% — 147 — - 
au d slender rods are dilticult to transport, and arc liable to become twisted and bent ; 
though this objection can be partially removed by halving them ; and, as the posts 
arc light， they will spring more under the shock of rapidly moving loads. 
tlie width of roadway and the live load increase, the inferior limit of the 
double intersection may be lowered. 
The common idea that a double- intersection bridge should, for economy’s sake, 
have more panels than a single-intersection bridge of the same span and loading, is 
incorrect. 
The economic depth for a double- in ter section truss is about three feet greater 
than that for a single-in torsection truss of the same span, and number of panels. 
These investigations were made for highway bridges, but as the loads did not 
(liffev very greatly from tlio loads used in this treatise most of tlio conclusions arrived 
a ' will hold good here also. In regard to economic panel length and limiting length 
ot s P a u for single intersection trusses the author has derived tlie following results 
咖 le taking the calculations for tlio trusses, whose diagrams are given on Plates 
幻 "V 〜 XLII. First that for spaus above one liundrod feet iu length the longer 
tlle Panel up to the limit of twenty four feet, or perhaps more, the more economic 
design . But as one lias to be guiJocl somewhat by appeai.ances， H is better to 
av e five panels in all spans uiidei. 0110 hundred feet, and not to exceed a panel 
eugtli of twenty-four feet in any case. Web diagonals having a slope much flatter 
than toi'ty-five degrees do not have a graceful appearance. 
The panel lengths that the author would recommend are given ia Table I. 
And second that in railroad bridges the economic limit of single intersection 
musses is higher than iu liigh^vay bridges. This is because of tlic engine excesses, 
^hich are made, by placing a panel point between them, to simultaneously affect 
e ' Ve b members of the same system of triaugulation in double intersection trusseb. 
奶 16 economic limit for single intersection trusses in the system of bridges lieve 
よ eated is ( vido Table I ) one hundred and eighty feet for single track bridges, but 
eie would not be much waste of iron if this limit were raised to two hundred feet. 
, ' e au thoi’ has not made similar investigations for double track bridges, but would 
= ge that the economic limit would bo from ten to twenty feet less tlian for single 
l ' ac ^ bridges. The limits given in Chapter VI are merely to prevent tlie designing 
rUsses Wtlx too light diagonals and posts. 
addition to what precedes, the following general economic considerations 
吣 0uld al ways receive attention. 
Field riveting should be avoided as much as possible, and designs should be 
e so that all the parts will come together readily during erection. 
ド' し slioulcl be spaced with regularity, so as to facilitate the puncliiug of tlie 
by 出 — gmachi 臟 . 
out S e nerally better, in tlirougli bridges, to pack all but tlie end chord bars 
e of the posts, and reduce the width of top chord plate to a minimum* 
It j 
ch 13 always better to employ the apparently most economical depth of 
ä, ^ or instance, if there be a choice of using ten or twelve inch channels 
