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RE: Wood interior wall studs

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Andy -
I think you are giving the load duration factor too much credit.  For normal
height walls (8' to 12'), a Cd of 1.6 only increases F'c by 5% to 20% over
that of Cd=1.0.  The increase in the allowable stress is a function of le/d,
so it is less for taller walls.

I like the discussion you've started though, I usually use Cd of 1.3 with
the 5 psf lateral load and use the same load combinations you listed in your
original email.

Josh Comfort, P.E.
Golden, Graper & Burton, Inc.
1500 W. Fourth Ave., Suite 509
Spokane, WA 99204
(509)624-3224 (509)624-3225 Fax



-----Original Message-----
From: Andy Heigley [mailto:aheigley(--nospam--at)jgaeng.com]
Sent: Friday, July 13, 2007 2:14 PM
To: seaint(--nospam--at)seaint.org
Subject: RE: Wood interior wall studs


True.

But talking about impact loading, that has a duration factor of Cd = 2.0.

I like this discussion, and how it applies to the 2x4 studs. Let's face it,
if it is 2x6 studs it doesn't really matter unless you're dealing with the
same slenderness ratios.

By the way, not that it helps much but my stud height is just less than 10'
at 9'-10".

I understand the reasoning for the 5 psf lateral loading.  It makes sense to
have a provision for impact and internal pressure loads.  It would be
wonderful if we could come to a consensus on how it is applied in wood
design and the application of the duration factor.

Let's say you have a 4-story building... it is primarily supporting axial
load.  Let's say you are at 0.99 interaction ratio for axial loads only.
You then apply a minor out of plane bending load.  If you then increase your
allowable stresses by 60% then your interaction is going to drop lower
significantly, when generally combined bending and axial result in worse
interaction ratio.

I'm starting to think that what might be more reasonable is if you use the
load Combination of DL + .75L + .75(out of plane live loading), with a Cd of
1.0.  Or Design for 100% (DL + LL + out of plane) and a Cd of 1.6

What are people's thoughts on this?

And slam dancing = nominal impact loads.

-----Original Message-----
From: Andy Heigley [mailto:aheigley(--nospam--at)jgaeng.com]
Sent: Friday, July 13, 2007 12:46 PM
To: seaint(--nospam--at)seaint.org
Subject: RE: Wood interior wall studs

Actually the 5 psf is due to HVAC.

Here's and excerpt from the IBC Handbook:

        "According to BOCA/NBC, the requirements of this section, reproduced
from the 1999 BOCA/NBC section 1606.9, are intended to provide sufficient
strength and durability of the wall framing and wall finish, so that a
minimum level of resistance would be available to nominal impact loads that
commonly occur in the use of a facility and to HVAC pressurization."



Andrew Heigley, PE



-----Original Message-----
From: Haan, Scott M POA [mailto:Scott.M.Haan(--nospam--at)poa02.usace.army.mil]
Sent: Friday, July 13, 2007 4:25 PM
To: seaint(--nospam--at)seaint.org
Subject: RE: Wood interior wall studs

I think the 5 psf is intended for people slam dancing on the walls - not for
pressure from HVAC.  The IBC has the 5 psf in the Live Load section. The 97
UBC says the 5 psf is "L" and does not need to be applied with wind or
seismic.

I would use the 1.15 Cr and 1.0 for Cd for the 5 psf.

-----Original Message-----
From: Joseph R. Grill [mailto:jrgrill(--nospam--at)cableone.net]
Sent: Friday, July 13, 2007 12:16 PM
To: seaint(--nospam--at)seaint.org
Subject: Re: Wood interior wall studs

There is also repetitive member increases in bending.  15% usually, but the
code allows 50% if a wind load.
Joe Grill

        ----- Original Message -----
        From: Jordan Truesdell, PE <mailto:seaint1(--nospam--at)truesdellengineering.com>

        To: seaint(--nospam--at)seaint.org
        Sent: Friday, July 13, 2007 1:07 PM
        Subject: Re: Wood interior wall studs

        I do a lot of wood, and here's my take:

        The 5psf should be at a Cd of 1.6.  Why? Unless I have an
intentionally pressurized room (which is not the case here), there is no
possible way that common construction techniques can _maintain_ that kind of
pressure differential over an extended period of time, and if you have a
constant load (say, a commercial kitchen with doors that seal
tight...right...) making the pressure differential then the HVAC engineer
should already be providing makeup air in that area.  The building code can
call it a "live" load all it wants, but the actual duration of load will
never reach 10 years in the 50 year design life of the building.

        Roof live load should be given a Cd of 1.25; snow is 1.15.

        I don't recommend a loadbearing wall of 10' in height be made of
2x4s. Why? Because the walls will not be sheathed before the floor joists
are
set into place. The code limits the L/d ratio to 75 for construction, and a
10' 2x4 has an L/d of 80. The contractor cannot build the wall unless he
plans on bracing every stud without violating the code.

        And, for the record, I agree with Don that 2x4s 10 feet long look a
lot like spaghetti. And as a bearing wall they give me the willies. My
answer
to such a request is normally to do it as designed, or provide another PE to
seal off on the change with full calculations, and provide me with a full
release of liability should anything happen.  I've never been taken up on
the
offer.


        Jordan


        Andy Heigley wrote:

                Everyone:

                <!--[if !supportEmptyParas]--> <!--[endif]-->

                Thanks for your responses...

                <!--[if !supportEmptyParas]--> <!--[endif]-->

                Here are my responses to some of your questions back:

                <!--[if !supportEmptyParas]--> <!--[endif]-->

                *       I would design for LL and LLr if the wall were
supporting both the floor and roof loads.

                <!--[if !supportEmptyParas]--> <!--[endif]-->

                *       I am designing to ASD.

                <!--[if !supportEmptyParas]--> <!--[endif]-->

                *       I guess I'm a little leary of using the Cd of 1.6
for
this reason.  The duration factor is applied to both bending and axial
capacities.  Applying 60% more to the allowable axial stress makes a huge
difference.  And if you have a 4 story building, for example, you are going
to be approaching the capacity of the stud just due to DL and LL...  you
then
add a "little bit" of short term horizontal loading to the stud and increase
the capacities by 60% seems non-conservative.

                <!--[if !supportEmptyParas]--> <!--[endif]-->

                *       Scott:  I haven't found the rated wall design
reduction factors you've mentioned.  Can you tell me the code section that
is
in?

                <!--[if !supportEmptyParas]--> <!--[endif]-->

                *       10'-0" high 2x4's... exactly why I initially called
for 2x6 stud walls, but the GC is flipping out about it... I get the old,
"I've been doing this for 30 years, and never had to do this before"...

                <!--[if !supportEmptyParas]--> <!--[endif]-->

                <!--[if !supportEmptyParas]--> <!--[endif]-->

                Andrew Heigley, PE

                <!--[if !supportEmptyParas]--> <!--[endif]-->

                <!--[if !supportEmptyParas]--> <!--[endif]-->

                <!--[if !supportEmptyParas]--> <!--[endif]-->



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