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Understanding punching and its reinforcement according to EC-2

April , 10th 2023 | Author: Prontubeam (@Prontubeam_en) Read: 2342 times

After reading various articles of punching failure, all of them seem to agree that Eurocode 2 is not very clear on the calculation of punching reinforcement. In addition, during my professional life, I have realized that this calculation creates certain doubts. The objective of this article is to discuss and understand the EC-2, to remember certain rules and to open a debate on the subject. It is also reminded that Prontubeam has an online tool to calculate punching and an interesting article about the effect of the longitudinal reinforcement on the punching resistance.

First of all, it must be remembered (and that is often forgotten) that the punching formulation is based on the assumption that the load producing the punching is a uniformly distributed load. In section 6.4.1 (5) of the Eurocode it says it quite clear:

Figure 1. Chapter 6.4.1 (5) EC-2 – Punching calculation hypothesis

This may be interpreted in two ways:

·         Typical case of punching of a column – The load has to reach the column, normally through a slab, in a uniform way:

Figure 2. Uniformly distributed loads on a slab transmitted to the column

The presence of high concentrated loads should alert us, it might lead to a local shear failure in a section more similar to a beam, rather than (or in addition) to a failure perimeter. An interesting example is the pile cap, receiving a column on its upper face. In this case, it should be verified a punching failure considering a perimeter limited by the piles and a shear failure in a plane between the column and each pile:

Figure 3. Different possible rupture sections in a pile cap

·         When there is a concentrated punctual load, the distribution in the slab where is applied is done uniformly in all directions. However, if for example, there is a wall below, close to the load, creating an area of high rigidity, we must think of a shear failure and put aside the concept of failure perimeter:

Figure 4. Shear failure mode (not punching) created by a concentrated load close to a support/wall below the slab

However, it is important to remind that even in the case of a column supporting a slab, the load may not arrive completely uniform. This occurs, for example, when the spans are not uniform in all directions. For these cases, the EC-2 introduces the beta coefficient that is used to consider the moment that occurs because the load does not arrive 100% uniform. We are not going into detail in the calculation of this beta coefficient since it is complicated enough to write a complete article about it.

Once the calculation bases are clear, we have to define the control rupture perimeter and the resistance of the concrete. In this article we are not going into detail about how to calculate the resistance of concrete, we are going to focus on section 6.4.5 of EC-2, the case when punching reinforcement is needed. We recall that the EC-2 proposes the following formulation:

Figure 5. Punching shear resistance following the EC-2

Let's see this formula in detail and how to interpret the term related to the punching reinforcement:

Figure 6. EC-2 Punching shear formula explanation

The Eurocode calculates the reinforcement in a distance of 1.5d, which would make a compression strut with an inclination of about 33º, different from the classic 45º

Figure 7. Punching compressed strut following EC-2

In the formula above Asw it is the amount of reinforcement (cm2) for each perimeter. It is not specified in the Eurocode how to consider this reinforcement if each perimeter does not have the same amount of reinforcement. In the following image we see an example in which in each perimeter the number of bars is different:

Figure 8. Punching shear reinforcement perimeters – Variable shear bars on each perimeter

In this case, it seems more interesting to think in “total reinforcement” placed in each 1.5d instead of in each perimeter of reinforcement, that is, the term:

The question that arises is from where to start counting the reinforcement to determine A1.5d. Since the Eurocode explicitly says that reinforcements at a distance less than 0.3d from the loaded applied area should not be considered and that punching failure cone has the following shape, it seems reasonable to count the available reinforcement in 1.5d starting from 0.3d, this means from 0.3d to 1.8d:

Figure 9. Punching shear reinforcement dispositions following EC-2

The EC-2 specifies that the reinforcement that can be taken into account for punching is placed at a distance equal to or greater than 0.3d from the loaded zone. However, the American ACI code specifies that the first perimeter of punching reinforcement must be at a distance of less than 0.5d, so it seems appropriate to place the first perimeter between 0.3d and 0.5d from the edge of the loaded zone. Additionally, the EC-2 indicates that the distance between one perimeter and another must be less than 0.75d while in the ACI it is 0.5d:

Figure 10. Punching shear reinforcement dispositions following ACI-318

We are not going to discuss this difference between the EC-2 and the ACI since it could also be linked to the terms of the formulation or decisions made when drafting each code.

Before finishing the article, it is provided, as a summary, the different requirements that according to the EC-2 the punching reinforcement must comply with:

Figure 11. Summary of the punching shear reinforcement dispositions following EC-2

All the conclusions of this article must be contrasted before being used, checking the calculation hypotheses of each particular case and understanding the code that is being applied.

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Carlos Corral . MEng Civil Engineering from the Politécnica university of Madrid. Speciality: Structural engineer. Owner and programer of Prontubeam.com and Prontubeam.com/en.
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