Design of column strengthening with CF sheets - Strengthening of Concrete Structures with Adhesively Bonded Reinforcement - page 114

0 : 8 ? N u
N Rk

k k ? f ck (7.76)

The recommended system coef cients [ k 8 ] and [ k 9 ] for the level of loading proposed by
Müller [112] are

k 0 : 89 (7.77)

k 0 : 0044 (7.78)
Within the scope of a national technical approval, however, other values can be verified
experimentally by way of suitable creep rupture tests.
With the above simpli cations and the use of the partial safety factors

γ s on the
resistance side plus γ F on the actions side, we get the following as a condition for the
load-carrying capacity of the con ned column at the serviceability limit state:

γ LG and

1
γ F

k k ? f ck ? A c ? α cc ? f ck A s ? ε c2 ? E s

1
γ LG ?

2 ? E L ? t L
D

1
γ s ? f yk ? A s

f ck k ? ? ε juk ? A c (7.79) ?

Here, γ F is the weighted partial safety factor for the actions corresponding to the
contributions of the permanent and variable loads to the internal forces combination at
the ultimate limit state. Solving for the theoretical thickness of the fibre cross-section of
the confining reinforcement t L results in the following expression given in the DAfStb
guideline [1, 2]:

D
2 ? E L ? ε

1
k 1

t L ? γ LG ? γ F ? k k ? f ck

juk ?

f ck

f yk
γ s

A s
A c ? ε c2

A s
A c

? α cc ? f ck

j ? E s

(7.80)

j

?

This criterion primarily governs when the longitudinal reinforcing steel only makes a
minor contribution to the load-carrying capacity, i.e. in the case of low reinforcement
ratios in combination with high uniaxial concrete compressive strengths. However, in
the majority of the applications for which the DAfStb guideline [1, 2] is valid, provision
RV 3.10.3 of the guideline governs. This provision is based on experience and speci es
that a maximum of 10 layers of CF sheet material may be attached. Together, the two
criteria guarantee that the loads on the concrete remain within the domain of exper-
imental evidence. In addition, redistribution to the longitudinal reinforcing steel is
possible, which leads to lower concrete stresses and a rapid reduction in the creep
deformations.