Rain Down Spout 8” round inside diameter = 7.98” == area = 6.03 in^2 / 144 = 0.04189 ft^2 times total distance = 11.5ft. Therefore 0.482 ft^3 of water * 62.4 #/ft^3 = 30.1 # of water

8” downspout weighs 5.39 #/ft * 11.5ft between supports= 62#

Total Gravity load 92#
LRFD 1.2Dead Load	110.5# total gravity force 

Wind
8" x 138" = 1104 sq in x .66 (round factor) 728.6 sq in or 5.06 sq ft 
LRFD 1.6 W  = 1.6 x 42 psf = 67.2 psf;	5.06 x 67.2psf =     340 # wind force 

Provide 3 supports, top, middle and bottom
Maximum Wind per support is 340#/2 = 170 #
30" span of 12 gauge HS-Style unistrut will require 3 anchors per section
Each anchor must hold 170# / 3 = 57# in shear

Use a 30” to 36” length of 12 gauge HS-Style unistrut using 3 anchors per section with a maximum spacing of 5 ft between unistrut supports. 3/8” self taping