BRIDGE PIERS AND ABUTMENTS 230. Placing of Piers. The outline design of a long bridge which requires several spans, involves many considerations: (1) If the river is navigable, at least one deep and wide channel must be left for navigation. The placing of piers, the clear height of the spans above high water, and the general plans of all bridges over navigable rivers, are subject to the approval of the United States Government.
(2) A long bridge always requires a solution of the general question of few piers and long spans, or more piers and shorter spans. No general solution of thequestion is possible, since it depends on the required clear height of the spans above the water, on the required depth below the water for a suitable foundation, and on several other conditions (such as swift current, etc.) which would influence the relative cost of additional piers or longer spans. Each case must be decided according to the particular circumstances of the case.
(3) Even the general location of the line of the bridge is often determined by a careful comparison, not only of several plans for a given crossing, but even a comparison of the plans for several locations.
231. Usual Sizes and Shapes of Piers. The requirements for the bridge seats for the ends of the two spans resting on a pier, are usually such that a pier with a top as large as thus required, and with a proper batter to the faces, will have all the strength necessary for the external forces acting on the pier. For example, the channel pier of one of the large railroad bridges crossing the Mississippi River was capped by a course of stonework 14 feet wide and 29 feet long, besides two semi-. circles with a radius of 7 feet. The footing of this pier was 30 feet wide by 70 feet long, and the total height from subsoil to top was about 170 feet. This pier, of course, was unusually large. For trusses of shorter span, the bridge scats are correspondingly smaller. The elements which affect stability are so easily computed that it is always proper, as a matter of precaution, to test every pier designed to fulfil the other usual requirements to see whether it is certainly safe against certain possible methods of failure. This is especially true
when the piers are unusually high.
The requirements for supporting the truss are, fortunately, just such as give the pier the most favorable formation so that it offers the least obstruction to the flow of the current in the river. In other words, since the normal condition is for a bridge to cross a river at right angles, the bridge piers are always comparatively long (in the direction of the river) and narrow in a direction perpendicular to the flow of the current. The rectangular shape, however, is modified by making both the upper and the lower ends pointed. The pointing of the upper end serves the double purpose of deflecting the current, and thus offers less resistance to the flow of the water; and it also deflects the floating ice and timber, so that there is less danger of the formation of a jam during a freshet. The lower end should also be pointed in order to reduce the resistance to the flow of the water. The ends of the piers are sometimes made semicircular, but a better plan is to make them in the form of two arcs of circles which intersect at a point.
232. Possible Methods of Failure. The forces tending to cause a bridge pier to fail in a direction perpendicular to the line of the bridge, include the action of wind on the pier itself, on the trusses, and on a train which may be crossing the bridge. They will also include the max imum possible effect of floating ice in the river and of the cur rent due to a freshet. It is not at all improbable that all of these causes may combine to act together simultaneously. The least favorable condition for resisting such an effect is that produced by the weight of the bridge, together with that of a train of empty cars, and the weight of the masonry of the pier above any joint whose stability is in question. The effects of wind, ice, and current will tend to make the masonry slide on the horizontal joints. They will also increase the pressure on the subsoil on the downstream end of the foundation of a pier. They will tend to crush the masonry on the downstream side, and will tend to tip the pier over.