Eli5: How can a high rise building survive an earthquake?
In: Engineering
TL;DR: They’re engineered to withstand the forces anticipated from an earthquake.
Longer answer:
In the US, for example, the USGS has maps available showing the expected earthquake strength across the United States. The engineer takes these values for the basis of the earthquake.
The ground conditions affect how hard a given earthquake will hit. Rock is good, soft ground is bad. A geotechnical engineer will assess the site by a series of boreholes and provide a recommendation for the groin conditions to the structural engineer.
The structural engineer takes the above values and then selects the building lateral systems (that resist wind and earthquake loads) and designs the systems for the resulting forces.
In low seismic areas this is fairly simple as a straight force is applied to the building and members are just designed to resist that force.
In high seismic areas it gets more complex. The earthquakes get too large to just “resist”, instead you need to pick systems that can absorb energy from the earthquake by deforming in a particular way that doesn’t threaten the structure itself, and also make sure that if members do fail they fail elastically – for example you want beams to fail before columns, if a beam fails in bending you just have messed up floors that are all uneven but if a column fails you have a collapsed building. But you can’t just make the systems *too* strong or else they get too rigid and can’t absorb energy so although the system is stronger the loads are also bigger.
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