Iconic Dome Structure Sustains Moisture Damage
Defining the Des Moines skyline since 1886, the Iowa State Capitol Building is one of the nation’s most iconic statehouses. The Renaissance-style, rectangular structure has a commanding, gold dome in the center, fortified with four smaller domes at each corner of the building. Reaching a height of 275 feet above the ground floor, the central dome is constructed of iron and brick and is covered in 23-carat, gold-leaf gilding. The tissue-paper thin sheets are covered with a protective layer to seal them from the elements.
In 2005, the exterior of the dome was regilded. It wasn’t noticed at the time, but the interior structure was sustaining moisture damage — both from exterior leaks and from the humidity given off by large crowds inside the building. These leaks had led to rusting wrought iron, along with brick and mortar deterioration. When discovered several years later, Shuck-Britson, a subsidiary of Snyder & Associates, was engaged to perform the structural assessment and design to determine the best course of action for repairing and restoring the dome.
Numerous Challenges Lead to Creative Structural Retrofitting Solutions
Because of the complex construction and location of the dome, the Shuck-Britson team faced multiple challenges to define the best restoration solution. Foremost was the lack of access in reaching the 275 foot dome. Special scaffolding had to be assembled onsite to reach the interior and exterior without damaging the building from excessive loads. Additionally, full usage of the building had to be maintained throughout the structural assessment and reconstruction process. This required careful consideration for project staging, as well as the safety of capitol workers and visitors.
Our team’s assessment determined that the restoration and strengthening of the iron and masonry dome could be completed without noticeable modifications. The wrought iron was reinforced with plates and the existing rivets were replaced with high-strength bolts. The deteriorated masonry posed a more difficult challenge, however. It had to be replaced brick-by-brick in order to keep the building in operation and more import, avoid damage to the gold leaf covering the exterior of the dome.
Near the top of the dome, an attic separator consisting of a cellular metal deck, steel beams, and tie rods separates the upper dome from the building. To control the moisture levels and temperature, a rubber membrane was added to the attic separator to help mitigate the possibility of future moisture damage. This separator and membrane create a thermal barrier between the dome and the building. To access this space for future maintenance, a monitoring platform constructed from lightweight aluminum and fiberglass grating was added to the structural design.
Professionals from Shuck-Britson oversaw all structural retrofits, as well as provided construction administration for this extensive project. Funding for the dome repairs were provided from money left over from prior state bond issues.