Structural FailureS investigating failures, along with their consequences and resolutions
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n the wee hours of August 24, 2014 (3:20 am to be precise), most of the Bay Area was awakened by a 6.0 magnitude earthquake that lasted a lot longer than those sudden, one or two second jolts that happen from time to time and that residents have become accustomed to, if one can actually get used to such things. Most people probably thought, “Is this the big one?” When the ground shaking didn’t get any stronger and eventually stopped after 10 or 15 seconds, many thought “wow that was a big one, somewhere.” Those engineers with client responsibilities checked the USGS website to find out where “somewhere” actually was and then, if required, responded to the earthquake impacted area. Most people said a thankful prayer and rolled over and Figure 1. EQ response spectra (with assistance from PEER). went back to sleep. As much as California engineers study and safe building and will survive future earthquakes. design for major earthquakes, most have never The highly variable nature of seismic ground personally experienced motions and the location of individual buildings a large earthquake since relative to the fault rupture location make this an they occur so infrequently. unrealistic expectation. Second: if a building is The largest earthquake in designed to the building code, it won’t be damNorthern California in aged. That simply isn’t true. recent memory was the 1989 Loma Prieta earthSeismic Ground Motions quake. That was 25 years ago – anyone younger than 35 probably has no This 6.0 magnitude earthquake, at least by significant memory of the event and the actual California standards, would be considered ground shaking. Depending on where one hap- moderate to strong and therefore the area of pened to be, the shaking might not have made strong ground shaking was somewhat smaller much of an impression since the epicenter was than one might think. The peak ground accelabout 80 miles from San Francisco and the eration was in the 0.3g to 0.4g range, with a damage, while very significant in total, was highly maximum (geometric mean) spectral accellocalized. Before Loma Prieta, the previous “large” eration of about 1g (Figure 1). A generation earthquake was in 1906. If you live in the Los ago, an earthquake of this size was the stanAngeles area, major earthquakes occurred in 1971 dard Uniform Building Code (UBC) “Zone and 1994, also a long time ago by most standards. 4” event. Today, this earthquake, in terms of The significant size of the South Napa earth- acceleration, is approximately 30% lower than quake gives a reason to re-learn valuable lessons the current design basis earthquake for Napa from the past that have been forgotten, and and much of coastal California, including the prepare for the next one by incorporating the inland portions of southern California, except best current knowledge. It is best not to rush to for areas very near active faults. So this event judgment and develop a host of conceptual code could be considered as a likely event throughout change provisions to address observed damage. It California and something we should be ready is better to let things settle out and reflect on the for almost anywhere and at any time. most significant issues that might require a change This earthquake and the damage it produced can in public policy, including the building code. also serve as a lesson for people living in other This article discusses the seismic ground motions parts of the country where an earthquake of this from the South Napa earthquake and its effect on size is much closer to the design basis earthquake select commercial, historic, residential and indus- for which new buildings are being designed using trial facilities. Reports suggest that the earthquake ASCE 7-10, Minimum Design Loads for Buildings caused about $400 million in property damages in and Other Structures. This would include extreme a small area with a population of about 137,000 coastal Oregon, coastal Washington including the (Napa County, 2010 census). Seattle area, and the areas around New Madrid, There are two common misconceptions that Missouri and Charleston, South Carolina. In always seem to arise after an event of this nature these areas, a 6.0 magnitude earthquake will be a that should be dismissed. First: if a building sur- much less frequent occurrence than in California, vived undamaged in past earthquakes, it must be a but they will eventually occur.
Reflections on the 2014 South Napa Earthquake By John A. Dal Pino, S.E.
John A. Dal Pino, S.E., is a Senior Principal at Degenkolb Engineers in San Francisco, CA, and a member of STRUCTURE’s Editorial Board. John may be reached at jdalpino@degenkolb.com.
20 March 2015
