Members subjected to combined axial load and bending moment - Reinforced Concrete with FRP Bars: Mechanics and Design - page 160

COMMENTARY

The tensile rupture strain of GFRP bars exceeds 2%. Such an ultimate
strain would lead to unacceptably large deformations, if the full tensile
capacity of the bars were to be achieved. To avoid this, it is recommended
[2] that, for design purposes, the ultimate design strain not exceed a fixed
limit of 1%.

5.5.4 Limit on maximum spacing of
transverse reinforcement
As a result of the different characteristics between GFRP and steel, the
spacing (or pitch) of the transverse reinforcement, s max , should be limited to
the least of the following quantities:

• Least dimension of the column (same as ACI 318-11)
• Twelve longitudinal bar diameters (75% of the limit in ACI 318-11)
• Twenty-four tie bar diameters (50% of the limit in ACI 318-11)

COMMENTARY

The spacing between ties can be related to the diameter of the longitudinal
bars by a simplified model that assumes that the bar is a compressive member
simply supported between two adjacent ties. The lateral support provided by
the concrete cover is neglected as, at the point of failure, the loss of cover is
very probable. For such a member to reach a strain level of ε without buck-
ling, this condition must be upheld:

2
2

EI

π

≤ε

AE

(5.4)

s

where s is the spacing of the ties, I and A are the moment of inertia and the
area of the longitudinal bars, and E is their modulus of elasticity. For a solid
round bar of diameter d b , this can be rewritten as

d

π

s

b

=

(5.5)

max

4

ε