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| Main Span |
 Title text: Wind stress? Don't be silly. When has wind stress ever been a problem for a suspension bridge? |
Explanation
The "main span" of a bridge refers to the longest section between two support towers. A spherical, lighter-than-air balloon is used to provide an upward force in order to hold up this suspension bridge, allowing the main span to exceed the theoretical maximum length (estimated here as 3–4 km) that would be stable under the forces of tension and compression alone.
Typically, the main span of a suspension bridge would only have suspension cables above that section of bridge.It appears that the balloon is essentially being used as a replacement for where a central support tower would normally go, and providing essentially the same function. [actual citation needed]
The title text is a reference to the Tacoma Narrows Bridge. There's a well known video showing it collapsing as a result of wind stresses that matched the bridge's natural resonance frequency. This is often used in physics classes to demonstrate resonance, and in engineering classes as a cautionary tale. Note, though, that technically in that case might have been more fluttering than resonance.
While using a single balloon to support a structure at this scale would be incredibly impractical if not outright impossible,[citation needed], similar designs have been proposed and built. Most similarly, Pond de Singe was an art installation in Tatton Park, near Manchester UK, where a short bridge was suspended by helium balloons, although guests were not allowed to use it. Additionally, pontoon bridges, which float on water rather than in air, are somewhat common. More speculatively, active support, using active methods rather than passive strength to support structures too large to exist otherwise, has been proposed in speculative settings, but no major structure has been built with active support.[actual citation needed]
Transcript
[A diagram of a suspension bridge across a cross-section of a body of water is shown. The bridge has three spans. The left and right spans are held up using normal pillars that bury into the ground under the shallow portions of the water. The middle span has no pillar, and is held up by a large balloon tethered to the top of the tower by about a dozen cables. There are two-headed arrows above the main cables to the left and right of the left tower, to the left of the middle tower, and to the left and right of the cables from the balloon to the middle tower. There are mostly unreadable annotations to the right of the left tower (diagonally above the arrows), to the left of the middle tower, and to the left and right of the balloon. Above the balloon is a line segment whose length matches the width of the balloon with the annotation "d" in the middle. To the right of the balloon is an equation for d.]
[Caption below diagram:]
How to get past the 3-4 km limit on suspension bridge main spans.
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