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 1.54mAs
EQUATION:
eb = 0.20h +
(58)
b
and
EQUATION:
m = fdy/0.85 f'dc
(59)
where,
eb
=
balanced eccentricity, in
h
=
depth of rectangular section, in
b
=
width of rectangular section, in
As
=
area of reinforcement on one face of the
section, in2
For a circular section with spiral reinforcement, Figure 36b, the balanced
eccentricity is given by:
EQUATION:
eb = (0.24 + 0.39 ptm) D
(60)
and
EQUATION:
pt = Ast/Ag
(61)
where,
pt
=
total percentage of reinforcement
Ast
=
total area of reinforcement, in2
Ag
=
gross area of circular section, in2
D
=
overall diameter of circular section, in
(5) Compression Controls.
(a) When the ultimate eccentric load, Pu, exceeds the balanced
value, Pb, or when the eccentricity, e, is less than the balanced value,
eb, the member acts more as a column than as a beam. Failure of the
section is initiated by crushing of the concrete. when the concrete reaches
its ultimate strain, the tension steel has not reached its yield point and
may actually be in compression rather than tension. The ultimate eccentric
load at a given eccentricity, e, less than eb may be obtained by
considering the actual strain variation as the unknown and using the
principles of statics. However, equations have been developed by Whitney in
the paper Plastic Theory of Reinforced Concrete Design which approximate the
capacity of the column. These approximate procedures are adequate for
design purposes.
2.08-93
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