JOISTS, PLANKS, GUARD-TIMBERS, AND WOODEN HAND-RAILS.

Joists, planks, guard-rails, hand-rails, and all other timber portions of the structure shall be of long-leaf, Southern, yellow pine, or other timber which, in the opinion of the Engineer, is equally good and serviceable.

The sizes of the timber joists shall be such as to give the requisite resistance to bending, the effect of impact being considered; but no joist shall be less than three (3) inches wide or twelve (12) inches deep.

As a rule the depth of a joist shall not exceed four (4) times its width. Otherwise, the joists shall be properly bridged at distances not exceeding eight (8) feet.

They shall be proportioned by the formula

where M is the greatest bending moment in inch-pounds upon a joist, R is the intensity of working-stress in pounds, b the width of the joist in inches, and d the depth of same in inches.

Joists shall be dapped at least one-half (1/2) inch upon their bearings, and shall have their tops brought to exact level before the planks are laid thereon.

They shall be spaced not to exceed two (2) feet between centres; shall, preferably, lap by each other so as to extend over the full width of the floor-beam; and shall be separated half an inch, so as to permit the circulation of air. The outside joists, however, shall abut so as to provide flush surfaces from end to end of span.

Floor-planks for the main roadway shall be at least three (3) inches thick and from eight (8) to ten (10) inches wide, and shall be laid with one-quarter (1/4) inch openings. Each plank shall be spiked to each joist on which it rests by two (2) seven (7) inch cut spikes, the holes for which shall be bored in order to avoid splitting the timber, or else by two (2) seven (7) inch wire nails.