Where a low bridge is required over a navigable stream, some one of the various kinds of movable bridges described in Chapters IX and X must be used; but for all ordinary cases the rotating draw is the most suitable type.

Third. Best Span Lengths to adopt.— In many cases there will be no choice as to span lengths, which are liable to be determined by such conditions as the requirements of the War Department, obstruction of stream by piers, danger from washout during erection, etc.; but, where the designer has any choice in the matter, he should be governed by the principles of economy laid down in Chapter III, taking care, however, that he does not violate any of the principles of aesthetics given in Chapter IV, unless he be forced to do so by circumstances that are absolutely beyond his control. As stated in Chapter III, the greatest possible economy will exist when the cost of each pier is equal to one half of the cost of the trusses and lateral systems of the two spans which it helps to support. The determination of these economic conditions is, of course, a matter of cut and try; but after a few trials the economic span length can be approximated very closely. In making such calculations the trial weights of trusses and laterals can be found with sufficient accuracy by taking a span of known weight and computing therefrom the weights for the spans of the trial lengths by the following methods:

A. The weight per foot of the lateral system is directly proportional to the span length, provided that the superstructure is not changed in width, which is generally the case. Should the width be changed, the new weight will have to be modified accordingly, under the assumption that the weight varies about half as rapidly as does the width.

B. To find the truss weight  W'  per lineal foot of span of length  l'  from the corresponding known weight  W  of span  l,  the following approximate but quite accurate empirical formula may be used:

This will give approximately the weight per foot of trusses