Reinforced concrete buildings are widely used for their durability and strength. However, they are not immune to structural damage and collapse, especially during natural disasters such as earthquakes. Several factors contribute to their failure, including improper design, construction errors, and the effects of seismic forces. This article explores the most common types of damage and their underlying causes to highlight the importance of proper construction practices.
Common Types of Damage and Collapse
1. Sliding of Roofs off the Supports
When beams merely rest on walls or columns without sufficient anchorage, they are prone to sliding during high-intensity earthquakes. If the bearing length is insufficient, the beams may leave their supports entirely, leading to the collapse of the roof structure.
2. Falling of Infill Walls
Infill panel walls, placed between reinforced concrete columns, are at risk of overturning if they are not securely connected to the structural framework. This failure mode compromises the integrity of the building and can cause extensive damage.
3. Crushing of Column Ends and Virtual Hinging
During severe shaking, columns experience significant eccentric compressive stresses. As a result, the concrete at the column ends may get crushed and spalled off. This damage progresses inward, weakening the effective cross-section. Ultimately, the column ends act like hinges, making the entire framework unstable and susceptible to collapse.
4. Short Column Effect
Columns with infill walls featuring large openings become relatively short in their deformable section. These short columns are much stiffer than their adjacent columns and attract higher shear forces during seismic events. This increased stress can lead to severe diagonal tension failure, compromising the overall structural stability.
5. Diagonal Cracking in the Columns
Severe ground shaking induces large shear forces in columns, often resulting in diagonal cracking. When twisting forces (torsion) are also present, the cracks may develop in a spiral form, further reducing the load-bearing capacity of the column and increasing the risk of collapse.
6. Diagonal Cracking of Column-Beam Joint
Many times, diagonal cracks appear at the junction of columns and beams. This weakens the structural connection between these elements, impairing the overall strength of the reinforced concrete frame. Such damage can propagate rapidly under continued stress.
7. Pulling Out of Reinforcing Bars
If column reinforcement bars do not have adequate anchorage length or proper overlap, they may be pulled out due to tension forces during seismic activity. This failure occurs when the stress reversal in the column exceeds the bonding strength of the reinforcement, leading to significant structural deterioration.
8. Collapse of Gable Frames
Gable frames, commonly used in school workshops, gymnasiums, assembly halls, and cinema halls, are particularly vulnerable. Once a joint in a reinforced concrete gable frame fails, the entire structure may spread outward, as there is no secondary resistance mechanism. Without careful design and detailing, these frames are at high risk of collapse.
9. Foundation Sinking and Tilting
Buildings constructed on loose or soft soils are susceptible to foundation instability during seismic events. When the foundation sinks or tilts, it can lead to severe cracks in the superstructure and, in extreme cases, total collapse. This issue underscores the importance of proper soil analysis and foundation design before construction.
Conclusion
The collapse of reinforced concrete buildings is often preventable through careful design, proper construction techniques, and adherence to safety standards. Understanding the common causes of structural failure helps engineers and builders implement appropriate measures to mitigate risks. By ensuring adequate reinforcement, anchorage, and soil stability, the resilience of reinforced concrete buildings can be significantly enhanced, reducing the likelihood of catastrophic failures.