United States Capitol Dome Restoration and Rehabilitation Project
Tim Ariosto, MS Candidate, Penn State, 2010

"The U.S. Capitol Dome is iconic and is the most recognized symbol of our government. In 1793, President Washington had insisted during the original construction of the Capitol Building that it demonstrate the permanence of our federal government. We take great pride in our mission to preserve the Capitol Building to ensure that it continues to stand for generations to come."
- Stephen T. Ayers, AIA, LEED AP, Architect of the Capitol


The U.S. Capitol Building with its iconic dome has truly been a work in progress that has continued for more than 200 years. From the placement of the cornerstone by President Washington in 1793 to the many expansions, redesigns, and renovations that have taken place over the years, the job of maintaining this historic structure does not seem to have an end in sight. The most recent major renovations to the dome took place in the early 2000’s and included restoring the interior area of the dome that had suffered due to the effects of water leakage, rust, and corrosion, as well as repairing fractures in welds present along the inner and outer regions of the cast iron dome.

Figure 1: The US Capitol Rotunda. Source: Architect of the Capitol

Key Terms

  • Cast-Iron
  • Corrosion
  • Hydraulic Jack
  • Interstitial Space
  • Lead Abatement
  • Lifting Ring
  • Pitting
  • Sponge Blasting
  • Stripe Coating
  • Weld Repair


The creation of the iconic United States Capitol Dome is a work that has spanned over 200 years and involved the creativity of many individuals. After the new country was established at the end of the 18th century, work had to begin on establishing a capitol. George Washington selected an area along the Potomac River to be the seat of the new government. The site of United States Capitol Building was selected by by a french engineer by the name of Pierre Charles L'Enfant on the top of a hill over-looking what would become the capitol city of Washington, DC. Once this position was determined, it was necessary to pick a design for the building.

Figure 2: US Capitol Building in 1846 with original wood and copper dome. Source: Architect of the Capitol

A design competition was held, however, a consensus could not be reached on a suitable design. Finally, after the competition was closed, Dr. William Thorton submitted a design that involved a central section with a small, low dome flanked by a wing on either side for the Senate and House of Representatives respectively. This design was found favorable and plans were made to begin construction immediately. The cornerstone was laid by George Washington on September 18, 1793.

The construction process was filled with many challenges. Materials and workers had to be brought in from afar since the city had not yet been built when construction began. In addition to all this, the War of 1812 resulted in much of the progress that had been made by that point being destroyed. However, construction did continue when the war ended, and small changes were made throughout the design as time progressed. Several different architects took command of the project over the 40 years of construction, most notably Benjamin Henry Latrobe and Charles Bulfinch, under whose watch construction was completed in 1829.

As time progressed and the fledgling country grew, the building was no longer large enough to meet the demands of the government. In 1851, Thomas Walter designed a massive expansion to the House of Representatives and Senate wings of the building. Since this expansion more than doubled the length of the building, it became clear that the wood and copper dome that was present at the time no longer matched the proportions of the rest of the structure. Therefore, Walter also had to design a new, larger dome as well.

Building Description

Figure 3: Cross-Section of Capitol Dome drawn by Thomas Walter. Source: Architect of the Capitol with annotations by Tim Ariosto

Walter drew inspiration for his dome from some of the great elliptical domes of Europe, including St. Peter's in Rome and the Panthéon in Paris. This new dome was over three times the size of the original dome that had been in place. The brick and stone masonry walls of the building that the new dome would bear upon would not be able to support a load of that magnitude, so a creative solution had to be devised. Amid much controversy, Walter's design used 36 cast iron ribs bound together with horizontal bands to form a framework that cast iron plates would provide the skin for. As a building material, cast iron was still relatively unknown and untested at the time. However, there were several benefits to it's use that made it an attractive option for Walter's design.

  • Weight - Cast iron is a lightweight building material.
  • Cost - In addition to being inexpensive from a material standpoint, cast iron was also a good choice from a constructability standpoint. The ribs, which were made in approximately 10' sections, could easily be set in place using a crane and small crews. In fact, crews as small as 6-8 people were often used during assembly. (Stovner et al. 2000)
  • Firesafety - Cast iron was believed to be a fireproof material at the time. However, it is now known that the strength and stiffness of cast iron actually does deteriorate at high temperatures.
  • Thermal Performance - Cast iron performs well with respect to thermal expansion, which Walter knew would be an issue for a structure of this size. Interestingly, the US Capitol Dome was one of the first structures to be designed for this consideration. (Stovner et al, 2000)

Construction of the US Capitol dome was completed with the placement of the 19'-6" tall Statue of Freedom in 1863. From ground level to the top of the statue, the new structure rose to a total height of 290' and has a diameter of 96'.

Figure 4-5: Construction of new dome in 1858 (left) and 1859 (right). Source: Architect of the Capitol


Important Survey, Renovation and Rehabilitation Projects

1956 - Structural Investigation

Key Player: Seely, Stevenson, Value, and Knecht (SSVK)

In 1956, the engineering firm SSVK was brought in to perform a complete study of the dome's structure due to previous reports that bolts in the rib and hoop elements were loose. This study found that some corrosion and deterioration of the plate members at the Boilerplate level had occurred (Stovener et al., 2000). Two post-tensioned circumferential cables were added at that level to accommodate for the increased tensile stresses due to this corrosion.

1993 - Restoration of Statue of Freedom

Key Players: Cagley and Associates; Fine Objects Conservation, Inc.

A 1988 inspection revealed that the bronze Statue of Freedom at the top of the Capitol had developed several cracks in its pedestal as well as some serious pitting and corrosion issues throughout the rest of the statue. As a response to this, Cagley and Associates was brought in to design a method for safely removing the statue from its' seat at the top of the dome without causing any additional damage.

Cagley and Associates' plan involved slowly jacking the structure up from its pedestal, then placing it on a lifting ring and harnessing it in several other locations. This operation had to be performed very slowly and carefully since the engineers at Cagley could not be certain of how stable the statue would be once removed from the pedestal. From there, the statue was airlifted using a Skycrane Helicopter. The statue was then placed on a restoration platform on the Capitol grounds' East Plaza for a six month period while the restoration process was completed.

Fine Objects Conservation, Inc. was selected to perform the actual restoration of the bronze statue. Medium-pressure water blasting was used to remove caulk and corrosion. After this, the defects had to be corrected. The surface pits were filled using bronze plugs, larger gaps were filled with specially cast bronze plates, and rusted portions of the statue were removed. This same procedure was conducted on the pedestal at the top of the dome. Lastly, all the parts were painted with a dark "bronze green" paint to match the earliest records (Cagley, 1994).

The statue was lifted back into place on October 23rd, 1993. Since that time the statue is cleaned and minor repairs are made every two years to increase the life of the structure and prevent another large restoration from being necessary.

1994 - Water Drainage System Evaluation and Rehabilitation

In the early 90's, reports began to indicate that water infiltration into the dome had begun to be a problem during heavy rainstorms. As a result of this, the water drainage system was evaluated and repaired to help reduce this effect. (AOC, 2005)

1998 - Structural Investigation

Key Player: LZA

A comprehensive modern structural analysis was completed on the Capitol Dome by the engineering firm LZA to "evaluate the structural integrity of the Dome and to determine any repairs or maintenance items that may be required..." (Stovener et. al, 2000). The investigation conducted by LZA included a survey of the existing conditions, analysis of the structural system using computer software, and analysis of cracks using monitoring equipment. Several conclusions were developed based on this study as noted below.

Although some corrosion and deterioration was noted in some of the non-structural elements, the structure was found to be otherwise in good condition. In fact, the engineers at LZA found the structural system to be "a crowning achievement in the technology of cast iron" and "capable of resisting all reasonably anticipated loadings" (Stovener et. al, 2010).

The cracks in the plates along the outer dome were found to be a result of the thermal loading on the structure. As the structure experiences a positive temperature change, the cracks decrease in size. Conversely, when the temperature decreases the cracks increase in size. The change in crack size was found to be between .0002" and .0003" inches per 10°F, which would have no sizable effect from a structural standpoint (Stovener et. al, 2000).

1999-2003 - Lead Abatement

Key Players: Aulson Company Inc.; Rathgeber/Goss Associates

In the years since the dome's construction, the cast iron used on the dome had been repainted again and again, usually with what is now known to be dangerous lead based paints. Over time, the layers (as many as 21 in some places) had built up to as much as 3/8 - 5/8" thick (Innis, 2003). The presence of the built-up paint created additional stresses in the cast iron panels by restricting their movement as they underwent temperature based expansion and contraction cycles. Between 1999 and 2003, the Aulson Company was commissioned to take on a an extensive lead abatement and repainting project for the interstitial space between the two domes. This project presented several unique challenges to the Aulson Company.

Before the lead abatement process could officially begin, a large scaffolding system had to be erected to allow workers to enter the dome from the outside without having to pass through the interior of the space, which remained open to employees as well as the general public during that time period. In addition, a ventilation method had to be devised to bring fresh air into the interstitial space while work took place. Lastly workers had to spend as much as a half hour at the start of every shift taking measures to protect themselves from the harmful lead-based paint. This included wearing Tyvek overalls, gloves, safety harnesses, and full face-masks.

The scaffolding in the interior of the dome as well as in the interstitial space was designed by Rathgeber/Goss Associates. Among the primary considerations that were taken into account in the design of this scaffolding system were containing the lead abatement process, catching a person, tool, or cast iron piece that might fall, and finally providing a visual shield from the public eye (Strand, 2010).

The lead abatement program was performed using a method known as sponge-blasting. This method involves spraying a open cell, water-based polyurethane foam impregnated with abrasives, in this case aluminum oxide, on the painted surfaces. When the "foam" hits the surface being sprayed, the abrasive is exposed and etches off the paint. As the foam rebounds off the surface, the foam pulls the paint particles off with it. The residual product is then vacuumed and can be filtered to separate the paint from the foam. At that point, the foam can be used again. The work on the Capitol Dome proceeded with the blasting occurring during the day and vacuuming of the particles at night. The particles were then transported to an off-site recycling facility. The foam was generally able to be reused three times.

An additional challenge was presented in removing the paint from the exterior face of the interior dome. Since the interior face of this dome is painted with Brumidi's The Apotheosis of Washington fresco, care had to be taken to impact the dome as little as possible while removing the paint. A special scaffolding system was devised to allow workers to be suspended within inches of the surface of the dome. For this portion of the project, paint was removed using chemical paint removers and putty knives (Innis, 2003).

After all the paint was removed, all the cracks and defects in the interstitial space were documented and repaired. Afterwards all the joints were caulked, and a stripe coating - an extra coat on exposed areas such as corners and edges where paint can be easily worn away - was applied to the bolts, corners, angles, and cutouts. Three coats of paint were then applied. A base of an polyamide epoxy was used for the first two coats. For the final coat, an acrylic urethane finish was applied.

2002 - Investigation into Cast Iron Plates

Key Player: National Institutes of Standards and Technologies (NIST)

While the lead paint abatement and repainting job was ongoing, another team began work on establishing the best methods of repairing the cracks in the exterior dome. A variety of solutions were devised, and the potential pros and cons of each were presented to the Architect of the Capitol. However, a solution has not as of yet been selected for the repair of these failures.

Figure 6: A worker applies paint to the peristyle level of the exterior of the dome. Source: Architect of the Capitol

Future Renovations

Key Player: Simpson Gumpertz and Heger**

The next stage of the rehabilitation problem is a full-scale lead abatement program, repair, and repaint of the exterior of the dome. This will be a major project, since it will be important to contain the lead that is removed from the outside of the dome. The project is slated to begin in 2013, and last approximately 3 years. (Guilfoyle, 2010)

Routine Maintenance

In addition to the repair and rehabilitation projects involved in the preservation of the dome, there is also a regular maintenance routine that must be followed. This routine is mostly concerned with insuring that the water drainage system used by the dome is in good working order. Therefore, maintenance workers from the Capitol Sheet Metal Shop check the downspouts at the Peristyle Level, the drainage gaps along the balustrade on the boilerplate level, and the balcony drains near the base of the Tholos. At each of these points, they inspect the drainage route, remove any debris that is found, and make any repairs that are necessary (Guilfoyle, 2010). Performing this function on a regular basis helps protect the dome in between renovation projects.

Related Cases

The US Capitol Dome has become an iconic image for both the United States and the rest of the world. Although it's design was groundbreaking at the time, many states have modeled their capitol domes after it, using a similar cast iron design. This has created a wealth of opportunities to investigate how other cast iron domes have performed, particularly those who have not benefited from federal funding for renovation work.

Colorado State Capitol Dome

The Colorado State Capitol's cast iron dome, built in the late 1800's, has experienced significant rusting and corrosion over the years. The rusting is so severe that large pieces of iron, some weighing as much as 10 pounds have fallen from the dome. An architectural inspection team recently said that "the potential loss of strength as a result of deterioration is a significant hazard to the building and its occupants." (Harden, 2010) However, the Colorado State Legislature has struggled to find the funding to fuel the $30 million repair process. While the funds are being gathered, the dome continues to deteriorate.

New Jersey State Capitol Dome

The New Jersey State Capitol also uses a cast iron dome from the late 1800's. Repair and restoration work has always been completed on a case by case basis with little emphasis being placed on structural issues. However, in 1997, an conservation team that was surveying the dome in preparation for a re-gilding project noticed that much of the cast iron, including tension rods located within the columns that support the cupola were heavily rusted and could result in large parts of the cupola failing. The rehabilitation work on this project, which costed roughly $7.5 million, consisted of removing the exterior cast iron, repairing it, then replacing it. (NJ Building Authority, 1999)


Preservation of historical structures is a long process that extends over the life of the structure. Typically the work associated with the preservation falls into one or more of three different categories. First, some problems or defects can be fairly simply corrected, such as in the case of the post-tensioned bands added to the domes structure in the 1950's. Other times, a solution may be determined that works temporarily, but leads to a new set of problems that had previously been unforeseen. Examples of this is the widespread use of lead paint, which was a commonly used at the time of its application. However, years of added paint have required work to be undertaken just to remove it, when the paint itself was originally conceived as a method of protecting the structure. Lastly, and arguably most importantly, is the routine maintenance that helps extend that life of a structure. This maintenance prevents some of the problems like corrosion that may occur as well as provides an opportunity to catch new issues before they become a major problem.

Monumental government architecture, such as the US Capitol Dome, are not often built in our modern society. Therefore, it is more important than ever that the existing structures in this style are preserved for future generations. However, in times such as these of poor economic conditions as well as a high national debt, dealing with historic structures can often be a particularly large challenge. The engineering community has a responsibility to balance short term solutions that prevent further deterioration from occurring with sustainable, long-term solutions that will not have to be repaired in a short amount of time.


Architect of the Capitol. (2010) “Capitol Construction History”. http://www.aoc.gov/cc/capitol/capitol_construction.cfm. Accessed 9/27/2010.
  • This article gives a summary of the various construction projects associated with the US Capitol building ranging from beginning of the initial construction in 1793 to current ongoing renovation projects.

Architect of the Capitol. (2005) “Rehabilitation of the U.S. Capitol Dome”. http://www.aoc.gov/projects/dome_ren/index.cfm . Last Modified 11/29/2005. Accessed 9/27/2010
  • This article describes a rehabilitation project of the Capitol Dome that responded to reports regarding water penetration of the dome.

Siewert, McCowan, Bushey, Doherty, Christ, Kotecki, Olson, Myers, Hinshaw, Kiser, Hildebrand. (2002) “Weld Repair of the U.S. Capitol Dome. Report to the Office of the Architect of the Capitol.
  • This report is a follow on the 1998 NIST investigation and subsequent recommendations of the weld issues with the capitol dome. Included in the report is a description of repair done at the time as well as further recommendations to correct the problem.

Siewert, McCowan, Bushey, Robinson, Christ, Hildebrand. (1998) “Preserving a National Landmark”
  • This article conveys recommendations given by a NIST team in 1998 on how to best complete weld repair on the cast-iron shell of the Capitol dome. This repair was intended to prevent leakage rather than repair a structural deficiency.

Stovener, Velivasakis, Thater, and Thorten (2000). “Building Case Study: The Dome of the United States Capitol”. The Structural Design of Tall Buildings vol. 9 (2000). pp 183-199
  • This document presents a summary of the design and construction of the Capitol Dome followed by an analysis of the dome carried out by LZA Technology.

Journal Record (2000). “Most famous manmade landmark’ gets facelift.” Journal Record March 3, 2010.
  • This article describes the work done on the initial portion of a restoration process of the interior area of the dome to repair the effects of “decades of seeping water, rust and layer upon layer of paint.” In addition, the article gives a brief description of future phases of rehabilitation project.

Lovley, Erica (2010). “Costs rise as Capitol crumbles.” Politico 7/1/2010
  • This article describes the repairs that are still necessary within the US Capitol building (with the dome and elsewhere) and the challenges of enacting these repairs under poor economic conditions.

Woods, Robert. “Under the Capitol Dome.” Mechanical Engineering Magazine. June 2003. Retrieved from http://www.memagazine.org/backissues/membersonly/june03/features/capdome/capdome.html on 9/28/2010
  • This article gives an in-depth description of the structural system of the cast-iron dome of the US Capitol building as well as some insights to the controversies surrounding its design.

Guilfoyle, Matt (2010). “Dome Keepers.” Foundations & Perspectives: Spring 2010. pp. 10-13
  • This article describes the efforts taken by maintenance crews on the Capitol dome to preserve it while waiting on additional renovations.

Architect of the Capitol (2010). "Architect of the Capitol to begin painting capitol dome." AOC Press Release 6/18/2010
  • This press release discusses an ongoing painting project to preserve the Capitol Dome, and discusses future projects that will take place concerning the dome

Innis, Jack (2003). "U.S. Capitol Dome Restoration." CoatingsPro Magazine. March 2003.
  • This magazine article discusses the steps that head to be taken to remove the lead based paint from the interstitial space between the inner and outer domes.

Cagley, James R (1994). "Restoring Freedom at the Capitol Dome." Civil Engineering. Jun 1994. 64(6), pp 57-59.
  • This article details the major steps of the restoration process for the Statue of Freedom

Harden, Mark (2010). "Colorado Capitol's dome named to 'most endangered places' list." Denver Business Journal. 2/4/2010.
  • This article describes the deteriorating state of the Colorado State Capitol Dome as well as several other "endangered places" in Colorado.

New Jersey Building Authority (1999). "Renovation and Restoration of Historic Structures." Annual Report. Retrieved from http://www.nj.gov/njba/Report/Historic.pdf
  • This report describes the various rehabilitation projects completed by the New Jersey Building Authority.

New Jersey Commission on Capitol Budgeting and Planning (1997). "Commission Meeting Minutes from 3/21/2007". New Jersey Commission on Capitol Budgeting and Planning
  • The meeting minutes from 3/21/2007 give several insights into how major projects such as the rehabilitation of the New Jersey Capitol dome are approved.

Strand, Gary (2010). Personal Interview. Information gathered between 12/7/2010 and 12/9/2010.
  • Gary Strand, a PE who has been involved in some aspect of various Capitol Dome Restoration since 1990 offered several insights into his experiences.