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Contribution of longitudinal reinforcement in punching resistance

Decembre , 22th 2020 | Author: Prontubeam (@Prontubeam_en) Read: 822 times

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On January 14 (2020), a children's playground collapsed over an underground car park in Santander, Spain. Miraculously, no victims are found so far. If we look at the images, it can be seen the parking columns "crossing trough" the playground. This is known as punching failure. This type of failure has already been seen in other cases like in the collapse of a swimming pool or a car park. In the following image we see, on the left, the Santander playground and in the other two images two examples of punching failure.

Punching shear examples

Figure 1. Punching shear failure examples

It is true that the total collapse of the playground may not have been solely due to the punching, since when this phenomenon occurs, the efforts change and other forms of collapse can occur after this, but the punching is the origin of the failure. It seems that the possible origin of this failure was that the drainage was not correct and that the overload produced by water and wetlands exceeded design loads. This accident made us share this article with you:

In this article we would like to write about the effect of the longitudinal reinforcement of the slab on its punching resistance and we would also like to share with you an important constructive detail to keep in mind while designing the reinforcement of our slab and column.

As it is known, in the punching shear resistance formulae, following the EC-2, there is a parameter to consider the influence of the longitudinal reinforcement ratio. The following formula, extracted from the EC-2, shows this parameter:

Punching formula

Figure 2.Punching shear resistance extracted from the EC-2 – Longitudinal reinforcement parameter circled in red

This term is defined as the longitudinal reinforcement in tension per linear meter of the slab above the column where the punching is produced divided by the effective depth d. The Eurocode specifies that if the reinforcement quantity is not the same around the column, an equivalent quantity per linear meter has to be calculated as an average of the reinforcement in a width equal to the width of the column plus 3d on each side. We see that in the punching resistance formula this term is multiplied by two other values (100 and fck) ​​and then raised to 1/3. This means that the effect of the longitudinal reinforcement is not linear, although if we look closely, they look like almost the same. If we raise an X term to 1/3 it would have this form:

Figura 3.Gráfica de un valor X elevado a 1/3

As it can be seen, the tendency in the studied area is almost lineal (not completely lineal, with a difference of 0.07%).

However, there is another reason to increase the longitudinal reinforcement on the rupture area, especially the reinforcement crossing over the column footprint: if, for any reason, there was a punching failure (brittle failure), there would still be the longitudinal reinforcement resistance working in pure shear; they would be our last chance. This will not avoid our structure from failing, but it will probably avoid our slab from falling into the lower level as it will be hung by those longitudinal bars placed over the column area. For this reason, we recommend to ensure that, at least, two longitudinal bars on each direction are place in-between the column reinforcement, to provide an additional resistance after the punching shear failure is produced.

We have explained the contribution of the longitudinal reinforcement to the punching shear resistance and why we need these bars working in in pure shear once the punching failure is produced. The following graph show different examples of the punching shear resistance (Vrdc-concrete) of a 20cmx20cm column in a fck=40Mpa concrete slab for different thickness and different reinforcement quantities ([email protected], C16/[email protected], [email protected], C20/[email protected] y [email protected]). The graph also shows the shear resistance (VRds-steel) of the bars crossing the column footprint, considering 2 or 3 bars on each direction.

Punching-resistance

Figure 4.Longitudinal reinforcement effect in the punching shear resistance(*) and the shear resistance of this reinforcement

 

(*)These results take into account neither the minimum concrete punching resistance proposed in the EC-2 nor the possible shear reinforcement already in place. It provides only the resistance given by the formula presented in Figure 2.

This graph shows that for small slab thicknesses (200mm), the reinforcement used to reinforce the slab provides more resistance in pure shear than the concrete punching resistance without additional reinforcement. Therefore, in case of punching failure, our slab will be hung by this reinforcement working in shear. If the thickness of the slab is increased or if punching reinforcement is used, this will be no longer the case, we will need to add additional longitudinal reinforcement or to increase the existing longitudinal reinforcement diameter. We would need to add as much reinforcement as to resist the punching shear design load.

It is needed to ensure, at least, two bars on each direction passing through the columns reinforcement. This recommendation is also provided in the Model Code for concrete structures and it is also included in the EHE-08 (Spanish code). These codes confirm that two bars are needed but they don’t specify the dimeter of these bars.

Additionally, the American code (ACI) also confirms this proposal. The following picture shows, extracted from the ACI, that there are 2 bars on each direction passing in-between the column reinforcement. It also provides the anchor length outside the punching shear rupture concrete cone:

Figura 5.Longitudinal reinforcement arrangement ACI 318-19

The section 8.7.4.2.2 of this code (ACI) is presented here below:

“At least two of the column strip bottom bars or wires in each direction shall pass within the region bounded by the longitudinal reinforcement of the column and shall be anchored at the exterior supports.

The continues column strip bottom reinforcement provided the slab some residual ability to span to the adjacent supports should a single support damaged”

It is advisable to ensure that, over the column, in total, considering the reinforcement on both directions, there is enough longitudinal reinforcement to resist, in pure shear, the punching design load.

The aim of this article is to guide the engineer in their punching calculation but, in any case, the numbers presented in this article could be used as design numbers. All the information provided has to be verified by the engineer before using it in any design. We recommend the engineer to read the applicable calculation code and to ensure that all the requirements are fulfilled, as they can be more restrictive than the ones mentioned in this article.

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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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