Design of column strengthening with CF sheets - Strengthening of Concrete Structures with Adhesively Bonded Reinforcement - page 89

of regulations approved by the building authorities could only be carried out using
sprayed concrete designed according to DIN 18551 [118].
As for reinforced concrete columns strengthened with sprayed concrete neither the shell
of sprayed concrete nor the additional reinforcement in that shell can be arranged with an
interlocking structural connection, the load-carrying capacity in the load transfer regions
must be guaranteed by the confining effect of the links in the original column and in the
sprayed concrete. Owing to the requirements regarding the minimum distance of the
reinforcing bars from the existing concrete and the concrete cover, a layer of sprayed
concrete always results in a signi cant enlargement of the column cross-section.
Strengthening with sprayed concrete is labour-intensive and has the disadvantages
of dust, noise and moisture, and therefore should only be used when a substantial
increase in a column's load-carrying capacity is necessary.
For reinforced concrete columns designed according to the earlier editions of DIN 1045
there are some strengthening assignments that fall midway between redesign and
sprayed concrete. It is for these projects that wrapping with fibre-reinforced materials
is to be recommended. This method has already been used frequently in Europe and
elsewhere, often on the basis of design codes or guides covering the subsequent
strengthening of reinforced concrete compression members by means of wrapping
with fibre-reinforced materials. Such codes already exist in, for instance, the USA [119],
the UK [120], Canada [121], Switzerland [122] and Italy [123]. However, it should be
noted that none of these documents deal fully with the issues relevant to design. Instead,
the analyses required are limited solely to the load-carrying capacity of the cross-section,
which has already been the subject of an almost incalculable number of experimental
studies involving small-format specimens. Other aspects, such as how the load-carrying
capacity of the member differs from that of the cross-section, the deformation behaviour
of the highly stressed concrete over time and the loadbearing behaviour of the fibre-
reinforced materials depending on the duration of loading and the ambient conditions,
are not addressed in these publications.
The objective of the subsequent con nement of a compression member is to increase
either its load-carrying capacity or its deformation capacity. The latter is very important
in countries where it is necessary to improve the seismic behaviour of members and
structures that do not comply with the design codes based on the latest findings. In
contrast to the strengthening of compression members, seismic loads mostly involve
shear forces as well, which induce relatively high flexural stresses. For compression
members designed to be concentrically loaded, flexural stresses are caused by an
unintended load eccentricity, prescribed in the relevant standard, and second-order
theory effects. These must be considered in the design in addition to axial forces and are
intensi ed by the creep of the concrete. Owing to the minimal eccentricities, the
members involved are therefore mostly loaded by predominantly axial loads. The
applications covered by the DAfStb guideline [1, 2] are thus restricted to strengthening
members loaded by axial loads with a small eccentricity, which corresponds to the
projects encountered in Germany.
In accordance with the scope of the experimental findings to date, the intentional
eccentricity in the DAfStb guideline is limited to one-quarter of the column diameter.