body of the work, the differences, however, are all within the limits allowed in good 
practice. Although tlie principles employed in designing plate girder spans have 
been fully elucidated, no examples have been worked out or diagrams given, for the 
reason that time and space do not permit ; and there is no necessity therefor, because 
you have many examples of existing plate girder spans, wliicli do not differ funda- 
mentally from tlioso which would be designed by tlio methods of this book. Never- 
theless you will find that tlie latter will exceed the former in weight and efficiency. 
No special treatment; has been given for skew bridges, for none is needed ; the 
methods for designing them being precisely the same as those for designing other 
bridges. Whenever it is convenient to do so tlie panel length of a skew bridge 
should be chosen so that the shoe of one truss comes opposite tlie first panel point of 
tlie other truss, in order that the floor beams may be at right angles to the planes 
of the trusses, both for economical reasons and to avoid using single beam liaugers. 
This arrangement can often be made by shortening the panel length a little, and, if 
ifc be allowable, slightly changing the angle of the skew. Even if it bo impracticable 
to make this arrangement, it is usually better in skew bridges to advance the ends 
of the floor beams at one side of the bridge by one or even two panel lengths, if by 
80 doing the floor beams be shortened. 
The ton used m the following chapters is tho American or short ton of two 
thousand pounds: it will be found much more convenient tlian the long ton. 
It seems almost unnecessary to state, except for the benefit of foreign readers, 
that the gauge of track is 8^ 6^, that the distance between centre lines of inner rails, 
where there is a double track, is G r 2J", and that the width of the head of a rail is 
2 士 "， making the distance between centres of parallel tracks 9 f 10i". 
In making the designs on Plates XIV — XLII American iron was employed, tlie 
reason being, as can be seen from the next chapter, that tlio European channel 
sections do not have the necessary range iu weight and are with a single exception, 
limited to a depth of twelve inches. Carnegie's sections have been exclusively used, 
because not only does his company roll more iron than any other company in tlie 
United States, but they are also tabulated in a more convenient form than are any 
otlier sections. On this account I have copied from bis il Pocket Companion, M with 
kind permission of the Company, all the tables that I think will be of any use 
to you. 
It would be well for each engineer in this country, who lias anything to do 
^vith ironwork, to provide himself with the books of sections of all the manufactur- 
ing companies mentioned in Chapter II. 
Concerning tlie cost per pound of finished bridges of the American type I have 
made many inquiries both iu England and America. The American price is at 
present about 4 i cents f. o. b. : the English manufacturers with but one exception 
refuse to quote prices without seeing working drawings. This exceptional company 
halves the price about half a cent less or 4 cents gold. The freight charges from 
England and America differ only a few cents per ton. 
. As to where it is better to have bridges manufactured each engineer must 
judge for himself. My opinion is that for plate girder spans it would be more 
