Engineering Wonders of the World
Volume I
År: 1945
Serie: Engineering Wonders of the World
Sider: 448
UDK: 600 Eng -gl.
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424
ENGINEERING WONDERS OF THE WORLD.
In most buildings and engineering structures
it frequently happens that main beams are
continuous over columns or piers ; that second-
ary beams are continuous over the main beams ;
and that floor, deck, or roof slabs—which, in
reality are thin wide beams—are continuous
over both main and secondary beams.
Wherever such arrangements occur, the
relative positions of the tensile and compres-
sive stresses due to bending are reversed for a
short distance near each support, the tensile
stress being transferred from the bottom to
Fig. 15.—LONGITUDINAL SECTION OF HENNEBIQUE REINFORCED
CONCRETE BEAM.
stance. Adhesion between the concrete and
the steel, in spite of the stresses caused by
loading, is a matter of prime importance, and
without it the remarkable qualities of rein-
forced concrete could not be realized.
Fortunately, when steel bars have been
embedded in concrete, they are gripped so
firmly after that material has hardened that
there is no fear of slipping or other move-
ment. Numerous experiments made for the
purpose of determining the amount of the
adhesion between concrete and steel bars
show that the actual resistance
ranges from about 400 lbs. to
650 lbs. per square inch of the
steel surface in contact with
concrete.
Equality of expansion during
temperature changes is a very
important factor in the case of
materials used together; and
if there were any appreciable
difference between the expan-
sion of concrete and steel, the
Fig. 16.—REINFORCED CONCRETE MAIN BEAMS, SECONDARY BEAMS,
AND CONNECTING SLAB (HENNEBIQUE SYSTEM).
adhesion between the materials
might be destroyed in the event
the top, and the compressive stress transferred
from the top to the bottom. Consequently,
the bent-up bars represented in Fig. 15 are
just in the places where they can effectively
reinforce the concrete against tension in its
reversed position, while the horizontal parts
of the same bars are in the right place for
withstanding the horizontal tensile stress near
the lower surface in the middle portion of the
beam.
Before dismissing the subject of reinforced
concrete beam design, it is desirable to direct
Properties
of Concrete
and Steel.
attention briefly to three im-
portant physical properties of
concrete and steel, upon which
depends the possibility of
combining those materials in such manner that
their action is akin to that of a single sub-
of fire, as happens to terra-
cotta and kindred products when used to
protect ordinary steelwork from fire.
Nothing of the kind is to be feared in the
case of reinforced concrete, because the “ co-
efficients of expansion ” of concrete and steel
are virtually equal.
Another point to be considered is the extent
to which steel and concrete are stretched
when under tensile stress, as in the lower part
of the beam shown in Fig. 12.
To put the matter shortly, a steel bar of a
square inch section is stretched of
its original length by every pound of pull;
whereas a concrete bar of equal section is
stretched or fifteen times as much
as the steel. Since steel is able to stand a
very much greater tensile stress than con-
crete, the latter, in a reinforced concrete beam,