Chile Earthquake: Soft ground bites the hand that builds on it

Wow! We haven't seen tilted buildings since Niigata, Japan! We don't have any ground motion data yet, but I've been looking at the pictures.

PGV- peak ground velocity

PGA- peak ground acceleration

An earthquake such as this, which affects modern construction, and should be well-instrumented provides a good check on our building code assumptions, which all work the same throughout the world:

You pick a sufficiently rare earthquake (usually 1 in 500 years).

You calculate the ground motion in PGA.

You maybe double it for soft ground, and take away factors for ductility, etc, so that it does not put engineers out of business (great politics here!).

You translate it to an equivalent lateral gravity force, usually around 10% g. This has proven to be fairly good throughout history. This is equivalent to tilting the building arctan 0.1 or a few degrees. You use the body force to calculate deformation and stress.

You design so the building 'deforms gracefully'. In other words, it can be highly damaged, but does not collapse, so people can escape.

Is this a good system? I don't think so, and this earthquake might help bring about some necessary changes. The main principle is good, but seismic is a forced strain, controlled by PGV, and is not a body force controlled by mass. When the earthquake hits, inertial forces cause the top to remain still, and the bottom to move. If there is resonance, then the PGV at the top increases, but the soil can resonate as well.

This is the biggest weakness of the current codes as seen in the picture. Nobody takes soft soil seriously! And it all boils down to using the non-physical PGA. Under high strains, the soil starts to yield in a non-linear manner, which damps out high frequencies. And this is great for PGA, yeah! It cuts down peak acceleration. For years I've been reading on how much better it is to build on soft soil, rather than nasty hard rock, because of PGA.

Well, silly people, when that soil is yielding, it is softening up, and down-shifting the frequencies. This pumps up PGV to tremendous levels. We can expect an amplification of 10 to 100 times, and building code only gives maybe a factor of 2!

So, there we have our tilted buildings. Perfectly strong, but tilted, and this is not good for the people inside, especially if they have grand pianos!