Identification of Cracks

IDENTIFICATION OF CRACKS

Aim: To identify the mode of failure and to find whether it is repairable or not.

Objectives :

• Awareness of structural failures
• Distinguishing structural and non-structural failures
• To know how to observe a structural issue
• Understanding seismic failures

Q1 a)

Observations :

• Concrete crushed and the reinforcement is exposed.
• Concrete spalled beyond the reinforcement which may be a sign of rupture
• Beam top portion looks safe because of the flange advantage of the slab
• The crack develops from bottom to top means rebar yielding occurs in support bottom only

This crack will be formed due to the flexural failure of the beam. The flexural strength/bending strength or transverse strength is a material property defined as the maximum stress in a material just before it yields in a bending test. The flexural strength represents the highest stress experienced within the material at its moment of rupture. To overcome this type main steel is provided at the bottom/top of the beam. To repair the damaged beam, locate a welder or metal shop and angle iron, cut, and drilled to the length of the beam being repaired.

Q1 b)

Observations :

• Cracks along beam column-edge
• Cracks seem to have developed through the structure as it looks much wider.
• Beam looks separated from the column at joints
• Concrete is not severely crushed

This occurs due to construction errors and mistakes in the honeycombing in concrete. This type of failure can be avoided if attended to with much care. To avoid cracks use high slump concrete. Its beam pull-out failure is due to insufficient development length of reinforcement in joints. During an earthquake, the horizontal movement of the structure makes this condition worse and detached the beam member from the column. Providing anchorage length between two existing concrete is very difficult. So not repairable.

Q1 C)

Observations :

• Cracks are exactly at the joints of column-beam
• Cracks in beam developed through its entire width
• Slabs also developed more cracks along the beam width direction.

It’s a plastic hinge failure. The damage occurred only in the joint zones. The high internal forces developed at plastic hinges cause critical bond conditions in the longitudinal reinforcing bars passing through the joints and also impose shear demand on the joint core. Cyclic lateral loading and stress reversal at the joints leads to damage. Even the slab participated to its best to handle the tension. But unable to control the damage. The joint can be strengthened.

Q2 a)

Observations :

• Concrete damaged at top of the column
• Concrete damaged all around the column face.
• Beams are safe
• The location of the column looks at the edge of the building
• No walls are built along the column means it is probably on the ground floor, for parking car movement access.

It’s a soft story failure, it’s a plastic hinge formation. Due to the absence of infill walls on the ground floor, the plastic hinges forms at the top of ground-storey columns during lateral movement. Based on the development of crush of concrete, it can either be repairable or non-repairable base on the severity of the spalling of concrete.

Q2 b)

Observations :

• Concrete developed complete breakage diagonally
• Longitudinal reinforcements are intact
• Shear reinforcement spacing looks wider

It's due to the shear failure of the column. Inadequate shear reinforcement compels concrete to take all the shear stresses. Shear links are provided to columns based on the shear forces exerted on them. Further, the size of the column is increased in the direction of shear to increase the shear capacity.

Q2 c)

Observations :

• Column failed completely
• Failure at top of the column
• There are two slabs connected to the column, probably one at mid-height of the column
• The wooden piles might be post-failure support after the column failed, providing temporary propo to avoid the collapse of the entire structure.

It’s a short column failure. In short columns, failure takes place due to yielding and we define this phenomenon as crushing. Short columns are classified based on the slenderness ration, which is the ratio of the effective length and least dimension of the column. Failure of the column usually occurs due to any one of the following stresses developed in the column. The short column will fail only because of the direct compressive stress developed in the column.