718. The use of concrete in large bridge piers was at first confined to the hearting or backing of stone masonry shells. It was soon found, however, that in many cases the concrete was able to withstand the effects of frost and ice better than was the variety of stone available for building the masonry shell, and many important bridges are now supported by piers built entirely of concrete.
As an example of this use may be mentioned the bridge across the Red River1 in Louisiana, which has six concrete piers of heights from forty-four to fifty-three feet. The pivot pier is twenty-seven feet in diameter, with vertical sides. The draw rest piers are seven feet wide under the coping, nineteen feet between shoulders and twenty-six feet long over all. The sides have a batter of one-half inch to the foot. The coping is of limestone.
719. In the construction of the Arkansas River Bridge 2 of the K. C. P. & G. R. R., ten piers and two abutments were built of concrete. The piers varied in height from twenty to sixty-five feet, some of them containing over six hundred cubic yards of concrete. The entire work was completed in eleven months, although many difficulties were met. The concrete was composed of one part Portland cement, two and one-half parts coarse, sharp sand, and five parts of clean, broken stone.
The lagging for the forms was of two-inch yellow pine, surfaced one side and sized to one and three-quarters inches. On the straight part of the pier this lagging was laid horizontal and supported by four by six vertical posts set four-feet centers, posts on opposite sides of the pier being tied together by three-quarter inch bolts passing through one and one-half inch gas pipes spaced five feet vertically. The gas pipe was allowed to remain in the finished pier, the bolts being withdrawn.
1 George H. Pegram, Consulting Engineer. Walter H. Gahagan, Engineer for Contractors.
2 Engineering News, Aug. 25, 1898.
The lagging for the semicircular ends was of two by six with bevel joints, placed vertical, and supported at five-foot intervals by segmental ribs of double two by twelve planks. At the ends of the ribs were bolted short pieces of eight by eight inch angle irons with edge horizontal. These angle irons were, in turn, bolted to four by six verticals at the corners or shoulders of the pier.
720. The foundation piers of the Lonesome Valley Viaduct,1 thirty-six piers and two abutments, are entirely of concrete. The piers are four feet square on top with batter of one inch to the foot, and are from five to sixteen feet in height. The total concrete laid was 926 cubic yards at a contract price of about $7.00 per cubic yard. The piers were finished on top with a steel plate, four feet square and one-half inch thick, taking the place of coping stones. Where rock foundations were not found, the lower portion of the pier was given an increased batter to secure such a cross-sectional area at the bottom that the unit pressure on the earth did not exceed one ton per square foot. The Cost of the concrete for this work has already been given (§ 322).
Steel shells filled with concrete have been used to good advantage, especially for bridge approaches. Such shells are usually in pairs placed abreast, one under each truss of the bridge or viaduct. The two cylinders of a pair are usually connected by lateral bracing, and if desired in heavy work, this bracing may be inclosed in a concrete wall and thus protected from injury by running ice, etc. The thickness of metal in the shells need not be great, three-eighths of an inch usually being sufficient, though this depends on the height, the stresses, and the liability to injury. In soft ground requiring piles, most of the piles are sawn off below the limit of scour, or below the water line for land piers, but one or more may be allowed to project up into the cylinders and the concrete filled in around the heads, thus anchoring the pier. In foundations on rock if the cylinders require an anchorage, this may be provided with bolts fox-wedged or cemented in the rock and projecting into the cylinder. (For details of methods adopted in this class of construction, see " Bridge Substructure and Foundations in Nova Scotia," by Martin Murphy, Trans. A. S. C. E., September, 1893).
1 Gustave R. Tuska, Trans. A. S. C. E., September, 1895.
Where masonry piers are being destroyed by the abrading or expansive action of ice, or by other causes, concrete is successfully used to arrest such action, the entire pier being incased in a layer, one to three feet thick, of Portland cement concrete of good quality.
The piers of the Avon River bridge,1 originally built of ashlar masonry, failed entirely to withstand the deteriorating influences of an extreme range in tide coupled with the severe temperature of a Nova Scotia winter. Five of them were subsequently incased in concrete, as follows: A form was made of ten by ten inch spruce timber surrounding the ashlar masonry of the piers and forming a mold to receive the concrete and retain it in place until set. The thickness of the concrete casing was two and one-half feet at the bottom and one and one-third feet at the top, which was three feet above high water. The concrete was composed of one barrel Portland cement, one and one-half barrels clean sand, one barrel of clean gravel, and in it was placed by hand four parts of common field stone weighing from eight to twenty pounds each. This treatment was entirely successful in preventing further disintegration.
723. An efficient cutwater for bridge piers is made by placing old rails vertically on the upstream nose of the pier, anchoring them to the masonry and filling between with concrete, leaving only the wearing surface of the rail head exposed.
724. Pneumatic caissons are usually filled with concrete, the filling over the working chamber being carried up fast enough to keep the work above water as the caisson is sunk. The filling of the working chamber calls for special care in tamping under and around the longitudinal and cross-timber braces. A space of about three inches next the roof of the chamber is filled with a rich concrete, containing no stone larger than one inch, mixed quite dry and solidly tamped with special edge rammers.