Eli5 how do high rises and skyscrapers stay up and last so long?

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Most of these building are meant to last very long time and with maintenance and upgrades even longer. How do 30 floor plus buildings stay standing? Are the lower floors under significantly more stress then higher floors?

In: Engineering
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Steel and concrete are very strong, and are used in ways to distribute the weight. And yes, sometimes building have more support on the lower floors and/or the upper floors are smaller to reduce weight.

Any individual floor is under the same stress, the loading from that floor doesn’t go into other floors unless you’re stopping a column and need to transfer the loads to nearby columns.

The columns at the lower levels however carry a LOT more load. These have to carry the total load of all the floors above them. The engineer calculates the load that will go on the column (this is based on the weight of materials used and the expected building use – an office floor a lot less floor loading than high density storage, for example). These loads are all defined in building codes. They then select columns/walls with sufficient strength.

The biggest challenge for tall buildings is often wind and seismic loads – these get applied horizontally and need strong walls or vertical frames to carry the sideways loads down to the foundations. These structures can get enormous – it’s not uncommon to see concrete shear walls that are several feet think and filled with steel reinforcement to carry these forces.