Earthquakes are sudden, violent shaking of the Earth’s surface caused by disturbances beneath the crust, such as the movement of tectonic plates or the release of stress accumulated along geological faults. The energy released during an earthquake travels as seismic waves, causing the Earth’s surface to vibrate, often resulting in significant damage to structures and loss of life. While earthquakes are natural phenomena, the extent of their impact can be significantly mitigated through careful planning and construction. In this article, we will explore the reasons behind high casualties during earthquakes and the general guidelines for earthquake-resistant building design.
Reasons for High Casualties During Earthquakes
Urbanization and Multi-Story Buildings
As urbanization accelerates globally, particularly in seismic zones, cities have seen a surge in multi-story buildings. Rapidly increasing land costs and limited space often push developers to build taller structures. However, the growing demand for high-rise buildings has not always been matched by adequate attention to earthquake-resistant design.
In many urban areas, building codes exist to regulate construction, but these codes may not always incorporate seismic provisions or might not be enforced rigorously. This is particularly problematic in older buildings or in regions where urban development has outpaced the implementation of modern seismic safety standards. In such cases, even buildings with basic, ordinary designs are sometimes constructed without sufficient checks, leaving them vulnerable to collapse during seismic events.
Lack of Seismic Code Enforcement
A significant contributing factor to the high casualties in earthquake-prone areas is the lack of consistent enforcement of seismic design regulations. While seismic provisions may exist in municipal building codes, they are not always strictly followed. Furthermore, some local regulations may not cover newer construction techniques or may be outdated, leaving gaps in safety protocols. Additionally, when inspections are lax or non-existent, poorly designed buildings continue to be constructed, amplifying the risks to public safety.
General Guidelines for Earthquake-Resistant Design
To reduce the impact of earthquakes on buildings and their occupants, engineers and architects follow specific design principles that ensure buildings can withstand seismic forces. Below are some key guidelines that are crucial in earthquake-resistant design.
1. Drift Control
Drift refers to the lateral displacement of a structure during an earthquake relative to its total height or the displacement between individual storeys. Excessive drift can cause significant damage to non-structural elements such as partitions, windows, and ceilings, or even lead to the collapse of the building.
To minimize the risk of damage, buildings must be designed to limit lateral movement. The maximum storey drift under specified lateral forces should not exceed 0.004 times the storey height. This means that the amount of lateral displacement must be kept to a minimum, ensuring the building remains structurally stable and preventing excessive movement that could cause severe damage.
2. Separation Between Adjacent Units or Buildings
In multi-building complexes or multi-unit structures, adjacent buildings must have sufficient separation to prevent them from making contact during an earthquake. As buildings shift during seismic activity, the risk of them colliding increases. To avoid this, a safe separation joint should be included between structures.
The distance between adjacent buildings should be calculated as a specified factor times the sum of the calculated displacements for each building. Proper separation prevents damage from contact and allows each building to move independently without imposing undue stress on the adjacent structures.
3. Soft Storeys
A soft storey is a floor in a building that has significantly less lateral stiffness compared to the floors above it. This condition is common in buildings with large openings, such as those with parking on lower floors or buildings with open, column-free spaces. Soft storeys are more likely to collapse during an earthquake, as they cannot absorb the seismic forces transmitted from the upper floors.
To mitigate this risk, special measures are needed to increase the lateral strength and stiffness of soft storeys. This may include adding shear walls or reinforcing columns to ensure that the building can withstand seismic forces. For structures with soft storeys, dynamic analysis is recommended, which takes into account the effects of infills and inelastic deformations.
4. Foundation Design
The foundation plays a crucial role in earthquake resistance. Structures in seismic zones must avoid foundations that are prone to significant differential settlement, which could cause tilting or collapse during shaking. A foundation that shifts unevenly can lead to catastrophic structural failure.
To address this, foundations should be designed with interconnected ties that can handle axial forces caused by ground shaking. Spread footings or pile caps that are interconnected offer increased stability and can bear the additional forces resulting from seismic activity. A well-designed foundation prevents excessive settlement and ensures the building remains stable during an earthquake.
5. Projections (Tanks, Towers, Parapets, etc.)
Vertical and horizontal projections, such as tanks, towers, chimneys, parapets, and balconies, are often added to the exterior of buildings. These elements can pose a significant risk during an earthquake due to their mass and leverage. If not designed properly, they can detach or collapse, causing serious damage to the building and its surroundings.
All projections must be carefully designed to withstand seismic forces. Heavy top structures and overhangs should be minimized or avoided entirely, as they increase the load on the building during lateral movement. Structural integrity must be ensured for any projection to prevent failure during seismic events.
6. Shape of the Building
The shape and symmetry of a building are critical factors in earthquake resistance. Buildings with very slender designs or asymmetrical shapes tend to experience torsion, or twisting, during an earthquake. This can create extreme forces at the corners of the building, leading to structural failure.
To avoid these issues, buildings should be designed with symmetrical shapes and should avoid excessive slenderness. In addition, the placement of the building on solid, stable ground is essential—especially avoiding steep slopes or areas with filled soil, which may not provide adequate support during ground shaking.
7. Damping Systems
Damping refers to the process of reducing the amplitude of vibrations in a structure, typically through the use of materials or systems designed to absorb kinetic and potential energy. During an earthquake, buildings vibrate as seismic waves pass through them. The goal of damping is to reduce these vibrations, preventing excessive motion that could lead to damage.
Proper damping systems are vital for maintaining structural stability. They help to dissipate the energy from ground motion, preventing excessive building movement. In earthquake-prone regions, damping systems are incorporated into buildings to increase their resilience. The Indian standard IS 1893 (Part I) – 2002 outlines seismic design considerations, with more than 60% of India’s area classified as earthquake-prone, underscoring the importance of damping in earthquake-resistant construction.
Conclusion
In earthquake-prone areas, ensuring the safety and resilience of buildings is crucial to protecting lives and minimizing damage. By adhering to comprehensive seismic design guidelines—such as controlling drift, ensuring proper separation between structures, strengthening soft storeys, and designing strong foundations—engineers can significantly reduce the risks associated with earthquakes. Furthermore, proper construction techniques, rigorous enforcement of seismic codes, and the use of advanced damping systems play vital roles in mitigating the devastating effects of seismic events.