Punching shear & Reinforcement ...

Punching shear is a type of failure of reinforced concrete slabs subjected to high localized forces. In flat slab structures this occurs at column support points. The failure is due to shear. This type of failure is catastrophic because no visible signs are shown prior to failure. Punching shear failure disasters have occurred several times in this past decade. An example of punching shear failure can be see in image.

Flat Slab Failure due to Punching shear

Punching Shear in Flat Slabs

A typical flat plate punching shear failure is characterized by the slab failing at the intersection point of the column. This results in the column breaking through the portion of the surrounding slab. This type of failure is one of the most critical problems to consider when determining the thickness of flat plates at the column-slab intersection. Accurate prediction of punching shear strength is a major concern and absolutely necessary for engineers so they can design a safe structure.

General Mechanism of Failure

Conventional wisdom does not apply when considering the mechanism of a punching shear failure; in a slab system with a concentrated load or at a slab column connection, the loaded area is not actually pushed through the slab as shown in Fig. Punching shear failures arise from the formation of diagonal tension cracks around the loaded area, which result in a conical failure surface as illustrated in Fig.

Misconception of Punching Shear Failure

Correct Representation of Punching Shear Failure

Punching shear is a phenomenon in flat slabs caused by concentrated support reactions inducing a cone shaped perforation starting from the top surface of the slab. Although generally preceded by flexural failure, punching shear is a brittle failure mode and the risk of progressive collapse requires a higher safety class in structural design.

Punching Shear Design

The design to prevent punching shear failure proceeds as:

1. Check if the concrete is strong enough alone;
2. If not, check if the amount of reinforcement is reasonable;
3. Design reinforcement if reasonable, if not, change form of structure.

The weight of a slab supported on a column induces shear stresses in the slab. These stresses, if sufficient and where additional reinforcement is not provided, would result in the column ‘punching’ through the slab. This punching shear is similarly induced in the footing on which the column bears.

Although punching shear can be relieved by localised thickening of the concrete with downstand beams and enlarged column heads, the construction of flat slabs offers many advantages. A consistent head space can reduce the overall height of a building and provide significant time and material savings.

Ancon Shearfix
Used within a slab to provide additional reinforcement around columns, Ancon Shearfix is the ideal solution to the design and construction problems associated with punching shear.

Changing the form of structure includes deepening the slab, making the column larger, introducing drop panels or flared column heads. There is also the possibility to adapt foreign codes of practice which are more liberal! The reinforcement put in is usually vertical and traverses the potential failure line. Of course, we don’t know where the failure plane might be, so we must reinforce each possible failure plane.

Various punching failure zones

Research has also been conducted in the past to develop an understanding of why punching shear occurs and how to prevent it. In recent years, the finite element method has been applied to analyze punching shear failure problems.  It can be used to develop an analytical model for the punching shear failure analysis of reinforced concrete plates. Furthermore, it has been discovered that punching shear can be prevented by increasing the depth of the concrete floor slabs, or by increasing the diameter of the columns supporting the floor.

Building Collapsed due to punching failure