6"x6" post strength - Structural engineering general discussion (2024)

Most P/T wood is Southern Pine. Assuming #2 grade, the
Fc = 525 psi and E = 1.2. L/d ratio is 13

Allowing for buckling, allowable compressive stress is roughly 475 psi, Area is 30 sq in, so the allowable force in the post is 14,300 lb. However, if the post is a stronger grade, the strength goes up quite dramatically.
The first thing you should do is check the grade stamp.

The flexing could be due simply to drying, because it is unlikely that the house is loaded to anywhere near design load.

Is the construction new?How have you determined the 30,000 lb load?Is the end attached to the floor?Is the load centric?

Timber Construction manual specifies Euler's formula with a safety factor of 2.727, that is;

Sigma(allowable)=0.3*E/(L/d)**2
0.3*E/(L/d)**2=P/A
=P/d**2

solve for d,d=5.26"okuse d=5.5 nominal dim

checking normal stress;
Sigma=P/A
=30,000/(5.5)**2
=991 psi

requires Compression Parallel to Grain "Fc" > 991
the only grade that meets this is Dense Select Structural
see http://www.southernpine.com/designvalues2.htm
I have never seen this grade used in residential application, so one can suspect the post is undersized.

AITC Column Tables provide a calculated load that can be supported based on column size, length, and material properties see https://www.aitc-glulam.org/TechnicalInfo/tables.asp?type=Column

In America, typical 6x6 pressure treated (PT) are southern yellow pine (SYP).I agree with "Trussdoc", bow is drying and actual loading is much less that design.(Straight 6x6's are difficult to find)

I would consider just adding an additional 6x6 next to the existing post bolt the two together.

I found this link,
http://www.wwpa.org/pdf/TN9.pdf