Stud Wall Framing Analysis 2

[boo1]

A thread was deleted on 2x stud wall construction. To continue the discussion the following is provided.

Analyze studs for combined axial and bending loading.

Methodology
1.Determine lumber design values, Fb, F’b, Fc, F’c, E, E’
2.Determine stress from load cases.
3.Check critical buckling.
4.Use interaction equation to proportion member.
5.Check span for bending using components and cladding pressure.

http://web.usna.navy.mil/~greg/en442/files/NDS Tables.xls

Axial Load, La
*Load case: W + D + Lr will govern,
La=W + D + Lr

Bending Load, Fb
*Load case: w (MWFRS)
Fb=(w*L**2)/(8*s)

Critical column buckling stress, FcE1
FcE1=(KcE*E’)/((Le/d)**2)

Interaction equation to check member capacities for combined loading.
((fc/Fc')**2)+(fb/(Fb'(1-(fc/FcE)))lessthanequal 1.0

Component and cladding pressures
zone 4
zone 5

Using this methodology for residential homes I thought #2 2x4's (16"oc) typically acceptable to 10' in length and the 2x6's (16"oc) limit was ~12'.

Comments?

 

[CSEllc]

The problem I see with you assumption is that you are using #2 lumber. Typically, the best you will find for 2x4's, in my area anyway, is STUD grade.

Based on this the Fb for a DFL or a SPF stud is 675 psi.
E for DFL Stud is 1,400,000 psi or 1,200,000 psi for SPF.

 

[boo1]

Agree that stud grade is most common. I specify and use #2 or #1 for the increase in strength and diminsional stability. Homedepot ad local lumer yards carries the higher grade lumber in both white woods and SYP.

Using Fb SPF stud 675 psi
Adjusted value
Adjustment Factors
•Cd=1.6 (Fb, Fc)
load duration
•Cr = 1.4 (2x6) & 1.5 (2x4), (Fb,)
repetitive member
IBC, Table 2306.2.1
•Cf = 1.5 (Fb)& 1.15 (Fc)
size factorUse with Hem Fir
Factors included in values for SYP
•CL = 1.0
lateral stability, assume wall sheathed
•F’b = 675 psi (1.6)(1.5)(1.5) = 2,430 psi allowable

 

[SperlingPE]

10'-0" span for 2x4's and 12'-0" for 2x6's is my general rule of thumb (I specifiy SPF no. 2 or btr).
Does anyone check the deflection of a stud? Limit deflection to L/600 for backing up masonry?

 

[AlohaBob]

Yes No

I don't think you need to be so rigorous for deflection out of plane. I think the L/600 prescription is for in-plane deflection design for lintels. Use L/240 usually and provide a joint at the base of the wall.

For GWB inside I use L/240.

 

[CSEllc]

My deflection criteria varies depending on what is there.

For example, solid stud wall L/240 is ok. If the wall is two story hign and full of doors, windows, etc. I am a little more stringent on my deflection criteria. I limit my delection at that point to about 0.75" max.

 

[SperlingPE]

Is the use of a larger deflection (L/240) due to the construction of the brick portion of wall, the nature of the load (short term wind), or both?

 

[AlohaBob]

The L/240 corresponds for current code and Live Load = Wind and brittle wall finish. See IBC Table 1604.3

The joints developing a crack horizontally isn't such a serious consideration. I would suppose since gravity would tend to keep it minimal in size. Not so though for a vertical or diagonal crack.

 

[ricklambert]

The FL building code allows a stress increase for stud walls with plywood sheathing one the exterior surface and gyp board on the inside. For 2x4 studs the factor is 1.5, for 2x6 its 1.4 and for 2x8 its 1.3

This code recognizes that the cladding on the face of the studs provides some composite action. Check this out. I would like to know if anybody has seen any such provision in any other codes.

RL

 

[AlohaBob]

The 2000 IBC specifically allows duration of load stress increase factor for wood. For wind this is 1.6. That code specifically acknowledges the design criteria of the NDS and incorporates it in its entirety. A good wood design textbook is still indispensible for basic design principles. There are factors that increase the carrying capacity of the member. Most importantly, the unbraced length. The sheathing prevents the stud from buckling out of plane in the weak axis. So its capacity sheathed is much greater. There's the capacity of wall studs to share load when a concentrated load is applied. Hence, a repetitive member stress increase factor applies too.

So you are on the right track to use the allowable increase factors. Without them, it is difficult to use wood in typical non-commercial construction.

 

[boo1]

Rick, so the Florida load combinations is in lieu of the 1.15 repititive load is 2313.3

http://www.sbcci.org/Florida_Building_Code/changed/ch-23/sec-2313.pdf

http://www.sbcci.org/Florida_Building_Code/changed/ch-16/sec-1609.pdf

 

[CSEllc]

Check out this Tech Note published by the WWPA.

http://www.wwpa.org/pdf/TN1.pdf

According to this publication, assuming 2x4 DF-L STUD grade at 16"oc, with an 8 ft. wall height, and a 20 psf wind load. Than maximum allowable axial load on the wall is approximately 1100 pounds.

 

[boo1]

see link at:

http://www.cwc.ca/pdfs/Tall_walls-e.pdf

 

[pdig]

2004 FBC allows a .7 factor on C&C wind loads. Note: you cant apply a 1.6 duration factor to your E values in calculating deflection. No other factors I can find apply to deflection calculations--ie. stress factors.
The deflection criteria of L/240 for a 9' 2x4 SPF #2 wall stud is not going to work unless some other factors are allowed. Any insight?

 

[boo1]

Do use FBC or ASCE-7 code for your designs?

 

[SacreBleu]

The usual deflection ratios are far too conservative for wood stud walls. We can easily go taller than 12' with 2x6 walls. Since the IBC is designed to confuse with endless factors, and to avoid lawsuits by halving allowables of EVERYTHING, it is better to get a grip on reality by doing the analysis by say, UBC, first.
Typically, we specify #3, stud, or construction grade for 2x4 up to 8'-1", and 2x6 up to 10'-1". Above that, we specify #2 grade because our usual stud wood species is Hem-Fir, which has relatively low allowables.

 

[boo1]

Sacreblue, the #3 studs are such poor quality in our area you cant frame a straight wall. We are in a high wind area 130-140 mph. With the combined loading of bending and the axial roof loading the stud grades often dont work. Multi story the first floor stud size also requires analysis.

ricklambert, your post above indicated including the drywall interior in the wall composite action. We never include the drywall in our designs in walls or ceilings. I have never seen the required attachment schedule followed for interior surfaces.

 

[SacreBleu]

boo1,
I hear you. We only have winds at 90 mph (3-sec gust, Exp. C).
The #3 studs are fairly straight, only because they are Hem-Fir. The framers here say the Douglas-Fir Larch #3 studs are not straight at all.
We never calculate composite action, but I suppose I am in "unscientific" territory by stating that deflection ratios as recommended are very conservative, and frequently makes it impossible to come up with reasonable designs (If a developer usually builds 3-story apartments with 2x4, he would go ballistic if I require 2x6). I suppose the interior wall board helps reduce deflection a smidge, and gypsum wallboard is not considered by my local peers to be brittle finish (as opposed to lath and plaster).

 

[boo1]

A three story loading with 2x4s on the first floor scares me. We use 2x6s on the first floor of most two story homes and walls over 10 feet height.

I never allow the developer to control the design, remember who is liable!

 

[SacreBleu]

I don't allow the developer to "control" the design. The developer has enough experience to know that 2 x 4 studs work in my area. If I used 2 x 6 instead of 2 x 4 at either 16, 12, or 8" on center, that would be not intelligent. After all, we have the software to design these walls for the various loads and height combinations.