TEC154 2010S, Class 29: Structures (2) Overview: Admin: * Because of copier troubles in the science division office, I do not yet have copies of the reading for Friday. Expect them outside my door this afternoon. Sorry. * While we touch on this reading on Friday (and you need to submit questions), our focus will be the feminist readings from last week. * John Simon, Jr.'s Every Icon is now available in the iTunes App store. * Bad design story: Lights in the CS Display Cases * This class will be transcribed (approximately) by SamR. * Paul Harding, who is teaching a short course on fiction, just won the Pulitzer prize. Reminder: Cast Iron * 20% of cast iron is carbon * It's very brittle (see the pipe that broke when hit with a hammer) * More like glass than steel * But doesn't rust very easily Another material: Concrete * Slide: The Pantheon * Reminder: Concrete is not a new material. It was known to the Romans * Joke: "We have no photographs from the time, but there's an understanding that it was similar to what you see here." * Note: Like many Roman materials, concrete takes compression, not tension * We often wrap concrete around steel * Why? Concrete acts as fire protection for steel * Steel won't burn * But it can melt, so the concrete insulates * Steel also helps the concrete * Consider a concrete "beam" with weight in the center * Compression on the top - no problem * Tension on the bottom - cracks * Solution: We put a steel bar across the bottom. The bar takes tension * Reinforced concrete! * The piece of steel is called "rebar" Two important engineers: Maillart (1872-1940) and ... * Let's trace the evolution of how engineers thought about building bridges * It's not all straightforward comptuation * There's a lot of trial and error * Maillart is Swiss, and the bridges he build are in Switzerland * First example, one in which he was Junior Engineer (not his design) [Approximatley 1899] [Built in large town] * At this point, 1/4 bridges failed. * Lots of money spent on making it look strong and pretty * This bridge supported by a three-hinged arch * Slide: Comparison of no-hinge arch and three-hinged arch * You can make a bridge supported by a single arch. But if you have any changes, you tend to get cracks at the crown (top) or abutment (ends) * If you put hinges at those places (both abutments and the crown), you avoid cracks. * Maillart liked the shape, and wanted to replicate parts * But his first bridge had a different domain * A small town, less traffic, less money * Slide: Scaffoliding * Slide: Cross-section of traditional bridge * The arch is about 1 meter thick. That requires a lot of scaffolding! * Scaffolding is expensive (about 30% the cost of a typical bridge). * Slide: Cross-section of Maillart Bridge * Cool idea: Instead of a big thick piece, we can use a hollow celled thing ("box") which gives similar support, but is lighter. * Slide: Photograph of bridge leading in to small town at base of mountains * Slide: Diagram of cracks in that bridge * Diagram on board: What happens when you put weight on a three-hinged arch * Cracks appear a bit in the facade * But they're not really in a weight-bearing part * Maillart's solution: In the next design, cut out the part of the facade that was cracking - * Slide: Diagram of that * Slide: Travanessa bridge * Also in the mountains * "built on a showstring" * Note: You can't easily calculate where the stresses will be on one of these bridges * The Swiss test each bridge by sending heavy trucks across and measuring what the deflection is. * The bridge no longer exists: A landslide took out about 1/3 of the twon as well as the bridge * Note there's a real elegance to the bridge * Next slide: Sketch of Maillart's next bridge, his longest bridge. Over a very deep valley. * Problem: How do you build scaffolding for such a thing. * Again, "A very light and airy design" * Interesting debate: Was Maillart successful because he had a good builder of scaffolding, or was it that his designs were so light that scaffolding was easy (or both)? * Next slide: The scaffolding for this thing: Built out from sides * Question: How do you deal with the fact that it's flexible at the top? * Think about the hinged board from last time * It can flex a bit there (crossing rebar between the two halves) * They can flex a bit to bend * They can also take compression * Question: If three flex points are good, why not more? * It is no longer stable * Question: If things want to collapse to an inverted arch, why not build it that way? * Then your main force is tension, rather than compression. * And that's how suspension bridges are built. * Question: How do you secure the bridge at the ends? * You need something solid (such as bedrock) * And, fortunately, it's easy to get to the bedrock in the mountains. * Question: What does cutting out the stuff at the sides do? * It's mostly for looks/aesthetics * Slide: More scaffolding. * Slide: Maillart's final bridge (1940) * Built for a small village of approximately 50 inhabitants * Intended mostly to bring cows to market * Need to use a topographical map to find out how to get it * Story about a producer of a coffee table book on bridges who wanted a photograph of the bridge, spent a day trying, and gave up. * Intended for foot traffic and cows, not really for cars. * Question: Explain cracks in facade again * Demonstration: Reinforced concrete * A two-by-four cut into seven pieces with a rope running through it * Wood simulates concrete * Rope simulates rebar * It will go over a span * Shows what happens with reinforced concrete * If you put weight on reinforced concrete, it will droop a bit until the steel begins tight * So you get cracks. * Can you deal with this problem? * Yes: You put tension on the steel/rope, and it holds better * "Prestressed concrete" * Came into common use in the 1940's (after WWII) * Coming up: U. Finsterwalder and Christian Menn, practitioners of the use of presetressed concrete * Slide: Diagram of what happens with reinforced concrete * Cracks are okay; lots of things today are built with reinforced concrete * As long as the cracks aren't visible from more than an arm's length, it's probably okay * If you enter the science building from the South entrance, look up at the ceiling at the border between old and new parts (right by the free-standing display) ... you'll see cracks * Slide: Diagram of how prestressing works * Photograph: Framework of prestressed concrete (w/o concrete) * Rebar makes a cage * Prestressing tendons are inside * You pour in concrete * Let it dry * Then use a jack to githten the tendons * At Roberts Theatre, you can see in the T-beams where this happened * Lots of slides zoom by * Using prestressed concrete you can build the bridge w/o building scaffolding * Christian Menn * Most of his bridges are in Switzerland * Slide: An arch bridge * Note: Swiss defense system: Every bridge needs to be easy to blow up. * Show's part where this can happen * Slide: Cantelever construction * Six lanes of traffic * 1 KM long * Last bridge, the Ganter * A difficult engineering problem * One side prone to landslides * The other side is moving (about 1 cm per year)