By DEB YOUNG, AIA, SARAH BERSETH, P.E., AND SEAN COTTON, P.E., M.ASCE
A multidisciplinary restoration project returned the iconic Minnesota State Capitol to its original stature while updating architectural and engineering systems for 21st-century functions. Weaving state-of-the-art technology into a building constructed in 1905 while also preserving the building's historical fabric required innovation and cross-disciplinary thinking from the entire team.
The five-year-long historic restoration of the Minnesota State Capitol created a community amenity that feels virtually brand new-yet retains the architectural splendor of an earlier era. The project, spearheaded by multidisciplinary architecture and engineering firm HGA, of Minneapolis, touched virtually every aspect of the capitol, from inside to outside, dome to basement.
The 1905 structure was designed by architect Cass Gilbert, who was inspired by the Beaux-Arts style of architecture displayed at the 1893 World's Columbian Exposition in Chicago and who would later go on to design the United States Supreme Court Building. Gilbert's design featured an exterior of white Georgia marble and St. Cloud granite and a three-level dome inspired by Michelangelo's Basilica of St. Peter in Vatican City. It was listed on the National Register of Historic Places in 1972.
With its prominent location on the Capitol Mall, surrounded by other government buildings, monuments, and memorials, the Minnesota State Capitol is one of the most visible and identifiable buildings in downtown St. Paul and the pride of the Twin Cities. A public landmark that attracts thousands of visitors annually, the grand structure is also a working building, housing chambers for the Minnesota Senate and House of Representatives, the Minnesota Supreme Court, the office of the state attorney general, the office of the governor, and private legislative offices.
By the 21st century, the structure had reached a critical point of deterioration, including areas of crumbling exterior marble; antiquated mechanical, electrical, and plumbing (MEP) systems; inadequate life-safety systems; insufficient public areas; and water-damaged and deteriorating decorative paint and murals throughout the interior.
Although small renovation projects had been undertaken throughout the building's century of use, they were done in piecemeal fashion-with limited budgets and timelines-resulting in varying levels of work quality, styles, and regard for historic character. The infrastructure had been patched and repaired time and again, essentially masking the issues with bandages. Parts of the electrical and mechanical systems were still original from 1905 and were badly deteriorated, inefficient, and difficult to maintain. The exterior of the building was no better off, with a leaking roof and large pieces of stone in danger of falling off the facade.
A significant political push for a comprehensive renovation project began in 2007, and this concept envisioned a large underground addition under the Capitol Mall to accommodate infrastructure and increase workspace. The plan proved contentious and the project was not funded, but funding was set aside for additional study and asset preservation, and a deeper analysis of how to solve the structure's problems began.
The next major push for renovation began in 2012. This time the scope of the project was determined via 11 workshops that were open to all stakeholders, including legislators, the governor's office, the press, the Minnesota Historical Society (MHS), and lobbyists. When the workshops were complete, three goals for the restoration project had been established. The first was to improve functionality to ensure the capitol would effectively serve its current government functions and continue to do so for the next 100 years. The second was to ensure that the building would be safe and accessible, which would be accomplished by upgrading life-safety systems, providing secure mechanical systems and infrastructure, and complying with guidelines for universal accessibility. The third was to preserve and restore the architectural integrity of the structure and the essential design elements related to interior details and exterior stone.
HGA had been involved in the structure's renovations and various studies for nearly 10 years and proved a natural fit to lead the full restoration. Although the Minnesota Department of Administration's Real Estate and Construction Services was the primary client overseeing the project, the restoration involved many additional design partners, including architects, engineers, commissioning professionals, interior designers, technologists, landscape architects, lighting designers, planners, sustainability experts, workplace strategists, preservationists, historians, artists, and craftspeople.
Valuable insight was provided by the client, strongly invested public agencies, and special interest groups, including the MHS, the Minnesota Department of Labor and Industry, the Minnesota Department of Human Rights, the St. Paul Building Trades Council, the Capitol Area Architectural and Planning Board, the State Capitol Preservation Commission, the Cass Gilbert Society, the Minnesota State Historic Preservation Office, the Minnesota House of Representatives, the Minnesota Senate, the Minnesota Supreme Court, the Minnesota Office of the Attorney General, the Minnesota Department of Public Safety, and the Minnesota Office of the Governor.
To systematically prioritize the project's goals, the design team identified four historic zones within the capitol to determine where infrastructure could be safely inserted with minimal disturbance to the architecture:
- Zone one included areas with historic stone, artwork, and/or ornate decorative painting. This was the highest level of preservation and was to remain undisturbed.
- Zone two included areas with historic character for which a combination of preservation and restoration would be needed.
- Zone three included previously re-modeled areas, such as offices, that exhibited little evidence of historic or character-defining features.
- Zone four encompassed areas that had not been previously occupied and could be renovated as needed to meet the engineering requirements. Zone four spaces included parts of the basement as well as the mechanical/electrical rooms.
The design team pored over hundreds of drawings of the capitol-original Gilbert drawings as well as those related to subsequent renovations, when available-to gain a deep understanding of the original systems and inform its strategies for integrating new systems into the existing historic fabric.
The structural floor systems throughout the capitol fall into three general categories of masonry arches:
- Segmental tile arches. This system consists of hollow clay tiles, typically 6 or 8 in. thick, arranged in a shallow arch profile spanning between supporting beams. (See the photograph at the top of page 43.) This is found primarily over the basement, where the appearance of exposed surfaces is not critical.
- Flat tile arches. This system consists of hollow clay tiles, which are trapezoidal in shape, typically 12 in. thick, and laid flat between supporting beams. The tiles' shapes along with a center keystone create the arching action between the supporting beams. (See the illustration on page 43.) This is found primarily in the original office spaces. The flat soffit created by this arch system was typically covered directly by plaster to create a finished ceiling.
- Guastavino timbrel arches (named for the Spanish architect Rafael Guastavino). This system consists of thin-typically 1 in.-solid tiles mortared together in multiple layers to create lightweight, long-span arch and dome structures. This is found primarily in the main public corridors of the capitol (see the photograph at the bottom of page 43) as well as the inner dome ceiling over the rotunda. In some locations, the initial layer of tile was glazed and laid in a decorative pattern to produce an exposed finished ceiling. In other locations, decorative plaster was applied directly to the underside of the tile arches.
The segmental and flat tile arches generally span 6 to 10 ft between supporting steel beams and are topped and leveled with concrete fill. These tile floor systems were a form of early fireproof construction common at the beginning of the 20th century. Special tile units were also placed around the supporting beams to protect the steel framing from fire.
The steel floor framing consists of rolled I-beam and channel sections with typical depths varying from 8 to 24 in. The beams and girders are supported on a combination of brick and stone masonry bearing walls and interior steel and cast-iron columns. The steel columns are built-up Z bar shapes, consisting of four Z sections riveted to a web plate. The cast-iron columns were either round or square.
Roof framing is also a combination of steel and clay tile framing. Typical roof areas exhibit "book tile" construction, consisting of 3 in. thick, hollow clay tile units supported on steel T sections spaced approximately 18 in. apart on center. The roof of the house, senate, and supreme court chambers is framed with riveted steel trusses, infilled with book tile framing.
The main dome of the capitol comprises three separate structures:
- The outer dome is marble over a multiwythe backup wall of hollow brick with embedded steel tension bands. A large, trussed-steel tension ring resists thrusts at the base of this dome, which is the second-largest self-supporting marble dome in the world, the first being at the Basilica of St. Peter.
- The center dome is actually a conical steel structure supporting the heavy stone- and steel-framed lantern at the top of the dome. It is constructed from steel column ribs that connect to the tension ring at the base of the outer dome.
- The inner dome, visible from within the rotunda, is of Guastavino design and was finished with decorative plaster. Its ceiling has a diameter of 60 ft, with a thickness of only 4 in. at the crown.
To create better connections between the public and the legislators and to meet requirements for universal accessibility, the team reorganized portions of the capitol's interior layout. Exiting was improved by extending and adding stairs. The exterior steps were reconstructed to increase safety, and a new sprinkler system was installed as well as code-compliant fresh-air intakes and an updated heating, ventilation, and air-conditioning (HVAC) system.
Of the 10 existing stairways in the capitol, many were noncontiguous and did not terminate at exit levels. As part of the life-safety upgrades, the design team extended five of the stairways to exit levels and added one additional, fully enclosed exit stairway.
The extension of the stairs required careful demolition and shoring for temporary support while reconfiguring the supporting framing to create floor openings for the extensions. The design team detailed the new and extended stairs to be similar in architectural style to the existing ones but did not attempt to replicate all the historic details.
The chemical composition of the existing steel was tested to evaluate weldability because many new connections to the old steel would be required. As is common with structural steel produced in the early 1900s, significant weldability concerns were identified. High levels of phosphorus and sulfur were present in the steel. These elements tend to segregate and solidify into weak planes within welds, often leading to cracking as the welds cool or even much later as stress is applied.
To overcome this challenge, the design team detailed all the connections to be drilled and bolted in the field. In locations at which access to only one face of an existing beam was possible—for example, where the hollow clay tile floor arches remained on one side of a beam web—blind bolts were used; these do not require access to both faces for bolting. All the steel framing was measured in the field by the fabricator to ensure a proper fit, avoiding any significant modifications in the field.
Creating the new, fully enclosed exit stairway, located alongside a new service elevator, involved one of the most significant structural interventions of the project, requiring a four-story shaft to be cut through the building from the basement to the underside of the roof. The footprint of the floor opening exceeded one full structural bay, requiring that the existing floor framing be shored from the basement to the third floor. Shoring was removed as the floors were resupported on the new reinforced-concrete masonry bearing walls surrounding the stair and elevator shafts.
Another significant challenge was finding pathways through the building for various services, some of which required ductwork as large as 4 ft in diameter. Pathways and equipment locations had to preserve the historical fabric of the structure while allowing adequate access for future maintenance.
Most of the building's new mechanical and electrical systems were installed in the zone four portions of the basement. Using the basement for mechanical rooms enabled the team to make the modifications necessary to accommodate 15 new air-handling units (AHUs) and new electrical infrastructure. (See "Fresh Air and Clean Energy," on page 48.)
However, the basement's zone four spaces had extremely low heights—8 ft, 4 in. in most places—which limited the ability to route systems overhead in many locations. Additionally, all the basement mechanical rooms were surrounded by load-bearing stone masonry walls up to 5 ft, 2 in. thick. The mechanical, architecture, and structural teams worked closely together to find pathways underground and in the basement that enabled most of the building's new systems to be located in these zone four spaces, leaving the remaining basement space available for public use.
The stone foundation walls and piers throughout the basement are spaced closely together, and most range in thickness from 2 ft to more than 5 ft. Footings for the walls consist of tiered concrete spread footings that become wider with depth (like a wedding cake), and most interior footings only bear 2 ft below the finished floor. As a result, the width and depth of excavation that could occur in the basement without impacting the stability of the foundations was limited, leading the team to explore underpinning options.
To understand the parameters for underpinning, a geotechnical evaluation of the existing soils supporting the capitol was conducted. The existing soils were found to range from dense to very dense/fine-grained sands and silts. While this provided a high bearing capacity, it ruled out one proposed underpinning solution: soil solidification using permeation grouting below the footings. The ultrafine cement grouts used in such a process would not be able to sufficiently penetrate the dense/fine-grained soils.
The geotechnical recommendations thus included defining a "no-dig zone" that encompassed a volume of soil beneath the foundations that would need to remain undisturbed to maintain stability. This zone was defined as extending 3 ft horizontally from the edges of the footings and downward at a slope of 1:1 (horizontal to vertical).
At those locations at which the no-dig zone had to be violated, underpinning was required to lower the bearing elevations of the footings. A traditional pit underpinning approach was implemented in which short segments—generally 5 ft or less—were excavated and backfilled with concrete.
Given the cost and labor required for this type of underpinning, the team coordinated closely to minimize it. All the underground systems and the structure were incorporated into a 3-D building information model (BIM), including the no-dig zones and underpinning segments. Modeling the no-dig zones as physical elements enabled the modelers to include these areas in clash-detection analyses as well as a "fly-through" visualization.
Approximately 110 individual underpinning segments were installed. Settlement was monitored throughout the process to prevent unacceptable movements, which could have damaged the historic masonry walls and finishes above.
Once the basement challenges were resolved, pathways had to be established to distribute the building services ductwork vertically through the building. Because the primary public corridors that bisect the building were part of historic zone one, no new ductwork or piping could cross these spaces. The team's solution was to use rooms known as "the vaults" for the primary vertical shafts.
These locations had the advantage of being well distributed throughout the three wings of the capitol—on either side of the historic public corridors—and stacked vertically from floor to floor. The primary disadvantage was that their original use as fireproof vaults, as their name implies, for record storage meant they were constructed of brick walls at least 2 ft thick on all sides, and their floors comprised 16 in. thick solid brick arches.
Despite the inherent structural challenges of getting systems into, through, and out of the vaults, they proved to be the best possible choice for routing the ductwork. However, the cost associated with each new wall penetration was substantial. The mechanical and structural engineers collaborated closely to consolidate systems and minimize openings as effectively as possible while maintaining a clear load path and safe structure. This collaboration was aided by the BIM, which included a 3-D view of each vault, showing the four walls from roof to basement. The engineers could rotate the vaults in the viewer and stretch and adjust the views to get a clear understanding of everything inside the vaults that needed to be routed out. The architectural ceilings were also modeled and visible in these 3-D views, so the engineering team could ensure all ducts and piping exiting the vaults remained concealed above the ceilings. (See the illustration on page 44.)
Additional Insight: Preserving the Rotunda
Additional Insight: Fresh Air and Clean Energy
After this design was complete, the design-assist subcontractors began creating fabrication-level models, working in conjunction with the structural design model. In many cases, the consolidation of systems into single openings was a significant challenge for the subcontractors, but because the cost of each opening was so high, the existing structure needed to remain as intact as possible. The relationship between the design and fabrication teams often challenged both, but it ultimately resulted in the best possible solution for each vault.
Existing wall and floor openings were reused wherever possible, although many were initially inaccessible, and some were not well documented and only uncovered once demolition began. Hundreds of steel lintels were used to create the new pathways through the brick and stone walls. These ranged from conventional wide-flange beam lintels—with multiple beams located side by side to accommodate the thick masonry walls—to unique solutions dictated by existing conditions. These "one-off" conditions included built-up plate sleeves and double-height beam lintels. (See the illustration on page 46.)
Because the various hollow tile and brick floor systems in the capitol rely on arching action to resist loads, new floor penetrations also had to be carefully detailed. Smaller penetrations were sleeved with steel pipe or rectangular channel frames grouted into the masonry to maintain a load path for the thrust developed by the arches. Larger openings generally required the removal of arch segments between supporting beams.
Over time, the capitol's marble exterior had deteriorated due to water damage, freeze-thaw cycles, sunlight, and environmental pollution. In some cases, one could pull damaged stone pieces off by hand-a significant life-safety issue. Scaffolding was erected around the entire exterior of the capitol so the design team, subcontractors, and craftspeople could work safely.
The team used sounding hammers to determine the integrity of each piece of stone. Where necessary, the white Georgia marble was replaced; in other cases, portions of stones were removed and replaced with new pieces carefully created off-site using computer numerical controlled (CNC) machinery. At times the pieces still needed to be carved by hand on-site to meet the highest standards of reproduction.
Areas in which the marble had deteriorated or "sugared"-a condition in which loose granules form on the surface because of acidic moisture and multiple freeze-thaw cycles-were cleaned, and all the stone was repointed and cleaned. During this process, HGA created a database documenting the location and status of each stone, allowing the state to better maintain the marble exterior in the future.
In fact, restoring the exterior stone involved a highly detailed forensics process. Laser scanning and 3-D printing were used. Once the laser image was captured, it was sent to the fabricator as a stereolithography (STL) file. The fabricator used this file with its CNC machine software and ultimately cut the pieces to represent an exact match of the scanned file.
In some cases, the scanned file was sent to a 3-D printing company because the pieces had deteriorated over time and the laser was only able to capture what remained. This gave the team a good starting point, but modifications were needed so the fabricator could have a model that accurately represented what each piece originally looked like. To accomplish this, the stone trade partner applied clay to the 3-D-printed, full-scale model to represent the complete element. This was then sent to the fabricator for reference when completing the finishing touches.
Of the 30,000 stone units on the building, nearly every piece was touched, and about 60 percent received some level of general repair. An additional 5,000 pieces of new stone were installed. This part of the project, which began with one production carving studio, ultimately required the efforts of eight masonry contractors, including four additional fabricators from three countries, so the project could meet its schedule.
The existing exterior stairs on all sides of the capitol plaza act as a roof for occupied spaces below. This roof of monolithic granite stair treads was supported on brick finger walls. Over the years, various water management systems had been implemented to attempt to capture and control the water infiltrating through the stone tread joints, but not all were effective.
As part of the project, the plaza leading to the stairs was to be waterproofed and restored, and as this was done, the stairs on the south, east, and west sides of the capitol were completely dismantled and rebuilt. (The stairs on the north side of the building had been previously rebuilt.) Removing the massive granite treads allowed the team to design new structural "lids" over the basement, beneath the steps. These lids were constructed of concrete slabs on composite metal decking, supported by the brick finger walls.
The top portions of the finger walls had deteriorated from years of moisture infiltration and freeze-thaw cycles and were removed down to the bearing elevation of the new slabs-on-deck. Stepped cast-in-place concrete curbs were formed above the deck on top of the existing walls to resupport the treads.
The lids and the curbs were completely waterproofed to prevent as much future water infiltration as possible, and the lids slope toward interior drains just in case. After waterproofing, the original stair treads were reinstalled, except where previous damage required replacement treads. The street in front of the capitol, Aurora Avenue, was then permanently closed and transformed into a public pedestrian promenade.
To ensure that the capitol functions as a modern facility, the team also added 40,000 sq ft of meeting and reception space, exhibit spaces, and public education classrooms; relocated caucus rooms closer to the legislative chambers; and expanded public seating throughout the building. Dining options were expanded, including a second-floor lunch counter for the public and legislators, and free Wi-Fi was made available throughout the capitol.
In large part, the extensive restoration work completed for this project is invisible. The public does not necessarily see the complex accomplishments—the architectural and engineering innovations that made the building safer, more secure, and more accessible while preserving its historic characteristics. The team approached the project as forensic scientists, peeling away the layers to discover what lay beneath the historic stone and marble and determine the best steps forward for a full restoration.
In August 2017 there was a three-day grand opening celebration for the capitol. As of that date the interiors were complete and the building was fully occupied, but some site work was ongoing. The last of the site work was designed last year and constructed this year; it includes a universally accessible sloped walkway connecting the new pedestrian walk in front of the building with the mall, directly to the south. The grand opening for this part of the project was held in August.
Today, the Minnesota State Capitol enjoys the conveniences of a modern, state-of-the-art public building. Balancing innovative solutions and high-impact interventions with technical restoration, the design team seamlessly integrated improvements into the historic fabric. Gilbert's masterpiece has stood the test of time and will continue to inspire future generations-providing Minnesota with an architectural landmark to last another 100 years.
Deb Young, AIA, is a senior project manager, Sarah Berseth, P.E., is a mechanical engineer, and Sean Cotton, P.E., M.ASCE, is a structural engineer at HGA, which is headquartered in Minneapolis.
PROJECT CREDITS Owner State of Minnesota, Minnesota Department of Administration, Real Estate and Construction Services Owner's representative (for interior work) CPMI Cost Planning & Management, Eagan, Minnesota Owner's program manager (for interior work) MOCA Systems Inc., Salt Lake City Geotechnical engineer Braun Intertec Corp., Minneapolis Architect and engineer HGA, Minneapolis (civil engineering; landscape architecture; architecture; interior design; structural, mechanical, electrical, and telecom munications engineering; audio-visual, lighting, and sustainability design; project management) Consultants to HGA Schooley Caldwell Associates, Columbus, Ohio (historic planning); Wiss, Janney, Elstner Associates Inc., Minneapolis (conservator and technical architectural support); Luken Architecture, Minneapolis (historic structures report consultant); Schuler Shook, Chicago (lighting consultant); Summit Fire Protection, St. Paul, Minnesota (fire protection/life-safety consultant) Construction manager J.E. Dunn Construction, Kansas City, Missouri Design-assist subcontractors Harris Companies, St. Paul (mechanical subcontractor); Gephart Electric Co. Inc., St. Paul (electrical subcontractor); Crenshaw Lighting, Floyd, Virginia (historic lighting restoration) Building stone (marble) supplier/trade partner Polycor Inc., Quebec City Stone procurement/facilitator Twin City Tile and Marble, Eagan Stone installer/carver Mark 1 Restoration Company, Dolton, Illinois Stone masonry contractors/fabricators Advanced Masonry Restoration, St. Paul; Italmarble Pocai S.r.l., Massa, Italy; Cutting Edge Stone, Alpharetta, Georgia; Tennessee Marble Co., Friendsville, Tennessee; Costa Paolo & C., Carrara, Italy; Traditional Cut Stone, Mississauga, Ontario, Canada; Art Cubus International Inc., Sherbrooke, Quebec, Canada; Grazzini Brothers & Co., Eagan Site stone (granite) supplier Cold Spring Granite, Cold Spring, Minnesota Structural steel TEK Steel Fabricators Inc., East Bethel, Minnesota; Bauer Custom Welding, St. Paul Roofing Berwald Roofing Co. Inc., North St. Paul, Minnesota Site work Carl Bolander & Sons Co., St. Paul Decorative painting Conrad Schmitt Studios Inc., New Berlin, Wisconsin Leaded glass restoration and fabrication Gaytee Palmer Stained Glass Inc., Minneapolis
Civil Engineering, December 2019, © American Society Of Civil Engineers. All Rights Reserved