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tlie structure in any case, for bridges are not proportioned to resist the blows of 
derailed trains. 
It is true that higher pressures than thirty pounds are sometimes recorded, but 
they extend over very limited areas : on this account the empty bridges are propor- 
tioned to resist from fifty to forty pounds per square foot according to the length of 
span. C. ölialer Smith Esq. C. E., one of the highest American authorities upon 
bridge building, proportions all his bridges under two hundred feet span to resist a 
pressure of fifty pounds per square foot, and considers that thirty pounds upon tbo 
loaded bridge will be large enough for all greater spans. 
But as tlie upper lateral systems of through bridges and the lower lateral sys- 
tems of deck bridges are not affected by the wind pressure upon the train, the author 
considers that empty spans from 200 to 250 feet iu length should be proportioned 
for forty-five poniuls per square foot, and all greater spans for forty pounds. 
In reality these figures have not been exactly adhered to in making tlie designs 
for Uiis treatise, because tlie author lias considered it better to reduce the intensities 
of wind pressure gradually than to change them suddenly by decrements of üve 
pounds per square foot. 
Theodore Cooper Esq. C. E” the author of the best American bridge specifica- 
tions, provides for a wind pressm.o of 150 pounds per lineal foot for upper lateral 
bracing in through bridges and lower lateral bracing in deck bridges. This is a 
rather small allowance for a country visited annually by typhoons. In preparing 
Table XIII tlie author used 150 pounds for spans of 100 feet and under, from that 
to 200 pounds for spans between 100 find 200 feet and from 200 to 240 pounds for 
spans from 200 to 800 feet as tlie pressures per lineal foot for upjier lateral bracing. 
The pressures per lineal foot ou trusses only for tlie lower lateral systems were calcu- 
lated to be from 2Ü0 pounds for spans of 100 feet to 320 pounds for those of 800 feet 
for empty bridges ; and from 170 pounds for spans of 100 feet to 240 pounds for 
those of 800 feet for bridges covered by the moving load. 
The pressures per lineal foot upon tlie upper lateral systems with an intensity of 
thirty pounds are about 00 pounds for spans of 100 feet and under, from W to 180 
pounds for spans between 100 and 200 feet, and from 130 to 180 pounds for spans 
between 200 and 300 feet. 
The method of calculating these pressures was fully explained in the last chapter. 
A portion of tlie leeward truss is protected by the train, but no deduction should be 
made oil tliis account, because the surfaces of tlio cliannclsi being coucavo towards 
tlio wind, tcml to increase tlie intensity of pressure : indeed it is well in figuring 
areas to allow an inch or two of extra width to compensate for this concavity. In 
this particular system of bridges the effect of wind pressure on bottom chord tension 
need not be considered in spans under ono hundred and thirty feet in length. 
Wind loads upon empty bridges are treated as moving, for it is possible for one 
part of a bridge to be protected while the remainder is exposed ; besides the centre of 
a whirlwind lias a motion of translation which would cause the pressure to really act 
as a moving load. This method of treatment affects principally the lateral rods aucl 
