Members subjected to combined axial load and bending moment - Reinforced Concrete with FRP Bars: Mechanics and Design - page 163

I
I

E
E

( )

FRPRC f

= 0.80 + 25 ρ 0.80 +ρ 25 (5.16)

f st

steel RC s

column

Substituting Equation (5.9) and an average of ρ f = ρ st = 2.5%, after
rounding, Equation (5.16) can be simplified as

E
E

f

I = 0.40 + 0.30 I ≤ 0.70 I (5.17)

column g g

s

Similarly to ACI 318-11, lateral deflections resulting from service lateral
loads may be computed by a linear analysis using 1.4 times the flexural
stiffness defined in Equation (5.17).

5.5.6 Slenderness effects
The definitions of non-sway and sway frames adopted here for GFRP RC
conform to ACI 318-11. In fact, a frame can be considered non-sway (i.e.,
braced) when the column end-moments due to second order effects do not
exceed 5% of the first-order end-moments.
Sway column: According to ACI 318-11 [3], the effects of slenderness for
compression members in a sway frame may be neglected when the slender-
ness ratio ( kl u / r ) is less than 22. Mirmiran et al. [15] showed that for GFRP
RC columns not braced against side sway, this limit should be reduced to 17.
This value is therefore recommended as the new threshold for neglecting
slenderness effects for a GFRP RC column free to sway.
Non-sway column . The effects of slenderness in a steel RC column in a
non-sway frame may be neglected if

kl
r

kl
r

M
M

≤

≤

u u 1

= 34 -12 40 (5.18)

max 2

where M 2 is the larger end-moment and M 1 is the smaller end-moment. M 1
and M 2 are factored end-moments obtained by an elastic frame analysis
and the ratio M 1 / M 2 is positive if the column is bent in single curvature and
negative if bent in double curvature.
Taking into account the conclusion by Mirmiran et al. [15], Equation
(5.18) can be modified so that the effects of slenderness in GFRP RC col-
umns may be neglected when

kl
r

kl
r

M
M

≤

≤

u u 1

= 29 -12 35 (5.19)

2

max