Worried about acceleration?

Ambraseys first law of earthquake
engineering, Imperial College, 1965.
Solid line good, dashed line, bad

In the old days engineers who were old timers (back then, I was a new kid) used to talk about horizontal ground acceleration as the principal source of   disturbance, damage, and failure in structures. Then in the late twentieth century we had some medium sized earthquakes like San Fernando and Northridge down in LA, with these big sharp accelerations, one g or more, twice as much as what experts like George Housner and Nathan Newmark, whom we all respected, had said would be likely in earthquakes; these leaders had led us to believe (we said to ourselves) that we didn't have to worry about more than 0.5 or 0.6g, that's about all the geophysics of the earth could produce and even my prof, Nick Ambraseys, (I've got it her somewhere in my 1965 lecture notes at Imperial College, something about stress drop and velocity and limits.) So everyone, seeing these high accelerations began to waffle around: There must be something wrong about this record. Or: just hold on here, my structure, it doesn't care about high frequency vibration, it worries about longer period motion. 


And then the vision of those old WW2 army barracks being pulled down comes back, that long noisy gradual fall.

Cal Tech engineer Tom Heaton, whose style is much in the solid practical tradition of George Housner or Nick Ambraseys, suggests that designing for peak ground acceleration or for targeted response frequencies, could even be dangerously misleading:

While I understand that many structural fragilities are described in terms of PGA, its use only leads to dangerous mischaracterization of earthquake risk....5% damped response spectral acceleration (sa) is also a common parameterization of shaking intensity. While SA is certainly more useful than PGA, there are serious concerns about using it to predict structural demand. In particular, sa is based on a linear analysis of a structure about its undeformed state. However, there will always be significant ductile yielding prior to catastrophic failure of a structure, and it is more meaningful to use parameters that better characterize a structure that is in its highly deformed state. For example, structures that yield plastically have much lower effective stiffness and much higher effective damping than is typically assumed in current practice. 

Though some code-bound engineers resist this, it was easy for me to keep in mind, I was designing earth dams, and Professor Ambraseys had shown us that the natural period of big earth dams was rather long, maybe a second or so for a hundred footer (H=100 ft, Vs=250 fps):

T=2.6*H/Vs

and even then, as soon as some significant deformation began to develop, the damping of the movement shifted way up to 10 or 20 percent and.....well, you just never saw those theoretical loads of more than 0.5 g or so even up at the dam crest where you worried the most, as we will see later when we look the behavior of a dam up in the area of the San Simeon 2003 earthquake and also in a couple of designs I did in the late 1960s, in Chile and in California.

So though there was a lot to be said for this point of view that peak ground acceleration would only hurt when the structure was made to be very rigid,  the problem was this: with Diablo, they said it was very strong, or, as one guy scrambling to defend existing shutdown nuclear power plants in Japan put it to me last month, it was built like a brick shithouse. And having looked at PG&E design philosophy over the years (beginning with Bodega Bay) I think that was  actually the way that they tried to build those old plants, or were told to build them that way by the company president who, as the twentieth century rolled on, was now a lawyer or a PR man or a financial guy, certainly not an engineer like the old days. So maybe Diablo Canyon was strong and rigid, though I never made a study of the subject except for looking at those photos of everything bolted and welded and tied down:

Structural bracing at Diablo Canyon. At my age I'd be more afraid of a slip 
and fall on that slick floor.

And the joker was that the stronger they built them, the stiffer they got, and now today what PG&E and the Japanese worry about, or are made to worry about most, is exactly those same short snappy accelerations, supposedly yanking pipes off their supports or cracking massive concrete with doubled up bracing which had been put in to resist the newly imposed Hosgri Fault, more later on that.

But what Ambraseys told us (top of this entry) that what we should worry about first and most is the fundamental behavior of the material itself, adding it was pretty simple and turning to lift up that outside sliding blackboard and sketching on the underlying one,  as if to say that this is the first and most important thing you should know.

A solid idea. But a decade later it would get me into a lot of trouble when I began to work with Tom Leps on old hydraulic fill dams.