The Mankato free press. [volume] (Mankato, Blue Earth County, Minn.) 1879-1924, 1912-04-26

be strengthened and states the cor.
responding loads that will be carried
safely. That is, the simple beam u
inches by IL inches square and "a feet
between points of support will carry
safely concentrated load Of one and
one-half tons at its center, while If the
same beam be made into King post
beam by passing underneath the beam
a one inch steel rod. which is made
fast al either end Of the beam. and in
sorting II single post under the load at
the center of the beam. then such a
beam will carry about two and three
quarter times as much. or load of
three and three quarter tons. safely,
while IL the same beam be built into
a King truss beam the load concen-
traced at the center may then be in
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
II well built there should be no fur
Their cost for repairs. That is, the first
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 DefOrmed steel rods imbedded
in the concrete are used for strength
amino tho Tho win "a
these materials into concrete and p.ac-
ing 1t I'm the forms are extremely SIM
pie matters after they are once we'll
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 out where it 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, G--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 to be strengthened with
concrete beams. This type is known
as the concrete # beam from it's resem-
blance to the capita! letter. They are
placed adjacent To. each other. the dis
tance from center to center depending
upon the load which the bridge is ex-
pected to carry.
The best type of concrete culyerts for
spans from ten feet up to thirty feet is
the steel beam incased in concrete. as
shown In Fig. 1. 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 l beams are incased in
concrete to protect them from rustiDg
Sometimes they are simply painted or
more often not painted at all, and what
would be a 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. 80, Highway Bridges and Gull
yefts," issued 10y the Office of public
roads. United States department of
agriculture.
No road can be called good road
that Is dotted with broken, WORKOUT
and unsafe wooden culverts and
bridges. such as are encountered on
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