Fig. 125.
PIERS.
258. The thickness of a pier may be considered either as depending upon the weight of the superstructure, or as resisting the thrust of arches or braces. For the first requirement, very little thickness would suffice; for the second, it may require to be considerable. The objection to thick piers is the expense, and the contracting too much the water-way; the benefit, a large bearing surface, and in stone bridges where there are several continuous spans, a saving of centring; as where the piers are not able to resist the thrust of the arches, they must all be carried up at once.
259. Piers supporting truss bridges, require very little thickness provided a good foundation is obtained. The following table shows the sufficient dimensions for the piers of wooden or iron trussed bridges, when the masonry is good. (See First Class Masonry, specification, Chap. IV.) From ten to twenty feet in height the batter is assumed at one twelfth; from twenty to fifty feet in height at one twenty-fourth.
| Span. | Length of bridge seat. | Width of seat. | ||
|---|---|---|---|---|
| 20 to | 40 feet, | 20 feet, | 4 | feet, |
| 40 to | 60 feet, | 20 feet, | 4½ | feet, |
| 60 to | 80 feet, | 22 feet, | 5 | feet, |
| 80 to | 100 feet, | 23 feet, | 5½ | feet, |
| 100 to | 125 feet, | 23 feet, | 6 | feet, |
| 125 to | 150 feet, | 24 feet, | 6½ | feet, |
| 150 to | 200 feet, | 24 feet, | 7 | feet. |
260. Upon the form of the up-stream end of the pier, or the starling, depends, in a considerable degree, the contraction of the water-way. In sluggish water the form is not of much importance, but in swift flowing rivers a great deal depends upon the choice. The forms in use are the rectangle, the rectangle terminated by right-lined triangles, and the same terminated by curved-lined triangles, and finally the ellipse.
The latter is that which causes the least disturbance to the water, but is also the most costly.
The effect of gyration at the shoulder, deserves notice, as it may be the cause of the ruin of the foundation when the bottom is of yielding material.
River beds being porous, springs work up through them with a force equal to the whole depth of water; and whenever there is a means of escape for such, its pressure will act upwards against any structure that comes within its reach; and if four or five feet deep, is capable of moving enormous weights. Such springs gave a great deal of trouble at the foundation of the United States Dry Dock, at Brooklyn, N. Y. When checked in one place they burst up in another, and to proceed with the work it was necessary to allow them a passage through which to flow.