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Re: Tall stud wall framing / IRC rant

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Oh, I don't disagree that the table is accurate. I believe that it is misleading. Its no different that advertising a mortgage rate of 2%, and putting an early payoff penalty, 5 discount points, and a 25% LTV ratio in the fine print.  I don't want the table removed - I don't want folks whining that they have to come to me because they need me to tell them that a 2x6 wall is okay for a 12' gable end.

It is my opinion that the table is unclear.  The limitations on material used are given in terms of Fb' and E, and Fb' requires the application of up to two adjustment factors (Cf and Cr) to the base Fb.  Values of Fb and E, as far as I know, are not provided in the IRC or at the point of sale, so there's no way for a contractor to verify whether the table applies. I would argue that most building inspectors would not be able to find the values and properly apply the adjustments, even if they happened to have a copy of the NDS in their truck when the go to the job site.  Allowing only 6' of tributary load from a floor or roof is practically a non-loadbearing condition; why not just say "non-loadbearing" in the title? How many roof spans have you seen in the past year that are less than 10' (1/2span+1' overhang)?

My point is that the IRC should give tables and values that are appropriate for residential construction. To argue that the 5th percentile residential contractor will be able to sort out *and calculate* engineering data to determine the appropriate grades and species, and for the 5th percentile building official to be able to verify this information in the 30 minutes they have to inspect 4000SF of residential framing is asking way too much. This table is misleading at best, and dangerous at worst.

There's no question that the builder, in this case, has it wrong. But after failing the inspection, he went back to the code and - even after reading it "carefully" - he still got it wrong.  And this is a builder with two full time, in-house designers with architectural experience building custom homes in the top 10-15% of the local market.  The designer was quite indignant about the failed inspection, too. Its his argument that they've built many walls very similar to this (it's from one of their popular "base" plans which they customize) and have never had a problem.  The builder has only been around for about 10-12 years, and we have not had anywhere near a code wind, snow, or seismic event in that time (15y, 40y, and 130y respectively, I believe). They really don't understand that the load criteria is 50 years. They can't be sued after 10 years (statue of repose in VA, if I remember correctly), and if they didn't have to design beyond that point they wouldn't.

BTW did you know that if you use the tables in the IRC to design your rafters, and you put a collar tie at the upper third of the rafter, all of the spans in the IRC tables must be multiplied by a factor of 0.5? Its right there in the code, but I have yet to find a framer who has known that. These guys (and gals) are not math or code wizards. Lets not pretend that they are.



Scott Maxwell wrote:
Jordan,

I am confused.

Why exactly are you ranting at the IRC/IRC folks?

Is there anything technically wrong with what is in the table (when
accounting for the limitations/footnotes) when compared with an
"engineered design"?  In other words, if you calc stuff out for
situation that meets the limitations that you listed (i.e. snow load of
25 psf or less, trib width of less than 6 ft, Fb less than 1310 psi [is
that supposed to be Fb, Fb' or Fb*?], etc), would 2x6s work for a 18
foot tall wall?

Is the table unclear?  And by this I mean that if someone reads ALL the
information in the table it reasonable to understand that they might come
to a different conclusion than someone else.  I don't consider something
unclear just because the limitations are not listed in big (24 point),
bold letter but rather in footnotes that might be small.

I guess that if I am understanding correctly, you issue is not that the
information in the table is wrong (at least that is not what you seem to
say), but rather than because is has such a limited practical use, it
should not be in the code cause in this situation it made your life
difficult because someone did not read all of it.  So because of that, you
want it gone from the code.

Let's look at if from the other side.  Let's say we get ride of it from
the code.  Now, say there is a project somewhere that is being designed
per the IRC (i.e. prescriptively) that DOES fit that criteria.  In this
situation, 2x6s would now NOT be permitted (this all assumes that the
information is technically accurate).  So, on this project the client
would have to pay for 2x8s when technically 2x6s would work.  Now, we have
just reinforced that good old mantra that contractors love to change:
"Boy, is this way over engineered" (whether true or not).

I guess in the end I am wondering if you want the IRC (and other model
codes) to put a big note in the front of the code in big, bold letters
that reads: "Idiots, this includes people who are otherwise smart that
just don't read all the requirements, shall not use this documents".  To
be frank, this would also put a lot of engineers out of business as there
are lot of people, engineers included, who don't always read all the
"instructions".

So, I think you should be ranting at the foreman and designer.  After all,
the foreman appear to have been WRONG.  The IRC apparently does _NOT_
permit the use of 2x6s in your particular case (you mentioned 30 psf snow
load as well as trib width of 17 feet, I believe)...altough s/he is
correct that there are situations (albit very limited) when the IRC does
permit it.

Now, if I am wrong in that you believe the information in the table (even
when factoring in the limitations) is inaccurate when compared with
a comparable engineered design, then maybe the code needs to be changed.

Regards,

Scott
Adrian, MI


On Sat, 27 Aug 2005, Jordan Truesdell, PE wrote:

  
I had a meeting today in which  a residential foreman  argued that an
18' tall stud wall needed only be 2x6 studs on 16" centers according to
the table in the IRC. Well, I'd had this discussion with the designer a
week previous, where I showed him the output of an analysis that
indicated a 2x8 was necessary for this framing. The designer also
thought that 2x6 was okay, but didn't reference the IRC.

I looked it up tonight, just to see if the IRC was sharpening their
pencils a little much again, and found that the table, R602.3.1 to be
exact, does indeed show 2x6s for an 18' stud wall.  Curious, I read the
notes at the bottom and found the caveats listed to be almost
ridiculously restrictive, such that the designs are practically unusable
in a good bit of the country. Snow load must be less than 25psf - I
assume this is ground snow load, since there is no formula in the IRC to
convert to a roof snow load (locally we're 30).  Fb must be > 1310psi.
Well, even with a repetitive and size factor added, No.1 SPF - the most
common stud material around me (and on the east cost, I suspect) comes
out to 1308psi. Add in an E of 1.6x10^6, and the list of lumber species
and grades gets mighty short - #2 S. Pine and Doug.Fir are the only
framing species that fit the criteria.  Here in the east there are no DF
studs, and S.Pine smaller than 2x8 is found only in treated lumber and
trusses without a special order.  Tributary width for vertical loads
must be less than 6'.  Unless you've got lots of interior bearing, or
are only checking gable end walls, that makes for pretty small rooms and
no trussed roofs. And if it weren't obvious from the material
properties, Stud grade studs are not permitted (nor are utility,
construction, or No 3).

It just makes me shake my head.  Even if this particular job didn't have
a 17' horizontal span without a sole-to-top plate stud, and if they
hadn't framed a two story wall by stacking two 9' tall walls, and if
they hadn't used 2x4s, and even if it wasn't backing up brick veneer,
there are still 5 different criteria that disallow the use of this
chart.  And yet, the chart says 2x6 on 16" centers works, so they figure
it's correct, and I'm full of *expletive* for telling them they should
have used at least 2x8s. Worst, with their openings they'll need 3/4T of
steel to fix the problem without packing the wall to thicker than
3.5".   It think the #2 S. Pine and Doug.Fir needs to be in the title,
and they may as well just say "non-bearing-walls only", with a little
note at the bottom for an exception for walls with tributary widths less
than 6' and snow loads of 25psf or less.

I know that moaning and complaining here won't get the code changed (and
I'm honestly not sure whose ear to chew on to try and get it changed),
but are the rest of you doing small jobs like this running into similar
problems? If so, how do we fix the code language so that mistakes are
less likely?  Even the best inspectors can't remember all the notes and
subnotes across all of the disciplines in a couple thousand pages of code.

Jordan

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