Wood Wall Stud Design

[abusementpark]

I have a question with regards to wind loading on a wall stud.

When designing a wall stud for out of plane wind loading, I always assume a Component & Cladding wind load along with gravity loads (Dead & Roof Live) for the roof with the revelant load combinations (i.e. D+W, D+0.75(Lr+W)).

For some reason, people in my office have been neglecting considering a positive wind load on the roof in addition to the wind load on the wall, which would increase the axial load in the stud.

My question is, if I were to start considering wind load on the wall and on the roof in the design of the stud, would I use Components & Cladding loads for both pressures. It seems too conservative to take C+C loads to design an element that is receiving that load from two surfaces. C+C loads represent the maximum intensity that can be expected over a small area. It seems very unlikely that could happen on both a wall and a roof at the same time. This idea is based what I learned from a professor at my school that is on the ASCE7 wind loading committee. He seems to like the idea of the summarily dismissing anything that collects load from two surfaces as MWFRS.

 

[paddingtongreen]

Does the stud have a tributary area so small as to make it support the intensified loads?

I disagree with your professor, the pressure on the corner of the wall and the suction on the corner of the roof are both caused by the increased air velocity at that corner, however I'm not sure it goes to the stud and purlin.

How do you get a positive pressure on the roof?

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[Lion06]

pad-

If your roof pitch is less than 45 degrees it won't see positive pressure per any of the tables in ASCE 7, but ASCE 7 also requires a minimum 10psf wind load. This to means you have to consider positive wind on the roof no matter what.

 

[WillisV]

True for now, though I believe the 10psf minimum is going away in ASCE 7-10.

 

[JAE]

C&C wind loads, both roof and wall, should be considered in the design of a stud as they do occur simultaneously.

So you would have both bending and axial in the stud at the same time, due to wind.

 

[dhengr]

The special wind loading reqr’mts. for components and claddings are primarily intended to assure their proper attachment to the structure, due to higher than normal +/- pressures, at the particular areas which are susceptible to these special loading conditions, (bldg. corners, eaves, parapets, and the like; as you said, relatively small areas). Those higher loadings are generally not intended to be applied to the primary structural or structural members. I wouldn’t say never consider this special condition of loading, but remember, studs are a repetitive type member, not a member acting alone, and they act with the sheathing, etc. The codes actually used to allow a 33% allowable stress increase when combining wind loading with other loads because of its short duration. The believe that the new load factors include this thinking, but also pay attention to our better knowledge of the actual wind loading pressures on various part of the bldg. Of course, you must consider all of the loads which can act on the studs at any given time or set of conditions. That is, axial loads and bending loads together, but not the special cladding intensities.

 

[ash060]

For increases check Chapter 3 of the 2005 NDS Special Design Provisions for Wind and Seismic

Depending on your stud size you can get a very large increase in Fb for (Cr factor) wind. You have to meet special criteria, but it is pretty standard and it will probably always be met except for some unusual circumstances.

For example, 2x4 Cr can be increased from 1.15 to 1.5.

 

[tadeng]

Component and cladding minimum pressure is 10 psf acting in either direction (+/-) normal to the surface.

In ASCE 7-05 Chapter 6 Commentary they give examples of components ...fasteners, purlins, girts, studs... So, C&C wind loads would be correct loading for stud design.

 

[JAE]

ash060,
Can you point me to where Cr = 1.5 in the NDS? I'm not seeing it. Both sections 4.3.9 and Table 4A show only the 1.15.

Were you perhaps looking at C_F?

 

[tadeng]

JAE,
Maybe ash060 was referring to IBC 2306.2.1 where in lieu of Cr =1.15 you can use the increases given in the chart, provided you meet the associated criteria.

 

[ash060]

Special Wind & Seismic Provisions. It is the thin book that comes with the NDS 2005 Set.

It is Table 3.1.1.1 under section 3.1.1 "Wall Framing".

I think that there is a later version of the special provisions, but I do not have it

 

[boo1]

http://www.awc.org/pdf/2001WindSeismicSupp.pdf

page 11 0f 33 showes Table 3.1.1.1 Wall Stud Bending Stress Increase Factors:
2x4 1.5

 

[JAE]

Got it - thanks. I had a copy of the 05 Wind supplement all along.

 

[DaveAtkins]

I agree that both roof and wall wind load in this case should be C & C.

But I CANNOT believe a positive wind load (pushing down on the roof) will ever be greater than snow load or minimum roof live load.

Am I missing something?

DaveAtkins

 

[abusementpark]

Well, it can govern in the bending + axial interaction in combinations like

D+W

D+0.75Lr+0.75W