Building Collapse Furthers Seismic Research
- September 4, 2013
The controlled collapse of a building at California State University, East Bay provided ground wave data that will help seismologists map the Hayward Fault.
By Catherine A. Cardno, Ph.D.
September 3, 2013—California is well known for its earthquakes, and much is understood about the faults that extend through the state. However, there are elements that remain elusive. It remains impossible to predict the arrival of an earthquake, and true maps of faults—even the most famous—that indicate precise ground motion activity are not easy to come by. That is about to change for the Hayward Fault, the fault line responsible for an October 21, 1868, earthquake estimated to have measured between magnitude 6.8 and 7.0 on the Richter scale. The controlled demolition of Warren Hall at California State University, East Bay on August 17 provided an opportunity for researchers to learn more about how to predict and prepare for earthquakes, enabling them to track the velocity and propagation of ground motion activity throughout the areas surrounding the fault. The collected data, captured by 600 seismographs, will be analyzed and used by the United States Geologic Survey (USGS) to map the subsurface Hayward Fault for the first time.
Bringing down a 13-story building with a massively reinforced, exterior concrete moment frame and interior steel frame is an extremely difficult and highly choreographed process, according to Mark Loizeaux, Aff.MASCE, the president of Phoenix, Maryland-based Controlled Demolition, Inc., and the principal in charge and lead blaster for the implosion.
The team needed to design the charges so that the minimum amount of explosives could be used to first incline the building away from the university library, located some 60 ft away across a two-lane road, and then bring the building down vertically so that the resulting debris could be accessed and removed with the type and size of equipment available to the main contractor, Oakland-based Silverado Contractors, Inc., Loizeaux says.
The building’s exterior moment frame “was extremely, extremely robust,” Loizeaux says. The exterior concrete columns were approximately 3 ft thick and 6 ft long, encasing wide-flange steel members. According to Loizeaux, there were 36 #14 vertical bars in each of the reinforced-concrete exterior moment frame columns, through-ties between every third bar going across the column, and stirrup space 6 in. apart from one another throughout the entire mass of the concrete moment frame. All this made it “very, very difficult” to displace the concrete and eliminate the exterior moment frame’s support without disrupting the charges placed on the interior, 3 in. thick, wide-flange steel columns, he says.
By placing the first implosion charges in the concrete columns located on the side toward which the building needed to lean, opposite the library, “we used the weight of the moment frame and the depth of the reinforced-concrete spandrel beams to create a cantilever in the structure,” Loizeaux says. Using “much longer delays than I would ordinarily use in a structure,” Loizeaux says, he was able to take out a row of concrete columns in the moment frame plus two rows of the interior steel columns in the first wave of charges. At this point, he says, he “let the building sit for about a second to let it rotate about the reaction line, which was actually not the row of columns closest to the library, but…the next row of columns in,” he says. The next wave of charges brought the building down vertically.
The implosion was managed, Loizeaux says, through a system of "very selective and limited predemolition" work, developed in conjunction with Amir Kazemi, P.E., a principal of the Hayward, California-based structural engineering firm FBA, Inc., and the project structural engineer for the implosion. The predemolition work included removing shear walls and partially notching or cutting columns, as well as adding four thrust-resistance walls at the base of the building. (The new walls controlled the location of the implosion's "major-motion floor," according to Loizeaux.)
Warren Hall is only 600 m from the surface trace of the Hayward Fault, according to Rufus Catchings, Ph.D., a research geophysicist for the USGS, who wrote in response to written questions submitted by Civil Engineeringonline.
As a result of the building’s location, Kazemi explained that “the timing of the preweakening had to be minimized to reduce the exposure to a possible large earthquake that would be strong enough to collapse the structurally compromised building.”
The last weakening of the building took place a mere hour and half before the building was brought down, Loizeaux notes.
The USGS study measured the small ground vibrations caused by the explosions, as that energy was transmitted to the ground via the columns, as well as the larger ground vibrations created by the fall of debris to the ground, Catchings explained. In all, 12,500 short tons of material were brought to the ground, according to Loizeaux.
To record the vibrations created by the fall of debris, the USGS deployed 600 seismographs placed in three arrays, Catchings said. The ground motion created by the building’s collapse probably measured between a magnitude 1 and 2 on the Richter scale, he said. Although such ground vibrations are inconsequential relative to large-magnitude earthquakes, the sensors were able to track their movement through a wide area.
“We want to use the cumulative data [we gathered] to better understand the fault zone and the potential shaking hazards associated with it,” said Catchings. “It is important to have some basic idea of how the various geologic terrains will respond to seismic shaking, although we cannot confirm that the shaking will be the same for all magnitudes of earthquakes.
“From the perspective of residents living in the area, knowing the width and geometry of the fault zone can help them understand the potential hazards they face, and they can take measures to reduce risk,” Catchings said. “From a scientific point of view, we need to know the P and S [primary and secondary] wave velocities and basin thickness information to develop more accurate models of ground shaking.”
The first of the three arrays of seismographs comprised a set of sensors placed no more than 200 m apart in concentric circles with a maximum diameter of 2 km around Warren Hall. The second was a linear array that extended approximately 25 km from the shore of the eastern San Francisco Bay to just west of the inland city of San Ramon. These sensors were placed at 400 m intervals once the configuration reached beyond the initial 2 km concentric circle placed around the building. The third configuration was a scatter array with sensors positioned primarily about 5 to 8 km from Warren Hall, though some were placed much further. Sensors were located as far away as Berkeley, 30 km to the north; Fremont, 17 km to the south; San Ramon, 20 km to the east; and Palo Alto, 30 km to the west and across the bay.
“We had multiple goals in setting up the arrays,” said Catchings. “One of the principal goals was to [perform] a comparative study of ground shaking and amplification within the soft sediments of the valley, within the fault zone, within the harder rocks of the hills, and along peaks in the hills, because three of these geologic setting are known to amplify seismic waves.” The sensors were tightly spaced so that seismologists would be able to systematically observe the differences in seismic wave propagation in various areas, and how the waves transitioned between those areas.
Researchers left the arrays in place for a few days in the hopes that they would “capture an earthquake or two,” as Catchings said. “Fortunately, there was at least one earthquake on the Hayward Fault during that time and several earthquakes in the general region—we hope to compare the propagation of the earthquake data with that from the implosion.”
The last five major seismic events on the Hayward Fault have occurred approximately 140 years apart, and because the last major event occurred in 1868 experts anticipate that another large event, measuring between magnitude 6.8 and 7.0 on the Richter scale, will occur at some point in the near future. While the creep experienced along the fault is well documented, experts were previously not sure how specific ground conditions would affect seismic-wave amplification in the well-populated area east of San Francisco Bay (See “Retrofit of Stadium Straddling Active Fault Moves Forward,” Civil Engineering, February 2010, pages 12-14).
Catchings anticipates that the first analysis of the raw data will be completed in a matter of months, although it will need to pass through the USGS’s publishing process before it can be publically released.
A video of the implosion is available via Controlled Demolition Inc.’s Youtube channel.