Take a look at the American Wood Council website for good online technical information and links to more specific sites: http://www.awc.org/technical/index.html

Mike - to properly design a load bearing stud wall (exterior) you need the following:

1.  Stud wall length
2.  Nature of sheathing on both sides (braced condition)
3.  Location of wall to determine necessary wind load speed.
4.  Wind Exposure category.
5.  Applicable building code
6.  Stud spacing
7.  Stud wood species and grade - to get the allowable axial and bending stresses.
8.  Axial load on the stud (dead, floor live, roof live, snow, etc)
9.  Proper combinations of the loads
10.  A check of the stud based on combined bending and axial loads.

With the IBC Residential code (and with most model building codes) - you can look up studs in their tables that condense the above items, but you do need to know what wind zone you are in and the species/grade to get the stud checked properly.

JAE, I appreciate your reply.  Some of the information that you have provided is enlightening, however, the essence of our discussion was more an exercise in academics.  The intent was not to design a stud wall, but to quantify the difference between a similarly framed stud wall with 2x4 vs. 2x6.  Also, the question was posed about the specific compressive strength of a given piece of lumber.  I recall seeing a video of a destructive compression test on a 2x4 stud.  I was hoping to quantify a typical failure pressure for such a piece of lumber.  
I see now that it might not be a simple mater of putting a figure on it, as many factors contribute to strength of a given wall or piece of lumber.

The AWC site proved to offer little relevent information, however, it had other interesting information.

Thanks.

Mike

Disclaimer..... please don't use this for design it is only a guidline and depends on many things..... but for academic purposes

1 2X6 doug fir stud in a 9' interior wall with normal floor loading is worth approx. 7400 lb

1 2X4 doug fir stud in a 9' interior wall with normal floor loading is worth approx. 2800 lb

Assuming you are referring to exterior residential stud wall application.  There are three areas to consider in the wall performance: 1 Structural response 2 Thermal performance 3 Moisture control.

The studs are sized interactively sized using: Bending Load, Critical column buckling, and Interaction equation to check member capacities for combined loading.  (Ignoring seismic loading and response).  Typically both 2x4 and 2x6 can be used in residential wall construction up to 10' sill height.  The 2x6 is used in sill height up to 12-13' depending on loading.

Comparing the whole-wall thermal performance of different systems to each other wood-frame construction you must consider the thermal short of the studs and corners.  The most commonly used calculation procedures for conventional residential wood-frame construction tend to overestimate the actual field thermal performance.  A typical 2x4 wood stud wall 16-in OC yields a 9.6 R-value and 2x6 wood stud wall 24-in oc yield 13.7 R-value.  

I prefer the 2x4 wall construction, as the cost is lower and you don't loose the extra 2" perimeter to the stud wall.  I add 3/4" or 1" insulating foam sheathing to the exterior to increase the insulation decrease air infiltration rate and provide a water drainage plane.  The 2x6's are dimensionally more stable but if the 2x4's are good quality they should be ok.

The Fir stud grade values are:
Grade       Fb  Ft   Fv    Fcp    Fc    E
Stud, Fir  650 425  90    520    825    1100000

(I use #2 grad studs in my projects)

Boo1, you might want to check your calculations regarding the allowable height with a 2x4.  Based on my experience, exterior wall constructed with 2x4 DFL "Stud" grade spaced at 16" on center are actually overstessed by a few percent on a normal 8 feet wall.  This is using combined loading, vertical floor loads + lateral (components and cladding) wind loads.

In my opinion, any wall over 8 feet should be 2x6.

Florida's current code bodies allow it see:
http://www.sbcci.org/Florida_Building_Code/changed/ch-23/sec-2308.pdf
SSTD-1099 recommends it.

I don't use stud grade lumber.  Primarly becouse the diminsionally unstabilty.  

I would love to learn more. Can you please send me a reply to BSolana@yahoo.com and i will send you my spread sheet.  

The ADS manual for Engineered Wood Construction Structural Lumber Supplement Table 5.2a and 5.2b gives axial compression allowables for 2x4 and 2x6 lumber of various grades and various lengths.  The table assumes that the studs are braced along the 2" nominal width and the limiting buckling is the 4" and 6" nominal width.  Table 5.2a is for duration 1.0 and 5.2b is for duration 1.15 (snow).  For No. 2 Spruce-Pine-Fir the allowable load in pounds are

Length   2x4   2.6

8 ft    3000  8710
10 ft   2010  6700

As far as the framing controversy on 2x4 vs 2x6 studs the 2000 IRC Table R602.3.1 shows the prescriptive allowable length of wood stud walls based on the number of floors supported.  In essence it says that a 10 ft tall 2x4 stud wall 16" o.c. can support a floor and a roof for their "model" structure.  2x6 walls 24" o.c. can be prescriptive up to 14 ft without engineering.

Rockengineer,

Your examples from the ADS Manual seem to relate only to axial loads.  

CSEllc,

Your comments talk about combined axial and bending (wind loading).  

Could you both clarify this?  It seems that two different loading schemes are being discussed.

Exterior wall studs should be designed to resist both vertical (Live loads, dead loads, snow loads, etc....) and horizontal (lateral) loads, such as wind.  In theory, there is no difference in the design of exterior wood wall studs or light gage metal studs for example. (Let's neglect the difference in design properties or materials, etc....) You still need to design the studs for all the loads.  

Based on this, if you analyze a standard exterior 2x4, 8 feet (92-5/8" precut studs with double top plate and sole plate) high wall DF-L stud, spaced at 16" on center, in a "standard two story center hall colonial type residence, you will find that the exterior wall studs are overstressed about 5%.  This assumes 40 psf LL, 15 psf, DL, and 30 psf snow and 80 mph, exposure B  "components and cladding" wind pressures.

RockEngineer,

about 15 years ago, I did a loading chart of built up 2x4 and 2x6 posts as well as solid 4x4, 4x6, and 6x6 posts, ranging in height from 8 feet to about 22 feet.  Based on my charts (or what is left of them), the allowable axial loads you listed for SPF are about the same I determined using DF-L.

RockEngineer and tfl have boot posted similar figures which relate to the discussion that I was having with a colleague.  This is all great information.  I'm quite amazed at the relative increase in load bearing that a 2x6 has over a 2x4.  With a 50% increase in width (and thus cross section area) there is almost 300% increase in load bearing capacity.  This is a suppressing figure, but I'm guessing that it's based on the increase in lateral stability of the member.
Thanks for all the info.

Mike