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.