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