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Restoration Work Begins On U.S. Capitol Dome

Exterior rendering, which features a scaffolding system around the DC Capitol Dome
A scaffolding system comprising industry-standard components will completely surround the dome so that the repair work can be completed. Architect of the Capitol

A two-year restoration project is now under way on the cast-iron dome of the U.S. Capitol building in Washington, D.C.

November 5, 2013—The nation was a different place when work began on the cast-iron dome of the U.S. Capitol building in Washington, D.C. In 1855, Abraham Lincoln had yet to be elected president, the Civil War was still on the horizon, and slave labor supplemented the construction team. Eleven years later, the dome was complete and the Civil War over. (See the two-part History lesson column titled “The United States Capitol: Forming One out of Many,” Civil Engineering, July 2009, pages 44-47, and August 2009, pages 42-45, as well as “Examining the Capitol Dome,” Civil Engineering, October 2000, pages 38-43 and 78.) Much has changed in the United States since then, but the dome remains an iconic and defining element of the city’s skyline.

Despite the recent economic downturn and continuing recovery, the office of the Architect of the Capitol (AOC) announced two weeks ago that a two-year restoration project on the U.S. Capitol’s dome and its decorative elements would commence this month to repair the cracks that have formed due to age and weather damage. Once the repairs are complete, the exterior of the dome will be repainted.

The cast iron dome was originally designed by the fourth architect of the capitol, Thomas U. Walter, and completed by bolting together 8,909,200 lb of ironwork at a total project cost of $1,047,291, according to material available on the AOC website. Captain Montgomery Cunningham Meigs, of the U.S. Army Corps of Engineers, was the initial supervising engineer for the project and responsible for its construction, funding, and logistical challenges. He designed the system that enabled the construction team to erect the dome, according to the AOC.

“Without Walter’s vision and Meigs’ revolutionary approach to logistics planning and an innate ability to overcome engineering challenges, the Dome as we know it today may never have been built,” wrote the current architect of the capitol, Stephen T. Ayers, FAIA, LEED AP, in a blog post for the AOC on October 22.

To lift the cast iron pieces for the dome into place, Meigs designed scaffolding with a triangular footprint that rose from within the Capitol Rotunda without unduly loading the floor’s unsupported center, according to the AOC. A steam engine located atop the Capitol’s roof and two 80 ft long “sticks” that acted as a mast and boom from the top of the scaffolding provided lifting power capable of moving 20,000 lb of material at a time as the dome was constructed in place. Pieces of wood salvaged from the previous, low-profile, copper-covered wooden dome fueled the steam engine.

The exterior of the new cast-iron dome was largely completed in 1863, when the final section of the Statue of Freedom, which tops the dome, was lifted into place. The interior, however, was not completed until 1866, when Constantino Brumidi finished his 4,664 sq ft circular painting the Apotheosis of Washington on the rotunda’s ceiling 180 ft above the floor level.

As time has passed, pinholes have formed in the Statue of Freedom and water has infiltrated those holes as well as cracks and open joints that have formed elsewhere in the dome, according to the AOC. Such leaks have compromised the dome’s protective paint coatings and caused its ironwork to rust. 

Night exterior rendering of the DC Capitol building where the dome is being repared

Most of the restoration work will occur at night and on weekends
to minimize disruption to both Congress and the visiting public.
Architect of the Capitol

“The advanced age of the cast iron coupled with the corrosive effects of weather have created more than 1,000 cracks and other deficiencies,” Ayers wrote in the blog. “If not addressed soon, these problems will create safety concerns and could cause irreparable harm to the priceless art found within the Dome, including the famed Apotheosis of Washington.” The current restoration work has been discussed and planned for the past decade, according to Ayers’ post.

A method termed “lock and stitch” will be used to repair each of the cracks, according to Ayers’ post. In this method, highlighted in this AOC video, metal pins are carefully drilled into a crack to “stitch” it together, and then the pins are sanded so that they are flush with the original surface. A “lock” is then placed over the top of the mended crack, to pull together its edges and provide additional strength to the repair.

Despite the age of the cast iron, “There is no structural work required in the restoration of the dome,” said Kristy A. Long, RA, CFM, LEED-AP, the project executive for the AOC, in a written response to questions posed by Civil Engineering online.

“The dome’s structural system is fully intact,” Long said. “The biggest challenges are the logistics and industrial processes since we are working on top of a historic and occupied building, and the potential unforeseen conditions within the cast-iron material that will not be revealed until the existing paint is removed.”

A scaffolding system utilizing industry-standard components will completely surround the dome so that the repair work can be completed, according to Eugene Poole Jr., an architect and project manager for the AOC, who wrote in response to written questions posed by Civil Engineering online. The scaffolding will extend from the base of the dome up to the base of the Statue of Freedom, according to the AOC’s website.

Most of the restoration work will occur at night and on weekends to minimize disruption to both Congress and the visiting public.

Inside the Capitol, a white catenary system will be installed within the rotunda to catch any falling debris while the restoration work is completed. This canopy will encircle the outer edges of the rotunda without blocking the view of the Apotheosis.

The restoration of the stone and cast iron located at the dome’s base was completed last fall.



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