— 167 — 
The io sultan t moment is about 
K = 25 \/2i = 35.0 inch tons 
c °Uespoadiug to a pin. As the horizontal components of tlie initial tensions 
may be taken aa balancing each other, the calculated value of H is too high, so we 
a y conclude that a pin will be strong enough. Had the counter stress been 
l ar g e r， it would have been necessary to use double counters close together at 
lr eefc and spread apart at their upper ends, so as to reduce the size of tlie middle 
u j C t} ^ n, because it is bad practice to let tlie single counters pull eccentrically 
The portal pins may be proportioned by assuming tlie lever arm of tlie stress 
a P° l， tal rod to be 1*"， and tlie stress (vide Table VI.) 10.298 + 1.875 = 12.173 
y making the moment approximately ^ x 12.17 = 18.7 inch tons, correspond- 
n o to a 2|” piu, foi* tlie column for lateral pins is to be used in this case. 
The stress at eacli bearing of the vibration rod connection to the upper lateral 
is (vide Table VI.) ^ (6.205 + 1.250) = 8.78 tons ; and tlie lever arm may 
. 6 assum ed as 2+"， making the moment 2} x 8.73 = 8.39 inch tons, correspond- 
ln g to a 2^ pi n . • 
. diameter of the bolt for coimectiug tlie vibration rod to tlie intermediate 
U U may be calculated ； but, if it be assumed to be 1 it will have abundant 
^ Provided that the round iron of the eye be properly flattened so as to 
c l ^ce the bending moment. 
如 V e “ical pins will be required for tlie lateral rod connections at tlie ends of tlie 
g . le ^ l owe r lateral struts nearest each eud of the span. As tlie lateral rods are 
& 8 e， they must be attached to the middle of tlie pins. To reduce tlie bending 
j e ^ ie centres of bearings must be brought as closely together as possible. The 
the f> ^° SS ^^ e is six iuches, raakirig the beruliug moment SP t where 2P is 
Yj olea ^ e ^ worJcing stress on the rod. The several values' of 2P as founcl from Table 
87. le 83.867, 25.058 and 14.881 tons, making the corresponding moments 50.8, 
21.57 inch tons and the corresponding diameteirs 8 备 ", 3 士 " and 2 聲 " • 
ftp . ? aie 110w ready to proceed with tlie ** Bill ol* Iron,” in making which, close 
^ximatioa g of lengths are allowable. 
tlie 1 . は US l )re P are the blauk form recommended in Chapter XVII., then turn to 
g 0 ^ given iu Chapter III., aiul till out the form, proportioning as we 
0 f thl ( °^ a ^ S w ^ i08e sizes have not been previously determined. The filling-out 
little ，抑 “ denomimited “ Main Portions is a very simple matter, and needs but 
diacr 0 X ^ aila ^ 01 i« It is to be noticed tliat the lengths of tlie chord bars and main 
ai ]f t/ 1 S 】 mVe ^ een increased by three feet to allow for the weights of the heads, 
Ur»«“ ） SG 〇 1 a 11 at liustal)lo ro.ls by five feet to allow for the woiglit of tlie eyes, 
s ， aua adjusting-nuts. 
It iu *" o l °nping of members having some similar dimensious is to bo observed, 
hi 卿 C0ns i(l era ble economy of labor if one li is to estimate ou many bridges. 
“• 〇11 ⑴。 0l 】t the last verticiil column, tlio tables of weights of flat, round and square 
Ueai* the eiui of Chapter II. will be found of great assistance. 
