Earthquakes have always been a powerful natural force, and as our cities grow, they pose an even greater threat. One of the biggest dangers during an earthquake is the collapse of buildings. But why do buildings fall during these events, and how can we prevent it?
If you’ve watched disaster movies, you might think buildings collapse because the ground shakes violently or splits apart. However, that’s not entirely true. Most buildings aren’t directly on fault lines, and the tectonic plates that shift during an earthquake are much deeper than a building’s foundation.
The real reason buildings collapse is more complex. Architects and engineers use models to understand how buildings respond to earthquakes. These models, even when simplified, help predict a building’s behavior using principles of physics.
Most building collapses during earthquakes aren’t caused directly by the quake itself. When the ground moves, it displaces the building’s foundation and lower levels, sending shock waves through the structure and causing it to vibrate. The intensity of this vibration depends on two main factors: the building’s mass and its stiffness.
Stiffness is determined by the building’s materials and the shape of its columns, and it’s largely influenced by height. Shorter buildings are usually stiffer and shift less, while taller buildings are more flexible. However, building shorter structures doesn’t always prevent shifting, as shown by the 1985 Mexico City earthquake.
During the Mexico City earthquake, many buildings between six and fifteen stories tall collapsed, while shorter buildings nearby remained standing. Interestingly, taller buildings over fifteen stories were also less damaged. The midsized buildings that fell were shaking more violently than the earthquake itself.
This is due to a concept called natural frequency. In an oscillating system, frequency refers to how many cycles of movement occur in a second. A building’s natural frequency, determined by its mass and stiffness, is the frequency at which it tends to vibrate. Increasing a building’s mass slows its vibrations, while increasing stiffness speeds them up.
In Mexico City, resonance occurred when the earthquake’s seismic waves matched the natural frequency of the midsized buildings, amplifying their vibrations and causing more movement than the initial quake.
Today, engineers work with geologists and seismologists to predict earthquake frequencies at building sites to avoid resonance-induced collapses. They consider factors like soil type, fault characteristics, and data from past earthquakes. Low-frequency motions can damage taller, flexible buildings, while high frequencies threaten shorter, stiffer structures.
Engineers have also developed techniques to absorb shocks and limit deformation. Base isolation uses flexible layers to separate the foundation’s movement from the building, while tuned mass dampers counteract resonance by oscillating out of phase with the building’s natural frequency to reduce vibrations.
Ultimately, it’s not just the strongest buildings that will stand during an earthquake, but the most intelligently designed ones. By understanding and applying these principles, engineers can create structures that are better equipped to withstand the forces of nature.
Build a simple model structure using materials like straws and clay. Use a shake table (or a homemade version using a tray and marbles) to simulate an earthquake. Observe how different designs and materials affect the stability of the structure. Discuss how mass and stiffness influence the building’s response to the simulated earthquake.
Using a set of tuning forks or a similar setup, demonstrate the concept of resonance. Show how matching frequencies can amplify vibrations. Relate this to how buildings with certain natural frequencies can be more vulnerable during earthquakes. Discuss how engineers use this knowledge to design safer buildings.
Research a real-world earthquake event and analyze why certain buildings collapsed while others did not. Prepare a presentation to share your findings with the class, focusing on the role of building design, materials, and natural frequency in the outcomes observed during the earthquake.
Create a model of a building with a base isolation system using materials like rubber bands and foam. Test its effectiveness by simulating an earthquake and compare it to a non-isolated model. Discuss how base isolation can help reduce the impact of seismic waves on a building.
Conduct an interview with a structural engineer or a seismologist to learn about the latest technologies and strategies used in earthquake-resistant building design. Prepare questions in advance and share your insights with the class, highlighting how these professionals apply scientific principles to enhance building safety.
**Sanitized Transcript:**
Earthquakes have always been a significant phenomenon, and they have become more dangerous as our cities have expanded, with collapsing buildings posing one of the largest risks. Why do buildings collapse during an earthquake, and how can this be prevented?
If you’ve seen many disaster films, you might think that building collapses are caused directly by the ground shaking violently or even splitting apart. However, that’s not entirely accurate. Most buildings are not situated directly on fault lines, and the shifting tectonic plates are much deeper than building foundations.
The reality of earthquakes and their effects on buildings is more complex. To understand this, architects and engineers use models, such as two-dimensional arrays representing columns and beams or simplified representations of a building’s mass. Even when simplified, these models can be quite useful, as predicting a building’s response to an earthquake primarily involves physics.
Most collapses during earthquakes aren’t directly caused by the earthquake itself. Instead, when the ground moves beneath a building, it displaces the foundation and lower levels, sending shock waves through the structure and causing it to vibrate. The strength of this oscillation depends on two main factors: the building’s mass, concentrated at the bottom, and its stiffness, which is the force required to cause a certain amount of displacement.
Stiffness is influenced by the building’s material type and the shape of its columns, and it is largely a matter of height. Shorter buildings tend to be stiffer and shift less, while taller buildings are more flexible. You might think that building shorter structures would minimize shifting, but the 1985 Mexico City earthquake illustrates why this isn’t always the case.
During that quake, many buildings between six and fifteen stories tall collapsed, while shorter buildings nearby remained standing. Interestingly, buildings taller than fifteen stories were also less damaged, and the midsized buildings that collapsed were observed shaking more violently than the earthquake itself.
This phenomenon relates to something known as natural frequency. In an oscillating system, frequency refers to how many cycles of movement occur within a second. A building’s natural frequency, determined by its mass and stiffness, is the frequency around which its vibrations tend to cluster. Increasing a building’s mass slows down its natural vibrations, while increasing stiffness makes it vibrate faster.
In the case of Mexico City, an effect called resonance occurred, where the frequency of the earthquake’s seismic waves matched the natural frequency of the midsized buildings. This amplification of vibrations led to greater movement than the initial displacement.
Today, engineers collaborate with geologists and seismologists to predict the frequency of earthquake motions at building sites to prevent resonance-induced collapses, considering factors such as soil type and fault characteristics, as well as data from previous earthquakes. Low frequencies of motion can cause more damage to taller, more flexible buildings, while high frequencies pose a greater threat to shorter, stiffer structures.
Engineers have also developed methods to absorb shocks and limit deformation using innovative systems. Base isolation employs flexible layers to separate the foundation’s movement from the rest of the building, while tuned mass damper systems counteract resonance by oscillating out of phase with the natural frequency to reduce vibrations.
Ultimately, it’s not just the sturdiest buildings that will remain standing, but the most intelligently designed ones.
Earthquake – A sudden and violent shaking of the ground, sometimes causing great destruction, as a result of movements within the earth’s crust or volcanic action. – During an earthquake, the seismic waves can cause significant damage to poorly constructed buildings.
Buildings – Structures with walls and a roof, such as houses, schools, or factories, designed for a specific purpose. – Engineers must ensure that buildings are designed to withstand natural disasters like earthquakes.
Collapse – The sudden failure of a structure, often due to excessive stress or inadequate design. – The collapse of the bridge was attributed to the lack of proper maintenance and design flaws.
Vibrations – Oscillations or repetitive motions of particles or structures, often caused by external forces. – The vibrations from the machinery were analyzed to ensure they did not exceed safe levels for the building’s structure.
Stiffness – A measure of a material’s resistance to deformation under an applied force. – The stiffness of the beam was increased to prevent excessive bending under load.
Frequency – The number of complete oscillations or cycles per unit time of a vibrating system. – The natural frequency of the structure was calculated to avoid resonance with external forces.
Resonance – A phenomenon that occurs when the frequency of an external force matches the natural frequency of a system, resulting in large amplitude oscillations. – Engineers designed the building to avoid resonance with the expected wind frequencies.
Engineers – Professionals who apply scientific and mathematical principles to design and build structures, machines, and systems. – Engineers play a crucial role in ensuring that structures are safe and efficient.
Structures – Arrangements or organizations of parts, often referring to buildings or other constructions designed to support loads. – The structures were tested for their ability to withstand seismic forces.
Design – The process of planning and creating something with a specific function or intention, often involving technical drawings and specifications. – The design of the new bridge incorporated advanced materials to enhance its durability.
