"Then the Earth Shook and Trembled..." The Washington National Cathedral in the Aftermath of Mineral

By MATTHEW FARMER1, CORTNEY FRIED1

1. WISS, JANNEY, ELSTNER ASSOCIATES, INC.

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Abstract

"Then the Earth Shook and Trembled..." The Washington National Cathedral in the Aftermath of Mineral

As a result of the Mineral, Virginia earthquake on August 23, 2011 and associated aftershocks, a number of historic masonry structures throughout the District of Columbia sustained varying degrees of damage. Among the numerous sites impacted by the earthquake, the Washington National Cathedral sustained some of the most significant, visually striking, and potentially dangerous damage. The Cathedral, whose construction began in 1897, is an unreinforced stone masonry structure in the Gothic Revival style characterized by an abundance of flying buttresses and of carved stone cantilevered spires and pinnacles at various elevations over its 300 foot height. A number of these elements partially collapsed during the earthquake and many more shifted position. The prevalence of damage to and movement of very heavy stone units and assemblies at great elevation raised grave concern as to the safety of occupants and passersby, both in the immediate aftermath of the earthquake and in the longer term. Immediately after the earthquake, the Cathedral staff solicited assistance from several sources with prior experience and familiarity with the Cathedral and its construction. Once imminent falling hazards were removed or stabilized, the process of developing a repair program began; however, the great costs of accessing and repairing these elements brought to the fore fundamental questions about whether work should be confined to the damaged elements, whether only the damaged elements or all similar elements should be strengthened, and how best to develop practical criteria for strengthening that reflected the limited funds available.

Given the magnitude and complexity of the Cathedral structure, simple and direct seismic assessment approaches were needed to: evaluate the dynamic characteristics of the types of elements that were damaged; quantify the transfer functions from the ground to the elevations at the base of these various elements; and measure the reliable existing capacity of these elements were needed. The assessments implemented relied heavily on a series of small scale analytical models of the more significantly damaged ornamental elements that were developed to calibrate the actual damage relative to the shaking at the site, and thus back into the transfer functions which were also quantified instrumentally. Capacities of the bed joints at which so much of the documented damage occurred were assessed via load tests, and seismological information was also accessed to provide some understanding of the intensity of shaking during the earthquake relative to what is thought to be a design event. The analysis models provided a better understanding of critical portions of this complex unreinforced masonry structure with respect to behavior when subject to earthquake ground shaking without development of a more copmlex global model that would have itself introduced many uncertainties. They also enabled development of targeted repairs to vulnerable ornamental elements that are “tuned” to their risk of future instability. This presentation will highlight the initial engineering response, temporary stabilization measures, our assessment methodology, and the development of a restoration program to address the broad range of deficient conditions identified at this historic national monument.

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Researchers should cite this work as follows:

  • MATTHEW FARMER; CORTNEY FRIED (2014), ""Then the Earth Shook and Trembled..." The Washington National Cathedral in the Aftermath of Mineral," https://datacenterhub.org/resources/12885.

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