## Monday, February 18, 2008

### Seismic Analysis

My next series will be on seismic analysis. I wrote a fair piece of that Wiki article, and I've been involved with the trade for many years. For those purists, I'm referring to civil seismic analysis, which is the study of soils and structures to seismic shaking.

The prime motivator for the start of seismic analysis was the observation that buildings shimmered and shook in earthquakes, like a dancer on the good juice! As well, it was noted that buildings on soft soils would act extra bad, if both the soil and building had matching resonance frequencies.

Thus, seismic analysis started down the road of studying resonance, and never really got off that streetcar! And what better way to study resonance than to muck up everybody's minds with eigenvalues! This has been designed as mental torture for young engineers, and filters them out by killing them! You young'uns might have to learn this crap, but it is not conducive to good thinking. I have met many people who spout eigenvalues, and eigenvectors, and have no clue what they are talking about.

Early seismic analysis was also contaminated by the California experience, where most historic earthquakes were long rumbly things that totally activated the soil basins, and thus resembled long sinusoids of varying frequencies. They could characterize these sinusoids with the simple mechanical contrivance of a series of oscillators. Thus, you could measure the maximum motion of a 1 Hz oscillator, a 5 Hz one, etc. Since engineers love everything mechanical, you could actually build one of these 'response spectrum' boxes, and they were used for years in nuclear power plants.

I'm just going into these ancient assumptions, because they are still used today, simply because they are 'tradition'. If you are ever in a position to be snowed by a fancy seismic analysis, simply ask what are the underlying assumptions, and how do they relate to modern science? They will just die!

In this series, I will attempt to do something never done before by an engineer on a blog: I will start a legitimate seismic analysis from scratch, using programs I've never used before! I take the risk of looking like a complete fool, which is something that the 'distinguished' people would never do. I'm only keeping a 'live blog' to motivate me to do something that seriously hurts my brain!