Resonance You must exert a force on an oscillator to keep it oscillating
Resonance The wiggler exerted a force on the elastic string to keep it vibrating. When the wiggler’s frequency (read out) matched the natural frequency of the elastic string (or a whole number multiple), then the harmonic pattern became visible.
A swing in the park When you push a friend on a swing and your pushes are in sync with his/her movement, then your pushes resonate with the motion of the swing. If you push when your friend is furthest from you, then your pushing is not resonating with the motion of the swing.
A guitar or violin or piano When you strum a guitar, the strings vibrate. The box near it begins to vibrate at the same frequency. It is resonating.
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Design Failure: John Hancock Building, Boston Background 60 story floor-to ceiling reflective glass panels Glass panels started blowing out from the beginning of construction (1973). At times more than 1/3 of panels were out. Cause not certain due to legal agreement of “nondisclosure in perpetuity” Design satisfied all governing codes Some information gleaned from construction grapevine Window design Double-glazed, doubled pane panels Lead spacer at edges to separate panels Design allowed for view of continuous glass
Design Failure: John Hancock Building, Boston Brainstorming Large wind storm correlated to panel blowout Lateral deflection of structure due to wind effect not verified in tests Twisting motion (short direction) observed by tenants (had to be damped out) 600 tons of lead dampers added near roof
Design Failure: John Hancock Building, Boston Failure Cause Analysis Glass panels were correctly installed Thermal stress cycles induced by compression / expansion of air between panes Most panes cracks found in outer panes first Lead solder overconstrains glass causing premature fatigue Replace all 10,344 panels with single pane glass
Tacoma Narrows Bridge Next slide to see very short video of the destruction of the bridge.