COMBINATIONS OF STRESSES.

In the girders of plate-girder spans and of deck, open-webbed, riveted-girder spans, the only stresses that need to be considered are those caused by the live, impact, dead, and centrifugal loads.

The trusses of through-bridges will be affected by the live, impact, dead, direct wind, and indirect wind loads; and in exceptional cases also by the centrifugal load. The trusses of deck-bridges will be affected by all of these loads. In no case will the traction load affect the trusses of bridges to such an extent as to require consideration; consequently the only provision for traction load required in through and deck bridges is adequate rigid bracing to carry it from the track to the trusses without subjecting any portion of the structure to an improper loading, as, for instance, the flanges of cross-girders to horizontal bending.

In bridges of all kinds the various loads herein specified shall be combined without any reduction; but in trestles, more especially very high ones, it will be legitimate, when combining the stresses from the various loadings, to reduce some of them or even to ignore some entirely, in order to avoid proportioning for any highly improbable or impossible combination of loads. For instance, when a trestle is situated near the middle of a sharp curve or near the apex of two heavy rising grades, it would be incorrect to assume a high velocity of train. In such cases as these the element of individual judgment in combining the stresses from the various loads and in assuming the sizes of the latter cannot well be eliminated.

BENDING ON TOP CHORDS.

For combined direct stresses and bending on chords, the moment is to be computed by the compromise formula