The Bennington evening banner. (Bennington, Vt.) 19??-1961, 1912-07-09

quarter times ms much, or a load OF
three and three-quarter tons, safely,
while If the same beam be built into
a King truss beam the load concen-
trated at the center may then be III
creased to nine tons.
The best culverts and smaller bridges
are built Of REINFORCED concrete. The
cost is greater than for wooden or pipe
constructions in the first instance, but
If well built there should be no fur
cost is the last cost, while durability
and safety are secured from the out
set
Good materials, consisting of crush
ed stone or gravel, sand and portland
cement and water, are required for con-
crete Defo2med steel rods imbedded
in the concrete are used for strength
ening the cover spans. The mixing of
these materials into concrete and plac-
ing 1t in the forms are extremely sim.
pie matters after they are once well
understood. but nevertheless should not
be undertaken by one who is unfamil-
lar with the use of concrete.
There are three general kinds of con
crete culverts, which are known as the
concrete box type. They are built for
the smaller sizes up to such as have an
opening about four feet by six feet
wide. For sizes above that the floor is
usually left ont where il is not needed
to protect the foundation, or paving
may be substituted for it. The floor
and side walls are constructed of con
crete with or without metal reinforce
ment, usually without such REINFORCE
mentS. The proportions of concrete
used for the floor and side walls are
usually. /, 3. 6-that is, one part by
measure of portland cement. three
parts by measure of sand and six parts
by measure of crushed stone or gravel.
For spans above ten or twelve feet the
cover needs 10 be strengthened with
concrete beams. This type is known
as the concrete T beam from its resem-
blance to the capital letter. They are
placed adjacent t. each other. the dis
lance from center to center depending
upon the load which the bridge is ex-
pected to carry.
The best type of concrete culverts for
spans from ten feet up to thirty feet is
the steel beam incased in concrete, rs
shown in Fig. l. Here the concrete
floor is designed to carry the load
across the span from one beam to an
other, while the steel | beams carry
the load from one abutment to the 9th
er. 1t often happens that the culverts
are built on yielding foundations and
that the abutmentS sometimes settle.
causing cracks that would be danger
GUS in some types of culverts, but it is
the ability Of the steel | beam type TO
withstand such conditions as these just
mentioned that makes it the best type
to build. The beams are incased In
concrete to protect them from rusting.
Sometimes they are simply painted or
more often not painted at all, and what
would be n permanent bridge is al
lowed to rust out for lack of proper
care.
For details In regard to the methods
of designing and building concrete
bridges attention is called to bulletin
No. 30. Highway Bridges and Cul-
verts," issued by the office of public
roads, United States department OF
agriculture.
No road can be called good road
that is dotted with broken, wornout
and unsafe wooden culverts and
bridges. such as are encountered on