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0.12 Ocpffc

Fig 6.40. Verification of Eq. 6 with test results

0.12 Ocpffc

Fig 6.40. Verification of Eq. 6 with test results

6.4.2.3 Punching shear resistance of labs with punching shear reinforcement

In ENV 1992-1-1, Eq. 6.34, it was assumed that the contribution of punching shear reinforcement to the total shear capacity can be accounted for by

Adding this contribution to the punching shear capacity of a similar slab without shear capacity, according to Eq, 6.31, would gives the total punching shear resistance.

A first important question is if the summation principle of a concrete component and a punching reinforcement component is valid anyhow. In the descriptive model of Kinnunen and Nylander [15] for slabs without punching reinforcement the tangential compressive strain at the bottom face of the slab is the design criterion. A punching shear reinforcement would then only be able to limit the rotation of the kinematic punching shear mechanism and as such reduce the compressive strain in the critical area, so that the failure load is increased. A further argument against the taking into account in full of the reinforcement according to Eq. 6.34 is that it is hard to find adequately anchored punching shear reinforcement at both sides of a critical crack Therefore the punching shear reinforcement is not yet yielding when the concrete contribution is at its maximum. One can also say that due to the vertical movement of the punching cone, concrete component has already a reduced value at the moment that yielding of the shear reinforcement has been reached.

In literature two types of proposals are distinguished in order to cope with this phenomenon. One group of researchers, such as Moe [15], Pranz [6], Herzog [16] Petcu [17], Kordina,/Nolting [18] propose efficiency factors for the contribution of the shear reinforcement ranging from 0.80 down to even 0.25. Others, like Elstner/Hognestadt [19],and Regan [20], propose to use the summation principle, however with a reduced concrete contribution (efficiency factors ranging from 0.6 to 0.8). Fig. 6.41 shows a comparison of test results (punching failures) from Gomcs [22, 23], Yitzakhi [25] and Regan [24] with the formulation

where Vc is the concrete contribution (punching shear capacity of similar slab without shear reinforcement) and Vs is the contribution of the yielding steel.

In the prENV 1992-1-1:2001, the formulation according to MC'90 has been chosen, but with effective design strength of the shear reinforcement which depends on the slab depth, in order to account for the anchorage efficiency. This means that

VRd,cs = 0,75 vRd,c- ud + ZAsw fywd,eff sina (6.39)

where vRd,c according to Eq. 6.32b a inclination of the shear reinforcement fywd,eff design strength of shear reinforcement, according to fywd,eff = 250 + 0,25d < fywd (Mpa). ZAsw shear reinforcement within the perimeter considered.

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