一 118 — 
the r n ; ) fc .! 1 panel point should be 
The following important fact should never be forgotten iu designing this member 
一 “if the pins pass through the holes in tlie struts and lie as nearly as possible to 
the centre of tlie span, the distances between tlieir centres should be I plus the allow- 
able play of tlie pin in the hole of an eyo-bar or l + ^ n . n 
Let us anticipate a little and consider the case of the bottom chord strut designed 
in chapter XVIII : its gross sectional area is 5. 7 square inches and its effective area 
2.8 square inches, so that ^ may be take equal to \ . 
Let us assnme l = 22 feet, ancl E = 28,000 ,000 pounds or 14,000 tons. 
Then tlie elongation at the [ n r panel point is 
レ 
n 
— 2 — 
5x22 ザ X 
0.0471 
— I} 
14,000 
With an evon number of panels the elongations for tlie various pin holes, begin- 
ning at tlie one next to but not at the miciale of tlie span would be 
0.047 ff , 0.094" , 0.141 ff , 0.18 8 " , 0.235 ^ , 0.282" &c. 
And with an odd number of panels they would be 
0.024 ", 0.071 ^ , 0.118 " ， 0.105 0.212^, 0.259, 0.306 ^ &c. 
But in connection with this investigation there is another point which must bo 
considered ； viz. that when the bridge is empty the dead load should generally be 
great enough to put tlie chord strut in tension near tlie ends of tlie span, in order to 
prevent botli vibration and undue stresses on the chord bars at these places. 
Whether this condition exist can be determined by finding tlie dead load stress 
at the middle of the span, dividing it by five and comparing the ratio, which this 
quantity boars to the actual sections of the chord bars at this place, with the ratio ^ . 
If tlie former be the greater the strut will be in tension, and all will be right. 
Otherwise it may be necessary to reduce the amount of tlie elongation of those two 
or three pin holes nearest the ends of the span ； because, supposing that the strut were 
not in tension when the bridge is empty, as soon as an engine covers one or two 
panels at the end of the span, the chord bars in these panels will bo subjected to a 
greater intensity of stress than will those in tlie other panels, and if there be any 
play in the strut eyes which is not already taken up by the dead load, these chord 
burs may be stretched more than the allowable amount p , even before the strut 
comes into action as a tension member ; but, if the play be taken up by the dead 
load, and the chord pauols near one end of tlio span be strained more than tlie others, 
the strut will immediately begin to do its share of tlie work as 8001 a as the live 
load is applied, and none of tlie chord bars will be overstrainod. 
Tlie danger of overstraining the chord bars of tlie end panels necessarily increases 
with the ratio n or that of ~h • These ratios increase with the length of span, but 
fortunately the ratio of dead load to total load also increases, causing the danger of 
overstraining to diminish. 
For example in the 30CK span on plate XLII the clead load stress at the middle 
panel of the bottom chord is about 141 tons, which, ctividecl by five gives 28. 2 square 
