Members subjected to combined axial load and bending moment - Reinforced Concrete with FRP Bars: Mechanics and Design - page 173

ε cu ε cu

ε f1 = 0
ε f2 = 0
ε f3 = 0
ε f4 = 0

ε f1 = 0
ε f2 = 0
ε f3 = 0
ε f4 = 0

P n

y

y

φ = 0.75

0.80φPn

ε cu

ε f1 = 0

c bal

ε f2

ε f3

ε f4 = ε fd

φ = 0.65

M ny

φ = 0.65

φ = 0.65

ε f1

ε f1 = ε fd
ε f2 = ε fd
ε f3 = ε fd
ε f4 = ε fd

ε f2

ε f3

ε f4 = ε fd

Figure 5.8 Rectangular section P n - M ny diagram .

5.8 COLUMNS SUBJECTED TO AXIAL
LOAD AND BIAXIAL BENDING

A column is subjected to biaxial bending when equally significant bend-
ing moments act in two orthogonal directions. For the design of an FRP
RC rectangular cross section subjected to moments about two axes, the
same approach valid for steel RC appears to be applicable. Two analysis
methods are herein proposed. They are the reciprocal load method and
the load contour method. Both methods refer to a three-dimensional (3-D)
failure surface to describe the interaction of the nominal axial load and the
nominal biaxial bending moments. For example, the interaction surface for
compression combined to biaxial bending is illustrated in Figure 5.10.