Earthquakes are natural phenomena that pose a significant risk to buildings and infrastructure, especially in regions prone to seismic activity. The severity of ground shaking during an earthquake can vary from minor tremors that are felt but cause little or no damage, to strong shaking that can cause widespread destruction. The challenge for engineers is to design buildings that are capable of withstanding these tremors while also balancing safety with cost. This article explores the principles and philosophy behind earthquake-resistant building design and how engineers strike a balance between safety and economic feasibility.
The Earthquake Problem
The severity of earthquake shaking at any given location can range from minor to strong, with each type having distinct frequencies. Minor shaking, which occurs frequently, may only cause slight discomfort, whereas strong shaking, though rare, can lead to catastrophic damage. For example, annually, about 800 earthquakes of magnitude 5.0-5.9 occur worldwide, but only about 18 earthquakes fall within the magnitude range of 7.0-7.9. This stark contrast raises a critical question for engineers: should buildings be designed to resist the rare, strong earthquakes that occur once every 500 or even 2,000 years, even though the average lifespan of a building is only 50 to 100 years?
Designing buildings to withstand such extreme events can be prohibitively expensive. On the other hand, designing buildings without considering these rare but potentially devastating quakes could result in disastrous consequences for public safety. Therefore, a middle-ground approach is necessary—one that ensures buildings can resist moderate shaking while minimizing the costs of designing for rare, extreme events.
Earthquake-Resistant Buildings
Rather than aiming to create “earthquake-proof” buildings that can resist even the strongest quakes without damage—an unrealistic and costly goal—engineers design buildings to be earthquake-resistant. Earthquake-resistant buildings are designed to withstand the shaking associated with earthquakes, even if they suffer some damage. The primary goal is to prevent collapse, ensuring that the building remains standing so that occupants can evacuate safely and that the structure can be repaired afterward. Such buildings prioritize the safety of people and property, preventing a disaster while managing the costs of construction.
Earthquake Design Philosophy
The philosophy behind earthquake-resistant design can be summarized as follows:
- Minor Shaking: During frequent minor tremors, the building’s main structural elements (those that carry vertical and horizontal forces) should remain undamaged. However, non-load-bearing parts, like decorative walls or facades, might incur minor, repairable damage.
- Moderate Shaking: Under moderate shaking, the main load-bearing elements might suffer repairable damage, while other parts of the building may sustain more significant damage, potentially requiring replacement after the earthquake.
- Strong Shaking: In the rare event of strong shaking, the main structural elements may sustain severe damage, even to the point of being irreparable. However, the building should not collapse. The goal is to ensure that the structure remains standing to facilitate evacuation and allow for property recovery.
The building’s performance during earthquakes is designed to be resilient in a way that minimizes operational downtime and repair costs. After minor shaking, the building should quickly return to full functionality with only small repair costs. After moderate shaking, repairs will restore the building to operational status. However, after a strong earthquake, the building may no longer be functional, but it should still stand, enabling safe evacuation.
Performance Objectives and Consequences of Damage
The severity of damage in a building during an earthquake directly affects its functionality afterward. After minor shaking, damage should be minimal, and the building should be fully operational. With moderate shaking, repairs to key structural elements may be necessary, but the building will be restored to working condition. In the case of strong shaking, the building may suffer severe damage and may need significant repairs or even decommissioning, but it should not collapse. Ensuring that the building remains standing after strong shaking is vital for public safety.
Certain buildings require additional protections due to their critical roles. For example, hospitals, fire stations, and other emergency response centers must remain functional even after an earthquake. These buildings should be designed to withstand much stronger shaking, ensuring that they sustain minimal damage and can operate immediately after the event. Similarly, critical infrastructure such as dams and nuclear power plants must be designed with additional safety measures to prevent catastrophic secondary disasters, such as flooding caused by dam failure.
Damage in Buildings: Unavoidable but Manageable
An important aspect of earthquake-resistant design is recognizing that some damage during an earthquake is inevitable. Contrary to common belief, the presence of cracks or other signs of damage does not automatically render a building unsafe for occupancy. In fact, some damage is both acceptable and necessary to prevent more significant structural failures.
Cracks, especially in concrete and masonry buildings, are common during earthquakes. Some cracks are considered acceptable, as long as their size and location do not compromise the structural integrity of the building. For instance, in a reinforced concrete frame building with masonry filler walls, cracks between the columns and the walls are typically acceptable. However, diagonal cracks that run through the columns are dangerous, as they can undermine the building’s ability to carry vertical loads and could lead to collapse.
The goal of earthquake-resistant design is to ensure that the damage that does occur is of an acceptable type and magnitude and that it happens in the right places. This approach is similar to the use of fuses in electrical systems. Just as fuses are designed to sacrifice small parts of the electrical circuit to prevent larger failures, earthquake-resistant buildings are designed to sacrifice some of their components to prevent total collapse.
Acceptable Damage and Ductility
One of the key properties that earthquake-resistant buildings must exhibit is ductility. Ductility is the ability of a material to deform without breaking, allowing it to absorb and dissipate the energy from an earthquake. This is the opposite of brittleness, where materials break easily under stress, like chalk. For example, a steel pin can bend repeatedly without breaking, demonstrating ductility, while chalk breaks easily under pressure, showing brittleness.
In earthquake-resistant design, ductility is crucial for the main structural components of the building. A ductile building can sway during an earthquake, absorbing the shaking forces and preventing collapse. This allows the building to withstand significant shaking, even if some damage occurs. Engineers design buildings to ensure that damage is concentrated in areas where ductility can absorb the energy without compromising the building’s overall stability.
Avoiding brittle failures is essential for earthquake resistance. Brittle materials or components are prone to sudden failure, leading to catastrophic consequences. In contrast, ductile structures bend and absorb energy, which helps to prevent such failures. A well-designed earthquake-resistant building ensures that the damage is predictable and occurs in specific, well-detailed areas that will not compromise the structure’s overall integrity.
Conclusion
Designing earthquake-resistant buildings involves a delicate balance between safety, functionality, and cost. Engineers aim to create structures that can resist the varying intensities of earthquake shaking without being overly expensive or wasteful. By designing buildings that can sustain damage but not collapse, they ensure that human life is protected, property is safeguarded, and communities can recover after an earthquake. Through careful planning, material selection, and adherence to seismic design principles, buildings can be made to endure the forces of nature, all while providing a safe and habitable environment for their occupants.