Skyscrapers are among the most sophisticated structures ever engineered by mankind. These giants rising up from the concrete jungle must withstand the force of gravity, wind, temperature, and human use. But for engineers in earthquake prone areas, there is an additional challenge of making sure that a large flexible structure does not come crashing down when the ground beneath it starts to shake.
Modern earthquake resistant skyscrapers are the result of decades of research into structural dynamics, earthquake engineering, materials, and computer modeling. Instead of brute force, modern engineers are discovering that there are more effective ways to deal with the forces of an earthquake. Ways that allow a building to flex and bend without collapsing.
## Why Do Skyscrapers Move During an Earthquake?
An earthquake does not push a skyscraper sideways. The ground moves during an earthquake, and the building's foundation moves with it, while the upper floors resist the movement due to inertia.
This causes dynamic loads to be applied to the structure.
A skyscraper that is completely rigid will not be able to withstand these forces. Flexibility is a virtue in skyscraper design, as it allows the building to absorb some of the energy of the earthquake. This does not mean that engineers are trying to make buildings that do not move during an earthquake, but rather that they want to control the movement.
## The Importance of Structural Dynamics
The study of structural dynamics plays a large role in modern skyscraper design. Engineers must understand how a building will react to different frequencies of an earthquake, as well as its amplitude and duration.
A building has a set of natural frequencies at which it vibrates. If an earthquake excites one of these frequencies, the building will be subjected to large dynamic loads.
Engineers use numerical models to study the effects of an earthquake on a building, including:
- Lateral displacement
- Story drift
- Acceleration
- Stresses
- Torsional response
- Energy dissipation
- Failure modes
Using Finite Element Analysis and nonlinear time-history analysis, engineers can simulate how a skyscraper would react to real-world or computer generated ground motions.
## Base Isolation: Decoupling the Building from the Ground
One of the most effective ways to protect a building from an earthquake is to separate it from the ground. This is done using specialized isolation devices that allow the ground to move freely underneath the building.
There are several different isolation technologies, including:
- Lead-rubber bearings
- Elastomeric bearings
- Friction pendulum systems
- Sliding isolation systems
The use of base isolation is most common in low- and mid-rise buildings, although there are some specialized isolation technologies that can be used on skyscrapers, depending on the design.
## The Rise of Damping Technology
A similar approach to isolating buildings from the ground is to dissipate the energy of an earthquake using special dampers. Dampers work on a similar principle to automobile shock absorbers, and can be used in a variety of ways to reduce the forces acting on a building.
There are several different types of dampers, including:
### Viscous Dampers
Viscous dampers use a fluid to dampen the movement of the building. These dampers can absorb a large amount of energy during an earthquake.
### Friction Dampers
Friction dampers dissipate energy by sliding against each other.
### Metallic Dampers
Metallic dampers utilize the predictable failure characteristics of metals to absorb energy from an earthquake before other, more critical, structural elements in the building are damaged.
The principle behind metallic dampers is simple: allow specialized replaceable structural elements to fail before other, more important, elements of the building.
## Tuned Mass Dampers
One of the most famous technologies used in skyscraper engineering is the tuned mass damper (TMD). A TMD is essentially a large mass connected to the building with springs and dampers. The mass of the TMD is then tuned to the natural frequency of the building, so that it acts as a damper for the building's movements.
While tuned mass dampers are most commonly associated with reducing the effects of wind on buildings, there are other advanced damping technologies that can be used to reduce the effects of an earthquake. The giant ball and springs system at the top of Taipei 101 is an example of a TMD.
## The Structural System Still Matters
While advanced technologies such as TMDs, base isolation, and damping systems can be effective at reducing the effects of an earthquake, they are not a substitute for a good structural system. The structural system is the foundation of any skyscraper, and engineers must carefully consider which system to use based on a variety of factors.
Some common structural systems used in skyscraper construction include:
- Shear wall reinforced concrete buildings
- Steel moment resisting frame buildings
- Braced frame buildings
- Composite buildings
- Tube buildings
- Core-outrigger buildings
The core of a skyscraper is often one of its most important structural elements, as it provides resistance to lateral forces such as wind and earthquakes. Outriggers can be used to extend the resisting moment of a building's core by connecting it to other structural elements such as perimeter columns.
## The Role of Performance-Based Seismic Design
Traditional building codes specify certain design criteria that a building must meet in order to be considered safe. Modern skyscrapers often utilize performance-based seismic design (PBSD) when designing a building's structural system.
PBSD allows engineers to define specific performance objectives for a building, rather than simply following a set of design criteria.
For example, a building could be designed to be able to continue operating after an earthquake, or to ensure that its structural elements are not damaged beyond a certain threshold.
Engineers can then use computer modeling to determine if the building will meet the desired performance objectives.
PBSD is particularly useful for complex skyscrapers that utilize unconventional structural systems.
## Artificial Intelligence and Earthquake Engineering
The rise of artificial intelligence is introducing a new field of study into earthquake engineering. Machine learning algorithms can be used to analyze large amounts of data, including ground motions, structural response, material properties, and computer modeling results.
Some potential applications of artificial intelligence in earthquake engineering include:
- Predictive analytics
- Structural optimization
- Damage detection and identification
- Structural health monitoring
- Damping system optimization
- Structural design
In the future, artificial intelligence could be used to analyze thousands of structural designs in order to find the most optimal one for a given set of conditions. However, it is important to note that AI is not a substitute for engineering judgement. Safety-critical structural systems still require physical testing, appropriate building codes, and professional engineering experience.
## Smart Buildings that Can Sense When They Are Damaged
Modern skyscrapers are becoming more sophisticated, not only in their ability to resist earthquakes, but also in their ability to monitor and assess their own structural integrity. Smart buildings can utilize a variety of different sensors to monitor ground motions, building accelerations, and structural deformations.
During an earthquake, these sensors can be used to detect damage to a building. Engineers can then use this information to determine what inspections and repairs are necessary after an earthquake.
## Materials Are Changing Too
The skyscrapers of the future will not only be smarter, but stronger. Researchers are constantly working to develop new materials that can be used to make buildings more ductile, durable, and capable of withstanding large deformations during an earthquake.
Some of the materials currently being researched and developed include:
- High-performance concrete
- Ultra-high-performance concrete
- Fiber-reinforced cementitious composites
- High-performance steel
- Shape-memory alloys
- Advanced composites
One promising field of study is self-centering structures. These structures are designed to return to their original position after an earthquake, which can reduce the amount of damage to the building.
## The Future: Buildings that Can Adapt
The future of earthquake engineering may involve buildings that can actively respond to an earthquake. Imagine a skyscraper with hundreds of sensors that can detect ground motions during an earthquake. When an earthquake occurs, the building's computer system could analyze the ground motions and determine what adjustments to the building's dampers would be most effective at reducing the forces acting on the building.
This approach to earthquake engineering goes beyond passive structural design by allowing buildings to adapt to the ground motions of an earthquake. Such a system would be much more effective at reducing the forces acting on a building, but it would also be much more complex and require extensive testing.
The reliability of such a system is of utmost importance, as any failure in the sensors, computers, or dampers could lead to catastrophic consequences.
## The Goal Is More Than Preventing Collapse
The goals of earthquake engineering have changed over time. In the past, the main concern of engineers was preventing buildings from collapsing during an earthquake. While preventing collapse is still an important consideration, modern engineers are also focusing on damage control.
A building that continues to stand after an earthquake may still require extensive repairs before it can be used again. This can be especially problematic for skyscrapers that house offices, hotels, and other important infrastructure. Modern engineers are taking a more holistic approach to earthquake engineering by considering not only life safety, but also damage control, recovery, and economic impact.
## Conclusion
The science of earthquake resistant skyscrapers is evolving, and the skyscrapers of the future will be fundamentally different from the ones we see today. Structural systems, dampers, base isolation, materials, performance-based design, artificial intelligence, and structural health monitoring are all coming together to form a new generation of earthquake resistant skyscrapers. These skyscrapers will be smarter, stronger, and more capable of withstanding the forces of an earthquake. For civil engineers, this is an exciting time as we move towards a future where skyscrapers are no longer passive structures, but rather active participants in the battle against earthquakes.
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