going to say if you go into the dictionary load bearing is going to say weight, not really wind or seismic or shear..I'd say a bit of a stretch but I could go there....but wouldn't that then also apply to the plywood on the outside of a VA building used for shear? Or for some reason we don't care because it is on the outside

 

[classicT]

Good point...I'm going to say if you go into the dictionary load bearing is going to say weight, not really wind or seismic or shear..I'd say a bit of a stretch but I could go there....but wouldn't that then also apply to the plywood on the outside of a VA building used for shear? Or for some reason we don't care because it is on the outside

Load-Bearing: supporting much of the weight of the overlying parts of a building or other structure.

I'd say that the plywood is one part of the overlying weight being supported.

Studs transfer the weight of cladding, roof, etc. vertically, transmitting the load downward (gravity, compression load) to the sole plate and foundation.

Purlins carry the weight of cladding, transmitting the load horizontally (acting similar to a beam) to columns, that then carry loads down to the foundation.

I will say, load bearing does not only mean gravity loads. Yes, in common sense, that is how we think of it. But most elements in a wall will have both gravity and uplift forces (compression and tension).

 

[Yankee Chronicler]

Why would we care about the support of an unrated floor made out of fuel?

I don't write the code, Sir -- I just have to deal with it. My guess is that we want the floor to stand up long enough for the people to get out of the building.

If you have any of the old BOCA codes in your office (ideally from the late 1960s and early 1970s), open 'em up and look at the construction types chapter and tables.

From the 2021 IBC Commentary -- Code:

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.

Commentary:

Buildings of Type III construction are typically constructed
with both combustible and noncombustible
materials. The exterior walls are required to be non-
combustible with load-bearing exterior walls required
to have a minimum 2-hour fire-resistance rating (see
Table 601). Exterior nonload-bearing walls are not
required by Table 601 to have a fire-resistance rating
but must comply with the provisions of Table 705.5.
The elements within the perimeter established by the
exterior walls (i.e., floors, roofs and walls) are permitted
to be of combustible materials. An example of a
typical building of Type III construction is a structure
having its exterior walls constructed of concrete,
masonry or other approved noncombustible materials,
but with wood-framed floor, interior wall and roof construction
(see Commentary Figure 602.3). The structural
members of a building of Type IIIB construction
are not required to have a fire-resistance rating, with
the exception of the exterior load-bearing walls.

 

[steveray]

I don't write the code, Sir -- I just have to deal with it. My guess is that we want the floor to stand up long enough for the people to get out of the building.

If you have any of the old BOCA codes in your office (ideally from the late 1960s and early 1970s), open 'em up and look at the construction types chapter and tables.

From the 2021 IBC Commentary -- Code:


Commentary:

Just odd that you know specifically that the exterior walls were not for fire spread (in and out) but you don't have any actual backup on "supporting the floor"...

And, of course the commentary posted, misses the FRTW exceptions....

 

[steveray]

Load-Bearing: supporting much of the weight of the overlying parts of a building or other structure.

I'd say that the plywood is one part of the overlying weight being supported.

Studs transfer the weight of cladding, roof, etc. vertically, transmitting the load downward (gravity, compression load) to the sole plate and foundation.

Purlins carry the weight of cladding, transmitting the load horizontally (acting similar to a beam) to columns, that then carry loads down to the foundation.

I will say, load bearing does not only mean gravity loads. Yes, in common sense, that is how we think of it. But most elements in a wall will have both gravity and uplift forces (compression and tension).

So they would be primary frame or secondary members? Because primary frame would take us back to zero (T601) and secondary would require individual encasement....

[BG] SECONDARY STRUCTURAL MEMBERS. The following structural members shall be considered secondary members and not part of the primary structural frame:

1. Structural members not having direct connections to the columns.
2. Members of the floor construction and roof construction not having direct connections to the columns.
3. Bracing members that are not designated as part of a primary structural frame or bearing wall.

704.4 Protection of Secondary Structural Members​

Secondary structural members that are required to have protection to achieve a fire-resistance rating shall be protected by individual encasement protection.

 

[classicT]

So they would be primary frame or secondary members? Because primary frame would take us back to zero (T601) and secondary would require individual encasement....

[BG] SECONDARY STRUCTURAL MEMBERS. The following structural members shall be considered secondary members and not part of the primary structural frame:

1. Structural members not having direct connections to the columns.
2. Members of the floor construction and roof construction not having direct connections to the columns.
3. Bracing members that are not designated as part of a primary structural frame or bearing wall.

704.4 Protection of Secondary Structural Members​

Secondary structural members that are required to have protection to achieve a fire-resistance rating shall be protected by individual encasement protection.

Well, considering that almost every PEMB I have seen has the purlins attached directly to the columns, then I would say that purlins are not secondary structural members. Thus, they are part of the primary structure.

Structural members not having direct connections to the columns.

Thus, 704.3 may apply.

2021 IBC 704.3 Protection of the Primary Structural Frame Other Than Columns​

Members of the primary structural frame other than columns that are required to have protection to achieve a fire-resistance rating and support more than two floors or one floor and roof, or support a load-bearing wall or a nonload-bearing wall more than two stories high, shall be provided individual encasement protection by protecting them on all sides for the full length, including connections to other structural members, with materials having the required fire-resistance rating.
Exception: Individual encasement protection on all sides shall be permitted on all exposed sides provided that the extent of protection is in accordance with the required fire-resistance rating, as determined in Section 703.

 

[steveray]

Well, considering that almost every PEMB I have seen has the purlins attached directly to the columns, then I would say that purlins are not secondary structural members. Thus, they are part of the primary structure.

Thus, 704.3 may apply.

2021 IBC 704.3 Protection of the Primary Structural Frame Other Than Columns​

Members of the primary structural frame other than columns that are required to have protection to achieve a fire-resistance rating and support more than two floors or one floor and roof, or support a load-bearing wall or a nonload-bearing wall more than two stories high, shall be provided individual encasement protection by protecting them on all sides for the full length, including connections to other structural members, with materials having the required fire-resistance rating.
Exception: Individual encasement protection on all sides shall be permitted on all exposed sides provided that the extent of protection is in accordance with the required fire-resistance rating, as determined in Section 703.

But also note #3:

1. Bracing members that are not designated as part of a primary structural frame or bearing wall.

So if the purlins do not brace the frame, they might meet #3......Just splitting hairs on individual encasement...Which would be ludicrous to build a slightly smaller building in some limited cases...That is what always gets me is the minor differences (If any) in H&A which seems like a ton of "give up" of the combustibles in IIIB vs. IIB....

 

[classicT]

But also note #3:

1. Bracing members that are not designated as part of a primary structural frame or bearing wall.

So if the purlins do not brace the frame, they might meet #3......Just splitting hairs on individual encasement...Which would be ludicrous to build a slightly smaller building in some limited cases...That is what always gets me is the minor differences (If any) in H&A which seems like a ton of "give up" of the combustibles in IIIB vs. IIB....

When do purlins not laterally brace the columns? Diagonal bracing exists as well, but is intended to resist racking.

Totally agree with what you said, but also making the case that purlins are clearly part of the primary structure in a PEMB.

 

[Paul Sweet]

Way back in the "good old days" building codes had fire limits, which were in the densely built-up areas of cities. Buildings within the fire limits had to be of fireproof or "ordinary" construction, which both had rated exterior walls and class A roofs to prevent conflagrations. Fireproof construction would be similar to IBC type I construction. Ordinary construction had masonry exterior bearing walls and wood frame interior construction.

I think it was in the late 70s or early 80s that the BOCA code, and probably the others as well, rewrote several sections to be less prescriptive and more performance oriented. The fire limits were taken out of the codes and fire ratings & limits on unprotected openings for exterior walls based on separation distance (present OIBC 705) were introduced. Ordinary construction was changed to noncombustible/combustible, and later to Type III.

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.

 

[steveray]

Way back in the "good old days" building codes had fire limits, which were in the densely built-up areas of cities. Buildings within the fire limits had to be of fireproof or "ordinary" construction, which both had rated exterior walls and class A roofs to prevent conflagrations. Fireproof construction would be similar to IBC type I construction. Ordinary construction had masonry exterior bearing walls and wood frame interior construction.

I think it was in the late 70s or early 80s that the BOCA code, and probably the others as well, rewrote several sections to be less prescriptive and more performance oriented. The fire limits were taken out of the codes and fire ratings & limits on unprotected openings for exterior walls based on separation distance (present OIBC 705) were introduced. Ordinary construction was changed to noncombustible/combustible, and later to Type III.

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.

But I am looking for the intent of the 2hr exterior bearing walls in IIIB.....If the entire guts of the building can burn and likely take down those walls, what is the point? And why is it so close to IIB in what seems like a much less fire safe building...?

 

[Yankee Chronicler]

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.

 

[Frank Lloyd Left]

Having spent the better part of my career in Type IA land and only recently in Type II, I find Type III baffling.

 

[Paul Sweet]

Type III is fast becoming an archaic building method (if it isn't already).

Steveray, I believe the 2 hour masonry exterior walls were meant to contain a fire to the building where it originated, and protect a building from fires in nearby buildings. Joists were firecut on the ends so they wouldn't pull the walls down when the interior burned and fell down.

 

[Penguin87]

Non-bearing exterior walls fire rating is dependent upon three things:

-construction type
-occupancy
-distance to nearby structures or property lines

See Table 705.5

For instance, if you have a zero-lot line Type IIIB building with a B occupancy, then 1 hour exterior fire rated wall along and within 10' of the property line.
If said wall is a structural bearing wall, then it needs to be a 2-hour fire rated wall per 601.

 

[Penguin87]

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."