The division between adjacent voussoirs should be clearly marked on the face, and additional joints may be indicated if desired, by lines in a plane approximately perpendicular to the line of pressure. Such lines are obtained by securing triangular strips on the inner face of the molds. When spandrel walls are used, these may be similarly marked on the face by horizontal and vertical joints. On long spans the spandrels should have expansion joints, and the coping and parapet, when of concrete, should also have vertical joints to provide for changes in length due to loading or thermal variations.
The arches over the spandrels should be provided with a waterproof covering, either of Portland cement grout or an asphalt mixture to prevent the percolation of water to the arch ring. Pipe drains should be provided to carry the water to a point over the piers where it may be discharged. Care should be taken that such pipes have their outlets so located that the drip shall not disfigure the wall. Open spandrels may be drained by pipes built into the arch ring at suitable places.
A good example of a highway bridge built of concrete without reinforcement is the monolithic arch spanning San Leandro creek, between Oakland and San Leandro, Cal.1 This arch has a five centered, elliptical intrados, with span of 81 1/4 feet, rise of 26 feet and width of about 60 feet. At the crown the thickness of the arch ring is 3 feet, the radius of the intrados 61 1/2 feet and of the extrados 88 feet.
As the arch rests directly on a bed of clay containing some gravel, the footings are made 30 feet wide, and they extend 5 feet below the creek bed. The lagging for the forms was of 2 by 6 inch scantling laid transverse to the axis of the structure or parallel to the axis of the intrados. The ribs of the centering were built of two 1 by 12 inch boards and the braces of 4 by 6 inch timbers, converged to three short 12 by 12 inch timbers supported by wedges bearing on 12 by 12 inch longitudinals.
When the haunches had been built up about one-third the way, as flooding of the work was anticipated, an arch ring one foot thick was first completed, the remainder being placed as a second layer. There is a parapet wall three feet six inches high on either side of the bridge. The spandrel walls show a solid face, and are paneled to bring out the outlines of the extrados and parapet. The centers were struck ten days after the completion of the second arch ring, and the settlement at the crown was about one and one-half inches. The forms contained 90,000 feet, B. M., of lumber, and 3,384 cubic yards of concrete were used. The Cost of the bridge was $25,840.00, or less than $8.00 per cubic yard of concrete. The contractors were the E. B. and A. L. Stone Co. of Oakland, Cal., and the plans were prepared by the County Surveyor's office of Alameda County, Cal.
The three span arch spanning a mill pond on Anthony Kill, near Mechanicsville, N. Y.,1 is worthy of notice on account of some peculiarities in the centering and because of the location of the plant on a side hill, so that the concrete was delivered on the work with very little labor. Two of the arches were of 100 ft. span, with rise of 20 feet, and the remaining arch was of 50 ft. span. The width Is but 17 feet, and the piers are founded on rock at a depth not exceeding 12 feet.
1 Described by Mr. William B. Barber, Engineering News, Aug. 27/1903.
1 Described in Engineering News, Nov. 5, 1903.
For the centering, piles were first driven, six feet centers, in bents ten feet apart, and the bents capped with ten by twelve inch timbers. Stringers of the same dimensions were then laid longitudinally, and eight by ten inch posts were erected on the longitudinals and spaced three feet centers. These posts, which were cut to proper length, so that their tops conformed to the curve of the intrados, were then capped with eight by ten inch timbers parallel to the axis of the intrados, and the lagging laid upon them transverse to this axis or parallel to the center line of the bridge. This lagging was of two thicknesses of one inch boards sprung into place and nailed, the upper layer being of dressed lumber to give a smooth surface to receive the concrete.
The concrete was of one part Portland cement, three parts sand, three parts gravel and three parts broken stone, except for the arch ring, in which but two and one-half parts each of gravel and stone were used. The concrete plant was so arranged that the stone could be passed from the crusher to the mixer by gravity. The concrete was delivered on the arch in cars of three feet gage drawn by cable. From fifty to sixty cubic yards of concrete were placed in ten hours with but nine laborers. 140,000 feet B. M. lumber was used in centers. The entire work consumed about 2,500 cubic yards of concrete.
The concrete bridge carrying the Illinois Central R. R. over the Big Muddy River furnishes an excellent example of a long span arch, built without reinforcement so far as the arch ring is concerned. The bridge is very fully described by Mr. H. W. Parkhurst, Engineer of Bridges and Buildings I. C. R. R., in Engineering News of Nov. 12, 1903. There are three spans, each 140 feet in the clear, with 30 feet rise above springing lines. The arch ring proper is five feet thick at the crown, but as the spandrels, which are built open over the haunches, have near the crown only a false opening on the face, the actual thickness of concrete at the crown is seven feet.
The piers and abutments already in place for the three Pratt trusses formerly in use, were surrounded with new concrete masonry, making the piers 21 ft. 6 in. wide at the top. As rock was found only at considerable depth, the piers rested on piles. To relieve the load on foundations as much as possible, as well as to avoid cracking, which would be likely to occur in heavy longitudinal spandrel walls from temperature strains, transverse spandrel arches were adopted. Since in case of derailment of trains these spandrel arches would be subjected to shock, the concrete in this portion of the structure was reinforced by a self-supporting skeleton structure built of steel rails. Longitudinal rails were laid horizontally, three feet center to center, connected at frequent intervals by one inch rods and held in place by vertical posts, which in turn rested upon transverse horizontal rails laid in recesses left in the arch rib.