Wood Joist

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Wood rafters can be sized by using this spreadsheet. Fill the yellow fields, as required. C coefficient shall be used to modify the Fb allowable stress.
Flectural allowable stress ( Fb ) for Southern Pine #2 are 1500 Psi for 4" member, 1250 Psi for 6" member, 1200 Psi for 8" member, 1050 Psi for 10" member and 975 Psi for 12" member.
Those figure shall be simultaneously multiplied by Cm for moisture, Cr for reputation, etc.

See following documents of NDS
Sections Property
Wood Beam

WoodJoist.xls23.5 KB