4 x 4 vs 6 x 6

[new land owner]

If I were to attach a 6 x 6 "foot" to the bottom of a 4 x 4 and then put in the foundation hole is there any real difference in the capacity to hold up a stucture?  4 x 4's are a much cheaper alternative.

[glenn kangiser]

Roughly - on the high side - 1000 psi per square inch of wood ----fiber stress. 

More than double on the 6x6. 

Here is some of John's free foundation info.

http://countryplans.com/foundation/index.html

[MountainDon]

A larger "foot" will spread the ground load out over a larger area, thereby reducing the PSI on the ground. However the load carrying capacity of the 4x4 portion of the leg will still be the load carrying capacity of the 4x4.

On the other hand you need to have more than a 4x4 or 6x6 contacting the ground under the post end in most types of soil. That's why you need a concrete footer in most cases. 16 x 16 x 8" thick is a good normal for a footing.

Does that answer your question?

[Don_P]

In this case the controlling factor is probably side grain compression of the girder the post is supporting, the top of the post crushing into the side of the girder. Usually limited to around 350 psi. The top of a 4x4 supporting a double girder is 10.5 square inches, multiply that by 350 and it'll get into crushing trouble if attempting to support more than 3675 lbs. A 6x6 in the same scenario would be good up to 5775 lbs. If a wood post is limited to a height of no more than 12 times its narrowest width then crushing is about the only concern. That's the easy way to engineer columns.

A 4x4 is really a mailbox or railing post and even then if it is notched the way many do it is often inadequate there. We had a death in my state a few years ago when a notched 4x4 railing failed. 6x6's and larger are safer support posts.

Don is right about needing an adequate footprint on the soil below to avoid sinking into it.

Off topic alert  Glenn the ~1,000 psi Fb number is used mostly in bending. The full name of that value is "extreme fiberstress in bending" The extreme fiber is the outermost fiber. In the case of a beam the extreme fiber in question is the fibers at the very bottom of the beam. As the beam bends those fibers want to tear. That is where the allowable value you quoted is used as a safe limiting factor in calculating the beam dimensions. It does come into play with taller posts.  This walks you through sizing a simple column to get an idea;
http://www.windyhilllogworks.com/Calcs/Simple_column.htm

I've set it up with a simple example but you can back out and change the inputs. Click "show result" and scroll down the the line that shows the loss of strength due to slenderness. Then go back up top and change the 6x6 to a 3.5x3.5 hit "show result" and look again at the slenderness strength loss. change the length of the post and the loads and keep playing with it noticing what happens. This is for a gravity load only. When the wind blows and enters a side load it gets much worse in a hurry. Your eyes are probably glazed from this much at the moment though.

The foundation link should mention bracing those posts adequately to avoid another tipping incident 

[new land owner]

I guess my question is that I am building a screen house this summer and I am thinking that I could get away with 4 x 4's instead of 6 x 6's.  My deck the screen house will be on will be 14' square.  My plan was to use three rows of 4 x 4 's with four in each row for a total of 12 posts.

This is my plan.