Cracks in beams | Flexure crack and Shear crack

In any structural system crack arises because of stress in the structural system. Failure of a reinforced concrete beam in flexural shear will not result in a sudden failure. But yes gradually it will decrease the flexural strength and result in failure of structure. In reinforced concrete beams, cracks occurs because of flexural and shear stress.

A beam is a structural element that is capable of withstanding load primarily by resisting against bending.

Beams generally carry vertical gravitational forces but can also be used to carry horizontal loads (e.g., loads due to an earthquake or wind). The loads carried by beams are transferred to columns, walls, or girders, which then transfer the force to adjacent structural members and finally to the ground.

There are three types of crack which are predominantly seen in concrete beams:

Flexural cracks or tensile cracks

When beams are subjected to high flexural stress and low shear stress. The cracks which are formed as a result of flexural stress is called as flexural crack. These cracks can be controlled by providing proper tension reinforcement. Generally flexural stress occurs at the middle span of the beam where flexural stress is maximum.

Shear Cracks

Near supports where the shear stress is maximum, shear crack is generated. The presence of shear stress reduces the strength of concrete in compression as well as in tension. Shear cracks are formed at a 45 degree inclination to the horizontal axis. It can be controlled by providing transverse reinforcement.

Flexural – shear crack

When a flexural crack occurs in combination with the shear crack the crack is called as flexural shear crack.

Let us take an example of a simply supported beam subjected to a uniformly distributed load. For this case the value of maximum bending moment occurs at the mid span and the bottom face of the beam is under tension because of which tension crack or flexure crack occurs. But at supports the value of shear force is maximum and thus results in the shear cracks. The cracks generated at a zone in between the maximum bending moment and maximum shear force is flexure shear cracks.

Check this post to know the design philosophy of a reinforced concrete beam.

Beam design

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