Floor joist span tables are tabulated based on allowable deflection limits of L/360. Floor live loads range from 30 psf for sleeping areas to 40 psf for other occupancies. Ceiling joist span tables are tabulated based on allowable deflection limits of L/240.
Typically a user will know the span and desired spacing for a given application. The user can select a species, size, and grade for trial use. Using the Design Values for Joists and Rafters, the user determines the modulus of elasticity and bending design value of the species, grade and size of lumber. Bending design values are already adjusted for load duration and repetitive member factors in the Design Values Supplement. With this information the user enters the span table with the given modulus of elasticity and determines the allowable span for an estimated size and spacing. If the allowable span is greater than that required for the application, the species, grade and size selected should then be verified for bending strength. At the bottom of each table, required bending design strengths for specific joist spacing are tabulated. If the bending design value selected from the Design Values Supplement is greater than the tabulated required bending design value, then the species and grade chosen is adequate. If the bending design value is less than the tabulated value then the user can either select a deeper member, decrease the joist spacing or select a higher lumber grade, thus a higher bending design value. For either option, modulus of elasticity has to be re-evaluated to ensure the deflection limits are maintained. Linear interpolation for intermediate design values is permitted.
More details available in the tutorial