You could also just pull it off the the NCMA section properties for a 16" wall. I=((15.625)^4)/12; A=15.625^2
(I/A)^0.5 = 4.5105 in^4.
NCMA says 16" fully grouted (or solid) units have an ravg of 4.51 in^4.
In plan, I placed two blocks on top of each other and calculated the Ix for the section buy using the parallel axis therom and the given Ix for an 8" hollow wall. came up with an rx of 25.67. area of 1 8x16 block is 39.9 in^2 and Ix is 444.22 in^4
It is unreinforced AND ungrouted? That doesn't sound very good. You should at least grout it solid. Either way, if it is ungrouted, your r is still coming up too high. You forgot to take the square root of I/A. Are you using 2-8" blocks to make the pier or are you using 1-16" block?
bldgdesign: You got too many mistakes in your calcs.
firstly, if it is fully grouted, two block area is An=244in^2, secondly, the Ix of two block is Ix=4967 in^4,
thirdly after Ix/An, you need square root it to get r=4.52.
I am checking someone elses drawings. They used two 8x16x8 blocks placed side by side which makes a square 16x16. These are trailer support piers, gravity load only. Hollow, dry stacked and face bonded.
I did forget to hit the square root key on the calculator. Didn't change much in my check, though. allowable stress dropped 1/2%. New r is 5.07.
yes this is to determine axial load capacity. the r is used in the slenderness reduction portion of the equation.
