Members subjected to combined axial load and bending moment - Reinforced Concrete with FRP Bars: Mechanics and Design - page 172

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

x x

φ = 0.75

0.80φPn

ε cu

ε f1 = 0

c bal

ε f2

ε f3

ε f4 = ε fd

φ = 0.65

M nx

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 5.7 Rectangular section P n - M nx diagram.

When multiple levels of reinforcement are considered, the ϕ-factor is com-
puted based on the reinforcement level subjected to the largest tensile
strain.
Per ACI 318-11 steel tied columns, the design compressive strength of a
GFRP RC column should be limited to 0.8ϕ P n . For the case of GFRP RC,
this limit is extended to include spirally reinforced columns.
Typical P-M diagrams are shown in Figure 5.7 through Figure 5.9. The
interaction diagrams in Figures 5.7 and 5.8 are built for the same rectangu-
lar section with bending moment around the strong axis (x-axis) and weak
axis (y-axis), respectively. The interaction diagram in Figure 5.9 is relative
to a circular section having the same concrete area and amount of GFRP
reinforcement of the rectangular one.