Short Answer:

The major failure modes in RCC buildings during earthquakes include column failure, beam-column joint failure, soft storey collapse, shear failure, and foundation failure. These failures occur due to sudden ground shaking that puts stress on structural components beyond their capacity.

Such failures often result from poor reinforcement detailing, inadequate ductility, weak connections, or design errors. Understanding these failure modes is important for improving earthquake resistance by ensuring proper construction, correct load transfer, and adherence to seismic design standards.

Detailed Explanation

Major Failure Modes in RCC Buildings During Earthquakes

Reinforced Cement Concrete (RCC) buildings are designed to resist both vertical and horizontal loads. During earthquakes, buildings experience sudden horizontal ground movements that create unpredictable forces. If these forces are not properly managed through design and detailing, the structure may fail. Understanding the major failure modes helps engineers and builders to prevent damage and design safer structures.

1. Column Failure
   Columns are vertical load-carrying elements that support beams and slabs. During earthquakes, if the column is weak or improperly detailed, it may fail by crushing, buckling, or breaking. This is known as brittle failure, which is dangerous because it gives little warning before collapse.

• Reasons:
  • Insufficient confinement reinforcement
  • Poor quality concrete
  • Overloaded columns due to soft storey or design mistake
  • Short column effects (walls partially restricting the column)

2. Beam-Column Joint Failure
   The beam-column joint is a critical zone where beams and columns connect. If this joint is not properly reinforced, it may crack or break during an earthquake, leading to progressive collapse.

• Reasons:
  • Inadequate anchorage and stirrups
  • Poor construction practices
  • Lack of seismic detailing
  • Overloading during horizontal shaking

3. Soft Storey Collapse
   Soft storey failure happens when the ground floor is much weaker than the upper floors, usually because it has fewer walls or columns (e.g., parking or open commercial space). During shaking, this floor sways more and collapses.

• Reasons:
  • Sudden change in stiffness
  • Lack of lateral support
  • Poor shear resistance
  • No special strengthening of ground floor

4. Shear Failure in Beams or Columns
   When horizontal forces act on a member and it fails by cracking or breaking along a diagonal path, it is known as shear failure. This is a sudden failure with little warning and is dangerous in earthquakes.

• Reasons:
  • Insufficient stirrups or transverse reinforcement
  • High seismic forces not resisted by design
  • Poor quality or thin sections

5. Torsional Failure
   This occurs when the building twists due to an uneven plan layout or irregular shape. This twisting motion causes stress concentrations in some parts and can lead to local or global failure.

• Reasons:
  • Asymmetrical plan
  • Uneven mass distribution
  • Discontinuous vertical elements

6. Foundation Failure
   If the foundation is not designed for seismic loads or the soil is loose or weak, the entire building can tilt or sink. This causes cracks in columns, walls, and beams.

• Reasons:
  • Poor soil bearing capacity
  • Uneven settlement
  • Lack of anchorage in foundation design
  • Liquefaction in loose, saturated soil during earthquake

7. Non-structural Element Failure
   Even if the main structure survives, non-structural parts like infill walls, parapets, ceilings, and cladding may fall, causing injuries or damage.

• Reasons:
  • Weak fixing or anchoring
  • No seismic separation joints
  • Brittle materials

Preventive Measures
To avoid these failures, the following practices are essential:

• Ductile detailing as per IS 13920
• Proper confinement with stirrups and ties
• Uniform and symmetrical layout
• Strong beam-column joints
• Strengthening of soft storeys
• Foundation design based on soil investigation
• Seismic bracing and shear walls
• Use of quality materials and skilled construction

Conclusion

The major failure modes in RCC buildings during earthquakes include column failure, joint failure, shear cracks, soft storey collapse, and foundation problems. These failures can be prevented by applying proper seismic design, good detailing, and quality construction. Understanding these failure types helps in building safer and earthquake-resistant structures.