Seismic dampers are specialized mechanical devices installed within a structure to manage the kinetic energy generated by an earthquake. Their function is to protect a building or bridge by controlling movement and preventing structural failure or widespread damage during a seismic event. These devices act as a supplemental damping system, absorbing energy that would otherwise be absorbed by main structural elements like columns and beams. They enhance the structure’s resilience, allowing it to remain operational and safe after the ground motion subsides.
How Dampers Dissipate Earthquake Energy
Unlike traditional construction methods that rely on the strength and stiffness of materials, dampers function by dissipating the energy of the structure’s movement. An earthquake imparts kinetic energy into a building, causing it to oscillate and deform. Without dampers, this energy converts into strain energy that causes permanent, damaging deformation in the structural frame.
Dampers intercept this kinetic energy and convert it into a less destructive form, primarily thermal energy, or heat. The damper’s action is velocity-dependent or displacement-dependent. This means the faster the structure tries to move or the further it tries to deform, the more resistance the damper provides.
The energy dissipation mechanism allows the primary structure to remain largely within its elastic range, minimizing damage. This distinguishes them from conventional building frames, which often dissipate earthquake energy through inelastic deformation, sustaining damage to prevent collapse. By absorbing energy, dampers shield the main load-bearing elements, ensuring the building’s integrity and allowing for quicker post-earthquake recovery.
Major Categories of Seismic Dampers
Viscous Dampers
Viscous dampers are self-contained hydraulic cylinders that operate using the principle of fluid orificing. A piston moves through a chamber filled with a highly viscous silicone fluid. When the structure moves during an earthquake, the piston forces the fluid to pass through small openings, or orifices, inside the cylinder.
The resistance created by the fluid being forced through these passages dissipates kinetic energy, converting it into thermal energy. These devices are velocity-dependent, meaning the damping force is proportional to the speed of the piston’s motion. This characteristic makes them effective at reducing high-speed, high-amplitude vibrations while providing minimal resistance during slow, normal building movements like those from wind.
Metallic Yielding Dampers
Metallic yielding dampers, also known as hysteretic dampers, dissipate energy through the controlled plastic deformation of specialized metal components. These devices are constructed from mild steel or aluminum plates, which are intentionally made to be the weakest link in the structural system.
When a seismic event causes the structure to deform, the metal plates within the damper yield, or bend and stretch, in a controlled and predictable manner. This inelastic deformation is a highly effective way to absorb large amounts of energy. The main benefit is that the damper acts as a sacrificial fuse, protecting the main beams and columns from permanent damage, although the damper itself must typically be replaced after a major earthquake.
Viscoelastic Dampers
Viscoelastic dampers utilize layers of a solid, polymer-based material, often composed of rubber, asphalt, or specialized polymers, sandwiched between steel plates. These materials exhibit both viscous (fluid-like) and elastic (solid-like) properties. When the structure sways, the alternating layers are subjected to shear deformation.
The damper dissipates energy by converting mechanical strain energy into heat as the viscoelastic material is sheared. A key feature of these dampers is their ability to provide damping at small displacements, making them effective for mitigating not only earthquake forces but also everyday wind-induced vibrations. However, their performance can be sensitive to fluctuations in ambient temperature and the frequency of the vibration.
Where Seismic Dampers Are Installed
Dampers are strategically installed in locations where the structural movement, or inter-story drift, is greatest during an earthquake. This placement maximizes the energy dissipated by the devices. Engineers often integrate dampers into the building’s lateral force-resisting system, particularly within the bays of the structural frame.
Common installation methods involve placing the dampers diagonally, similar to a traditional bracing system, or in a chevron (inverted V) configuration between a beam and column. This ensures the damper is activated by the relative horizontal movement between adjacent floors. Strategic placement is often optimized to minimize both inter-story drift and peak floor accelerations across the building’s height.
In bridges, dampers are installed to control the movement of the bridge deck relative to the supporting piers or abutments. They limit both longitudinal and transverse displacements, which helps prevent the deck from moving off its supports during severe ground shaking. The use of dampers in both high-rise buildings and long-span bridges is a standard technique to ensure structural resilience and minimize post-earthquake repair costs.