All trusses shall be so designed as to admit of accurate calculations of all stresses, excepting only such unimportant cases of ambiguity as occur when two stiff diagonals are used in a middle panel.

In important bridges with steel stringers, all lateral bracing and other sway-bracing shall be rigid above and below , i.e. the sections must be capable of resisting compression, adjustable rods for such bracing being allowed only in towers of draw-spans and in the lower lateral systems of deck bridges; but, in cheap country bridges, the lateral and other sway diagonals may be adjustable rods.

The stiff diagonals of lower lateral systems, which shall be of double cancellation, shall be riveted rigidly to all the steel stringers where they cross them.

In the trusses of important bridges counterbracing the web shall be effected by using stiff diagonals, but in cheap bridges it may be done by using counters of adjustable rods.

All through-spans shall have portal bracing at each end, carried as low as the specified clear headroom will allow. The portal struts shall be riveted rigidly to the web or both flanges of the inclined end posts. Riveting portals to one flange only will not be allowed.

When the height of the trusses is great enough to permit, transverse, vertical sway-bracing shall be employed; otherwise, corner brackets of proper size, strength, and rigidity are to be riveted between the posts and the upper lateral struts.

Deck-bridges shall, as a matter of precaution, have sway-diagonals between opposite vertical posts of sufficient strength to carry one half of a panel-truss live load with its impact allowance; and the transverse bracing between the vertical or inclined posts at each end of span shall be sufficiently strong to transmit properly to the masonry one half of the total wind-pressure carried by the upper lateral system of the span.

The lower lateral systems of deck-bridges may be made of ad- justable rods in alternate panels, thus leaving every other panel unbraced,  and  forcing  the  wind-pressure  from  below  up