Exterior Wall Deflection Limits 1

[pthorley]

2012 IBC and IRC deflection limits have a new entry for exterior walls "with plaster or stucco finished" L/360. Previously there was just brittle (L/240) and flexible finishes (L/120). Does this deflection limit apply to in-plan and out-of-plane lateral loads? Does this limit only apply to traditional stucco or does it also apply to synthetic and EFIS stucco systems?

 

[msquared48]

With respect to your first question, yes, for all gravity and lateral loads. I have been doing that for years, so no changes here for me.

With respect to the second question, I will have to defer to others, but if it is termed a stucco, I would be prudent and use the L/360 limitation.

Mike McCann
MMC Engineering

 

[Ron]

pthorley....even though there was an L/240 brittle finish criteria before, there has long been a reference in the building codes (UBC, SBC, IBC) to ASTM C926 for stucco. As stated in the administrative portion of the code (Chapter 1), reference standards are to be treated as if they were included verbatim within the code. That said, ASTM C926 "Standard Specification for Application of Portland Cement-based Plaster"(Stucco), requires L/360 design criteria for the substrate to accept stucco. As with any provision of the code, where there is a conflict between two criteria, the more stringent will apply.

The serviceability criteria apply in all directions (at least in my opinion). If deflection is caused, without regard to the direction of application, it should be considered. Chances are that it will be critical in only one direction; however, you won't know that unless you check.

The criteria should also apply to all forms of stucco; although, the manufacturers of "one-coat" stucco systems are working under an evaluation report for code equivalence and they will argue that their systems are not required to meet the criteria of ASTM C926.

 

[pthorley]

Mike,

Thanks for your comments.

In Utah I believe that there are latex additives to stucco that are added for the climate. Also all stucco is back by 7/16 OSB. When you do out of plane locating to you just analyze the stud wall or the composite action with the studs and structural sheathing?

Thanks, Pthorley

 

[XR250]

My suspicion is that they are NOT referring to EIFS. You can tie that stuff in a knot and it won't crack.
Having said that, once you get too much below L/240, strength seems to begin to control the design.

 

[Ron]

pthorley....when analyzing for deflection limit criteria, the sheathing is not included in the analysis.
Latex additives are common in synthetic stucco and they are not added for the climate. They are added to impart better properties to the stucco, but it doesn't always work out that way.


EE...the coating used for EIFS does crack. It is simply a sand filled acrylic material that has somewhat better flexibility than conventional portland cement plaster, but as the acrylic weathers, it will crack.

 

[AELLC]

All this is nonsense. A typical 46' wide tract home with 2x4 Hem-Fir #2 grade studs, vaulted ceiling at 2.5:12 and 8' plate height at the bearing wall would have maximum non-brg wall height of 12.8', and with 90 MPH Exp C wind (IBC 2009) would have a deflection of 1.82", which is far greater than even L/120=1.28".
Yet we who reside in the real world know that the actual deflection is far less due to all those secondary considerations we don't include, such as the wallboard nailed to the studs. And what is their definition of "stucco"? Out here in the West we almost always use the "Western" or "California" one-coat (3/8" stucco) applied over 1" foam, which is surprisingly flexible.

 

[Ron]

AELLC....it is hardly nonsense when there have been millions of dollars in damage from lack of attention paid to such details; allowing excessive cracking, water intrusion at the cracks, exacerbated corrosion of lath, stucco falling off walls, substrate damage, people losing their life's savings because of the cost of repair....the list goes on.

One-coat stucco systems perform VERY poorly in areas of moderate to high rainfall.

 

[msquared48]

AELLC:

You obviously have not dealt with Condo construction and the lawsuits stemming from stucco. This is not an area to save money for the contractor or owner. There is not profit for you in paying his bills.

Mike McCann
MMC Engineering

 

[AELLC]

Ron and Msquared,
I am not advocating the use of one-coat stucco in regions of high rainfall etc. I am merely saying this new requirement doesn't allow us to design tract homes with 2 x 4 studs at 16" on center any more. How am I supposed to tell my clients (major homebuilders) the studs have to be a 12" on center now?
None of the one-coat stucco systems fail in the Phoenix area as far as I know unless it was a bad installation.

 

[Ron]

AELLC....there's nothing new here. The L/360 requirement has been in place for stucco since 1994.

 

[AELLC]

Ron:
What page in the UBC 1994 says that?

 

[AELLC]

Ron,

I bought a new-built tract home with 3-coat stucco exterior walls in North Las Vegas years ago. The stucco cracked horribly, and it wasn't because of L/120.

The 3 coats were applied in 3 consecutive days, and the City Inspector denied that fact, and the Board of Registration for Contractors was so corrupt it said the cracking was normal.

 

[jeffhed]

AELLC,
We have always considered wood stud walls 10'-0" and less to be covered under section 2308, conventional framing. IBC 2012 Table 2308.9.1 specifies that 2x4 studs can be used at 16" o.c. when supporting one roof, one floor and a ceiling (2 story) as long as they are not utility grade studs. In our area, DF #2 is usually used for exterior wall studs. Of course it doesn't go into what wind loading is being applied in that table, but buildings 2 stories and less have been framed for years around here with 2x4 studs 10'-0" tall.

 

[oldrunner]

For a long time many engineers were not paying attention to out-of-plane wind loads. Others were ignoring Exposure C and using Exposure B and others were ignoring uphill wind forces. And there is the 10 foot ceilings heights and some 18 to 20 foot ceilings heights. At two story houses we were seeing horizontal cracks at the 2nd floor levels or the other side of the house we were seeing horizontal cracking at mid height of the 2nd story houses.
Basically many were ignoring the wind load directly against the house and also ignoring the suction loading on the lee side of the house. I'm not sure what the real damage was in North Las Vegas, but it my impression that there were thousands of homes involved in lawsuits.

In our area hundreds of homes were involved and there was even a TV report with an expert explaining all of the problems. In our area, basically the local building department apparently were not checking the out of plane aspect or the use of the correct wind exposure.

There were many lawsuits - but one never hears of the settlements. It is my understanding that some of this practice is still going on locally - with stucco contractors coming back for repairs of maybe a perfect stucco application the first time.

The solution was really just deeper studs - 2x6's where 2x4's have been traditionally used and 2x8's to 2x10's for full two story walls - depending where the house was being located. On the flat in the middle of a tract versus at the edge of the tract on a bluff with a clear fetch of a couple of miles.

With the outburst of many new homes being prepared to be built - will we see these problems reappear or will the contractors continue to use bottom feeder engineers to design their tract houses and the local agencies look the other way so that these houses can be added to the tax rolls?

 

[Ron]

AELLC...I agree with you that there are numerous reasons why stucco cracks occur and most of them have to do with application; however, when the basic design premise is not followed, the flexibility of the structural system, without regard to the level of loading, is greater and contributes to cracking.

 

[Ron]

AELLC...I agree with you that there are numerous reasons why stucco cracks occur and most of them have to do with application; however, when the basic design premise is not followed, the flexibility of the structural system, without regard to the level of loading, is greater and contributes to cracking.

 

[AELLC]

Well I am still skeptical - I have never heard of stucco cracking because of wind. I looked at a house in Henderson NV where an engineer said the stucco cracked because the shear walls were insufficient for in-plane wind loads, but as it turned out, the stucco cracked because of differential settlement.
I just can't start specifying studs at 12" on center when they have been 16" on center for decades. I do use 12" on center when needed for some cases where interior walls are heavily loaded because they are the interior support of very long spanning 3-point bearing roof trusses, and cases where walls get very heavy roof plus floor loading.

 

[Ron]

AELLC...one consideration that is erroneously neglected is the differential deflection between stiff portions of the wall and more flexible parts. An example is the stud pack at windows as compared to the next stud out from there. There is often strain incompatibility and resulting cracking, whether wind induced or otherwise. The problem is worse with weaker sheathing (gypsum).

 

[pthorley]

Ron,
Referring to your 3 Jul 13 17:00 post. Why is the sheathing (interior or exterior) not included in the calculation for out of plane wall deflection?