The ground beneath buildings can shift violently during an earthquake, creating complex and dynamic forces that structures must endure. Structural engineering and seismology intersect to define the hazards and develop ways for buildings to remain functional and safe following a seismic event. The central challenge is mitigating the immense, rapidly changing forces generated by ground motion that push and pull on a building’s mass. Understanding the specific mechanisms of force transfer and resulting damage patterns is the foundation for designing structures that resist collapse and minimize damage.
How Ground Motion Translates to Structural Force
When the earth’s crust fractures, it generates seismic waves that cause the ground to accelerate and decelerate rapidly. A building’s foundation moves with the ground, but the structure’s mass, particularly the upper floors, tends to resist this sudden change in motion due to inertia. According to Newton’s first law, the structure attempts to remain stationary while its base is being pulled out from under it, generating powerful internal forces proportional to the building’s mass and the ground’s acceleration. These inertial forces are generated internally as the building’s mass resists the movement.
The resulting ground motion involves multiple wave types, each affecting the structure differently. Primary (P) waves arrive first, causing a rapid, low-amplitude vertical and horizontal push-pull motion. Secondary (S) waves and surface waves follow, generating more violent side-to-side and rolling motions that induce the most damaging horizontal forces. The building’s inherent flexibility determines its natural period—the time it takes for the structure to naturally sway back and forth.
A danger arises from resonance, which occurs when the frequency of the ground motion closely matches the natural frequency of the building. When this match happens, the amplitude of the building’s sway can increase dramatically, leading to amplified shaking and the potential for catastrophic failure. Taller, more flexible buildings have longer natural periods, making them more susceptible to damage from lower-frequency ground motions found in softer soil conditions. Conversely, shorter, stiffer buildings have shorter periods and are more vulnerable to the higher frequencies associated with shaking on hard bedrock.
Typical Types of Earthquake Damage
The inertial forces from ground motion subject structural elements to extreme stress, causing specific, recognizable patterns of failure. One common form of damage is shear failure, which results from horizontal forces pushing structural components in opposite directions. This stress causes diagonal cracks in columns and walls as the material fails to resist the sliding action, leading to brittle failure. Short columns, which are disproportionately stiff compared to adjacent elements, are vulnerable because they attract and must resist a larger share of the total shear force.
A dangerous failure mode is soft story collapse, which occurs on the ground floor of multi-story buildings. This floor may be designed with large, open spaces, such as commercial storefronts or parking garages, and lacks the robust shear walls or bracing present on upper floors. This deficiency creates a weak story with insufficient lateral resistance, causing the entire floor to fail and the floors above it to collapse in a “pancaking” effect. This mechanism was responsible for widespread destruction in past seismic events.
Buildings constructed too close to one another face the risk of pounding damage. This occurs when adjacent structures, often having different heights and natural periods, sway out of phase and physically collide. The impact can cause localized but severe damage to columns and floor slabs, potentially initiating a broader structural failure.
Ground failure can also compromise the foundation, most notably through liquefaction. Beyond the direct damage to the superstructure, this phenomenon occurs when saturated, loose granular soil temporarily loses its strength and stiffness due to increased water pressure caused by shaking. Foundations resting on liquefied soil can sink, tilt, or shift laterally, causing severe damage or collapse.
Designing Structures for Seismic Resilience
Modern seismic design focuses on controlling a building’s response to ground motion rather than simply increasing its strength to resist all forces. A primary strategy is ensuring structural ductility—the ability of a material to deform significantly without fracturing. Engineers design elements, particularly those made of reinforced concrete and steel, to bend and yield under stress, absorbing seismic energy instead of breaking suddenly. This prevents catastrophic collapse and allows the building to sustain damage in specific, replaceable components while maintaining its overall integrity.
To counteract lateral forces, structures incorporate elements like shear walls and bracing systems. Shear walls are rigid vertical elements designed to resist the horizontal forces generated by an earthquake, improving the building’s lateral stiffness. Bracing systems, such as diagonal members, serve a similar function by creating rigid triangles within the structural frame to prevent sideways deformation. These systems transmit the inertial forces from the upper floors down to the foundation.
For structures requiring superior performance and minimal damage, advanced systems like base isolation and damping are employed. Base isolation systems separate the building’s superstructure from its foundation using flexible bearings, often made of rubber or steel. These isolators lengthen the building’s natural period far beyond common earthquake frequencies, drastically reducing the forces transferred into the building. Damping systems, which act like large shock absorbers, are installed within the structure to dissipate kinetic energy from the shaking. These devices limit excessive movement and prevent resonance, ensuring the structure’s response remains controlled.