The nominal moment strength of a doubly reinforced
concrete beam can be found in the manner given below;
With reference to the equivalent stress diagram as per
Whitney,
Force of tension due to tension rebar;
T = (area tension rebar) * (stress of
steel)
T = A_{s}
f_{y}
It is required that tension steel has yielded when failure takes
place. This makes sure that the section is not controlled by
concrete.
Foce of compression due to concrete;
C_{C} = 0.85f_{C}^{'
}b a
acting at a/2 from the top edge
Force of compression due to compression
rebar;
C_{S} = A_{S}^{'} (f_{s}^{'
} 0.85f_{c}^{'
}) acting at distance "d^{'} " form the
top edge.
for equilibrium of the section;
Compression = Tension.
T
= C_{C}
+ C_{S}
The above equation will give the value
of "a", and hence the nominal moment strength is
calculated by taking the moment of both the compressive forces about
the centroid of tension rebar area.
M_{n} = C_{c} (d  a/2)
+ C_{S}
(d  d^{'})
To learn more about strength of reinforced concrete beam please
visit solved
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example 92,
example 93
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