Design of column strengthening with CF sheets - Strengthening of Concrete Structures with Adhesively Bonded Reinforcement - page 88

Fig. 7.1 The effect of con ning reinforcement
concentrically loaded, short (i.e. no or very little risk of buckling), confined columns
with a circular cross-section. However, using the results of the few tests with eccentric
loading available to him, he was already able to make a quantitative estimate of the
reduction in the axial force capacity caused by simultaneous bending. In contrast to
earlier editions of DIN 1045, the design concept based on Müller's work only allows
eccentricities amounting to one-eighth of the core cross-section lying within the
confining reinforcement, which means that the confining effect may only be assumed
for columns compressed over their full cross-section.
Müller also made a proposal for ascertaining the in uence of the flexural slenderness of
the con ned core which, however, did not find its way into any standard. It was not until
the studies of Menne [113] demonstrated that con ned columns with just moderate
slenderness ratios suffered a considerable decline in their load-carrying capacity DIN
1045 [114] was updated accordingly in 1975 [115].
Redesigning to DIN EN 1992-1-1 [20, 21] allows higher loads to be assigned to
reinforced concrete columns originally designed to DIN 1045 [94]. Therefore, in some
circumstances it is possible to avoid costly, elaborate strengthening measures. This can
be attributed to the lower partial safety factor for the portion of the load carried by the
reinforcing steel and the higher permissible compressive strains in the concrete, which
permit the reinforcing steel to be better utilized. Testing the concrete of the actual
member to establish its strength is another way of possibly avoiding the need for
strengthening measures. On the one hand, owing to the strength allowance and age
hardening, concrete strengths are often higher than the values of the strength classes
originally envisaged. On the other hand, knowledge of the material properties allows
lower partial safety factors to be used on the resistance side of the equation (see [116,
117], for example).
However, if a structure is to undergo major changes, e.g. the addition of an extra storey,
constructional measures to strengthen the vertical loadbearing members will be
unavoidable in most cases. So far, the methods dealt with here, such as wrapping
with fibre-reinforced materials, have not been covered by German standards or national
technical approvals and could only be used on the basis of individual approvals for
particular projects. In Germany, strengthening reinforced concrete columns on the basis