It is useful in this sense to study the periodic transformations and expansion of museums and performing arts centers in New York. Both of the present authors have served as structural engineers for these projects, including those of the Guggenheim Museum, the Museum of Modern Art (MoMA), the Metropolitan Museum of Art, and the Metropolitan Opera. MoMA, for example, has added buildings over time since its start in 1939, in the building designed by Philip L. Goodwin and Edward Durell Stone. Major expansions have occurred—in 1980–84 with César Pelli, in 2001–04 with Yoshio Taniguchi, and, most recently, in 2016–19 with Diller Scofidio + Renfro (DS+R). Buildings and a garden were added by Philip Johnson between 1950 and 1964. With each cycle, the choices to retain or replace reflected the prevailing taste. Taniguchi aptly captured that dialectic by framing the Johnson garden, painstakingly recreated, in his all-around crisp Modernism, while restoring the historic sequence of facades along the side of the 53rd Street entrance. DS+R’s addition to the west, including the tragic destruction of the American Folk Art Museum (designed by Tod Williams and Billie Tsien), extended this sequence to include the Jean Nouvel tower at the west end. Along the street, one can follow the history from Goodwin-Stone to Nouvel via Johnson, Pelli, Taniguchi, and DS+R.

From the engineers’ viewpoint, this history is also embodied in the multiplicity of structures, enclosures, and systems that emerged from an equally eclectic series of actors. While mostly invisible, there is a compelling story of New York construction, quite like that of the commercial construction of tall buildings noted above, which has been incorporated through these cycles. Both the Pelli and Nouvel towers are built in typical residential flat-plate construction, though the Nouvel tower’s jagged profile displays a jazzy version of Chicago-style diagonal bracing. The remaining museum sections are a mixture of concrete and steel framing. The Taniguchi expansion includes some mostly invisible nuances, both in the structure’s tatami-inspired geometry and the extreme minimalism in some places, including the carving of spaces through the Pelli tower to accommodate the reconfigured circulation. What is striking, and very New York, is the feeling inside the fully expanded museum of smoothed, frictionless space as one progresses through the invisible history of constructions.

In contrast, the multiple expansions of the Metropolitan Museum of Art are more clearly manifest as one moves around its labyrinthine interior. One of the museum’s charms is the difficulty of finding one’s way, proving a challenge to many young explorers. This palimpsest of architectures presents a very different experience from the MoMA’s “make it new” smoothness. Yet here, too, the embedded history of construction is little understood or documented. Morrison Heckscher’s excellent history of the sequence of buildings, from the those of Calvert Vaux to those of Kevin Roche, shows how each generation of expansion wraps around and both buries and loads the last. Depending on the architect, the corresponding constructions may have some historical significance or not. The records are limited. In one instance, in the 1960s, the lack of understanding and documentation of the Guastavino vaults installed at Charles McKim’s direction in the courtyard and gallery spaces of the northern Wing H (1904–13) resulted in their demolition. They were replaced by a steel and concrete floor for the expanded Egyptian exhibits. As has often happened in New York, a combination of lost craft knowledge and misplaced or nonexisting documentation has led to the demolition of these hidden cultural treasures. At the Metropolitan Museum of Art in particular, given its age and position at the edge of Central Park, a rich material culture and history is subsumed in the walls and floors. These—including the underground and back-of-house hidden mysteries, from water tunnels to vaulted foundations—could warrant a follow up to Heckscher’s history.

The Guggenheim Museum, designed by Frank Lloyd Wright and completed in 1959, represents an obvious landmark in many ways, including, in our opinion, structurally. Apart from the more conventional northeast portion of the building (where the stairs and elevators are located), the majority of the structure consists of nine cast-in-place concrete radial “web walls” (visible within the galleries), the continuously rising ramp slab (cast-in-place concrete as well), and the iconic facade spiral, which was constructed in concrete using the gunite method (fluid concrete that was sprayed onto mesh reinforcement, a method more often used to build swimming pool shells or sculpture).

The precise performance of the Guggenheim’s facade—both the portions hung from the web walls and simultaneously bracing the web walls—has not always been evident. While there is a clear structural-load path throughout and, by observation, the structure is rigid and laterally capable, in 2005–08, a structural team, led by Robert Silman, was asked to determine the cause of persistent cracks in the facade concrete that have vexed the museum from the outset. The analysis and fix ultimately provide a potent example of using modern analysis tools to assess, and then restore, historic structures. The critical feature of this approach is harmonizing modern digital design programs with archaic structures to ensure that one does not incorrectly ascribe modern properties and behavioral assumptions to brittle elements from an earlier time.

At the time of Silman’s analysis, large-scale digital scanning was relatively early in its use in buildings but had finally become available beyond academic domains. Given the importance and scope of the building, the team leveraged this new technology, scanning the entire building into an unimaginable digital cloud of data, which was then converted into a three-dimensional geometric image. They uploaded that image into structural modeling software and analyzed it as a solid concrete structure. This process took months to complete (months alone simply to clean up the data and “mesh” the elements in the model); it would take considerably less time today, as the technology has advanced rapidly in the 20-plus years since this analysis took place.

Over the long period of modeling, the building was simultaneously monitored under all external conditions on a day-by-day basis, with the ultimate intent of calibrating in situ movements with the computer model. Finally, simultaneously with both of these efforts, the team opened physical probes on the facade that unlocked the key to the ultimate design solution: discontinuities in the reinforcing at the circular facade. These discontinuities would not have been visible to any sort of scanning (even today), because the break in the reinforcing was obscured behind other steel elements.

By combining each of these steps, the team learned the cause of the cracking and then, most importantly, designed a fix that could be verified with confidence, based on the calibration of the digital model with actual behavior in the field. Any one of these approaches may have yielded a supposition or recommendation. Together, model, monitoring, and testing ensured that the fix (in this case, laminating the entire nonhistoric inside face of the facade with carbon fiber to add tensile strength) aligned with the original intent of the structure. Quietly, and hidden behind the finishes, this trifecta returned the structure to its original intent. The churn in this case was more one of the technological sort within a unique form.