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

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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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