The Virginia earthquake of August 23, 2011 occurred within a regionally well-established zone of earthquake activity, the "Central Virginia Seismic Zone". This area has produced small and moderate damaging earthquakes since at least as far back as the 18th century. The magnitude of the August earthquake was 5.7, and the epicenter was located in Louisa County, about 135 km southwest of Washington DC. Significant damage occurred to a number of structures in Washington, DC, including the Smithsonian Institute's Museum Support Center (MSC) and the Washington National Monument. Damage to both structures was unexpected. That is, there was a marked uptick in the shaking intensity and damages in the Washington, DC, region relative to other areas located much closer to the epicenter. This research is to investigate reasons that help explain this trend by performing detailed analyses of two important facilities that were damaged. The MSC, a large warehouse complex that serves as the main storage facility for the Smithsonian, is typical of many building systems in the eastern US. The Washington Monument is the world's tallest stone structure and the world's tallest obelisk, while also being of national historical significance. Preliminary analyses indicate that the damage to both of these structures is related to the interaction of key engineering factors, including their dynamic structural characteristics and the specific geologic and geotechnical (soil) conditions underlying the Washington, DC, area.
The study is a collaborative effort between researchers at Lehigh University, Virginia Tech, and the US Geological Survey (USGS). Research activities involve both structural and geotechnical tasks. The structural engineering activities include gathering perishable damage data from the MSC and Washington Monuments, performing field vibration tests to establish the dynamic characteristics of these two structures, and developing advanced numerical models. The main geotechnical tasks involve working with USGS to perform field tests to characterize the dynamic behavior of the sites, and developing a detailed numerical simulation of the earthquake shaking that occurred during the magnitude 5.7 earthquake. Collectively, the researchers will use the structural models, dynamic site parameters, and ground shaking simulations to perform detailed analyses of both facilities to explain the observed damages during the magnitude 5.7 earthquake. The findings will then be extended by using the models to simulate different earthquake scenarios to better understand and communicate the potential impacts of the magnitude 5.7 or an even larger magnitude earthquake occurring closer to Washington, DC, or other populated areas in the region. Results will be shared with engineers, stakeholders, and decision makers in the engineering community and beyond.
The two structures being studied represent a unique opportunity to analyze the behavior of east-coast US structures not designed to resist earthquakes. Our in-depth study will be insightful to the profession for establishing the vulnerability of eastern US structures to earthquake hazards and needs for renovation to ensure their resiliency. Of particular importance, this work will promote advancements in building codes and design procedures specific to the eastern US.
The Virginia earthquake of August 23, 2011, with a magnitude of 5.8 Mw, is the largest ground motion in the State of Virginia since the 18th century. Following this earthquake significant structural damage was reported in the Washington, DC area, despite being over 130 kilometers away from the earthquake epicenter (Mineral, VA). This research investigates potential causes of the observed damage in two important structural facilities in this region during this event. The first structure is the Smithsonian Institute's Museum Support Center (MSC), a large warehouse complex typical of many building systems in the eastern U.S. and the second one is the Washington Monument, the world's tallest stone structure, while also being of national historical significance. This study is a collaborative effort between researchers at Lehigh University, Virginia Tech, Clemson University, and the US Geological Survey (USGS). Research activities involved both geotechnical and structural tasks. The main geotechnical tasks involved working with USGS to perform shear wave velocity measurements at the sites of the MSC and the Washington Monument. The shear wave velocity profile of the soil column was created based on the test data used in various scenarios for transition of shear wave velocity to the bedrock level in order to account for the uncertainty associated with estimation of dynamic characteristics of the bedrock. A site response analysis was also performed to develop a detailed numerical simulation of the ground shaking underneath these structures during the 2011 Mineral earthquake. The structural engineering activities included gathering perishable damage data from the MSC, performing field vibration tests, establishing the dynamic characteristics of these two structures, and developing three-dimensional numerical finite element models calibrated based on the identified structural characteristics to explain the behavior of the structure during this earthquake. Collectively, the structural models, dynamic site parameters, and ground shaking simulations indicated that the damage to both of these structures was related to the interaction of key engineering factors, including their dynamic structural characteristics and the specific geologic and geotechnical (soil) conditions underlying the Washington, DC area. Bidirectional dynamic time history analyses of both facilities further explained the potential causes of the observed damage during this earthquake: combined effect of torsional and translational structural response in the case of the MSC, and dynamic amplification of the higher structural modes due to ground motion in the case of Washington Monument. Using the calibrated structural models, several other earthquake scenarios were also considered to better understand and communicate the potential impact of the 2011 Mineral, VA earthquake or a larger magnitude earthquake occurring in the Washington, DC area. The principal findings of this research are as follows: The Virginia earthquake inflicted significant damage to poorly-built unreinforced masonry and stone structures in the epicenter region. Little damage occurred in nearby populated regions such as Charlottesville and Richmond located 45 and 60 km away, respectively. However, a marked uptick in intensity and damage occurred in the Washington, DC area. Despite being more than 130 km away, maximum intensities were as high as those near the epicenter. Intensity and damage patterns appeared to be strongly correlated to regional geology. In general, most damage occurred along the Fall Line and eastward within the Coastal Plain. Only isolated damage occurred on Piedmont sites except where they were underlain by isolated pockets of soft sediments. In the Deep Coastal Plain the impedance contrast below the upper soil deposits and the stiff Tertiary sediment, and between the Tertiary sediments and the basement rock, causing a strong amplification of the ground motions across a wide period range included long and short period structure. Also, it is likely that the duration of shaking was likely increased greatly due to trapped and reflected waves. This corroborates with the damage that occurred at the MSC and the widespread damage that occurred throughout the Coastal Plan during the M5.8 earthquake. Importantly, current NEHRP/IBC simplified code procedures do not adequately capture site response at many DC area sites. Rather, they under predict the motions from the calculated site response analyses. This event illustrates the surprising seismic vulnerability of a densely-populated region such as the DC region from a relatively small distant CEUS earthquake. 3D structural analysis showed that the effect of combined torsion and translation played an important role in the response of the MSC during the earthquake, which resulted in a non-uniformly distributed damage pattern. Bidirectional time history analysis of the finite element model of the Washington Monument revealed a concentration of tensile stresses at the upper levels of the shaft and significant amplification of the acceleration from ground surface to the observation level, which are consistent with the observed damage in this structure. Fragility analysis of the calibrated structural model of the Washington Monument shows a high probability associated with initiation of more grout cracking under futuristic ground motions.
