Calculate the structural integrity of a floor as part of the design process for a house. Follow local building codes for foundations; these will determine depth of footings, reinforcing steel in concrete and other aspects of the foundation. Use 2-by-8-inch boards for sill plates, fastened to the foundation typically with bolts set upright in concrete and secured with nuts.
Figure the size, spacing and span of floor joists using tables of the International Building Code and American Wood Council. Determine the type of lumber to be used and find the span limits for various sizes of that wood. Base calculations on typical floor loads -- a live load or external weight of 40 pounds per square foot and dead load or weight of flooring and other components of 10 pounds per square foot.
Start span calculations using 2-by-8-inch No. 1 pine joists spaced 24 inches apart, the most common size and spacing. Plan to add interior posts and beams if that span, 11.5 feet, is too short for the width of the house or reduce spacing to 16 inches to get a span of 13.1 feet. Move up to 2-by-10-inch joists to get a span of 14.7 feet at 24-inch spacing.
Vary the size of joist lumber and spacing of joists to arrive at a workable calculation for the type of house; an open basement, for instance, will require wider spans and heavier joists than one with center posts and beams. Adjust the depth of sill plates to accommodate thicker joists, which must be secured to the sills with 16d framing nails, driven diagonally with a hammer, or with metal joist holders, U-shaped brackets fastened to the sills to hold joists.
Include X-bracing between joists, using two-by-four boards cut with 45-degree angles, to reinforce the floor structure. Fasten these with a hammer and 16d framing nails from the top of one joist to the bottom of the adjacent joist and staggered so two braces side by side form an X. Brace all joists between support points, such as basement walls or interior posts.