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Concrete and Rebar
Name: Herve
Status: Student
Grade: Other
Location: N/A
Country: United States
Date: June 2007
Question:
What is the purpose of steel in a
horizontal reinforced concrete beam? in a vertical
column? Does concrete need reinforcement more under
compressive or more under tensile stress?
Replies:
Hi Herve,
The purpose of steel reinforcing rods in any concrete is to
add strength when the concrete is stressed in tension.
Concrete is very strong in pure compression, but relatively
weak in tension. In a horizontal beam that is supported at
each end, a load placed in the middle causes the lower half
of the beam to be exposed to tensile stress, and the upper
half to experience compressive stress. Steel reinforcing
helps share the tensile stresses, resulting in greater
overall strength of the beam under load.
A vertical beam is mainly under compressive stress, and so
would seem not to need steel reinforcing. However, the
compressive stress can result in some local tensile stresses
that try to cause the beam to expand slightly. In this case
the steel acts to relieve these local "bulging" forces and
hold the beam together and prevent the propagation of small
cracks into larger, more dangerous ones.
Same purpose in both. Concrete has great compressive strength, but
relatively poor tensile strength. Steel adds the tensile strength
that concrete lacks.
Tim Mooney
The purpose of the steel reinforcement is basically the same in
any reinforced concrete beam. It is there to add more tensile
strength to the concrete.
Steel has tremendous tensile strength, yet its compressive strength
is not necessarily as great. Concrete is the other way around,
tremendous compressive strength (after all, it is basically composed
of rock, glued together REALLY well), but its compressive strength
leaves a little to be desired. After all, it is still just basically
rocks, glued together really well. Steel beams, or reinforcing bars,
can quickly overcome this disadvantage, and make for a very practical
building material.
Ryan Belscamper
Concrete is much weaker in tension than compression,
so it really needs the reinforcement of steel for use in tension.
But think through each situation...
usually any kind of stress results in some local tensile stress,
in some direction, at some part of the shape.
A horizontal concrete beam really needs longitudinal steel rods to increase
its bending strength.
The bending stress is caused by building loads resting on the middle of a
beam, between two vertical supports.
When bending stress pushes a straight beam to a slightly curved shape,
there is longitudinal compressive stress on the inside of the curve,
and longitudinal tensile stress on the outside of the curve.
So the reinforcement is always needed on the bottom across the middle,
and on the top of the ends if the beam is rigidly merged to the supporting
walls.
But usually they just use two layers, one on top and one on bottom, all the
way across.
The steel rods embedded in the end-mergings probably also prevents cracks
there
which would detach the beam from the supporting wall.
You can imagine that would be a catastrophic failure.
I guess that situation would be called shear-stress.
The vertical column:
A simple round column supporting a big load has mainly compressive stress,
but due to the elastic coefficients of the concrete (or any other
substance),
compressing it vertically makes it want to expand horizontally by a smaller
but similar percentage.
That stretches the concrete around the column's circumference.
With excessive loading, eventually some partially-vertical fracturing
occurs and pieces fly out sideways.
Then the whole column is weaker and it crumbles completely.
With moderate loading, the reason it cannot yield that way is the concrete's
tensile strength in the horizontal direction.
Which is not very great, of course.
So, steel screens wrapped circularly around, embedded inside the column,
add hoop strength to resist this mode of failure.
Steel sheets around the outside help too.
There are usually also embedded steel rods that run vertically.
I suspect they do not do much for compressive strength, if no
circumferential metal is there.
I think they are added mostly for bending strength, in case an earthquake
shakes the load side-to-side.
keep on thinking, Herve
Jim Swenson
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Update: June 2012
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