Bzinforoed Oonorete Box Culverts

if the sides and the top are rigidly connected, the actual moment in the side will be less than that given by equation 3, because the flexure of the top produces a moment in the sides contrary to that due to the horizontal pressure of the earth.

Decrease of Section toward Ends. The preceding design has been limited to a unit section under the track; but beyond the ends of the ties the load decreases, and hence toward the ends of the culvert the amount of reinforcement and the thickness of the con crete could be made less than under the track. This decrease would probably not pay except in a comparatively long culvert.

Shear in Concrete. Since the span is comparatively short, the shear is small; and hence no provisions need be made for resisting shear except to bend up part of the main reinforcing bars.

Bond Stress. All plain rods should have an embedment beyond the point of maximum moment of 50 diameters (see * 472).

Longitudinal Reinforcement. If the supporting power of the earth is not uniform, a box culvert may act as a beam spanning the weak places; and hence in such cases longitudinal reinforcement may be advantageous. Longitudinal reinforcement would also re sist the formation of contraction cracks, but the range of temperature for a culvert is comparatively small, and hence no great amount of temperature reinforcement (I 503d) is required.

Examples.

K.-C., M. & 0. Ry. Fig. 156 shows a 3- by 3-foot reinforced concrete box culvert employed by the Kansas City, Mexico and Orient Railway.* Notice that this culvert has the perpendicular head wall, but on this road all sizes above this have splayed wings at both ends.

Ry. Fig. 157 shows the 6- by 6-foot box culvert employed by the Lake Shore and Michigan South ern Railway.t Notice the splayed win:: at the up-stream end and the straight wings at the down-stream end. Notice also that there is no floor or pavement, but that there are cross walls or baffle walls which prevent scour and also act somewhat as struts to hold the side wall in position. This road finds that it is frequently called upon

to lower the waterway of its culverts, and as a floor or pavement interferes with this more than the cross walls has adopted the latter instead of the former. Notice that there are no shear bars in the roof. This road omits shear bars, because of the difficulty of getting them properly placed; but keeps the shear in the concrete within a safe limit.

1161.

C. M. & St. P. Ry. Fig. 158,* shows the cross section of an 8- by 6-foot open-box culvert employed by the Chicago, Milwaukee and St. Paul Railway as designed for a 16-foot fill; and Fig. 159 shows an 8- by 6-foot closed box culvert as designed by the same road for a 32-foot fill. However, the unit stresses were not assumed to be the same in the two cases. For example, in the roof for the 16-foot bank the stress in the steel was limited to 12,600 lb. per sq. in., and that in the concrete to 500 lb. per sq. in., while for the 32-foot bank the corresponding limits were 14,900 and 600, respectively; and for the side walls, the limits were 13,200 and 15,000 lb. per sq. in. for the steel, and 400 and 450 lb. per sq. in. for the concrete, respectively. Such discrimination is hardly justified by the indefinite character of the data (see 5 1148).

Fig. 160, page 586, shows the forms employed in the construction of the culverts shown in Fig. 158 and 159.

Illinois Central R. R. Fig. 161, page 587, shows the standard 8- by 8-foot box culvert of the Illinois Central Railroad.

The culvert shown has straight win:: at both ends, but this road also builds culverts with splayed wings and with head walls perpendicular to the axis of the culvert. Notice, in the right-hand portion of Sec tion XX, that bars are placed on both sides of the wing, the additional bars being in an ticipation of the extension of the culvert to accommo date a second track. All the reinforcement is corru gated bars (§ 465), and is 3 inches from the nearest sur face of the concrete.