C. B. & Q. R. R. Fig. 162 shows the cross sec tion, of a 20- by 20-ft. box culvert employed by the Chicago, Burlington and Quincy Railroad, and also the forms used in construct ing the culvert.* This road builds single box culverts of all dimensions from 4 by 4 feet to 20 by 20 feet; and, when a greater waterway is required than can be secured with a single box, builds a double box, one of which is shown in Fig. 163.t Fig. 163 illustrates the difficulty of securing in structures of such magnitude, such a distribution of the pressure upon the foundation as will prevent unequal settlement. This culvert was designed for a bank 30 feet high above the top of the culvert, and consequently the load was considerably more under the track than under the toe of the bank. . To secure a foundation under the body of the culvert that should decrease in area from the center toward the ends of the culvert, a solid 'concrete floor is laid over the entire width of the waterway under the track and a gradually increasing portion of the floor is omitted as the ends of the culvert are approached. This method of procedure was reasonably successful, although minute cracks showed that the ends of the culvert did not settle quite as much as the center. The wing walls must be designed to resist overturning, and the area of the base required for this purpose is so great as to make it impossible to secure a pressure per unit of area which shall be uniform under both the trunk of the culvert and the wings; and therefore it is the practice of this road to attach the wings to the body by a tongue and groove slip joint so that the two may settle independently. Sometimes temporary wings of piles and planks are put in, and after the culvert has settled, permanent wings of concrete are built; but this is not necessary unless the foun dation is very soft.
Highway Culvert. Fig. 164, page 590, shows the standard design for a highway culvert published by the company controlling the corrugated'bars (b and c, Fig. 28, page 236). The live load was assumed to be a 20-ton road roller.
This design was made for a fill of 2 feet of earth over the culvert; and it was assumed that the culvert top supported a prism of earth which was H ft. (in this case 2 ft.) wider than the clear span of the culvert (see paragraph 3, § 1148). The top, bottom, and sides were assumed to act as beams having fixed ends; and the flexure in the sides due to the bending of the top and bottom was neglected. Notice the longitudinal reinforcing rods near both surfaces of all four sides to prevent cracks due to unequal settlement. Notice also the reinforcement at the corners, and the provision for shear in the roof and the floor of the culvert.
Rail-Top Culverts. In the early use of concrete for verts, particularly before the principles of reinforced concrete were well understood, culverts were sometimes built with a considerable number of railroad rails in the lower sides of the roof slab, transversely across the opening. These rails acted
as beams and also as forcement in the lower side of the roof ;slab; but steel rails have too large sections to be efficient reinforcement and are not in the right form for economy, and sequently such construction has for the most part been discontinued in favor of ordinary reinforced concrete box culverts. However, it sometimes happens that it is necessary to provide a considerable waterway through a shallow embankment, in which case it is desirable to make as thin a roof for the culvert as possible; and under these circumstances rail-top culverts are still built. Fig. 165 shows the cross section of a rail-top cul vert on the Chicago, Milwaukee and St. Paul Railway, and also the forms used in the construction.* The rails, usually old ones, are placed upon their bases nearly in contact, and should extend at least 12 inches over the inside edge of the side walls. Usually about 2 inches of rich mortar, or concrete containing only fine stone, is placed below the base of the rail to protect the steel from corrosion; and usually at least 6 inches of concrete is placed above the top of the rails.
Sometimes when the distance between the upper side of the waterway and the base of the rail is limited to 18 or 24 inches, six or eight rails are set with bases down and as close together as possible under each rail of the track, and other rails are turned base up be tween them, thus making a nearly solid course of rails under each rail of the track.
Sometimes, instead of railroad rails, steel I-beams are used, which because of their greater depth are more economical as beams and permit•better embedment in the concrete.
Reinforced Concrete Box Culverts. Table 87, page 591, shows the estimated cost of the reinforced concrete culverts shown in Fig. 158 and 159, page 586, as given by the Committee.* The price of the concrete includes the cost of the excavation and of mixing and placing. The cost of the forms is figured at 21 cents per foot, B. M., for the original cost of the lumber, which it is as sumed will be used twice, making the actual cost of the lumber if cents per foot, B. M.; and the cost of labor is considered 3} cents per foot. • The following is the cost to the contractor of a 14- by 15 ft. box culvert 250 ft. long built near Kansas City, Mo., in 1905.t The -following is the cost of a reinforced concrete box Culvert to carry an irrigation canal under a creek, built in Montana in 19064 Rail-Top Culvert. The following is the cost of a rail-top culvert containing 113 cu. yd. of concrete and requiring 36 cu. yd. of excavation, built in Scranton, Pa., in 1907, for the Delaware, Lackawana and Western R. R. by contract.*