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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**