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  • Breaking Things Down and Building Them Up

    Posted by Michael Hiteshew on January 26th, 2016 (All posts by )

    First some demolition:

    There’s actually a lot more going here than meets the eye. First, a structural model of the building is created. Buildings and bridges are overbuilt, such that the structure is capable of supporting considerably more load than it’s actually required to hold. This allows for minor failures to occur during the construction and life of the building without it collapsing. Once the model is built, they determine what support members may be removed without collapsing the structure, taking it from a safety factor of 1.5 (50% stronger than necessary) to 1 (just strong enough to stand). The analysis is carried out or overseen by structural engineers.

    Next, charges are laid on some support members, like columns and beams, but not others. The idea is to leave parts of the building connected by steel girders to parts that will fall so they get pulled in that direction and fall on top of the pile. Gravity does the actual demolition, the charges just break the supports.

    Finally, the charges are detonated in a careful sequence. First are a series of weakening charges that remove the 50% of support safety margin, then the building is collapsed from bottom to top and (usually) from the center outwards to the periphery, with the back and sides being pulled into the center debris pile.

    Dams & Bridges:

    Below, you’ll see an example of dewatering a small reservoir prior to dam demolition. Notice all the silt that both comes out and is left behind. This is one reason all dams have a limited life. Unless the entire reservoir is dredged on a regular basis they will fill with silt.

    Marmot Dam Removal – What to do about the silt?

    Many small dams have outlived their original purpose. There are ongoing projects to remove them, both in the USA and Europe. Here, a small dam on the Raritan River in New Jersey is being removed as part of a larger project to restore the Raritan River.

    ….

    If you caught the reference to ‘sheet piles’ in the previous video, they are a wall of interlocking steel plates that are driven into the bed of the river under a dam. When water builds up behind a dam it increases the “pressure head” of the water, meaning the water pressure increases as the depth increases (.43 psi per foot of depth). The increased pressure will drive water into the bed of the river and under the dam, which can undercut it and wash it out. Here’s a video with Dr Ralph Peck that was produced for senior and graduate level civil engineering students describing the problem and methods used to prevent it. Dr. Peck is a minor legend in the civil engineering world both for his vast store of knowledge and his ability to communicate complex problems in a clear and understandable way.

    And finally here are a few quick time lapse videos of bridge construction:

    There’s something about a society that is building and rebuilding infrastructure that is a sign of health. It’s a society that is planning for the future, investing in it. It also a society that is learning; to organize, to solve problems in an effective way, and developing both theory and experience to accomplish that. It is a society that is alive and growing. I could drop you into any society on earth and based solely on your view of the infrastructure you could describe the level of civilization that exists there.

     

    11 Responses to “Breaking Things Down and Building Them Up”

    1. newrouter Says:

      Implosion brings down old Hulton Bridge
      http://www.wtae.com/news/implosion-brings-down-old-hulton-bridge/37644704

    2. Sgt. Mom Says:

      Those videos were so cool – especially the various views of the Houston building coming down. I could visualize the logic of the timed explosions, and appreciate how it all came collapsing down into the footprint of the original building.

    3. Gringo Says:

      Then we have the Tacoma Narrows Bridge collapse of 1940, which I first saw in a high school physics class.

      https://www.youtube.com/watch?v=j-zczJXSxnw

    4. Michael Hiteshew Says:

      Gringo, that bridge is a classic in failure analysis of bridge design. It’s also a good example of how waves can build towards resonant frequency and tear a structure a apart. Resonance is a prime concern in earthquake analysis for bridges and buildings, where different earthquake durations at different distances through different strata different give periods of motion. Then judgements have to applied to how likely or unlikely an earthquake event you wish to design for.

      Here’s two short videos on seismic loading features that were designed into the new SF-Oakland bridge:
      San Francisco-Oakland Bay Bridge East Span Seismic Innovations: https://youtu.be/1tKw36JEx90
      San Francisco-Oakland Bay Bridge New East Span: https://youtu.be/pEV6EKcGSb4

    5. Gurray Says:

      A similar thing happened with the London Millennium Footbridge. The crowd increasingly stepped along with the vibrations, and the positive feedback grew to cause it to sway. It was an example of crowd synchrony.

      120 years ago, Nikola Tesla patented his earthquake machine. It could send vibrations through structures at their resonant frequency. I remember watching them try it on Mythbusters on a bridge. It actually worked, but they claimed it was busted because the swaying was too faint. It seemed to me that if they had just waited longer it may have really started building up. I think what happened was the Mythbusters guys expected it to be just a joke, and then when it started working for real they were probably ordered to stop before any real damage was done.

    6. PenGun Says:

      Refrains from posting youtube vids of WT7 going down.

    7. Michael Hiteshew Says:

      I’d be interested in hearing all the charges going off. And seeing the evidence of detonated beams and columns. And evidence of blasting and fusing debris. Let us know when you’ve got all that.

    8. David Adair Says:

      The Tacoma Narrows Bridge was brought down by edge flutter.

    9. Mike K Says:

      “It could send vibrations through structures at their resonant frequency. ”

      I remember reading a very good short story based on this concept. I was a kid but still remember it. A guy was seeking to extort money and had a device in a building that was running back and forth on a track to get the building moving. I have no idea how accurate the story was but it was an interesting idea for a short story.

    10. Jonathan Says:

      Late to this, but cool post.

    11. Kirk Parker Says:

      especially the various views of the Houston building coming down

      Indeed. Having seen the fall of the Asarco (Tacoma) Smelter stack in real life, and the Seattle Kingdome on TV, I find it completely amazing how well the demolition folks manage to control most of these.