Background Knowledge - Strengthening Design of Reinforced Concrete with FRP - page 30

FIGURE 2.18 Column profile and cross section of Example 2.4.

Case B : P u = N u = 30 kips

N 30,000

2, 000 14

u

φ=φ× V 21 + λ fbd = 0.75 × ×+ 21 4,000 c cw

( )

2, 000 A × 12 g

×× 12 11.69"

= 14,496.36 lb = 14.5kips

φ< V 15 kips

c

d

S max == 5.85 in.(controls)

2

0.75 × 0.22 × 50 × 11.69

φ=φ+φ= + VVV 14.5 = 32.04 kips > V = 15 kips n c s u

5.5

2.4 INTERNAL REINFORCEMENT TO CONFINE RC COLUMNS
ACI 318-11 does not rely on increasing strength by means of core confinement using
internal reinforcement. However, as the load and deformation increase to an extent
that spalls off the concrete cover under axial compression, the code tries to com-
pensate for loss of strength from the spalled cover by introducing lateral reinforce-
ment that increases the strength by a comparable amount for concentrically loaded
columns. Since this confinement effect may only be effectively achieved in circu-
lar cross sections, ACI 318-11 limits that provision to spirally reinforced columns
(Section 10.9.3).

A
A

f
f

g c

ρ= 0.45 − 1

s

ch yt