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The author is of the opinion that, if he were to make three such designs for comparison, there would not be such great differences in the weights, because constructive reasons will cause the designer to use only a few different sectional areas in the chords of an arch; while Prof. Howe's students, who, as he states, made the calculations from which the tabulated ratios were determined, probably proportioned the section of each panel length of each chord for the greatest stress to which it could be subjected. This would be eminently proper in making such a comparison; but the results of the computations would not agree with similar results obtained by a bridge specialist.
It is difficult to make a proper comparison in respect to economy between arched and simple truss bridges, owing to the fact that the piers differ for the two cases; but a fair one can be obtained by assuming that steel braced piers are used to support the deck span.
The author has had occasion lately to design in complete detail for a British Columbia railroad a 260. ft. arch bridge, shown in Fig. 5, having a rise on the centre line of 59 ft., and to compute the exact weight of metal in same. For the sake of comparison, he has since designed according to the same specifications a 260 ft. deck-span, having a truss depth of 35 ft., resting on steel braced towers 36 ft, high. The total weight of metal for the arch design is 2,111 pounds per lineal foot, and that for the truss design, including the towers, is 2,542 pounds per lineal foot, showing a saving of about twenty per cent in favor of the arch.
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