# Bridge building question **URL:** \ **Category:** Factual Questions\ **Created:** [January 7, 2008, 6:06pm UTC](https://boards.straightdope.com/t/bridge-building-question/432609 "2008-01-07T18:06:04Z")\ **Posts on this page:** 5\ **Page:** 1
**Author:** ![ultrafilter](https://avatars.discourse-cdn.com/v4/letter/u/3d9bf3/32.png) [@ultrafilter](https://boards.straightdope.com/u/ultrafilter)\ **Post date:** [January 7, 2008, 6:06pm UTC](https://boards.straightdope.com/t/bridge-building-question/432609/1 "2008-01-07T18:06:04Z")
The airport in Columbus, Ohio has a couple runways that go over one of the roads in. The concrete is pretty thick, and the undersides are lined with some kind of metal. What other tricks do they use to make the bridges strong enough to support the weight of a commercial airplane? ---
**Author:** ![NinetyWt](https://avatars.discourse-cdn.com/v4/letter/n/cc9497/32.png) [@NinetyWt](https://boards.straightdope.com/u/NinetyWt)\ **Post date:** [January 7, 2008, 6:34pm UTC](https://boards.straightdope.com/t/bridge-building-question/432609/2 "2008-01-07T18:34:35Z")
One of the runways at Memphis has a grade-separation bridge. Their solution was to support the bridge by many many concrete beams set very close together. ---
**Author:** ![gotpasswords](https://avatars.discourse-cdn.com/v4/letter/g/c57346/32.png) [@gotpasswords](https://boards.straightdope.com/u/gotpasswords)\ **Post date:** [January 7, 2008, 6:35pm UTC](https://boards.straightdope.com/t/bridge-building-question/432609/3 "2008-01-07T18:35:14Z")
Same stuff they do to make bridges hold the weight of cars - it all comes to keeping track of how concrete has great strength under compression but has zero ability to be stretched without falling apart, and steel has great resistance to being stretched, and is pretty good at handling compression. ---
**Author:** ![robby](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/robby/32/11048_2.png) [@robby](https://boards.straightdope.com/u/robby)\ **Post date:** [January 7, 2008, 6:38pm UTC](https://boards.straightdope.com/t/bridge-building-question/432609/4 "2008-01-07T18:38:13Z")
They use [reinforced concrete](http://en.wikipedia.org/wiki/Reinforced_concrete). ---
**Author:** ![Santo\_Rugger](https://avatars.discourse-cdn.com/v4/letter/s/e95f7d/32.png) [@Santo\_Rugger](https://boards.straightdope.com/u/Santo_Rugger)\ **Post date:** [January 7, 2008, 6:43pm UTC](https://boards.straightdope.com/t/bridge-building-question/432609/5 "2008-01-07T18:43:36Z")
Prestressed concrete is poured with cables under tension embedded inside. When the tension is released, it puts the concrete in compression, so that said compression must be relieved before the steel rebar can be put into tension. This will be done on the bottom half of the section in question, as the bottom of the “smile”, underneath the neutral axis, is the part that will eventually end up under tension. > **[Prestressed concrete](https://en.wikipedia.org/wiki/Prestressed_concrete)** > > Prestressed concrete is a form of concrete used in construction. It is substantially "prestressed" (compressed) during production, in a manner that strengthens it against tensile forces which will exist when in service.: 3–5  > This compression is produced by the tensioning of high-strength "tendons" located within or adjacent to the concrete and is done to improve the performance of the concrete in service. Tendons may consist of single wires, multi-wire strands or threaded bars that are most com... > **[Stressed ribbon bridge](https://en.wikipedia.org/wiki/Stressed_ribbon_bridge)** > > A stressed ribbon bridge (also stress-ribbon bridge or catenary bridge) is a tension structure similar in many ways to a simple suspension bridge. The suspension cables are embedded in the deck, which follows a catenary arc between supports. As with a simple suspension bridge, the weight is taken by the suspension cables, but unlike the simple span, the deck or ribbon is stressed in compression, which adds to the stiffness of the structure (simple suspension spans tend to sway and bounce). The EETA: You can see the same effect on flatbed semi trailers, which actually hump upwards when they aren’t under a load.