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

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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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