Design of strengthening measures with externally bonded CFRP strips - Strengthening of Concrete Structures with Adhesively Bonded Reinforcement - page 35

Fig. 3.13 Calculation of force in shear wrapping due to tensile force in bending reinforcement

Where a shear strap is necessary, it should be designed according to Figure 3.13 for the
design value of the acting tensile force according to Equation 3.85:

F LwEd F LEd ? tan θ (3.85)

When using externally bonded reinforcement, an approach that lies on the safe side is to
design the force at the end strap for the maximum force in the flexural strengthening that
can be accommodated by the end anchorage according to Equation 3.86. This is because
a larger force cannot occur in the strip at the end strap except when the shear strap counts
towards increasing the bond force of the flexural strengthening. The force in the shear
strap is, however, superposed on the maximum force that can be accommodated at the
end anchorage according to Section 3.7.2.

F LEd f bLk ; max ? b L ? t L (3.86)

3.5 Fatigue analysis
When checking fatigue for non-static loads, the DAfStb guideline can be used to analyse
the bond of the flexural strengthening in the form of externally bonded CFRP strips. As
the carbon fibres exhibit virtually no signs of fatigue, only the bond needs to be checked
for fatigue when using CFRP strips. Besides the fatigue of the strengthening system, the
concrete, reinforcing steel and prestressing steel must also be checked according to DIN
EN 1992-1-1 [20] in conjunction with its National Annex [21].
To verify the bond of externally bonded CFRP strips, the DAfStb guideline calls for a
quasi-fatigue strength analysis by limiting the change in force in the strip at the concrete
element between cracks. This approach was developed in DAfStb publication 593 [10]
(q.v. [88]) on the basis of numerous tests [10, 89, 90]. The DAfStb guideline contains
both a simplified analysis and also a more accurate approach. In the former it must be
shown that owing to the bond stresses occurring as a result of the maximum load,
the elastic range of the bilinear bond stress

- slip model is not exceeded. In the latter, the