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Failure Modes in Concrete Beams: Flexural and Shear Failure

Failure Modes in Concrete beams

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Failure modes in reinforced concrete beams are classified into two major types: flexural failure and shear failure. The former occurs when the imposed load exceeds the flexural capacity of the materials of the beam, while the latter occurs due to deficiency in shear resistance between different materials of the beam.

These failure modes are further divided into different kinds of failure; the former is categorized into tension failure, compression failure, and balanced failure whereas shear failure is categorized into tension shear failure and compression shear failure. The nature and mechanism of these failure modes are different.

Some of them are favored in terms of design, but there are others that need to be avoided in order to prevent severe collapses of structures. For instance, the crushing of concrete and shear failure modes are undesired since they occur suddenly without any warning.

Flexural Failures

This failure mode occurs when the loads on the beam exceed its flexural capacity. The shear strength of the beam should be greater than its flexural strength otherwise the shear failure would occur before flexural failure. The flexural failure is divided into three types which are discussed below.

1. Flexural Tension Failure

Flexural tension failure initiates by yielding of steel reinforcement followed by crushing of concrete at compression side of the beam. It occurs when the beam is under-reinforced; the reinforcement ratio in the beam is low than balanced reinforced ratio as per ACI 318-14.

The signs of this type of failure are the development of cracks at the tension side of the beam which further extend to the compression side. These cracks are mostly vertical and located at the middle third of the beam. Great deflection is another sign of flexural tension failure.

In summary, the flexural tension failure happens gradually i.e. ductile failure and it is desired failure type in beam design process.

Fig. 1: Flexural Failure in Beam

2. Flexural Compression Failure

The flexural compression failure begins by crushing of concrete at compression side followed by yielding of steel at tension side of the beam. It occurs when the beam is over-reinforced which means the beam reinforcement ratio is greater than balanced reinforcement ratio as per ACI 318-14.

This type of failure is sudden and does not provide warning i.e. brittle failure. That is why it is not desired from reinforced concrete design point of view.

This type of failure can be prevented by avoiding over-reinforced concrete beam design or increasing compression strength of concrete by introducing steel reinforcement at the compression side or increasing the geometry of the beam.

Fig. 2: Flexural Compression Failure

3. Balanced Failure

It is a type of failure in which concrete crushes and steel yields simultaneously. It occurs when the amount of steel in the beam is equal to balanced reinforcement ratio as per ACI 318-14.

Shear Failure Modes

Shear failure occurs when the beam has shear resistance lower than flexural strength and the shear force exceeds the shear capacity of different materials of the beam. A shear load is a force that tends to produce a sliding failure on a material along a plane that is parallel to the direction of the force.

This type of failure is sudden and provides no warning i.e. brittle failure. The effective span to depth ratio of the beams and its size are important parameters in determining the type of shear failure.

Shear failure is an undesired type of failure and commonly stirrups are placed in the beam to prevent the shear failure. The shear failure mechanism is characterized by shear sliding along a crack in beam without shear reinforcement and yielding of stirrups in a beam with shear reinforcement. It is classified into the following forms:

1. Diagonal Tension Failure

Diagonal tension failure begins with the development of vertical crack (flexural cracks) at the bottom of the beam due to flexural tensile stress. Then, as the load on the beam increases, this crack growth both in width and length and bends in a diagonal direction as it moves to the upper part of the beam toward the loading point. After that, the last stage of shear tension failure occurs which is a sudden failure of concrete in shear.

This mode of failure is common in beams with low or no web reinforcement. Diagonal tension failure occurs typically in beams with a shear-span to depth ratio (a/d) greater than 2, but could occur also for lower values of a/d.

Fig. 3: Diagonal Tension Crack Development

2. Shear Compression Failure

Shear compression failure begins by initiation and development of cracks in the beam cross-section. Then, these cracks propagate and penetrate the compression zone of the beam, and the final stage of the failure occurs when the compressive strength of the concrete is exceeded.

The crushing of concrete takes place at the tip of the diagonal crack in the area around the point of load application. Fig. 4 illustrates the development of cracks at the location of shear compression failure in a beam.

Shear compression failure is mainly related to high amount of shear reinforcement. Finally, shear compression failure can occur in beams with span to depth ratio of less than four.

Fig. 4: Shear Compression Failure of a Beam

3. Splitting Shear (True Shear) Failure

When the shear span to depth ratio of a beam is less than one, splitting shear failure can be expected. Commonly, this type of beam is called deep beam in which loads are directly transferred to supports, and shear strength is much higher than in ordinary flexural beams.

Sometimes, failure in compression of the region adjacent the supports may occur instead of splitting shear failure.

Fig. 5: True Shear Failure or Splitting Shear Failure

4. Anchorage failure

Anchorage failure is the splitting of concrete along the longitudinal reinforcement (before compression failure can occur) due to small diagonal cracks. It occurs when the main reinforcement is not adequately anchored beyond the crack.

Read More:

Design of Rectangular Reinforced Concrete Beam

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