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Figure 6.13. Shear strength of non-prestressed members without shear reinforcement, comparison of test results with Eq.

Figure 6.13. Shear strength of non-prestressed members without shear reinforcement, comparison of test results with Eq.

It was however argued, that the equation

has two disadvantages the first is that it does not distinguish between persistent & transient loading combinations and accidental loading combinations, for which different safety levels apply (prEN 19921-1:2001 chapter 2.4.1.4 gives yc = 1,5 for persistent and transient and yc = 1,2 for accidental situations). Therefore the equation was modified by introducing the concrete safety factor explicitly.

The second is that the shear capacity goes to 0 when pi = 0.

Furthermore it was wished to have a simple conservative value for VRd,c for a first check of the bearing capacity. In many countries simple formulations have been used on the basis of

where fctd is the design tensile strength of the concrete and C is a coefficient. Practice in the various countries however is quite different because C varies in the range from 0,3 to 0,75. Considering the value of C it should be noted that this equation is a simplification of the rigorous one. To have general validity, even for rare but still possible cases, C should be based on the most unfavourable combination of parameters. That means that the governing case is a slab with a large cross-sectional depth d and a low longitudinal reinforcement ratio.

In his paper "Basic facts concerning shear failure", Kani (1966) showed that shear failures are unlikely to occur for longitudinal reinforcement ratio's smaller than 0,6%. However, his "shear valley" was based on beams with a cross-sectional effective depth of only d = 270 mm. For larger depths the critical value of p0 decreases. Therefore a number of shear failures reported in literature have been selected with large d and small p0 values, see Table 6.8.

Table 6.8. Determination of C on the basis of selected tests

Beam

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