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Section, Revised Dome Design for U.S. Capitol. Pen, Ink and watercolor by Thomas U. Walter, 1859.

Dancing by Dawn's Early Light

Did you know that the 15,000-pound bronze Statue of Freedom, standing atop the 9 million pounds of cast iron of the U.S. Capitol Dome, dances every day?

This motion has long been known. The fifth Architect of the Capitol, Edward Clark, replied to a query by stating, "The statue on the Capitol has a motion."

While her limbs don't move, the entire structure atop the U.S. Capitol moves in a slow circle every day. Despite this constant movement, the Dome still stands strong.

How is this possible?

Many Questions. One Answer.

The answer is in the Dome's design, created by Thomas U. Walter. Walter was the Architect of the Capitol from 1851-1865, during the extension of the U.S. Capitol to accommodate a Congress that was growing as the country expanded. His drawings showed that the original wooden Dome, clad in copper, would be too small and get overwhelmed by the mass of the extended building. However, Walter couldn't simply build a larger dome where the old one stood. First, he had to solve many intertwined problems. Walter found a single solution for all of these issues that was as elegant as the final structure he designed, but it also created the movement in the Dome.

Larger, but Lighter

Walter knew that any new dome would have to sit on the existing base, a ring of sandstone blocks atop the Rotunda. This opening was smaller in circumference than needed for a dome tall enough to be proportionate to the enlarged U.S. Capitol. The sandstone posed another problem: it couldn't bear the weight or the expansion forces of a larger dome made of traditional stone or masonry materials.

Walter had traveled to Europe and studied the large domes there, including the dome atop St. Paul's Cathedral in London, which weighs 132 million pounds — too great for the Rotunda walls to support. Additionally, the weight of the dome would thrust outward at the base, exerting expansion forces, much like pressing down on the top of an inflated balloon causes it to bulge. The most common solution is to surround the dome with heavy masonry reinforcements that contain these forces.

Dome construction from the East Front with the crane rising above the original sandstone Rotunda walls.

However, previous builders couldn't accurately calculate how much reinforcement was needed. Walter had also visited the masonry dome of St. Peter's Basilica in Rome, where this expansion force caused large cracks to form 200 years after construction, showing the weakness of stone, which is much stronger when being pressed on than when being pulled to expand. Ultimately, several tension rings had to be placed around the dome to stabilize it.

The sandstone ring at the base of Walter's new Dome lacked the mass needed to restrain such forces or the strength of the stone used in St. Peter's. If he couldn't find a way to keep the Dome from thrusting outward, the sandstone could also crack and even fail.

Hot High-Tech Solution: Cast Iron

Even if he could build a lighter Dome that sandstone could support, Walter would still have to control the expansion forces and place this towering, wider structure on a narrower perch. The solutions to these three problems were intertwined and relied on a material that had been in use for more than 2,000 years, but its use in constructing large buildings had only just begun when Walter started his design: cast iron.

Meigs' journal sketch of a bracket for the Dome columns and Walter's design for the bracket with the column and brick structure it connects.  

Anyone who has used cast iron cookware wouldn't consider it lightweight compared to modern steel or even aluminum pans, but Walter accurately estimated that his iron Dome would have a total weight that was approximately 7 percent of the similar-sized St. Paul's Cathedral dome. The weight reduction is the result of technological, architectural advances from the cast iron construction boom, such as cast iron columns that are much more slender than masonry supports.

Metal in Motion

In addition to allowing for lighter construction than masonry, cast iron also expands more than stone or brick when it is heated. The result is that any structure built with iron, or any metal, must accommodate for expansion and contraction when the metal gets cold. One of these accommodations that most people have encountered is expansion joints in bridges.

These joints often appear as metal strips running across the roadway, sometimes filled with rubber. By creating a space into which the metal structure of the bridge can expand, these joints are a way of avoiding the buckling and bulging that would occur without them. However, Walter surely didn't want rubber rings to stripe his beautiful design, so he would have to find another way to accommodate the motion caused by the expansion and contraction of the iron.

Before he did that, Walter still had to ensure that he could place his wider Dome atop the narrower Rotunda walls.

Ingenuity from Bottom to Top

Walter's design details 36 columns surrounding the base of the Dome that resemble the intricately carved stone monoliths of Greek and Roman temples. Unlike their ancient ancestors, however, Walter's cast iron columns would be light enough that they could stand outside the Rotunda walls on iron brackets attached to a brick structure built atop the sandstone base.

This makes the base of the Dome appear broader than the opening of the Rotunda while retaining the interior dimensions. The illusion continues below the bases of the cast iron columns in a structure referred to as the Dome "skirt." The iron skirt, which appears to be a solid base for the Dome, is actually a hollow structure that visually connects the new Dome to the original roof of the U.S. Capitol.

Dome construction from the West Front, showing the Dome skirt as a dark line below the Dome columns.

Inside the Dome structure, 36 cast iron ribs, corresponding to the exterior columns, stand on top of the walls, bearing the weight of the upper Dome. This weight is carried down the curved portions of the ribs and thrusts outward where they become vertical, mirroring the columns standing above the skirt. An ingenious compression ring designed by Montgomery C. Meigs, engineer of the U.S. Capitol during Dome construction, contains the thrust and converts the expansion to a compression force, pressing in on the ring of sandstone blocks that form the Rotunda walls.

Out of Many Pieces, One Dome

Cast iron allowed Walter to create a lighter and broader Dome that the existing foundation could support while controlling the expansion forces that threatened the traditional masonry domes he had studied.

However, still, it moved.

Although it doesn't happen often, temperatures in Washington, D.C., can drop to zero during the winter and rise to 100 in the summer. A 10-foot-long piece of cast iron heated by 100 degrees will grow by about 1/16th of an inch. This amount of expansion is manageable, but Walter had to accommodate it in his design.

Thomas U. Walter's drawings detail the many pieces of iron that are connected in a way that secures them and allows for motion. 

Rather than building in a few large expansion joints, however, Walter designed the iron structure to be assembled from many pieces of iron. They attach through connections that simultaneously secure them and allow a small amount of motion. Through these thousands of links between pieces, Walter's Dome absorbs most of the motion of the metal.

However, at dawn, while the western side of the Dome still faces the cold darkness of the fading night, the sun heats the eastern side. As the sun rises through the day, the southern side of the Dome rises in temperature faster than the shaded northern portion. At sunset, the eastern side is already cooling while the western side is absorbing the last heat of the sun before night falls.

This uneven heating is what causes the Statue of Freedom to move slightly toward the west at dawn, as the eastern side of the Dome expands, to the north at midday, and the west in the evening, as the sun sets. During the night, the Dome returns to a uniform temperature and a neutral position.

Dancing by Dawn's Early Light

Even on the hottest of days, however, it would be difficult to discern this motion. Mr. Clark's report stated that, "The statue on the Capitol has a motion... The entire length of the line of oscillation of the plummet from the eastern limit to the western limit is only four and a half inches."

It may be only a few inches, but still, as you admire the uniquely American ingenuity of Walter's cast iron design, you will know that this solid structure and the Statue of Freedom that stands atop it begin a dance at dawn every day.

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Comments

Michael Lilly (not verified)
THANK YOU; This is a history lesson I really appreciate. Your group, AOC, is greatly appreciated.

Kate Kelly (not verified)
Thanks so much for this terrific lesson in architecture, engineering, and art.

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