SEISMIC VULNERABILITY ASSESSMENT OF THE WASHINGTON MONUMENT

In 2011, the Washington Monument was subjected to ground shaking from the Magnitude 5.8 Mineral, Virginia earthquake, whose epicenter was roughly 80 miles from the National Mall in Washington, D.C. Shaking of the 555foot tall unreinforced stone masonry structure resulted in damage, most significantly to the pyramidion, the pyramidal construction comprising the upper 55 feet. The damage that was incurred necessitated closure of the monument. A post-earthquake damage assessment project employing rope access inventoried the damage over the full height of the monument and that information was used as the basis for the repair of the earthquake damage. The seismic vulnerability assessment of the monument addressed potential concerns relating to its behavior during future earthquakes. The assessment focused on the monument’s three distinct structural regions; the pyramidion, the shaft, and the foundation. Seismological study was then conducted to develop a science-based understanding of the shaking intensity that actually occurred during the Mineral event, and of the shaking that might someday occur during future MCE earthquakes from various other sources. Nonlinear Modal Time History analyses (also called Fast Nonlinear Analysis, or FNA) of the monument responding to the mathematical characterizations of ground shaking developed by the seismologists were also conducted. The analysis models were designed to capture the most important structural characteristics of the unreinforced masonry tower, soil-structure interaction, as well as the intricacies of the pyramidion construction, and explicitly modeled PV interaction at critical stone block interfaces in order to simulate the behavior of mortared joints. After the models were validated using the Mineral ground motions and the damage assessment, the effects of postulated MCE events were determined and the significance of the predicted damage was evaluated and compared to what occurred on August 23. Parallel “peer review” analyses were conducted in a different nonlinear analysis environment to confirm the validity of the primary study. The recommendations generated from the vulnerability assessment were incorporated into the repair project for the monument.