Earthquakes are intense shakings of the Earth’s surface, caused by the sudden release of stored energy in the Earth’s crust. This energy release generates seismic waves that radiate outwards from the source, causing the ground to vibrate. While earthquakes are natural occurrences, the collapse of buildings during these events is often a direct consequence of how structures interact with these powerful forces.
The Forces of an Earthquake
Earthquake energy translates into forces that impact buildings through seismic waves. P-waves, or compressional waves, are the fastest and cause initial vibrations, while S-waves, or shear waves, arrive next and cause more destructive side-to-side motion. Surface waves, including Love and Rayleigh waves, arrive last but can cause significant low-frequency vibrations, particularly affecting taller buildings.
Ground shaking, driven by these seismic waves, causes inertia in buildings. Inertia, the tendency of a body to resist changes in its state of motion, causes forces within a building as its mass resists ground movement.
Buildings experience lateral forces, which result from intense side-to-side shaking and create shear stress. Vertical forces, or up-and-down motions, also occur but are typically less damaging than lateral forces. Torsional forces, which are twisting motions, can develop, especially in buildings with irregular shapes or uneven mass distribution.
All buildings possess a natural frequency, or resonance, which dictates how the structure vibrates. If the ground motion’s frequency matches a building’s natural resonance, the vibrations are amplified, leading to larger oscillations and increased damage.
Building Vulnerabilities and Structural Failure
Structural weaknesses, often due to inadequate design and construction, contribute significantly to building collapse during seismic events. Buildings constructed with brittle materials like unreinforced masonry or concrete lacking sufficient reinforcement are prone to sudden failure. Ductile design, which allows materials to deform without fracturing, is crucial for absorbing seismic energy and preventing catastrophic collapse.
Weak connections between structural elements, such as beams, columns, and walls, can cause sections to detach during intense shaking. Concrete without proper steel rebar lacks the tensile strength to withstand the stretching forces induced by an earthquake. Poor placement of rebar means the concrete cannot effectively distribute the seismic forces, leading to cracks and potential collapse.
Several failure modes are common in earthquakes. Soft story collapse occurs in buildings where one or more floors, often the ground level with large open spaces like parking garages or retail areas, lack adequate shear walls or bracing. This creates a weak point that cannot resist lateral forces, causing the upper floors to fall onto the compromised lower level in a progressive failure known as pancaking, where floors stack one upon another, making escape difficult.
Adjacent buildings can experience pounding, where structures with different vibrational characteristics collide during shaking, causing damage to their frames. The short column effect occurs where columns of shorter height within a story attract disproportionately larger shear forces due to their increased stiffness. Inadequately designed, these short columns can suffer severe, brittle shear failure. Torsional irregularity, caused by an asymmetrical distribution of mass or stiffness, leads to twisting forces that concentrate stress on certain structural members, increasing damage risk.
Impact of Soil and Site Conditions
The ground beneath a building can significantly influence its response to an earthquake, often contributing to structural failure. One phenomenon is liquefaction, where saturated sandy or silty soils temporarily lose their strength and stiffness, behaving like a liquid. This loss of solid ground can cause buildings to tilt, sink, or collapse as foundations lose support. Liquefaction can lead to lateral spreading, where the ground slides horizontally, disrupting foundations and underground utilities.
Certain soil types can amplify seismic waves, a process known as seismic amplification. Soft, unconsolidated sediments increase ground motion intensity compared to bedrock, making shaking more severe for structures on such sites. This means a building on soft soil might experience stronger shaking than one on solid rock from the same earthquake.
Earthquake-induced ground instability can lead to landslides or mudslides, undermining foundations and causing widespread destruction. Buildings on unstable slopes are particularly vulnerable. These site-specific geological conditions vary and can significantly impact a building’s survival, regardless of its construction quality. Understanding the local soil characteristics is a primary consideration in earthquake-resistant design.