BRIDGE PIERS The selection of the site of the bridge and the arrangement of the spans, although important in themselves, do not properly belong to the part of the problem here considered; therefore they will be discussed only briefly. The location of the bridge is usually a compromise between the interests of the railroad or the highway, and of the river. On navigable streams, the location of a bridge, its height, position of piers, etc., are subject to the approval of engineers appointed for the purpose by the United States Government. The law requires that the bridge shall cross the main channel nearly at right angles, and that the abutments shall not contract nor the piers obstruct the waterway. For the regulations governing the various streams, and also reports made on special cases, see the various annual reports of the Chief of Engineers, U. S. A.
The arrangement of the spans is determined mainly by the rela tive expense for foundations, and the increased expense per linear foot of long spans. Where the piers are low and foundations easily
secured, with a correspondingly light cost, short spans and an increased number of piers are generally economical, provided the piers do not dangerously obstruct the current or the stream is not navigable. On the other hand, where the cost of securing proper foundations is great and much difficulty is likely to be encountered, long spans and the minimum number of piers is best. Sound judgment and Large experience are required in comparing and deciding upon the plan best adapted to the local conditions.
Within a few years it has become necessary to build bridge piers of very great height, and for economical considerations steel has been substituted for stone. The determination of the stability of such piers is wholly a question of finding the stresses in frame struc tures,—the consideration of which is foreign to our subject.