Concrete Pipe Culverts.—Reinforced concrete culvert pipes are sometimes made from 18 to 48 inches in diameter, and in lengths from 4 to 8 feet. They usually have a hoop reinforcement, as shown in Fig. 97, passing near the interior surface at top and bottom and near the exterior surface at the sides, the reinforcement being bent to circular form and the pipe made in oval form with the larger diameter vertical. Concrete pipe is also sometimes made with a double reinforcement, one line near each surface. Table X11 gives dimensions recommended by the Iowa State Highway Commission for circular pipe with double reinforcement.
The load to be carried by a culvert under an embankment may usually he taken as equal to the weight of embankment. immediately above the culvert, and the live load carried by the roadway considered as distributed through the fill. For pipes in trenches the weight of filling is partly borne by the sides of the trenches. A study of pres sures on pipes in trenches has been made by Professor Marston at the Iowa State College, and the very interesting results published in a bulletin of the Engineering Experiment Station of the College.
A uniform horizontal earth pressure over the whole width of a pipe produces positive bending moments at the top and bottom sec tions and negative moments at the ends of the horizontal diameter which are each equal to where W is the total load and d the diameter of the pipe. The pipe must he uniformly supported over its whole width in this case. If it is supported only at the middle, as when laid in a flat trench, the moments at top and bottom will he about doubled. In laying pipe the bottom of the trench should he rounded to fit it, being cut a little deeper under the middle, so that the bottom is free, not quite touching the soil, and letting the pipe rest upon the soil at the sides. Depressions should also be dug for the sockets to prevent the pipes being supported at the sockets and thus subjected to longitudinal bending.
Pipe should be laid from the down stream end with the sockets upstream. It is also desirable to give a slight crown to the grade of the culvert to provide for possible settlement.
180. Box Culverts.—Rectangular culverts are commonly used for sizes too large for pipes. These may be open boxes consisting of a slab top resting upon sidewalls, or closed boxes, in which a bottom slab connects the bases of the side walls and distributes the load over the foundation soil.
Stone box czdrerts have been extensively used in the past, but are now being superseded by reinforced concrete; but where suitable stone is available, they may often be found satisfactory and eco nomical.
The side and end walls should be built of stone at least 6 inches thick, laid in cement mortar, and with frequent headers extending through the wall. The walls should extend downward sufficiently to obtain good foundations and to he safe from frost. The floor of the culvert between the side walls should be paved with stone, unless it is of material which will resist erosion.
The width of opening for stone box culverts is limited by the dimensions of the cover stones available and is never more than 4 or 5 feet. The cover stones should have a thickness al least one-quarter of the width of opening, and should have a bearing of about 1 foot upon each wall.
Concrete box culverts are somet lines constructed with a reinforced slab top resting upon side walls which may or may not be reinforced. The design of short bridges of t his type has been discussed in Chap ter IX. Where many culverts are to be constructed, it is common to adopt specific loadings and work out standard forms and dimensions to be used. Such standards have been adopted by many railways and State highway- departments. Table XXXI shows dimensions suit able for ordinary highway culverts 5 to S feet in span, to carry the loadings used in Section 156. The steel is to be placed 1 from the lower surface of the slab.
Closed box culverts of reinforced concrete are frequently used for small openings, as they require less headroom than arched openings and are easily applied when open ings are too large for convenient use of pipes.
The stresses in such a culvert cannot be ac curately determined on account of the indeter minate character of the loads. A load applied up on the top of the culvert produces an equal upward thrust upon the bottom of the culvert, as shown in Fig. 9S, which causes a moment tending to bend the top and bottom slabs inward and the sides outward.
Let b= widt h of culvert; h= height of sides; w =uniform load per foot; =bending moment in top and bottom slabs; bending moment at middle of sides; = bending moment at corners; =moment of inertia of sections of top and bottom; I2= moment of inertia of sections of sides.