wood tolerences?

Question

I have a Gambrel house with a 12' 11 1/2" span on the first floor the area on the second floor above this is a bedroom there is no wall above it. this span is from the gable end to a center stairway.

can someone tell me the weight bearing value per linear foot for (3) pine 2 x 8's? (3) oak 2x 8's (1) pine 8x8?

Answer 1

For #2 grade southern pine 2 x 8, allowable bending stress, Fb = 1380 psi, all. shear stress, Fv = 175 psi.

A = 1.5" x 7.5" = 11.25 in^2

Thus for (3) 2x8's, S = 4.5 (7.5)^2/6, = 42.2 in^3
M = Fb * S = 58236 # inch = 4,853 # ft

M = wl^2/8, where l = 13 ft
w = 8 M/l^2 = 230 #/ft

Answer---> 230 #/ft

Check shear. 230 #/ft * 13'/2 = 1495 #
Fv A = Allowable Shear = 1969 # --> OK

To compare any other species or grade, multiply 230 #/ft by the ratio of Fb/1380 psi.

For a #2 souther pine 8 x 8, allowable bending stress, Fb = 850 psi, all. shear stress, Fv = 165 psi.

S = 7.5 (7.5)^2/6 = 70.3 in^3

M = Fb * S = 59,755 # inch = 4,980 # ft

M = wl^2/8, where l = 13 ft
w = 8 M/l^2 = 236 #/ft

Answer---> 236 #/ft

Negligable difference in the pine values.
Select the cheaper of the 2.