Unfortunately, the "Powers" at Calcutta have not yet seen fit to give these layouts consideration; and it seems probable that the time and gray matter expended by the author in evolving this solution of a knotty and interesting problem will be wasted—at least from a pecuniary point of view—hence, in order that such waste may not be total and permanent, he is now presenting to the engineering profession the results of this economic study.

Comparative Costs of Various Types of Movable Spans

The collection of the necessary data concerning the quantities of materials in movable spans has been no easy task, because the designers and builders of such structures seldom publish the total weights of metal involved, nor, what is equally important, the division of the said weights into various logical groups, such as moving span, towers, counterweight trusses, etc. Furthermore, such records as can be collected need to be carefully plotted and compared on some logical basis, since different bridges are designed for various specifications and often under dissimilar conditions.

The analyses of all these weights have been prepared for this economic investigation by the author's assistant engineer, Mr. Shortridge Hardesty. Comparisons have been made with great thoroughness between vertical-lift bridges and both the heel-trunnion and the Brown balance-beam, single-leaf bascules; firstly, because it was possible to secure the fullest data concerning these types; secondly, because, for many layouts, the logical choice would be one of them; and, thirdly, because it is a comparatively simple matter to contrast them fairly and definitely. Swing spans and deck bascules with underneath counterweights have also been investigated, but questions of types of piers, distance from grade to water line, aesthetic considerations, and other factors affect the comparisons so largely as to make the results considerably influenced by the personal equation of the designer and by the conditions of each individual case. Railway bridges have been used as a basis for the most part, because the best records available deal with that class of structure, and because there is less variation in them than in highway bridges.

For the vertical-lift bridge the author had at hand the records of some thirty cases designed in his office, supplemented by complete curves of weights of towers for different heights thereof and for various weights of moving spans. The weights of the different machinery groups, such as ropes, sheaves, equalizers, operating machinery, etc., were plotted in terms of the weight of the moving span and the height of lift. Curves of average weights were then drawn for each group. The same was done for each of the items of structural metal. These various curves, after being first drawn in terms of the weight of the moving span, were replotted in terms of the weight of a fixed span of the same length and carrying capacity. [The]