Thursday, September 12, 2013

Physics of building

Walking through the city of London can leave one with little doubt about how far we have come where building is concerned.  The building sites in the city do not appear to be much bigger than the buildings they contain, the roads are not filled with trucks carrying building materials or disposing of the old building also taken down in its own footprint.

The skyscrapers that are emerging are impressive.  The concrete and steel structures all go up the same way.  Led by the central concrete core which is put up first, followed a few floors behind by the steel, then a few floors behind by the glass.  Its a slick operation.  Being built of glass means you can see the structure underneath as it goes up and indeed once up.

The strange thing about most of the buildings is that the steel used at the top of the building is the same as the steel at the bottom of the building.  I you take a 30 story building the weight at the base must be fairly significant as it carries all 30 floors.  But the beams at the top only need to carry the roof and the last floor.  Yet the steel structure appears uniform and the thickness of the steel uniform too.  Either I do not understand the physics or the steel used at the top is 30 times too strong for what is needed.

The only explanation I can come up with is to do with other properties of the structure.  It may just be that to create rigidity against lateral motion steel needs to be of a certain thickness and strength.  Therefore what I witness in the structure represents the strength needed for lateral strength (which may be uniform across the height of the building and not the compression strength which is needed most at the base.

More research required.


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