Definition of Under Reinforced Section And Over Reinforced Section
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Definition of Under Reinforced Section And Over Reinforced Section
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Under-Reinforced Beam Section
If less amount of steel used in the
section than that of a balance section is called under reinforced section. In such condition
the stress the stress in steel will reaches its concrete remain below the
permissible stress. Under such condition the beam will fails due to over stress
in the steel. The steel provided in the beam is such that steel reach the
limiting values of strain prior to concrete. This results in yielding of the
steel and the steel could yield till it attains the ultimate strain at which
point it breaks. It is important to notice that it is the yielding stain that
is considered as failure strain in steel and not the ultimate fracture strain.
The steel still has to undergo strain hardening, softening and then fracture.
In under reinforced section, the depth of actual Na is less than the critical
Na.
i.e.; Na<Nc.
Reinforced concrete beam
sections in which the steel reaches yield strain at loads lower than the load
at which the concrete reaches failure strain is called under-reinforced sections.
Every
singly reinforced beam should be designed as under-reinforced sections because
this section gives enough warning before failure.
Yielding
of steel in under-reinforced beam section does not mean the structure has
failed, as when steel yields, excessive deflection and cracking in beam will
occur before failure which gives enough time to occupants to escape before the
section fails.
The
failure in under-reinforced beam section is due to the concrete reaching its
ultimate failure strain of 0.0035 before the steel reaches its failure strain
which is much higher 0.20 to 0.25.
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| Under Reinforced Section And Over Reinforced Section |
Over Reinforced
Section
Reinforced concrete beam sections
in which the failure strain in concrete is reached earlier than the yield
strain of steel is reached, are called over-reinforced
beam sections.
In an over reinforced section, the percentage of steel provided is greater than the
balanced section. So, the actual neutral axis shifts downward i.e., n>nc. In this section, stress in
concrete reaches its permissible value while steel is not fully stressed.
Concrete is brittle and it fails by crushing suddenly. As steel is not fully utilized,
the over reinforced section is uneconomical (steel is much costlier than
concrete). The various features of over reinforced s section are:
(i)
Concrete is fully stressed while steel is not (i.e., the stress in concrete is
at its permissible value σcbc but
stress in steel is less than σst).
(ii)
The actual neutral axis is below the critical neutral axis i.e., n > nc.
(iii)
The percentage of steel is more than the balanced section, so the section is
uneconomical.
(iv)
Sudden failure.
If more
steel used in the section than that of a balance section is called over reinforce section. The steel is not
fully stressed to its maximum permissible tensile stress, which the
comprehensive stress in concrete at extreme fiber reaches its maximum
permissible value in this case the beam will fails initially due to over stress
in concrete.So, in over reinforced section, the depth of actual Na is greater
than the critical Na.
i.e.; Na > Nc
Xumax Equation:
Xumax / 0.0035 = d-xumax / (0.87fy/Es + 0.002),,,
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