Design of column strengthening with CF sheets - Strengthening of Concrete Structures with Adhesively Bonded Reinforcement - page 112

unstrengthened column, meaning that we must assume significant activation of the
confining reinforcement. Therefore, this approach is unsuitable. On the other hand,
ACI 440.2R-08 [119] contains recommendations for limiting the concrete stresses to 65%
of the uniaxial compressive strength and the steel stresses to 60%of the yield strength. This
level of stress is already reached with moderate concrete strengths and customary
longitudinal reinforcement ratios under the rare load combination, which means many
column strengthening measures would not comply with this criterion. It is also already
known from the creep tests carried out on confined compression members that levels of
loading that led to concrete stresses far in excess of the uniaxial compressive strengthwere
able to be carried permanently over test periods of several years (see [140). Therefore, the
provision in ACI 440.2R-08 [119] is not suitable either.
Verifying compliance with a maximum thickness for the con ning reinforcement as
required in the DAfStb guideline [1, 2] permits a level of loading on columns
strengthened with a wrapping of CF sheet which is based on the load-carrying capacity
of the unstrengthened reinforced concrete column. This procedure corresponds to the
provisions of the earlier editions of DIN 1045 for compression members with con ning
helical reinforcement. The equation for the helical reinforcement ratio given in the 1988
edition of DIN 1045 [94] is attributed to Müller [112]:

A w ? β Sw δ ? 2 : 3 ? A b 1 : 4 ? A k ? β R A s ? β S (7.66)

A w π ? d k ? A sw
s w

(7.67)

A k π ? d k
4

(7.68)

where:
A W cross-sectional area of helical steel reinforcement distributed over the column length
d k diameter on centre-line of helical bar

A sw bar diameter of helix
s w pitch of helix
β Sw yield strength of helical reinforcement
A b total cross-section of compression member
A k core cross-section of compression member
A s total cross-section of longitudinal reinforcement
β R characteristic concrete compressive strength
β S steel stress at 2 ‰ compressive strain.
Müller de ned the permissible level of stress in the con ned compression member such
that at the serviceability limit state (rare load combination), max. 80% of the calculative
load-carrying capacity N u of the unconfined reinforced concrete cross-section without
considering the ultimate strain could occur:

core diameter

=

N u A bn ? β R A s ? β S (7.69)

where A bn is the net concrete cross-section.