Identification of Cracks

Q1). What is the mode of failure or damage of the beams in the figure above? Are the damages repairable? Elaborate

1a).

Answer:

Observations:

• Concrete is 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 advantange from slab.
• The crack develops from bottom to top means rebar yielding occurs in support bottom only.

Mode of Failure:

• Its form of flexural failure due to reversal of stress during earthquake.
• During stress reversal, support top experiences compression and bottom experiences tension.
• Top compressiom is managed by slab advantage. Whereas bottom tension was not satisfied by the provided reinforcement.
• Since the reinforcement got yielded, it cannot be repaired.
• This crack is formed due to flexural failure of a beam.
• the fluxural strength known as modulus of rupture, bending strength or fracture strength is a material property defined as the stress in the material just before it yields in a flexure test.
• the flexure stress represent the highest stress experienced within the material at the moment of rupture.

1b).

Answer:

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.

Mode of Failure:

• Its beam pull out faialure due to insufficient developmetn length of reinforcement in joints.
• During eathquake, the horizontal movement of structure makes this condition worse and detached the beam member from column.
• Providing the anchorage length between two existing concrete is very difficult. So not repairable.

1C).

Answer:

Observations:

• Cracks are exactly at the joint of Column-Beam.
• Cracks in Beam developed through its entire width.
• Slab also developed more cracks along the beam width direction.

Mode of Failure:

• It's a plastic hinge failure.
• The damage occurred only in the joint zone first of all.
• Cyclic lateral loading and stress reversal at the joints led to the damage.
• Even the slab participated to its best to handle the Tension but cracked by undergoing tension with its falnge condition and unable to control the damage.
• This damage can be repaired by strengthening the joint.

Question2).

2a).

Answer:

Observations:

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

Mode of Failure:

• It's a soft storey failure, due to formation of plastic hinge.
• Due to absemce of Infill walls at the ground floor, the plastic hinge 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.

2b).

Answer:

Observations:

• Concrete developed complete breakage diagonally.
• Longitudinal reinforcements are intact.
• Shear reinforcement spacing looks wider, that is lesser shear rebar is provided for the column.

Mode of Failure:

• It's a pure shear failure.
• Inadequate shear reinforcement compels concrete takes all the shear stresses.
• The size of the columns is increased in the direction of shear to increase the shear capacity.
• Shear failure occurs when the acting shear force exceeds the shear capacity of the corresponding section of the structural menber.
• shear force acts along a plane parallel to the direction of force and hence , producing a sliding failure on beam.
• shear failure is considered as brittle failure as it does not provide any warning before complete failure.
• Shear reinforcement must be provided as per codal provisions provided by IS 456:2000.
• Provide adequate shear links.
• high grade concrete deformed bars must be used for proper anchorage.

2c).

Answer:

Observations:

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

Mode of Failure:

• It's a short column failure.
• An additional slab or a beam connected to a column in mid-height leads to this effect.
• This leads to failure especially if the connection is unintentional and not accounted in design.
• The failure occured is not repairable.
• Short columns are commanly subjected to axial loads, lateral loads and moments , short columns under the action of lateral load and moments undergo lateral moments and bending
• under such circumstances when the stresses in steel and concrete reach their yield stress material failure happens and rcc column failures . this type of failures is also called as combined compression and bending failure.
• short column effect generally occurs in the building resting on the slopping ground.
• The performance of short column during an earthquake is poor , this is because a tail column and short column of some cross section move horizontally by amount , howeever a short coulmn is stiiff than tall column hence it attracts larger earthquake forces.