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Re: FW: Re: FW: Re: 2x4 stud walls in High Wind areas

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Scott,

Note that i indicated previously that the "SBCCI" Wind Loads committee was
the first to endorse this approach.

I think we'll have to agree to disagree on this one.

Buddy

********

From: Scott Maxwell <smaxwell(--nospam--at)engin.umich.edu>
To: "SEAINT Listserver (E-mail)" <seaint(--nospam--at)seaint.org>
Subject: Re: FW: Re: FW: Re: 2x4 stud walls in High Wind areas

Buddy,
I don't mean to be harsh, but I don't think that the approval of the ANSI
committee that develops the WFCM would do too much without a similar
approval of the ANSI committee that develops the loading criteria used in
the current model building codes (in otherwords, ASCE 7) or of the model
building code agency/agencies themselves (ICC, UBC, BOCA, NFPA, etc).
It likely carries as much force as if AISC put in their steel
specification (another ANSI based document) that the likelihood is that
wind would not occur at the same time as full roof snow load or if ACI put
some such in ACI 318. The point is that loading and load combinations (as
I understand it) are the providence of ASCE 7 and to a certain degree the
model building code groups (although it is becoming solely the providence
of ASCE 7 with the NFPA 5000 and newer editions of the IBC).
Thus, unless you are aware of some exception from within the model
building code(s), it would seem that you maybe giving conflicting advice
to practicing engineers.
Now, where you may be "OK" is if there is some exception in the IRC
(International Residential Code) that may not be in the IBC. This I don't
know since I am not too familiar with the IRC at this time.
HTH,
Scott
Ypsilanti, MI

On Tue, 17 Dec 2002, AWC Info wrote:
> Scott, we've had this same debate in various forums. In a nutshell, this
was
> a decision made by the SBCCI Wind Loads Committee in the development of
> SSTD-10, and the logic was carried over into AF&PA's Wood Frame
Construction
> Manual (WFCM) for One- and Two-Family Dwellings with approval of the ANSI
> committee that develops the WFCM.
>
> Buddy
>
> ************
>
> From: Scott Maxwell <smaxwell(--nospam--at)engin.umich.edu>
> To: "SEAINT Listserver (E-mail)" <seaint(--nospam--at)seaint.org>
> Subject: Re: FW: Re: 2x4 stud walls in High Wind areas
>
> Buddy,
>
> Where you lost me was the following:
> "For high wind design, the assumed roof loads will not be present (which
> modifies case 6):"
>
> How do you justify this modification other than "it is highly unlikely
> that maximum wind will occur at the same time as maximum roof loads (i.e.
> snow or rain ponding"?
>
> My point is that while I won't personally disagree too much the above
> rationale in most locations (more on that in a moment), I am _NOT_ aware
> of any exception in the codes (IBC, ASCE 7, BOCA, UBC, etc) that allows an
> engineer's judgement to override the load combinations listed in the
> codes. In otherwords, I don't know of an exception to D+.75W+.75Lr that
> says "unless the engineer thinks that it is unlikely that the wind load
> and roof live load will occur at the same time". As unlogical as it may
> seem, the code _IS_ saying that you must apply the wind load and snow roof
> load at the same time.
>
> Thus, the worst case (in many cases) would be for a stud from a non-shear
> wall bearing wall, if such an animal really exists, (i.e. not part of
> lateral system) when the stud has gravity load applied to the tune of
> D+.75Lr and has the added effect of .75W from C&C loading.
>
> An yes, I am aware of what the C&C loading is, yet feel that many others
> do often confuse it with the MWFRS loading. I have always understood the
> difference to be that MWFRS loading is more of a system loading for the
> lateral system of the building. This means that it is not really applied
> to individual members directly, but rather to the overall lateral system
> that then indirectly loads individual members. So, the MWFRS loading
> would not be used as a direct loading in bending on a stud or roof joist.
>
> The C&C loading on the other hand _IS_ meant as a member loading, which is
> why the tributary area is part of it. The smaller the trib area, the
> higher the C&C loadings because this means that the probability that the
> member with a smaller trib area will actually see the peak velocity
> pressure (which will vary over the surface of the structure...in
> otherwords, even though we think in terms of one nice uniform pressure,
> the truth is that the overall surface of the structure "sees" a fairly
> wide variation of local pressures that we then "average" into a uniform
> pressure for the entire surface...then the smaller the area that is looked
> at, the higher the average pressure becomes because the variation becomes
> more limited as the area considered in reduced).
>
> Now as to the rationale that you state in general...I agree that in many
> areas of the country (and world for that matter) the likelihood that the
> full wind load will occur at the same time as the full roof live load is
> rather smaller. But, there are some locations where this is not so true.
> There are many places where they have snow on their roofs about 50% of the
> year, which means that the likelihood of the maximum wind hitting while
> loaded with maximum (or darn near it) snow increases dramatically. Thus,
> even if you ignored the fact that none of the codes would seem to allow
> such a deviation from the specified load combinations, it would still not
> seem logical in many situations to do so.
>
> HTH,
> Scott
> Ypsilanti, MI
>
> On Mon, 16 Dec 2002, AWC Info wrote:
> > The C&C wind load is a instantaneous peak pressure on 1 or 2 studs
(small
> > effective wind area). Looking at ASCE 7-02:
> >
> > 1. D + F
> > 2. D + H + F + L + T
> > 3. D + H + F + (Lr or S or R)
> > 4. D + H + F + 0.75(L+T) + 0.75(Lr or S or R)
> > 5. D + H + F + (W or 0.7E)
> > 6. D + H + F + 0.75(W or 0.7E) + 0.75L + 0.75(Lr or S or R)
> > 7. 0.6D + W + H
> > 8. 0.6D + 0.7E + H
> >
> > For discussion, ignore H (soil pressures), F (flooding), R (rain), T
> > (temperature effects), and E (earthquakes).
> >
> >
> > 1. D
> > 2. D + L
> > 3. D + (Lr or S)
> > 4. D + 0.75L + 0.75(Lr or S)
> >
> > 5. D + W
> > 6. D + 0.75W + 0.75L + 0.75(Lr or S)
> > 7. 0.6D + W
> >
> > 8. 0.6D
> >
> > For stud design, it is clear that Cases 1 & 8 are non-limiting.
> > For high wind design, the assumed roof loads will not be present (which
> > modifies case 6):
> >
> >
> > 2. D + L
> >
> >
> > 3. D + (Lr or S)
> > 4. D + 0.75L + 0.75(Lr or S)
> >
> > 5. D + W
> > 6. D + 0.75W + 0.75L
> > 7. 0.6D + W
> >
> > In the case of a stud supporting a roof:
> > 2. D
> > 3. D + (Lr or S)
> > 4. D + 0.75(Lr or S)
> > 5 D + W
> > 6. D + 0.75W
> > 7. 0.6D + W
> >
> > Case 2, 4 and 6 are non-limiting.
> >
> > 3. D + (Lr or S)
> > 5 D + W
> > 7. 0.6D + W
> >
> > Case 3 limits stud design for gravity.
> > Case 5 limits stud design for wind downward pressure. C&C for axial or
> > bending, MWFRS for axial and bending.
> > Case 7 limits stud design for wind uplift. C&C for axial or bending,
MWFRS
> > for axial and bending.
> >
> > In the case of a stud supporting a floor and roof, it's a bit more
> > complicated, however, it was assumed that case 2 or 4 would control, not
> > case 6. If case 6 controls, then the influence of W is greater than Lr
or
> > S. For individual cases, it should be checked, but using the same logic
as
> > above... C&C for axial or bending, MWFRS for axial and bending. When
> > considering whether to use C&C or MWFRS with gravity loads, one should
> > consider the influence area of the wind and the gravity loads (i.e. a
C&C
> > wall load is on a small area and will generally be distributed over
> several
> > studs in a wall, a design floor live load is on a small area and will
> > generally be distributed over several studs in a wall.) Our feeling was
> > that it is highly unlikely to have a maximum gravity load and a maximum
> stud
> > load occur at the same time. In addition, our current design procedures
> are
> > fairly conservative for studs... they are treated as braced columns. To
> > really conducted this analysis properly, one needs to know the axial and
> > bending capacities of the sheathed wall assembly. The repetitive member
> > increase, C_r=1.5, for bending addresses only part of the problem. MOE
> > values used in the column equations could be increased by 1.8 or more,
but
> > that hasn't been approved yet.
> >
> > Hope this helps.
> >
> > Buddy Showalter, P.E.
> > AF&PA/AWC
> >
> >
> >
> >
> > ***********
> >
> > From: Scott Maxwell <smaxwell(--nospam--at)engin.umich.edu>
> >
> > To: "SEAINT Listserver (E-mail)" <seaint(--nospam--at)seaint.org>
> >
> > Subject: Re: 2x4 stud walls in High Wind areas
> >
> > Buddy:
> >
> > I am going to play dumb for a moment (something that I have some natural
> >
> > talent in <grin>).
> >
> > How can you design the studs for bending from only Components and
Cladding
> >
> > wind load and not include axial loads from gravity loads? I would think
> >
> > that axial loads from gravity loads should be checked in combination
with
> >
> > either MFWRS winds loads _OR_ C&C loads. The only difference would be
> >
> > that with MFWRS winds loads, it is possible (even highly likely) that
> >
> > there will be some additional axial loading (either tension or
compression
> >
> > depending on the roof configuration) due to the MFWRS loads (i.e.
suction
> >
> > on the roof or downward thrust on the roof), where as for C&C loads you
> >
> > are only considering the wind loads on that one member (which means that
> >
> > the C&C wind loads for bending would like be significantly large due to
> >
> > the smaller tributary area).
> >
> > I will look at that article that you mentioned because it might explain
> >
> > it, but I am curious as to an explanation for what you stated.
> >
> > Thanks,
> >
> > Scott
> >
> > Ypsilanti, MI
>

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