Why do buildings fall in earthquakes? - Vicki V. May

Summary

Earthquakes pose a significant risk to buildings, especially as cities grow. While it's often assumed that the shaking ground causes buildings to collapse, the reality is more complex. The true danger arises from how buildings' mass and stiffness interact with the seismic waves, leading to vibrations. In some cases, buildings can resonate with these waves, amplifying their movements. Solutions like base isolation and tuned mass dampers help mitigate this risk. Engineers work with experts to predict seismic impacts and design smarter buildings that can withstand the forces of earthquakes, ensuring safety during such events.

Takeaways

• 😀 Earthquakes are terrifying events, and their destructive impact on cities is worsened by the risk of building collapses.
• 😀 Building collapses during earthquakes aren't directly caused by the ground shaking but by how the building responds to seismic forces.
• 😀 Most buildings are not located on fault lines, and the ground's movement during an earthquake affects the foundation and structure rather than the fault itself.
• 😀 Engineers and architects use models to predict how buildings will respond to earthquakes, based on physics principles like mass and stiffness.
• 😀 Building collapses during earthquakes are usually caused by the building vibrating violently due to ground movement, which affects its structural integrity.
• 😀 A building's mass and stiffness play crucial roles in how it oscillates during an earthquake. Stiffer buildings are less likely to shift than taller, more flexible ones.
• 😀 Shorter buildings tend to be stiffer and more resistant to shaking, while taller buildings are more flexible and can shift more during an earthquake.
• 😀 The 1985 Mexico City earthquake demonstrated that mid-sized buildings (6-15 stories) are more prone to collapse due to resonance, a phenomenon where seismic waves match the building's natural frequency.
• 😀 Resonance amplifies a building's vibrations when the frequency of seismic waves matches its natural frequency, leading to catastrophic shaking.
• 😀 Engineers now predict earthquake motion frequencies at building sites to prevent resonance-induced collapses by considering soil and fault types, along with past earthquake data.
• 😀 Innovative systems like base isolation and tuned mass dampers help reduce the risk of building collapse by absorbing shocks and preventing resonance from amplifying vibrations.

Q & A

What is the primary cause of building collapses during earthquakes?

Building collapses during earthquakes are primarily caused by the displacement of the foundation and lower levels of the building, which sends shock waves through the structure, causing it to vibrate back and forth.

Why is it a misconception that earthquakes directly cause buildings to collapse?

It is a misconception because most buildings are not located directly on fault lines, and the shaking caused by tectonic plate movements occurs much deeper than the building's foundation.

What two main factors determine the strength of a building's oscillation during an earthquake?

The two main factors that determine the strength of a building's oscillation during an earthquake are the building's mass (concentrated at the bottom) and its stiffness (the force required to cause displacement).

How do the height and flexibility of a building affect its behavior during an earthquake?

Shorter buildings tend to be stiffer and shift less, while taller buildings are more flexible and tend to shift more during an earthquake.

What happened during the 1985 Mexico City earthquake that challenged the idea that shorter buildings are always safer?

During the 1985 Mexico City earthquake, mid-sized buildings (6–15 stories) collapsed due to violent oscillations caused by resonance, even though shorter buildings nearby survived and taller buildings suffered less damage.

What is the concept of natural frequency in relation to buildings and earthquakes?

Natural frequency refers to the rate at which a building vibrates naturally, determined by its mass and stiffness. If the frequency of seismic waves matches the building's natural frequency, it can cause the building's vibrations to amplify, leading to greater displacement and potential collapse.

What is resonance, and how does it affect buildings during an earthquake?

Resonance occurs when the frequency of seismic waves matches a building's natural frequency, causing the building's oscillations to amplify with each wave, much like pushing a swing at the right moment, making it swing further than the initial displacement.

How do engineers predict the impact of earthquakes on buildings?

Engineers work with geologists and seismologists to predict earthquake motions based on factors such as soil type, fault type, and data from past earthquakes. This helps them design buildings to avoid resonance and minimize damage.

What is the effect of low and high-frequency seismic waves on different building types?

Low-frequency seismic waves tend to cause more damage to taller, more flexible buildings, while high-frequency waves pose a greater threat to shorter, stiffer buildings.

What engineering innovations are used to prevent building collapse during an earthquake?

Engineers use systems like base isolation, which uses flexible layers to isolate the foundation's displacement from the building, and tuned mass dampers, which reduce vibrations by oscillating out of phase with the building's natural frequency.