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Placing these two expressions equal to each other, and solving for h, we find: Assume that a concrete culvert is covered with 6 feet of earth weighing 100 pounds per cubic foot. Assume a live load on top of the embankment equivalent to 500 pounds per square foot, in addition; or that the total load on the top of the concrete slab is equivalent to 1,100 pounds per square foot of concrete slab. Assume that the concrete slab is to have a span (1) of 4 feet. Then the total load W on a section of the concrete slab one foot wide will be 1,100 X 4 = 4,400 pounds. Assume that the stone is sandstone, with an average ultimate modulus of 525 pounds per square inch (see Table XII), and that the safe value R is assumed to be 55 pounds per square inch, or 144 X 55 pounds per. square foot. Substituting these values in the above equation for h, we find that h equals 1.29 feet, or 15.5 inches. The above problem has been worked out on the basis of the live load which would be found on a railroad. For highways, this could be correspondingly decreased. It should be noted that in the above formula the thickness of the stone h varies as the square root of the span; therefore, for a span of 3 feet (other things being the same as above), the thickness of the stone  equals 15.5 X4-=13.4 inches. For a span of 2 feet, .the thickness should be 15.5 X 4 -- = 11.0 inches. Owing to the uncertainty of the true transverse strength of building stone, as has already been discussed in the design of Offsets for Footings (see sections 181-183), no precise calculation is possible; and therefore many box concrete culverts are made according to empirical rules, which dictate that the thickness shall be as follows: For a 2-foot span, 10 inches; For, a 3-foot span, 13 inches; For a 4-foot span, 15 inches. These values are slightly less than those computed above. Although a good quality of granite, and especially of bluestone flagging, will stand higher transverse stresses than those given above for sandstone, the rough rules just quoted are more often used, and are, of course, safer. When it is desired to test the safety of stone already cut into concrete slabs of a given thickness, their strength may be computed from Equation 8, using the values for transverse stresses as already given in Table. Xii. A box concrete culvert with a stone top is generally limited by practice to a span of 4 feet, although it would, of course, be possible to obtain thicker stones which would safely carry the load over a considerably greater span. Therefore, when the required concrete culvert area demands a greater width of opening than 4 feet, and when this type of concrete culvert is to be used, the concrete culvert may be made as illustrated in Fig. 74, by constructing an intermediate concrete wall which supports the ends of the two sets of cover-stones forming the top. A section and elevation of a double box concrete culvert of 3 feet span and a net height of 3 feet, is shown in Fig. 74. This figure also gives details of the concrete wing walls and end concrete walls. The double box concrete culvert illustrated in Fig. 75 has two spans, each of 4 feet. The stone used was a good quality of limestone. The cover-stones were made 15 inches thick. The ends of a concrete culvert are usually expanded into end concrete walls for the retention of the embankment. For the larger concrete culverts, this may develop into two concrete wing walls which act as retaining concrete walls to prevent the embankment from falling over into the bed of the stream. An end concrete wall is especially necessary on the upstream end of the concrete culvert, so as to avoid the danger that the stream will scour the Pig. 75. Double Box Concrete culvert, 4 by 3-Foot bank and work its way behind the concrete culvert concrete walls. The end concrete wall is also carried up above the height of the top of the concrete culvert, so as to guard still further against the washing of earth from the embankment over the end of the concrete culvert into the stream below.

Are You in Litchfield New Hampshire? Do You Need Concrete Cutting?

We Are Your Local Concrete Cutter

Call 603-622-4441

We Service Litchfield NH and all surrounding Cities & Towns