until the span is fully loaded; the latter being considered to act as a continuous girder over four supports.

Case No. 5. Greatest direct stresses, on the chords that carry the live load, from wind load when the bridge is open.

Case No. 6. Greatest direct stresses, on the chords that carry the live load, from wind load when the bridge is closed and wholly or partially loaded.

Case No. 7. Greatest indirect wind-load stresses or transferred-load stresses on the lower chords when the bridge is closed and wholly or partially loaded.

The first combination of these stresses includes Cases No. 1, No. 2, No. 3, and No. 4, and gives the greatest stresses for all truss members from combined live and dead loads, for which combination the regular specified intensities of working-stresses are to be used. It is to be noted that wherever the load for Case No. 2 increases the total stress on any member, its effect is to be considered; but wherever the said load decreases the total stress on any member, its effect is to be ignored. The reason for this is that the amount of uplift is a purely arbitrary assumption, which possibly may never be realized. This method of treating the uplift-load stresses causes errors on the side of safety, which do not add materially to the total weight of metal in the structure, and which tend to strengthen the lighter members of the trusses.

The second combination of these stresses includes all seven cases, but it is to be noticed that the only truss members affected by the wind loads are the inclined posts at ends and over drum, and the chords which carry the live load. In this second combination it must not be forgotten that the metal is to be strained thirty (30) per cent higher than in the first combination.

For the lateral systems the following conditions are to be considered:

For upper lateral systems of through-bridges and lower lateral systems of deck-bridges—

Case  No.  1. Greatest wind-load stresses when span is swinging.

Case No. 2. Greatest  wind-load stresses when span is closed