Physics
Resonance Avoidance
Quick fact
The Tacoma Narrows Bridge collapse in 1940 is a classic example of resonance—and why resonance avoidance is now a fundamental principle in structural design.
Why this is interesting
Have you ever seen a singer break a wine glass with just her voice? That's resonance—a beautiful but dangerous phenomenon. How do engineers prevent skyscrapers and suspension bridges from shaking themselves apart?
Read the full explanation
Understanding Resonance Avoidance
Resonance occurs when an external force pushes a system at its natural frequency, causing vibrations to grow larger and larger. Think of pushing a child on a swing: if you push at the right moment each time, the swing goes higher. Resonance avoidance is the opposite—it's about ensuring that never happens unintentionally. Engineers achieve this by making sure the system's natural frequency is far from any likely excitation frequency, by adding energy-absorbing materials (damping), or by physically isolating the system from the source of vibration. For example, tall buildings are designed to have natural frequencies well above or below earthquake frequencies, and car engines use rubber mounts to isolate vibrations from the chassis.
A deeper explanation
The mechanism behind resonance avoidance relies on controlling the forced oscillation equation. When an oscillatory force is applied at or near a system's natural frequency, energy accumulates and amplitude can grow to dangerous levels. To avoid this, engineers use three main strategies: (1) Detuning: altering the system's stiffness or mass to shift its natural frequency away from expected excitation frequencies (e.g., adding stiffeners to a bridge). (2) Damping: converting vibrational energy into heat through viscous, frictional, or hysteretic mechanisms (e.g., shock absorbers, tuned mass dampers). (3) Isolation: inserting flexible elements that prevent the transmission of vibrations from the source to the structure (e.g., rubber pads under machinery, base isolators in buildings). Resonance avoidance matters because it prevents structural fatigue, noise, and catastrophic failure—it is why your car's engine doesn't shake itself loose and why buildings survive earthquakes.