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martynr | 3 years ago

From experience of NZ quakes, wooden buildings seem to survive well.

Aside from the flexibility of wooden vertical load bearing posts, the typical weather board / external wooden slat construction which is widely used in New Zealand allows for the narrow boards to slide independently under the lateral shaking forces. The external walls can deform into trapezoid shapes more effectively - hence a higher threshold for catastrophic damage and better future usability.

Brickwork chimneys, internal plasterwork and the subsurface conditions are another story but a least the dwelling is still functional :/

discuss

order

WalterBright|3 years ago

> the flexibility of wooden vertical load bearing posts

I know you didn't write this, but lots of people think that the flexibility works because it "absorbs" the energy of the earthquake. It does not. What it does is spread a short sharp move into longer but lower amplitude move. The energy imparted is the same.

wiredfool|3 years ago

Ultimately, it's: Ma + cv + kx = 0, the differential equation for damped harmonic motion. M, C, and K are matricies/tensors that can change over time as damage occurs, x, dx/dt, and d2x/dt2 are time derivative vectors of your node DOF (xyz, + 3 rotations). Your boundary conditions are that the nodes connected to the ground have a forcing function xg = f(t).

If everything is linear elastic, you can take the principle components/eigenvectors and get fundamental modes for a first approximation. Once you hit plastic deformation, things go non-linear and get more interesting.

So, Wood:

1) Less stiff, so KX means less force due the ground motion. 2) Less mass, so the MA term is less.

However, wood also tends to be used in smaller structures, so your on the left side of the resonance peak, were first mode structure motion is greater than the forcing function (but not into the resonance peak)

Steel tends to be used for really tall buildings, where the earthquake doesn't excite the fundamental modes, so the ground basically wiggles the bottom of the skyscraper, but the top is nearly unaffected. (the right side of the resonance peak)