A DESIGN OF STRUCTURES 251 
Under condition (b) ‘the exposed area is— 
pe a Sq. Ft 
The face area of the upper flange . F : ; : + 226 
The face area of the lower flange and floor system . . . 420 
One and a half times the area of the verticals . ; ‘ . 240 
One and a half times the area ofthe diagonals . . . 265 
Total . : . 1150 
The distributed wind load at 30 lbs. per square foot is therefore 
16 tons. The rolling wind load at 300 lbs. per foot-run is 20 tons. 
_ Hence the total wind load under these conditions is 36 tons. 
In estimating the above areas reference may be made to an existing 
bridge, or since, in this case, the wind stresses will only affect the lower 
booms, the scantlings of the other members of the main girders may be 
calculated and their actual areas used. 
3 The two bottom flanges of the main girders which constitute the 
- booms of the wind girder are 18 feet centre to centre. Having found 
the total wind load, the stresses due to it under both conditions (a) and 
(b) can be calculated. 
Maximum and Minimum Stresses.—All the stresses under each con- 
dition of loading are now tabulated. The maximum and minimum stress 
in each bar and the ratio MUM stress i. next determined. The 
maximum stress 
maximum stress in a bar is the greatest stress whatsoever in one direc- 
tion which can come on it. The minimum stress is the least stress in 
the same direction, or should the stress reverse, the greatest stress 
in the opposite direction. In the latter case the minimum stress is 
tive. 
In finding the maximum and minimum stresses, however, care must 
be taken that they are the values between which the stresses actually 
alternate. Two examples taken from the stress sheet will be here 
considered. ; 
Bar 10, tension boom. The maximum stress will occur when 
the bridge is fully covered by a train and is made up of dead 
load stress = +184°9, live load stress= +190°9, and wind load stress 
= +37°5, total 413°3 tons. The minimum stress occurs when the, 
bridge is quite empty and no wind blowing, and is +184°9 tons. The 
oe, will evidently alternate between these values, and their ratio 
is +044. 
Bar 40, web member. When the bridge is empty, the stress in this 
bar is +6°6 tons. As a train rolls on the stress steadily decreases 
until the front of the train reaches the panel, when it has become 
+ 6°6 — 161 = —9°5 tons. It now begins to increase until, the rear of 
the train having just passed the panel, it reaches a positive maximum of 
+6°6 + 22°7 = + 29°3 tons, decreasing again to +6°6 tons as the train 
rolls off the bridge. The stress therefore alternates between a maxi- 
Bet gee tons and a minimum of —9°5 tons, and their ratio 
is -0°32. 
Working Stresses.—The safe working stress in a member depends not 
only on the maximum stress in it, but also on the range through which 
the stress alternates. Various methods and formule, based chiefly on 
& 
