制震装置(Seismic Dampers): 建物用ショックアブソーバー

What Seismic Dampers Do

Every time a building sways during an earthquake, it stores and releases energy. Without mechanisms to dissipate that energy, oscillations persist and grow, potentially driving the structure to failure. 制震ダンパー地震エネルギーを吸収・減衰させ、構造物の揺れを低減するために建物に設置される装置。粘性ダンパー、摩擦ダンパー、同調質量ダンパーなどの種類がある。 systems address this problem directly: they convert the kinetic energy of structural motion into heat, reducing oscillation amplitudes and protecting the building's structural members from overstress.

The analogy to automotive shock absorbers is apt but incomplete. Car shock absorbers smooth out road-induced vibrations in a single direction at relatively small displacements. Seismic dampers must handle far larger forces and displacements in multiple directions, survive extreme loading in seconds, and continue functioning reliably for decades between major earthquakes. The engineering challenge is substantial, but the rewards — dramatically reduced structural response and building contents damage — justify the investment for critical and high-value structures.

Viscous Fluid Dampers

Viscous fluid dampers are the most widely deployed seismic energy dissipation technology. They operate on the same principle as hydraulic cylinders: fluid forced through orifices or valves converts mechanical energy into heat through viscous flow resistance. The devices are typically installed diagonally within structural bays, connected to the structure at both ends. When the structure drifts horizontally, the damper piston moves, forcing fluid through orifices and generating a velocity-proportional damping force.

The force generated by a viscous damper follows a power law: F = C × v^α, where C is the damping coefficient, v is the velocity of movement, and α is an exponent typically between 0.3 and 1.0. Linear dampers (α = 1.0) provide force proportional to velocity. Nonlinear dampers (α < 1.0) provide larger forces at low velocities and relatively smaller forces at high velocities, a desirable characteristic that limits peak forces while maintaining effective energy dissipation at intermediate response levels.

A major advantage of viscous dampers is that their force is out of phase with the structural displacement. When displacement is at its maximum, velocity is zero and the damper force is zero. Peak damper force occurs when velocity is maximum — near the neutral position. This phase relationship means damper forces do not add to the peak demand on structural members at maximum displacement, allowing columns and beams to be smaller than would otherwise be required.

Viscoelastic Dampers

Viscoelastic dampers use rubbery polymer materials that exhibit both viscous (fluid-like) and elastic (spring-like) behavior when deformed. These materials are sandwiched between steel plates; when the structure moves, the material is sheared, dissipating energy through internal molecular friction while also providing a restoring force. They are typically installed as diagonal braces or between structural elements within the floor system.

The energy dissipation capacity of viscoelastic materials depends on temperature, frequency, and cumulative deformation. At low temperatures, the material stiffens; at high temperatures, it softens. High frequencies of cycling degrade performance differently than low-frequency seismic response. Designers must carefully characterize the material properties across the range of expected service conditions to ensure adequate performance. Despite this complexity, viscoelastic dampers were successfully installed in the original World Trade Center towers and later in other major structures.

Yielding Metallic Dampers

Metallic yielding dampers exploit the energy dissipation that occurs when steel or lead deforms plastically. The simplest form is the added damping and stiffness (ADAS) device: multiple X-shaped steel plates connected between adjacent structural elements. During an earthquake, the plates bend and yield plastically, dissipating substantial energy while maintaining a relatively stable force level. Because the force is limited by the yield strength of the metal, these dampers protect the main structure from overload.

Buckling-restrained braces (BRBs) have become one of the most widely adopted seismic energy dissipation systems. A steel core brace is surrounded by a casing filled with concrete or grout that prevents the core from buckling in compression. The core yields in both tension and compression, providing symmetric, stable hysteretic energy dissipation. BRBs effectively replace conventional steel braces that would buckle under compressive seismic loading, providing far superior ductility and energy dissipation. Their visible presence within the building frame makes inspection and condition assessment straightforward.

Friction Dampers

Friction dampers dissipate energy through sliding between surfaces under controlled normal force. The design challenge is maintaining consistent friction coefficients over the building's lifetime despite temperature changes, humidity, and surface oxidation. Brass on steel interfaces with carefully controlled clamping force have been used successfully in several major installations. Friction dampers are inherently simple, reliable, and capable of large displacements, but require periodic inspection to verify that clamping forces remain within design tolerances.

Tuned Mass Dampers

Tuned mass dampers (TMDs) take a different approach: rather than dissipating energy, they transfer it from the primary structure to a secondary mass that oscillates out of phase. A large mass — sometimes hundreds of tons — is suspended from the top of a tall building on springs and connected to the structure through dampers. When the building oscillates at its natural frequency, the TMD mass oscillates at the same frequency but opposite phase, exerting forces on the structure that counteract the primary oscillation.

TMDs work most effectively for wind-induced vibrations where loading is narrowband and predictable. For earthquakes, whose energy spans a broad range of frequencies, TMDs are less effective but still contribute to overall response reduction. The Taipei 101 skyscraper contains the world's largest TMD — a 660-ton steel sphere suspended by cables near the top of the building, designed primarily for wind response but also contributing to seismic performance.

構造物の共振地震波の周波数が建物の固有振動数と一致したときに生じる、建物の揺れの増幅現象。低層建物は高周波数の波と、高層建物は低周波数の波と共振しやすい。 and Supplemental Damping

The fundamental vulnerability that 制震ダンパー地震エネルギーを吸収・減衰させ、構造物の揺れを低減するために建物に設置される装置。粘性ダンパー、摩擦ダンパー、同調質量ダンパーなどの種類がある。 systems address is 構造物の共振地震波の周波数が建物の固有振動数と一致したときに生じる、建物の揺れの増幅現象。低層建物は高周波数の波と、高層建物は低周波数の波と共振しやすい。 — the amplification of response that occurs when ground motion frequencies match a building's natural frequencies. Supplemental damping systems increase the effective damping ratio of the structure from the typical 2-5% critical damping of bare structural systems to 15-30% or higher, drastically reducing the resonance amplification. At 20% critical damping, peak response may be reduced to 20-30% of that experienced at 2% damping.

耐震設計地震力に耐えられるよう構造物を設計する手法。現代の耐震設計は、大地震における構造被害はある程度許容しつつ、倒壊の防止と人命の保護を目指す。 standards such as ASCE 7 Chapter 18 provide procedures for analyzing buildings with supplemental dampers, accounting for their nonlinear force-displacement behavior, velocity dependence, and temperature sensitivity. Performance-based design methods allow engineers to explicitly demonstrate that damper systems achieve target performance levels, justifying their cost through quantified risk reduction.

Real-World Applications and Performance

The 2011 Christchurch earthquake provided crucial evidence for damper performance. Buildings equipped with buckling-restrained braces and viscous dampers performed substantially better than comparable code-minimum structures, with several remaining fully operational after the earthquake. Post-earthquake inspections found yielding and deformation concentrated in replaceable damper elements rather than primary structural members — exactly as designed.

The Torre Mayor skyscraper in Mexico City incorporates 98 viscous dampers installed diagonally within its structural bays. The building survived several significant earthquakes after its completion in 2003, with measured accelerations significantly below those of nearby conventional buildings, demonstrating real-world performance matching engineering predictions.

The economic case for 制震ダンパー地震エネルギーを吸収・減衰させ、構造物の揺れを低減するために建物に設置される装置。粘性ダンパー、摩擦ダンパー、同調質量ダンパーなどの種類がある。 systems rests on lifecycle cost analysis rather than initial construction cost. Reduced structural member sizes can partly offset damper costs. More importantly, reduced earthquake damage translates directly to reduced repair costs, shorter post-earthquake downtime, and lower business interruption losses. For hospitals, emergency operations centers, and data centers, post-earthquake operability is worth enormous premiums.