The engineering goal in seismic zones is not to construct a truly “earthquake-proof” building, which is nearly impossible, but rather a structure that is highly resilient and can withstand severe ground motion without collapsing. Earthquakes create powerful inertial forces within a building, resulting from the structure’s mass resisting the sudden, violent acceleration of the ground. These internal forces, proportional to the building’s mass and the ground’s acceleration, are the primary cause of structural damage. Modern seismic design manages these forces by controlling how the building interacts with the ground, how its skeleton deforms, and how energy is dissipated.
Managing Ground Motion Through Foundation Design
The first line of defense against seismic forces involves decoupling the building from intense ground movements or securing it deeply into stable earth. Base isolation is one highly effective method, involving specialized flexible components installed between the foundation and the superstructure. These isolators, often made of laminated layers of steel and rubber, act like giant shock absorbers.
Their primary function is to shift the building’s natural frequency of vibration away from the dominant, high-energy frequencies characteristic of earthquake ground motion. By reducing the structure’s stiffness, base isolators lengthen the building’s natural vibration period, preventing the structure from resonating violently. When the ground moves laterally, the isolators flex, allowing the foundation to move with the earth while the building above remains relatively stable. Lead-rubber bearings incorporate a central lead core that provides additional damping and recentering capabilities.
For structures that cannot be isolated or where soil conditions are poor, fixed deep foundations are used. In areas prone to soil liquefaction—where saturated soil temporarily loses strength and behaves like a liquid—the foundation must bypass these unstable layers. Engineers drive deep piles or construct piers down to bedrock or a stable stratum to anchor the building. This design ensures the support system is rooted firmly, protecting it from movement and loss of bearing capacity in the upper soil layers.
Enhancing Structural Flexibility and Strength
Beyond the foundation, the structural skeleton is engineered for both strength and controlled flexibility, a concept known as ductility. Ductility allows the building to undergo significant deformation without brittle failure, absorbing seismic energy through intentional yielding of materials. Specialized materials, such as high-strength, ductile steel, are used in framing to ensure components can bend and stretch without snapping.
Two primary structural components resist the lateral forces generated by an earthquake: shear walls and moment-resisting frames. Shear walls are rigid, vertical panels, typically made of reinforced concrete, that run the full height of the building. They resist horizontal forces by acting like deep, stiff beams, transferring the lateral load down to the foundation. Their placement is often symmetrical, surrounding elevator shafts and stairwells, preventing twisting.
Conversely, moment-resisting frames are designed to be more flexible, using rigid connections between beams and columns that allow the frame to sway in a controlled manner. These frames resist movement through the strength of their joints, maintaining a 90-degree angle even as the structure temporarily deforms. Often, a hybrid system combines the high rigidity of shear walls with the controlled flexibility of moment frames to provide multiple pathways for resisting and dissipating energy.
Dissipating Seismic Energy with Damping Systems
To actively manage and reduce the energy transmitted into the structure, engineers integrate damping systems that function much like a car’s shock absorbers. These devices dissipate the kinetic energy of the earthquake-induced motion, converting it into heat that harmlessly leaves the system. This prevents the frame from having to absorb all the energy through potentially damaging structural deformation.
Viscous dampers are a common type, consisting of a piston moving through a cylinder filled with a highly viscous fluid, such as silicone gel. When the building moves, the fluid is forced through small orifices, creating resistance proportional to the velocity. This fluid resistance converts mechanical energy into heat, which is then dissipated into the air, reducing the intensity of structural oscillations.
Another strategy uses friction dampers, which convert kinetic energy into heat using the friction generated between sliding plates. These plates are pressed together with a specific force, and when the seismic force exceeds this clamping force, the plates slide, dissipating the energy. Both viscous and friction dampers can be installed diagonally within the frame, or in conjunction with base isolation systems, providing supplemental energy dissipation and significantly reducing forces on the beams and columns.
Optimizing Building Geometry and Non-Structural Elements
The overall shape and layout of a building fundamentally influence how seismic forces are distributed and resisted. Buildings with simple, symmetrical, and regular geometries—such as square or rectangular plans—perform significantly better than irregular shapes like L or T configurations. Irregularity can cause non-uniform stiffness and mass distribution, leading to torsional forces that twist the building and concentrate stress in corners.
A particular vulnerability is the “soft story,” which occurs when one floor—often the ground level with open space for parking or retail—is significantly less rigid or strong than the stories above it. Because the open ground floor lacks sufficient shear walls or bracing, large lateral forces concentrate there, causing the story to fold and the upper floors to pancake down. Engineers mitigate this by increasing the strength and stiffness of the columns and adding specialized bracing or shear walls on the ground floor to ensure cohesive movement.
Finally, non-structural elements—everything not part of the load-bearing frame, like cladding, ceilings, and utility equipment—must be secured to prevent internal injury and disruption. Heavy components, such as air conditioning units and electrical gear, are anchored firmly to the floor slabs to prevent sliding or overturning. Proper detailing of partitions and exterior cladding is necessary to accommodate the expected sway of the building so they do not fail during a seismic event.