Design of column strengthening with CF sheets - Strengthening of Concrete Structures with Adhesively Bonded Reinforcement - page 113

This is effectively equal to limiting the structural response that can be assumed for the
confining effect. The level of loading suggested by Müller [112] makes use of the
provisions in the 1959 edition of DIN 1045 [145], which were intended to prevent the
concrete shell outside the helical reinforcement from becoming detached as a result of
cracking at the serviceability limit state. This rule was valid up until 2001 and can be
regarded as proven, because no damage has been discovered to date.
The Equation 7.37 specified for the design value of the axial load capacity of a column
wrapped with CF sheet was simpli ed for the required limit to the degree of

strengthening.
θ 1 : 0 (7.70)

α 1 1 : 0 (7.71)

sin 2 ? π ? θ

1 1 : 0 (7.72)

2 ? π ? θ

θ c θ t 1 : 0 (7.73)

Only the con ning effect of the CF sheet is used for the compressive strength of the
con ned concrete:

2 ? E L ? t L
D

f cck f ck k ? ? ε juk (7.74)

The theoretical load-carrying capacity (characteristic value) of the uncon ned
reinforced concrete column to DIN EN 1992-1-1 [20], considering the strain compati-
bility, is

N Rk A c ? α cc ? f ck A s ? ε c2 ? E s (7.75)

where:
A c concrete cross-section
α cc reduction factor for uniaxial concrete compressive strength in structure: α cc = 0.85
f ck characteristic uniaxial concrete compressive strength
A s total cross-section of longitudinal reinforcement
ε c2 value of permissible compressive strain in concrete according to DIN EN 1992-1-1
[20] Tab. 3.1; taking into account the favourable effect of the creep of the concrete
is permitted for small eccentricities according to DIN EN 1992-1-1/NA (NCI) [21]
section 6.1 (3)P
E s modulus of elasticity of longitudinal reinforcing steel: E s = 200 kN/mm 2 .
Comparing the characteristic value of the theoretical load-carrying capacity of the
unconfined reinforced concrete column to DIN EN 1992-1-1 [20, 21] with the
stipulation by Müller [112] leads to the following expression in the range of permissible
longitudinal reinforcement ratios, which is only approximately linearly dependent on
the compressive strength of the concrete: