Considered to be one of the most beautiful libraries in the world, the George Peabody Library at Johns Hopkins University in Baltimore houses 300,000 rare and historic volumes. The structure, fittingly described as a “cathedral of books,” features five stories of ornamental cast-iron balconies surrounding a 61-foot-high atrium flooded with natural light. Rising from its black-and-white marble floor to a massive glass laylight and decorative ceiling, the library is stunning to behold. Behind the scenes, writes Jessica Adler in JHU Engineering magazine, a complex network of iron trusses above the laylight—virtually invisible to visitors below—spans the library space and supports a massive skylight above.”
When it was built in 1878 under the direction of architect Edmund G. Lind (1829-1909), this then state-of-the-art building relied on regionally made, pre-fabricated cast iron for architectural ornament and columns and on wrought-iron trusses for structural fortitude and fire resistance. Bartlett-Robbins and Company fabricated the Neo-classical designs in cast iron, while the original iron trusses were made at the Phoenix Iron Works. Its original glass skylight had been replaced at some point with plastic (polycarbonate) panels that later failed.
Assessing the Structure and Construction Challenges
The polycarbonate panels began to lift in high wind. To protect one of the world’s most important collections of 19th-century volumes, a permanent fix was needed. Johns Hopkins suggested replacing the failed panels with insulated glass as a long-term solution for use on the skylight and to improve its thermal efficiency. It was imperative to safeguard the building’s historic fabric as well as the rare books. There were concerns about the structural safety of using insulated glass, which would potentially weigh more than the cast- and wrought-iron structure could support. Further, the size of a structural reinforcement had to be considered, as a larger frame risked diminishing the quality and amount of light admitted by the skylight and laylight. The skylight measures 25 x 75 feet for a total of 1,875 square feet.
Johns Hopkins commissioned 1200 Architectural Engineers, a firm based in Alexandria, Virginia, to perform a structural analysis and feasibility study. According to company partner John Matteo, PE, FAAR, during their investigation it became apparent that the building was well-built and efficient when it was constructed in the 19th century, and it had an engineering system of trusses that was worth protecting. As such, they did not want to weld or drill into the historic iron frame. Designing the construction sequence would be just as important as finding the right solution for strengthening the trusses.
With these concerns in mind, Johns Hopkins turned to Ziger|Snead Architects, in Baltimore, noted historic preservationists. Firm principal, Steve Ziger, FAIA, and project architect Dan Carter understood the importance of keeping the rehabilitation “light handed.” Architect, engineer, and contractor partnered together to devise a means of strengthening the truss system, protecting its historic fabric, and maintaining the distinctive ambience created by the skylight and laylight—all while keeping the library open to patrons during the construction process.
Engineering a Reversible Solution
Key to the discussions and analysis among client, architect, engineer, and contractor was the principle of reversibility. Conservators and preservationists seek to intervene in ways that allow the construction history of the building to remain interpretable for future generations. John Matteo reiterated that, “whenever possible, repairs should be done in such a way that they can be altered at a later date, assuming that future changes and associated interventions will be possible while leaving the original historic fabric intact.”
Nevertheless, the building finds itself in the 21st century where, in addition to needing to support increased loads from a heavier, insulated glass skylight and the introduction of new catwalks, our understanding of the environmental demands from wind, snow, and unbalanced snow loads has increased well beyond that from the time of original construction. John Matteo explains that they used a combination of traditional hand calculations and finite element computer analysis to both evaluate the changing impact on the historic trusses and to design the new reinforcement system. Steve Ziger explains the historic truss system has its own beauty as well as function; as the analysis and consideration of solutions progressed, “they felt like they were reaching across history” from the 21st to the 19th centuries to arrive at the right solution. There was a lot of back and forth discussions in a process that played out over two years from inception to project completion. Ultimately, Matteo says, “the team designed a pair of inverted triangular trusses, set alongside the existing framework, with diagonal tie rods or stays to a new steel saddle that cradles the historic iron truss at mid-span. The tension in the stays would be resisted by the horizontal compression strut at the top of the new truss. Ultimately, the new and existing loads continue to be supported by the original cast iron columns.” The work sandwiched the historic iron trusses with compression-fit, steel bolts and plates. The reinforcement is completely reversible.
The Construction Sequence
Establishing a viable construction sequence was critical to the safety of the library and its occupants because of the weight of the steel and glass, the protection of the historic books, and the limited space available for access and storage of building materials during the process. Matteo noted, “To accommodate the need to deliver materials through an occupied building and the challenge of installation within the geometric constraints of the attic space, strengthening components were limited in size to no more than 8 feet in length.”
First in the construction sequence was the installation of debris netting. The next step involved building catwalks for the truss reinforcement, infrastructure that would stay in place for later mechanical and skylight maintenance. Work platforms were installed sequentially, to limit construction phase loadings on the historic structure, while the reinforcement work is installed one truss at a time. The project finished with the installation of the new insulated glass panels.
Both Matteo and Ziger credit the contractor, Rockville, Maryland-based Grunley, with helping strategize the construction sequencing and planning for the demands of the project’s means and methods.
Recognition of the Project
Baltimore Heritage took notice of this project’s important preservation work; on June 13, 2019, the project will receive a Preservation Project Award. Johns W. Hopkins, executive director of Baltimore Heritage, said of the award-winning project, “The Peabody Library is one of the most extraordinary historic interior spaces in the country, and the restoration of the central skylight does the library justice. No simple pop-on transparent dome here. Behind the gorgeous refurbished skylight lies a structural steel framework that is engineered with precision and is as meticulous and well executed as the glass itself.”
See It In Person
To see the atrium, check the hours of the George Peabody Library at peabodyevents.library.jhu.edu/about-the-library/
And if you are in Baltimore, the rooftop bar at the nearby Hotel Revival has a great view of the exterior skylight.