A reinforced concrete building gets damaged and collapses due to many reasons such as sliding of roofs, falling of walls, crushing of columns, short column effects, diagonal cracking, foundation sinking and tilting etc.
Types and Causes for Damage and Collapse of Concrete Buildings
The following types of damages are quite common in reinforced concrete buildings:
1. Sliding of Roofs off the Supports
Where the beams simply rest on walls or columns, they are bound to slide when the intensity of earthquake exceeds the frictional resistance and many times leave the support and fall down, particularly if the bearing length is small.
2. Falling of Infill Walls
The infill panel walls in between reinforced concrete columns overturn outside the framework if not tightly held or connected with the frames.
3. Crushing of Column Ends and Virtual Hinging
During severe shaking, the column ends are subjected to heavy eccentric compressive stresses under which the concrete gets crushed and spalled off from the outer surfaces. In repeated cycles, the damage progresses inwards, thus the effective section gets very much reduced. Both the column ends virtually start behaving as pins and the whole framework collapses like a mechanism.
4. Short Column Effect
When infill walls with wide openings are attached to the columns, the portions of the columns that will deform under lateral seismic loads become very short as compared to their normal height. Such short columns become much stiffer than other columns and attract much larger shear forces under which they get severe diagonal tension which may lead to the failure of the column.
Read More: Short Column Effect
5. Diagonal Cracking in the Columns
Columns are subjected to diagonal cracking due to large seismic shears caused under severe ground shaking. If the building undergoes the twisting action as well, the crack may take a spiral form reducing the load capacity of the columns severely.
6. Diagonal Cracking of Column Beam Joint
Many times diagonal cracking occurs through the junction of the columns with the beams which seriously impairs the strength of the frame.
7. Pulling Out of the Reinforcing Bars
Where the anchor length of the column bars or overlaps between the longitudinal bars are not adequate for developing full tensile strength of the bar, they are often pulled out due to tensions caused in the column under reversal of stresses.
8. Collapse of Gable Frames
Reinforced concrete gable frames, often used for school workshops, gymnasia and assembly halls, and cinema halls, have a tendency of spreading out with no secondary resistance available once a joint fails. These are often found to fail and collapse unless very carefully designed and detailed.
9. Foundation Sinking and Tilting
Sinking or tilting of foundations of columns due to seismic shaking occurs in loose soft soils and can lead to severe cracking of the superstructure and even collapse.