From broken marble panels to bent steel to structural shifting, the Washington Monument suffered a devastating hit last summer from the relatively modest earthquake that shook North America from Georgia to Canada, a new engineering report shows.
Photos: Wiss, Janney, Elstner Associates Inc.
|Engineering teams surveyed the exterior damage to the Washington Monument over 10 days in September and October.|
Damage to the iconic structure includes through-and-through cracks in seven-inch-thick stone, large chunks of spalling, gaps and cracks where daylight is visible and water is leaking in, new breaks in old repairs, damage to the lightning protection system, damage to joints, lateral movement of the exterior, and more.
Those are among the findings of a new Washington Monument Post-Earthquake Assessment prepared for the National Park Service by Wiss, Janney, Elstner Associates Inc., of Emeryville, CA, and Tipping Mar Structural Engineering of Berkeley, CA.
|Extensive cracking included one crack that extended the full four-foot-four-inch height of a marble panel and through its seven-inch thickness.|
The 33-page report offers a fascinating look at the monument’s structural history, a sobering overview of its current damage, and a glimpse at its future repair needs.
The magnitude 5.8 quake occurred Aug. 23, 2011, and was followed by several aftershocks. The temblor was centered 3.7 miles below ground at Mineral, VA, 84 miles southwest of Washington, D.C.
The intra-plate quake was the most widely felt in U.S. history, due to the geology of the U.S. Eastern seaboard, Wiss Janney reported.
|The quake dislodged historic repairs of cracks and other damage.|
Such a quake would normally carry little potential for structural damage, Wiss Janney reported.
However, the report added, the Washington Monument’s unique aspect ratios and manner of construction “may have rendered it uniquely vulnerable to ground motion associated with an earthquake.”
Anatomy of a Monument
Planning and construction of the Washington Monument unfolded over more than 50 years—before and after the Civil War—under a series of private and government sponsors from 1833 to 1888, according to Wiss Janney, citing a 2004 history published by the NPS.
The federal government did not take over the project until 1876. When it did, the Army Corps of Engineers reviewed the project and recommended structural modifications—including removing six feet of previous construction—to strengthen the foundation.
The final obelisk design consists of a 500-foot stone masonry shaft built on an 80-by-80-foot foundation and topped by a 55-foot-high, steeply sloped pyramidion. The shaft is largely hollow with “archaic iron interior framing” that supports the elevator and stairs, the report says.
|Engineering teams removed large chunks of spalling during their review, to prevent the pieces from falling.|
Restoration projects were completed in 1935, 1964 and 1997-2000.
Among the damage noted in the Wiss Janney assessment:
• New cracking of the marble pyramidion panels, including one crack on the west elevation that spans the full height of the four-foot, four-inch panel and extends all the way through its seven-inch thickness.
• Re-cracking and other damage to historic repairs. Noting that the 1999 restoration project documented 30 panels with cracks, the new assessment found that many of these formerly repaired areas now have additional cracking or spalling.
• Spalling was observed in “a significant number of panels” in the pyramidion. “These spalls were often much larger than the typical panel corner spalls, and in contrast cracked along two faces of the stone, thus wrapping around the pyramidion corner,” the report said. The spalls were often noted near deformed or broken rods of the lightning protection system.
• Cracking was also noted in areas of the rib units that serve as vertical supports for the exterior marble panels. In addition, most of the tie beams in one level of the structure showed vertical cracks at the connection with the center rib units.
• Signs of displacement and lateral movement were seen in “multiple locations” throughout the upper shaft and pyramidion.
• Gaps due to missing mortar or vertical cracks were reported in walls above the 450-foot level, allowing water infiltration. Some gaps were present before the quake, but others opened along old repairs or areas where the sealant failed.
The report makes general recommendations for types of repairs, noting that repair details must await more detailed investigation and analysis. Among the recommendations:
• Repair the cracking of the marble pyramidion panels, to restore their integrity and protect against water damage;
• Salvage and re-insert spalled material wherever possible, fastening it to the substrate with mortar or stainless-steel anchors;
• Replace deteriorated spalling material with new marble Dutchmen;
• Install stainless-steel plates to repair cracking at rib-bearing haunches;
• Complete a variety of repairs to address cracking of tie beams, displacement of various components, and damage to joints; and
• Install a new lightning protection system, and repair the elevator rails and framing and mechanism.
Finally, the report advises a seismic evaluation to assess remaining vulnerabilities and the need for future seismic strengthening.