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CDX in Type IIB exterior wall

vegas paul

vegas paul

Silver Member
Joined
Oct 17, 2009
Messages
495
Location
Salina, KS
Need opinions...

Type IIB construction, the exterior wall is a steel moment-frame wall, designer is proposing CDX plywood as in-fill as a backer for the culture-stone facade.

IBC 603.1 allows fire retardant plywood in Nonbearing Exterior Walls. Now, the CDX is not fire retardant - so that's an automatic no-no in my book, but even if it were fire retardant, isn't the exterior wall a bearing wall even if the sheathing has no contribution to the bearing capacity? In other words, if the CDX were on fire, doesn't that reduce the bearing capacity of the steel moment-frame?

Any plywood (retardant or not) allowed in these walls? I can't see how, but please comment!
 
603.1, exception 1.2 allows fire retardant wood in nonbearing walls where no fire rating is required. Extending the possibilities of what the flaming wood would do to the bearing capacity of the structure is cheating.
 
texasbo - If the wall IS bearing, but the sheathing is not part of the bearing element, is it still allowed?
 
I agree with you VP, I don't think that whether or not the ply contributes to the bearing makes any difference, it's a bearing wall. Plus, like you said, if the CDX isn't fire-retardent treated....well, it's already out.
 
vegas paul said:
texasbo - If the wall IS bearing, but the sheathing is not part of the bearing element, is it still allowed?
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If it's not part of the bearing wall assembly, and is installed as an exterior wall covering per 603.1 Number 10, and 1405, I think it would be permitted.
 
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It can only be included if (and that's a big IF) it can be considered as part of the exterior wall covering system - definitely not as the wall itself.
 
The non-structural plywood backing for the skin is NOT part of a bearing wall and its loss will have no impact on the performance of the steel frame.

A bearing wall has a well defined meaning in the building code and a steel moment frame is not a bearing wall structural system.

If you have a bearing wall structural system you cannot seperate the bearing wall from the sheathing that is a part of the wall.
 
Mark K said:
If you have a bearing wall structural system you cannot seperate the bearing wall from the sheathing that is a part of the wall.
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Mark: are you saying that a Type I or Type II building with a CMU exterior bearing wall could not have an exterior veneer with combustible componentsi?
 
I had a similar situation about a year ago. After much research I came the the conclusion that the plywood (OSB in my case) is permitted since it is part of the exterior cladding per 603.1 #10.

I've cut and pasted below a portion of the letter I put together for the designer.

In section 1405.4, 7/16 OSB is acceptable for exterior veneer that is attached to a non-combustible backing (steel studs, and no fire resistance rating required per table 602 and 1405.4 #2) in Type 1 construction. Per the definition of “exterior wall covering” (definitions 1402), it states that a wall covering is a material used for a water resistive barrier, and this definition includes the term “veneer” (7/16” OSB per 1405.4).

Since the OSB will be used as a backing for the required moisture barrier, then our department will accept that the use of the 7/16” OSB is technically by definition an exterior wall covering that is providing a “weather-resisting barrier” (1402), and conforms to the combustible materials use as permitted by section 603.1 #10.
 
I am saying that in making the determination as to the fire rating that it is inappropriate to consider the sheathing as part of a bearing wall. This is based on the statement that the sheathing and studs are not supporting vertical loads.

I have read articles on the performance of steel construction during fires and the fuel load from the sheathing is not going to impact the ability of the steel frame to remain standing.

I am not an expert on fire ratings but I am an engineer and am very familiar with what is and is not a bearing wall.
 
Mark, I agree completely. However, the OP also asked if the sheathing would be acceptable if it WAS applied to a bearing wall.

Mark K said:
I am saying that in making the determination as to the fire rating that it is inappropriate to consider the sheathing as part of a bearing wall. This is based on the statement that the sheathing and studs are not supporting vertical loads.I have read articles on the performance of steel construction during fires and the fuel load from the sheathing is not going to impact the ability of the steel frame to remain standing.

I am not an expert on fire ratings but I am an engineer and am very familiar with what is and is not a bearing wall.
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steveray said:
Mark,What if the sheathing is resisting wind and seismic loads? Are those not also loads requiring "bearing"?
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Definition of bearing wall includes vertical loads only.
 
Sorry, just read that definition! I was getting "structural" and "force resisting" confused with "bearing". I do not believe that combustible sheathing could be a component of any force resisting system (bearing or non) in a non-combustible building. As for cladding or exterior wall covering, that might take some more convincing....

The more I read the more I doubt what I have been taught.... Table 601 also only speaks of gravityloads.....can that be???
 
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To Texasbo:

When I talked about sheathing being integral with the wall I was refereeing to sheathing that is a part of the lateral system of the building. If you had CMU bearing walls any sheathing would be acting as a veneer and would not contribute to the lateral system.

As I read the code exemption only applies to non-bearing walls.

To steveray:

ASCE 7 which is referenced from the IBC recognizes both bearing wall and building frame systems to resist wind or seismic loads. The difference is whether they support significant gravity loads. Bearing can occur in many ways but when talking about bearing walls it is clear the focus is on gravity loads.

In reality a non-bearing wall can resist some fairly small amount of gravity loads and still be considered a non-bearing wall. The exact amout has been debated by structural engineers but it does not appear that that is relevant to this discussion.

Regarding your belief regarding not allowing combustible sheathing on non-bearing walls all I ask is that you enforce the code as it is written and not as you think it should be. If you believe the code is wrong understand the rational for the existing provisions and if you still disagree then work to change the code.

The primary focus on gravity loads is tied to the code emphasis on life safety. By this they mean that as long as everybody can get out of the building safely the code does not require that the building remain standing or be salvageable. Also it is assumed that you will not have an extreme wind or earthquake at the same time the building is totally on fire. A building on fire should still be able to deal with the small normal wind loads.

There is no guaranty that multiple extreme situations will not occur but if the codes were written to prevent all risk we could not afford to build most structures even if it were possible.
 
Thanx Mark,

I do understand that as I am reading it now, but that is NOT what the people at the state level teach here, we have been instructed that all force resisting systems in non-combustible buildings should be non-combustible. Hence my comment about doubting what I have been taught...

I would hope we are all still learning and teaching and sharing our perspectives on a daily basis as I believe that is the best way to gain a thorough understanding, and that is why I am here!

Thanks to all!
 
cladding is not part of a force resisting system. It's the pretty side of the building, not the structural side..
 
The OP said "Type IIB construction, the exterior wall is a steel moment-frame wall, designer is proposing CDX plywood as in-fill as a backer for the culture-stone facade." With a moment frame the exterior wall is non-bearing and doesn't resist any lateral loads. Fire-retardant treated plywood or OSB is allowable.
 
A Hard Nose Reply

NO

Why? other than FRT the exterior wall transfers and reacts to load even if not necessarily a vertical floor or roof load

Load Bearing also applies to Lateral, like wind and shear

If it is an exterior wall is surely transfers nad resists lateral wind load

So I'm Mean but only once in a while
 
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