FORM or CROSS SECTION. The dimensions on the top will depend somewhat upon the form of the cross section of the pier, and also upon the style and span of the bridge. The examples pre sented later give representative dimensions. Theoretically the dimensions at the bottom are determined by the area necessary for stability; but the top dimensions required for the bridge seat, together with a slight batter for the sake of appearance, usually gives sufficient stability.
In a sluggish stream, the form of the horizontal cross section is not of much moment; but in a strong current it is important.
The up-stream end of a pier, and to a considerable extent the down-stream end also, should be rounded or pointed to serve as a cut-water to turn the current aside and to prevent the formation of whirls which act upon the bed of the stream around the foundation, and also to prevent shock from ice, logs, boats, etc. In some respects the semi-ellipse is the best form for the ends; but as it is more expen sive to form, the ends are usually finished to intersecting arcs of circles or with semicircular ends. For an example of the former, see Fig. 143 (page 558); and of the latter, see Fig. 145 (page 559).
Above the high-water line a, rectangular cross section is as good as a curved outline, except possibly for appearance.
A cheaper, but not quite as efficient, construction is to form the two ends, called starlings, of two inclined planes. As seen in plan, the sides of the starlings usually make an angle of about 45° with the sides of the pier (see Fig. 144, page 558). A still cheaper con struction, and the one most common for the smaller piers, is to finish the up stream end, below the high water line, with two in clined planes which inter sect each other in a line having a batter of from 3 or 4 inches per foot, and to build the other three sides and the part of the up-stream face above the high-water line with a batter of 1 in 12 or 1 in 24 (see Fig. 141 and 142).
The portion of the pier above high water is sometimes built as two independent pedestals; and sometimes the pedestals are connected all the way up by a thin vertical wall, but sometimes only at the top by an arch.