Design Spectra

Wellington has been beset by some seismic activity over the past few days. I have been rather blasé about them, perhaps because I haven’t been anywhere near anything that I think is likely to be seriously affected. From Canterbury, we have a good sense of the buildings most likely to be hit hard – spindly concrete structures & brick. Conventional lightweight timber structures, where I spend most of my time, typically perform well. That sense is not scientific – what is scientific are “design spectra”. These are basically ground-structure interaction curves that plot the “Pseudo-Spectral Acceleration” (i.e. effective horizontal acceleration) against the period of a building. Without having crunched any particularly serious numbers, I have glanced at some data and drawn some “conclusions” – semi-educated inferences really.

There are quite a number of factors that go into the equations for a response spectrum. The likely ground acceleration profile, the building’s stiffness, its ductility, its damping and so on. The point is simply that there are a lot of variables contributing to the performance of any specific building.

Well, the first comparisons of our theoretical spectra with the real earthquakes we experienced are starting to be produced. Doubtless there will be closer and more thorough looks, but for now what has been produced shows that the effective acceleration applied to buildings by the recent Earthquakes was around 25% to 30% of the design standard for the most-susceptible building periods. This compares just a single variable from the echelon of factors involved in the design response spectra; crucially, the duration of the Earthquakes is not recorded on the graphs of PGA v. Building Period, which means that the total seismic energy put into a building may not be what’s implied by that 25% to 30%.

The upshot is that the level of shaking experienced by most buildings appears to be somewhere in what we call a “Service Level” earthquake, which means that the loads/displacements are of a magnitude where the design intent will be zero damage to the structure, and nothing more than cosmetic damage to anything attached to the structure. The early/preliminary reports imply that’s roughly where damage is, with a tiny handful of exceptions. Obviously inspections are ongoing and so things could all change, but from the comfort of my office I wouldn’t expect to see much structural damage emerging.

Crucially, it appears that one way or another, these earthquakes were below the so-called “Earthquake Prone” level of 33%. In fact, for low-period buildings (as EQP buildings tend to be) the effective level of shaking might have been as low as 10% – possibly less than the design wind loads. That means that these quakes, while alarming, have probably not really tested the ability of any structure to survive earthquakes.

All of which is a long way of saying: heed these events as a warning; try not to think they’re indicative of future building performance to any significant degree.

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