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(2) Pure Compression.
(a) The ultimate dynamic strength of a short reinforced concrete
column subjected to pure axial load (no bending moments) is given by:
EQUATION:
Po = 0.85 f'dc (Ag  Ast) + Ast fdy
(56)
where,
Po = maximum axial load, lb
Ag = gross area of section, in2
Ast = total area of reinforcing steel, in2
(b) A member subjected to pure axial compression is a
hypothetical situation since all columns are subjected to some moment due to
actual load conditions. All tied and spiral columns must be designed for a
minimum load eccentricity. This minimum design situation is presented in a
subsequent section.
(3) Pure Flexure. An interior column of a shear wall type structure
cannot be subjected to pure flexure under normal design conditions. For the
purpose of plotting a PM curve, the criteria presented for beams is used.
(4) Balanced Conditions.
(a) A balanced strain condition for a column subjected to a
dynamic load is achieved when the concrete reaches its limiting strain of
0.003 in/in simultaneously with the tension steel reaching its dynamic yield
stress, fdy. This condition occurs under the action of the balanced load,
Pb, and the corresponding balanced moment, Mb. At balanced conditions,
the eccentricity of the load is defined as eb, and is given by:
EQUATION:
eb = Mb/Pb
(57)
(b) The actual values of the balanced load and corresponding
balanced moment are generally not required. The balanced eccentricity is
the important parameter, since a comparison of the actual eccentricity to
the balanced eccentricity distinguishes whether the strength of the section
is controlled by tension or compression. The comparison of the actual
eccentricity to the balanced eccentricity dictates the choice of the
appropriate equation for calculating the ultimate axial load capacity, Pu.
(c) Approximate expressions have been derived for the balanced
eccentricity for both rectangular and circular members. These expressions
are sufficiently accurate for design purposes. For a rectangular tied
column with equal reinforcement on opposite faces, Figure 36a, the balanced
eccentricity is given by:
2.0892


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