Design of column strengthening with CF sheets - Strengthening of Concrete Structures with Adhesively Bonded Reinforcement - page 87

7 Design of column strengthening with CF sheets

7.1 Principles
As with other materials, triaxial compression loads on concrete lead to an increase in the
compression that can be accommodated in the direction of the largest principal stress.
Just a hydrostatic lateral pressure amounting to 20% of the uniaxial strength f cm of
concrete results in a doubling of the admissible compressive stress; and the admissible
deformations also increase considerably. In contrast to a speci c load applied in the
transverse direction, the effect of con ning reinforcement resulting from the prevention
of lateral strain is regarded as a passive lateral pressure. Owing to the large deformation
capacity of the reinforcing steel, the normal situation in compression members with
helical reinforcement, for example, is that the disintegration of the concrete micro-
structure leads to failure of the member (in a similar way to a triaxial compression test
with hydrostatic lateral pressure). If the confining effect is achieved by including
transverse reinforcement in the form of fibre-reinforced materials with a virtually linear
elastic behaviour, then the lateral pressure rises continuously until the confining
reinforcement fails in tension. Figure 7.1 shows a schematic representation of the
effect of CFRP wrapping compared with a cross-section containing confining steel
reinforcement and an unconfined section.
When it comes to describing the loadbearing behaviour numerically, a distinction has to
be made between the load-carrying capacity of the cross-section, which essentially
depends on the material properties and therefore can be described by tests (e.g. multi-
axial compression tests) on small-format specimens, and the load-carrying capacity of
the member, which besides the material properties is also dependent on the geometry of
the member and the loading. Only in the case of a concentric load on a short column, in
which the in uence of slenderness can be excluded, is the load-carrying capacity of the
cross-section equal to that of the member.
The development of the principles for designing con ned concrete members is
attributed to the French engineer Armand Considére [110, 111], who in 1902 patented
a method for casting concrete elements with a high axial compressive strength. The
particular feature of this method was that a metal helix, with closely spaced windings,
was placed around the core of the concrete member. On the basis of his experimental
studies, Considére formulated an initial addition function that considered the increase in
the load-carrying capacity due to the confining reinforcement.
As early as 18 September 1909, the

' Circular decree concerning the design of concrete

columns with confining iron bars valid for the Kingdom of Prussia permitted an

increase in the load due to the confining effect of helical transverse reinforcement
according to Considére

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s method. The effect of confining reinforcement was subse-
quently described in numerous publications.
In the German language the studies by Müller [112] and Menne [113] are the most
important. The design method in DIN 1045 (see [94], for example) for con ned
compression members was based on their investigations and remained valid and
unchanged for more than 25 years. Müller's work was primarily based on tests on

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