A floor system in a small building consists of wood planks supported by 2 in (nominal width) joists spaced at distance s, measured from center to center (see figure). The span length l of each joist is 10.5ft, the spacing of the joists is 16in, and the allowable bending stress in the wood is 1350psi. The uniform floor load is 120lb/ft2, which includes an allowance for the weight of the floor system itself.
Calculate the required section modulus for the joists, and then select a suitable joist size (surfaced lumber) from Appendix F, assuming that each joist may be represented as a simple beam carrying a uniform load.

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Planks Joists