Bzinforoed Oonorete Box Culverts

BZINFOROED OONORETE BOX CULVERTS. In recent years reinforced concrete has come into great favor for small culverts and also for larger culverts where the head room is not sufficient to permit the use of an arch. Generally the closed-box type is employed; but occasionally, when the foundation is very firm and not likely to scour, the open-box is used.

Indefiniteness of Data.

It is not possible to secure mathe matical accuracy in the design of the cross section of a culvert for the following reasons: 1. The law of the pressure of earth is not known (see 1 998 1011). The relation between the pressure and the depth of earth is not known for a homogeneous mass devoid of cohesion; and in any particular case the load upon the roof of the culvert varies greatly with the nature of the filling and the method of depositing it. It is possible that under certain conditions a considerable part of the prism of earth vertically above the culvert may be supported by arch-like action against the sides of the excavation; and on the other hand, it is possible that the culvert may support a mass of earth which is wider at the surface than the span of the culvert. However, it is reasonably certain that the higher the embankment, the less the proportionate load upon the culvert top, and that at some unknown height any increase in height will not increase the load upon the culvert.

2. The effect of the rail, the ballast, and the earth in distributing the live load parallel to the track is not known. It is reasonably certain that the live load is transmitted downward in diverging lines; but there is no experimental data as to the law of this distribution. However, since the weight of the maximum train is nearly the same as the weight of the largest locomotive, it is safe and reasonably correct to assume that the live load is uniformly distributed along the track and is transmitted vertically downward to the culvert top.

3. The effect of the tie, the ballast, and the earth in distributing the live load perpendicular to the track is not known. It is fre

quently assumed that the live load is carried down at a slope of horizontal to 1 vertical, from the end of the tie.

4. The effect of the live load upon the horizontal component of the earth thrust is not known. Some designers in computing the horizontal component of the earth pressure assume that the live load is equivalent to an equal weight of earth; while experiments seem to show that the live load does not materially affect, the hori zontal component (11008).

5. There are no experimental data as to a reasonable allowance for the effect of impact due to the motion of a railroad train. The allowance should be greatest for a short span under a shallow bank, and least for a long span under a high bank. The allowance by different designers varies greatly, some allowing 100 per cent for shallow banks and decreasing as the height increases; and a designer who is frequently quoted, either directly or indirectly, allows 50 per cent for impact on all banks up to 40 ft. high. It is probable that the effect of impact upon culvert tops is not very great, since (1) the elasticity and inertia of the earth neutralize a considerable part of the effect of impact, and since (2) experiments show that the repeti tion of the load develops cohesion, and hence part of the load will be carried by the beam-like action of the earth filling. Probably any allowance for impact, except possibly for spans of, shy, not more than 10 feet under banks less than 5 feet high, is largely an illusion.

6. The degree of restraint of the four sides of the reinforced con crete box culvert is not known. Some designers assume that the cover and the bottom have fixed ends, while others assume them to have free ends. Further, when the top and the bottom are assumed to be monolithically connected to the sides, the effect of the resulting moment is usually neglected in determining the resistance of the sides to the horizontal component of the earth pressure.