Earthquake engineering

Earthquake engineering aims to design structures that can resist seismic forces while remaining functional after an event. The primary performance objectives are life safety, damage control, and operational continuity, often codified in building codes. The fundamental approach involves analyzing the dynamic response of structures to ground motion, considering the structure's stiffness, mass, and damping. Key concepts include ductility, which allows structures to deform plastically without collapse, and base isolation, which decouples the structure from ground motion. ¹

Seismic design typically uses a response spectrum or time-history analysis to determine forces and displacements. Modern codes, such as ASCE 7 in the United States, prescribe minimum lateral forces and detailing requirements to ensure adequate performance. The philosophy is to allow minor damage in moderate earthquakes but prevent collapse in rare, severe events.

The modern field emerged after the 1906 San Francisco earthquake, which exposed the vulnerability of unreinforced masonry buildings. Early seismic codes in Japan and Italy focused on lateral force coefficients. The 1933 Long Beach earthquake led to the Field Act in California, mandating seismic design for schools. The 1971 San Fernando earthquake revealed deficiencies in welded steel connections, prompting research into ductile detailing. ²

Significant advances include the development of capacity design in New Zealand in the 1970s, which ensures that brittle failure modes are avoided by making ductile elements yield first. The 1994 Northridge earthquake exposed fractures in welded steel moment frames, leading to improved connection designs. Today, performance-based earthquake engineering (PBEE) allows for tailored designs that meet specific performance objectives, using probabilistic methods to quantify risk. ³

Seismic hazard assessment involves characterizing the ground motion expected at a site, considering fault sources, attenuation, and local soil effects. Probabilistic seismic hazard analysis (PSHA) combines these factors to produce hazard curves, which are used to define design ground motions. Deterministic scenarios are also used for critical facilities.

Soil-structure interaction (SSI) accounts for the influence of the foundation and soil on the structure's response. Soft soils can amplify ground motion, as seen in the 1985 Mexico City earthquake, where distant shaking caused severe damage on lakebed sediments. Site response analysis and geotechnical investigations are essential for designing foundations that mitigate liquefaction and settlement risks. ⁴

Common strategies include strengthening the lateral load-resisting system, using moment frames, braced frames, or shear walls. Ductile detailing, such as closely spaced stirrups in columns, ensures that structures can undergo inelastic deformation. Base isolation and supplemental damping devices, such as viscous dampers, are used to reduce seismic demand. ⁵

Innovative materials like fiber-reinforced polymers (FRP) are used for retrofitting existing structures. Seismic retrofitting of older buildings is a major focus, as many are seismically deficient. Techniques include adding steel bracing, concrete shear walls, or base isolators. The cost-effectiveness of retrofitting is often evaluated against the potential losses from future earthquakes. ⁶

Beyond conventional buildings, earthquake engineering addresses nonstructural components, such as ceilings, facades, and equipment, which can cause significant damage and economic loss. The 1994 Northridge earthquake caused billions in damage to nonstructural elements. ²

Historical structures, such as masonry monuments, require special consideration; techniques like shape memory alloy devices have been tested for their preservation. The 2010 Chile earthquake highlighted the performance of tall buildings with core walls, which performed well due to high ductility. ³

Seismic isolation is also applied to bridges and industrial facilities, and even to art objects in museums. The field also considers the impact of aftershocks and cascading hazards like tsunamis and landslides.

Base isolation

A seismic design technique that decouples a structure from ground motion using flexible bearings.

Ductility

The ability of a structure to undergo large inelastic deformations without collapse.

Response spectrum

A plot of maximum response of a single-degree-of-freedom system to ground motion as a function of period.

Liquefaction

A phenomenon where saturated soil loses strength and stiffness during shaking, behaving like a liquid.

Performance-based design

An approach that designs structures to meet specific performance objectives under various seismic hazard levels.