Preserving a National Landmark - Page 1 Page 2 - U.S. Capitol Dome Facts

By Tom Siewert, Chris McCowan, Roger Bushey, Bill Robinson, Tom Christ and Kevin Hildebrand A team of welding experts studied the best way to make weld repairs to the cast-iron outer shell of the U.S. Capitol dome The dome of the U.S. Capitol has been a focal point of the nation's capital city ever since the 1860s when workers used a steam-powered boom and derrick to hoist the ironwork into place. One of the most recognized landmarks in the United States, the nearly 140-year-old dome is undergoing the first phase of a rehabilitation program. Over the years, corrosion has built up at joints, leading to cracking of some of the castings that form the shell. Since the dome is a national landmark, the goal is to restore the castings to their original condition, replacing as few components as absolutely necessary. The major challenge is the castings were produced with 1850s' technology, so the composition is far different from current castings designed for weld repair. Thomas U. Walter designed the present Capitol dome in the 1850s. It was the second cast-iron dome in the world and is the world's largest iron dome. Primarily constructed between 1855 and 1863, it replaced an earlier wooden dome that was no longer in scale with expansions to House and Senate wings - expansions needed to accommodate legislators from states then recently added to the Union. A larger, masonry dome was ruled out because the existing Rotunda walls would not support the weight. However, calculations showed the Rotunda could support a cast-iron dome, which could be cast with cutouts in areas where material wasn't needed. In addition, cast iron was fire resistant and could be formed in complex shapes and erected in conveniently sized pieces. Recognizing heating and cooling cycles would subject the dome to movement, the designer included features to accept this movement. Although the majority of the dome, complete with its inner and outer shells and lower skirt, is composed of cast iron, wrought iron was used in a few places. The dome's main framing consists of 36 arched ribs that bear on 36 paired pillars. These, in turn, bear on 36 pairs of cast-iron brackets embedded in the Rotunda's masonry walls. Bands or hoops consisting of either cast-iron sections or wrought-iron riveted plates tie the ribs together at multiple levels. From the main rib framing, an elaborate arrangement of cast-iron brackets support the dome's outer shell, giving it its distinctive shape. Wrought-iron hangers or cast-iron brackets suspend the inner shell from the main ribs. Also suspended from the main ribs near the top of the dome is a shell of cast-iron grating to which the plaster base of the fresco titled The Apotheosis of Washington is applied. At the top of the dome, the 36 ribs converge into 12 that continue upward to support the Tholos and Lantern levels and the Statue of Freedom. More information, including Walter's elevation and cross-section drawings from 1859, is availale at the Web site of the Architect of the Capitol at www.aoc.gov. Getting Involved in the Restoration

In June 1998, a group of researchers from the National Institute of Standards and Technology (NIST) visited staff from the Office of the Architect of te Capitol to learn if their skills could help with the planned restoration of the Capitol dome. Also present were Richard Kadlubowski of Hoffmann Architects, the consultant for the dome rehabilitation, and A. J. Julicher, an independent structural engineer familiar with the weld repair completed six years ago to the cast-iron ring at the base of Freedom, the statue at the very top of the dome. Restoration is a priority because moisture is leaking into some of the interior areas of the building. The goal is to restore the dome to its original condition, with minimum replacement of castings. Therefore, welding is an important option for repair of cracks and corrosion damage. While touring the dome, we noticed the interior rib structure was in good condition,but the outer shell had some cracks and visible corrosion at a number of joints (where paint did not reach all the surfaces). The current moisture leakage problems are attributed to movement caused by expansion and contraction of the exterior shell and failing joint filler material between abutting plates. Most of the joints in the exterior skin are difficult-to-seal hairline joints. The leakage has led to corrosion at the joints of the outer shell and railings (about 1-cm-thick castings or wrought structural forms). The corrosion accumulates in the joints until they stress the component castings beyond what the mechanical fasteners can accommodate, leading to cracking of the shell panels and railing components. This allows more moisture to penetrate, which leads to further corrosion. During our tour, we saw a few weld repairs believed to have been made about 40 years ago. Some of these welds had cracks that appeared to originate in the heat-affected zone (HAZ) and then propagated further into the castings transverse to the weld. There was no documentation on the procedures used for these welds, but the shiny surfaces suggested they were of one of the nickel-rich compositions (commonly nearly pure nickel or a 55Ni/45Fe alloy) typically used on cast irons, while the bead shape suggested the welds were applied as a wide weave bead with a high heat input (leading to a wider and more brittle HAZ). A July 14, 1998, report from Lucius Pitkin Testing Laboratories indicated the castings were quite low in strength (7.8-18.8 ksi tensile strength). Current technology gray iron castings often have strength minimums of 210-280 MPa (30-40 ksi) although there are grades as low as 140 MPa (20 ksi) and as high as 420 MPa (60 ksi). While gray iron castings are not expected to have much ductility, a doubling of casting strength through technology improvements in the last 140 years means current castings can tolerate double the deformation of the castings in the dome simply from the absorption of elastic strain. This means the hasting repair technology in use today might not be optimal for the historical castings found in the outer skin of the dome. The Architect of the Capitol's staff had collected a team with structural analysis and corrosion expertise, and their consultants could over see most aspects of the repair operation. We were to investigate alternative materials and procedures for the weld repairs of the cracked panels of the outer shell. These alternative materials and procedures would be designed especially for the rehabilitation task (optimized for the properties of the castings used in the dome) and would be stable over time. In particular, we could look for innovative ways to reduce the tendency for cracking, both during the repair and into the indefinite future. Our team included Tom Siewert, leader of the welding activities at NIST; Chris McCowan, a NIST metallographer and metallurgist; and Roger Bushey of ESAB Welding and Cutting Products, a former chairman of the AWS A5J Committee on Electrodes for Cast Irons and member of the AWS D11 Committee on Welding of Iron Castings. The welds were made and tested for strength in the ESAB laboratory, then sent to NIST for macrographic and microstructural examination. Both groups were involved with the experimental design.

Fig. 1 -- Microstructure of the 1860's-era cast iron in the Capitol dome.

Evaluating the Problem

At NIST, we examined several sections of a railing from the original casting (Fig. 1), and confirmed the microstructure was a pearlitic gray cast iron. The microstructure contains type A graphite flakes (with some regions of graphite rosettes) in a matrix of pearlite (with some free ferrite), decorated with an interdendritic phase. This phosphorus-rich phase likely caused the low effective strength and low ductility of the dome material. This microstructure, together with the low strength and low ductility of the dome castings, caused us to question whether the materials and electrodes currently recommended and used to repair cast irons were appropriate for use on the special microstructures and properties found in the dome. For example, the nickel electrodes designed for joining cast irons (designated ENi-CI-A, according to AWS A5.15) are required to meet a specified room temperature yield strength of 262-414 MPa (38-60 ksi) and tensile strength of 276-448 MPa (40-65 ksi). Current tecnology ENiFe-CI-A (55Ni/45Fe alloy) electrodes have yield strengths between 294 and 434 MPa (43-63 ksi) and tensile strengths between 400 and 579 MPa (58-84 ksi). These strengths are quite appropriate for gray iron castings manufactured in strength grades ranging up to 400 MPa (60 ksi). For current technology castings, this high-strength filler material keeps the strength of the casting above a specified minimum and reduces the likelihood a repaired casting will fail in the weld repair through overload. For the castings found in the dome, the repair criteria are far different, focusing more on restoring the casting integrity (relatively low loads) and serving as a moisture barrier. After reviewing the previous repairs to the outer shell, we considered how to avoid a repeat of the cracking problems noted in these earlier repairs. The fundamental cause of the cracks in those welds was likely a result of the residual stresses that form as the weld cools. Degradation can appear immediately after welding (during or shortly after cooling) or after time (such as when seasonal thermal stresses and corrosion damage add to the residual stresses and exceed the strain tolerance of the castings). Therefore, the optimal filler composition would seem to be one near to or even below the strength of the castings. A low-strength filler would induce less stress in the casting due to the buildup of shrinkage stresses during cooling, and be able to selectively accept more of the strains developed during the weld repair and through the future service conditions. In fact, some steel structures have been built with filler materials that have lower strengths than the base plates. This strategy is known as undermatching and is applied in situations where more ductility is required. We searched for filler metals and techniques not normally used on cast irons, especially those that would produce welds lower in strength than the traditional nickel and nickel-iron compositions used. Using a lower strength, but higher ductility, filler materal for the dome castings is justified in that the repairs are not in structure-critical regions but simply restore the dome surface integrity.

Preserving a National Landmark - Page 1 Page 2 - U.S. Capitol Dome Facts