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  1. Home/
  2. NIVETHITHA NATRAYAN/
  3. Identification of Cracks

Identification of Cracks

Question: 1. What is the mode of failure or damage of the beams in the figure shown below? Are the damages repairable? Elaborate. Objective of the challenge:  To have an awareness or knowledge on, Structural failure issues Distinguishing structural and non-structural failure Understanding seismic failure Analyzing…

    • NIVETHITHA NATRAYAN

      updated on 19 Oct 2022

    Question: 1. What is the mode of failure or damage of the beams in the figure shown below? Are the damages repairable? Elaborate.

    Objective of the challenge:

     To have an awareness or knowledge on,

    • Structural failure issues
    • Distinguishing structural and non-structural failure
    • Understanding seismic failure
    • Analyzing whether the damage is repairable or not.

    a) Data:

    Observation:

    • Concrete crushed and reinforcement is exposed which may be the sign of rupture.
    • Bottom portion of the beam is damaged.
    • Top Portion looks safe because of the flange advantage from the slab.
    • The crack develops from bottom to top means the rebars yielding occurs in support bottom only.

    Answer:

    • It is the form of flexural failure due to the reversal of stress during earthquake.
    • Flexural stress, exerts both tensile and compressive force upon an object. This results in unequal distribution of forces among the fibers of the objects. The 'extreme fibers', i.e., the fibers at the surface of the object tend to experience maximum forces. Therefore, they are most vulnerable to breakage or rupture.
    • Here the top portion experiences compression and bottom experiences tension
    • It occurs when the beam is under-reinforced; the reinforcement ratio in the beam is low than balanced reinforced ratio.
    • Top compression is managed by the slab advantage where as bottom tension results in damage.
    • Since the reinforcement got yielded, it cannot be repaired.

    b) Data:

    Observation:

    • The crack is along the beam- column edge.
    • The concrete looks cracked widely but not crushed.
    • The beam seems like separate from the column.

    Answer:

    • It is beam pull out failure which is a failure mode in which the anchor pulls out of the concrete without development of the full steel or concrete capacity at the joint due to insufficient development length of reinforcement.
    • During earthquake the lateral movement of the building makes it worse and detach the beam from column
    • Under earthquake shaking, the beams adjoining a joint are subjected to moments in the same (clockwise or counter-clockwise) direction. Under these moments, the top bars in the beam-column joint are pulled in one direction and the bottom ones in the opposite direction. These forces are balanced by bond stress developed between concrete and steel in the joint region. If the column is not wide enough or if the strength of concrete in the joint is low, there is insufficient grip of concrete on the steel bars. In such circumstances, the bar slips inside the joint region, and beams loose their capacity to carry load.
    • Providing anchorage between two existing concrete is difficult. Hence repairing damaged joints is difficult, and so damage must be avoided.

    C) Data:

    Observation:

    • Cracks are exactly at the beam- column joint.
    • Cracks has developed in the entire cross-section of beam.
    • Cracks are clearly visible in slab of beam width direction.

    Answer:

    • It is a plastic hinge failure.
    • Plastic hinge effects are defined as the moment at which the entire cross-section has reached its yield stress. This is theoretically the maximum bending moment that a cross-section can resist. When this point is reached a plastic hinge is developed and the structure becomes an unstable mechanism.
    • Cyclic lateral loading and stress reversal at the joint led to the damage.
    • Slab has undergone tension but unable to control the damage.
    • Joint can be strengthened

    2. a) Data:

    Observation:

    • Damage is caused only in the column but not the beam.
    • Top portion of the column is damaged along the faces.
    • It seems like an exterior column as it exposed outside on the left side.
    • No walls are connected between the exposed column could be reason for ground floor open or parking area.

    Answer:

    • It’s a soft- Storey failure, which isa situation when the upper levels of a building are stiffer than the lower storey. This can result in undesirable performance of the building due to plastic hinge formation.
    • The absence of the infill wall, the lateral rigidity of the ground floor is lower than the upper storeys. Thus, this failure mechanism is triggered by earthquake.
    • Based on severity of concrete spalling, it is decided whether it is repairable or non-repairable.

    b) Data:

    Observation:

    • Concrete spalled diagonally.
    • Longitudinal reinforcement is not damaged.
    • Shear reinforcement spacing is wider.

    Answer:

    • Shear failure which are the most brittle mode of RC columns are caused by especially lack of lateral reinforcements. 
    • Due to insufficient lateral reinforcement the concrete is forced to take all the shear load and spalled.
    • Columns may be repaired by using one or more of the following methods: Encasement or enlargement of the column cross section (jacketing).

    c) Data:

    Observation:

    • Column damaged completely at the top.
    • There are two slabs attached to the column, lower one is probably at the mid height of the column.
    • Wooden support acts as the post damaged support to avoid entire collapse of the structure.

    Answer:

    • It is a short column failure.
    • Mid height slab added to that particular column crushed the concrete.
    • If a short column is not adequately designed for such a large force, it can suffer significant damage during an earthquake.
    • The damage is not repairable

     

     

     

     

     

     

     

     

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