REINFORCED CONCRETE CULVERTS The difference between a culvert and a bridge is not very clearly marked; a large cul vert may be called a small bridge, and a small bridge is but little different from a culvert. The two structures may best be distinguished by their purpose; a bridge is intended as a crossing over a stream or gulch, while a culvert is needed to allow a creek or ditch to pass under a road. A culvert serves as a drain or conduit, and is covered by the road embankment; for this reason it must be composed of a material which will not rot under the most adverse con ditions of alternate wet and dry periods. Bridges are often built of considerable lengths when an embankment with a small culvert would be much more desirable because of the greater stability and permanence of the cross ing and the reduced cost of repairs and renewals.
There are three distinct types of culverts: Box-culverts, which are rectangular in section; arch culverts, which have arched tops and in some cases arched floors; and pipe culverts, which are circular or elliptical in section.
Box culverts (see Fig. 56, A and B, and Fig. 57) were formerly built of wood or stone, and later of concrete, with covers of old rails; but reinforced concrete is now the standard con struction. Arch culverts (see Fig. 56, C and D, and Fig. 58) are built of concrete, either plain or reinforced, the more modern material having replaced stone masonry.
Culvert pipes of cast iron and clay products are still used, but concrete has invaded this field also, and it will probably surpersede the other materials.
Fig. 59 shows an outline plan for a standard railway culvert. Fig. 60 and Fig. 61 contain curves for the design of culvert tops and side walls, for various spans and lengths of fill. The cluwes in Fig. 60 give the thickness, amount of steel, size and spacing of reinforcing bars, and other information for designing the cover of culverts of various spans under various heights of fill. The method of using these curves is very simple. The height of fill, or dis tance from the top of the culvert to the track, is read on the top or bottom border of the plate, and the vertical line is followed to an inter section marked with the required span-length; from this intersection, the horizontal line is fol lowed to the right, and in the first column, the depth of the slab is given. The second, third, and fourth columns give the spacing of the re inforcing bars for three sizes respectively, and 1 inch square. The fifth and sixth columns explain themselves.
The curves in Fig. 61 give the required thick ness and reinforcement for the side walls of culverts; and they are used in the same way as the curves for culvert covers in Fig. 60. Plate 12 (A and B) shows views of typical railway culverts.
The following general instructions for the design of railway culverts are intended to sup plement the outline drawing, Fig. 59 and the curves in Fig. 60 and Fig. 61.