246 _ APPLIED MECHANICS 
It may be noted here that in American practice the ratio of depth to 
span commonly adopted is larger than is usual in British practice. 
224. Wind Pressure.—In what follows, the direction of the wind is 
supposed to be horizontal. In estimating the effect of wind pressure on 
a bridge, two alternative cases should be considered. (a) When the 
bridge is unloaded, and a wind pressure of 56 Ibs. per square foot is 
acting on it. (+) When a train is crossing, and a wind pressure of 30 Ibs. 
per square foot is acting on both the bridge and train, The wind 
pressure on the moving train forms a travelling load acting laterally on 
the structure. 
The area upon which the wind acts may be estimated as follows. For 
a single flat bar or a solid body like the floor system, the face area pre- 
sented to the wind may be taken. When two bars lie, the one directly 
behind the other, but from two to three diameters apart, the combined 
area may be taken as one and a half times that of a single bar. If, how- 
ever, the distance between them is relatively great, the combined area 
presented is twice that of one. For example, the area presented by two 
ties, one behind the other, in the same panel of a lattice truss, would be 
one and a half times the face area of one, but the total wind pressure on 
the two girders of the bridge would be twice that on one. In a plate 
girder bridge, however, the windward girder may be assumed to shield 
the leeward girder to an extent depending on their distance apart. The 
train surface may be taken as 10 square feet per foot-run, and the 
travelling wind load is then 300 Ibs. per foot-run. 
225. Wind Girder.—The lateral wind load is supported by a girder 
formed by bracing together two of A 
the main booms in a _ horizontal 
plane, usually that of the bridge 
floor. Sometimes the other two 
booms are also similarly braced 
together, then the two girders so 
formed share the load. 3 lib th 
If the bridge floor consists of continuous plating, this may be looked 
eeeee/fee#eeee® 
a.‘ 
e e 
Fig. 377. 
upon as forming the web of the wind girder. Frequently, however, a 
