Posted: 8/24/2010 7:17:45 AM EDT
How the hell is this thing not falling apart? Maybe it's magic. 
They add a small piece on ever other day it seems. You can see what that looks like by looking under it on the right side. I realize that the answer is probably really easy since it's working just fine, but it looks weird to me. 
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Physics, how does it work? Sunken foundation pillar, gravity and balanced load keeps it from tumbling. This is a pretty sporting assembly in any case. I'm betting they're going to build some supports, like you can see on the left side of the 2nd photo, once they get the balanced segment built out past the existing road-bed below... But yeah, that looks like a hairy project to work on. Edit: for clarity |
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Steel rebar is the primary reason it's not falling down.
Concrete is great in compressive strength, but is weak in tensile. The rebar is positioned along the beam in the places it experiences tensile load. ETA: You actually want to design in just enough rebar so that when a failure occurs, the concrete cracks first, rather than the rebar just snapping catastrophically. This allows people a chance to get the hell out of there. |
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Steel rebar is the primary reason it's not falling down. Concrete is great in compressive strength, but is weak in tensile. The rebar is positioned along the beam in the places it experiences tensile load. That's a big ass beam column with an enormous bar bell sitting balanced on top, so the problem is one much more complicated that whether rebar is included.. I would be interested to know the load conditions it's designed for during construction. It's a multi-million dollar element at risk of collapse from off design conditions caused by stupidity or nature. |
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Are you suggesting a bridge SHOULDN'T be able to hold itself up.   After it's assembled. While it's under construction those spans might need a little help to make sure something is left when it comes time to join the sections. When that happens, the distribution of internal loads is significantly different. |
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How the hell is this thing not falling apart? There's a guy in the vehicle on top. He drives back and forth, shifting weight, to keep it balanced. 
the whole time he's yelling, "whoooooaaaaaaaa, whooaaaaaaaahhhhhhhhhhh" I design bridges for a living. Those box beams, the rebar, prestressed strands, post tensioned strands, column, foundation, etc, are all designed for any unbalanced load. it's called cantilever construction. don't forget, you have the figure out a way to build whatever you design. the contractor is given the opportunity to improve the operation if he wants to, but the plans need to show something buildable. trust me, the intermediate stages are analysed and design the same as the final condition. a bridge like that is actually very easy to design, they have been around for a long time. one of the design loads is the temperature change from the bottom of the beam to the top of the beam from sunlight. you know how a load induces a deflection, well it works the other way also. |
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How the hell is this thing not falling apart? There's a guy in the vehicle on top. He drives back and forth, shifting weight, to keep it balanced.
the whole time he's yelling, "whoooooaaaaaaaa, whooaaaaaaaahhhhhhhhhhh" I design bridges for a living. Those box beams, the rebar, prestressed strands, post tensioned strands, column, foundation, etc, are all designed for any unbalanced load. it's called cantilever construction. don't forget, you have the figure out a way to build whatever you design. the contractor is given the opportunity to improve the operation if he wants to, but the plans need to show something buildable. trust me, the intermediate stages are analysed and design the same as the final condition. a bridge like that is actually very easy to design, they have been around for a long time. one of the design loads is the temperature change from the bottom of the beam to the top of the beam from sunlight. you know how a load induces a deflection, well it works the other way also. I respectfully disagree. You obviously have never designed anything that I have built.
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That last section? How does it get inserted? The column bends when they have it imbalanced. This lets them add the last section. Yes, concrete will flex. Slightly. the sections are grouted together but they have a keyed waffle pattern on them called "matchcast" a closure pour is done, a foot wide field pour to close up the sections |
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Steel rebar is the primary reason it's not falling down. Concrete is great in compressive strength, but is weak in tensile. The rebar is positioned along the beam in the places it experiences tensile load. ETA: You actually want to design in just enough rebar so that when a failure occurs, the concrete cracks first, rather than the rebar just snapping catastrophically. This allows people a chance to get the hell out of there. actually, what happens is that the mild steel rebar yeilds and starts stretching, making the beam crack and deflect to give a ductile failure, redistribute load to other members, etc if the beam is over-reinforced, then the concrete fails in compression at the top of the beam, with a sudden brittle fracture and failure if you need to, you can add lots of steel to the beam if you also put compression steel in the top of the beam to balance the tension steel, just as long as the tension steel yields first i put 130 prestressed strands in a beam once, it took 30 #11 bars in the top flange to insure ductility. that was a big beam, lemme tell ya |