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For centuries, architects have relied on the Vitruvian Virtues of “commodity, firmness, and delight” or in the Roman architect’s own words: firmitas (solid), utilitas (useful), and venustas (beautiful). For most of architectural history these three virtues were inextricably linked, and structure, an essential attribute of firmness, clearly contributed to both a building’s form and beauty. Over time, this linkage between architectural and structural form changed, and the linkage between the two is rarely evident in architecture today.
Spurring this change were innovations both ideological and material. The invention of the arch was perfected by the Romans and brought to sublime architectural expression by the builders of Gothic cathedrals. The emergence of structural steel allowed the form of a building to be decoupled from the structural frame because the exterior walls were no longer load bearing, giving architects new opportunities for exploring aesthetic boundaries. And today, the analytic tools available to structural engineers make it possible to design structures to make just about any shape conceivable withstand the forces of nature.
And over time, structural engineers have been successful at doing more with less. As buildings evolved from heavy stone structures to lightweight steel and reinforced concrete frameworks, live-load to dead-load ratios improved significantly. No longer was gravity load the only force a structure must resist, and facades needed to include braces or shear walls which architects employed to express structure with varying degrees of success aesthetically. Fashion has moved on, too.
However, the force of gravity is the same today as it was in Roman times, and the shape of a building is a significant influence on the efficiency of the structure supporting it. Although sustainable design rewards optimal use of resources, there are no metrics or standards to benchmark efficient use of steel or concrete like there are for mechanical, electrical, and plumbing systems. No LEED points are credited for an efficient structure.
Given this, is it meaningful to strive for a link between “firmness” and “delight” in contemporary design? Following are two case studies illustrating how structure-informed architecture yield practical but also aesthetically sustainable designs.
Both case studies are additions to historic buildings on dense urban sites in San Francisco, California. San Francisco is in an active seismic zone which played a key role guiding the architectural design in both cases.
Terrace Addition—Cathedral School for Boys
The Cathedral School for Boys is a K-8 private school operated in partnership with Grace Cathedral, a building of Gothic proportions serving as a keystone of the Episcopal Diocese of California. Both sit on Grace Cathedral Close, a full city block on San Francisco’s Nob Hill. The entire block is a local historic landmark, recognized in the San Francisco Planning Code. New interventions on landmark sites are subject to strict zoning controls and public scrutiny.
The original two-story school was built in the 1960s and employed a state-of-the-art concrete structural system consisting of waffle slab floors and reinforced concrete columns. The structure was stepped down following the sloping site which is underlain by bedrock. Non-structural wall panels were precast, post-tensioned concrete, unusual even today. Several alterations and small additions had been made to the building in decades past, including a partial-story concrete classroom addition on the downward sloping side with a concrete outdoor roof terrace.
The new project included remodeling all of the school classrooms and inserting a two-story 4,000-square-foot addition between the original school building and the cathedral, landing on the existing concrete terrace structure and avoiding the need for new foundations. The new addition serves the upper and lower schools as a “learning commons” and functions as the new heart of the school even though it is on the side of the building.
The existing terrace structure was robust enough to support a lightweight structure vertically—short-span steel framing was the logical (and perhaps only) choice. The existing school structure was almost robust enough to receive additional seismic load and only needed strengthening of a few existing concrete shear walls, which was accomplished by adding shotcrete bonded to them. The new lightweight concrete on metal-deck floor diaphragms transferred torsional seismic loads to the existing structure without the need for bracing or any structural elements that would interfere with the transparency of the wall facing the cathedral.
The addition needed to be light in weight, but also light in appearance with large windows to provide daylight into the addition itself as well as into adjacent original classrooms. Historic preservation precepts required an addition with a perceptible contemporary timestamp, one that would fit comfortably between two venerable concrete structures. A light prefabricated curtainwall achieved this goal, with pale green aluminum elements reminiscent of aged copper.
Furthermore, urban infill projects and existing building alterations pose a structural challenge not typically encountered in new construction. Locations for inserting new structural components are limited by existing conditions and should not conflict with architectural layouts, also constrained by existing construction. Two conditions were especially challenging on this project and required close collaboration between the architect and the structural engineer of record.
The first condition was the location of columns adjacent to the original structure to support the upper floor and roof of the addition. The new columns needed to be next to and align with the original columns to avoid new foundation work. The roof parapet, however, extended beyond the face of the original building as an important (and historic) architectural feature and interfered with the column-beam connection.
Carefully managed by Truebeck Construction, HSS6x6 columns were snaked up through openings created in the parapet from below. Roof beams were similarly inserted horizontally. Structural engineer, Murphy, Burr, Curry, devised details to connect the beams to the columns which also needed to deliver diaphragm shear forces to the existing roof structure. The connection protruded above the roof and was encased in concrete to become part of a bench along the perimeter of the existing roof play yard.
The second condition involved support of the new steel columns along the facade parallel to the cathedral.
The existing terrace was supported by a large upset concrete girder spanning over a ribbon of clerestory windows providing daylight to lower-level classrooms. The new steel columns of the addition landed on this girder acting as a transfer beam. The original concrete girder was heavy and lacked sufficient stiffness to ensure the windows below would not crack under the additional loads from the addition. Consequently, it was replaced by a W27 steel girder, which also needed to pick up the terrace load, now the lower floor of the addition.
Coordination between general contractor and the design team was essential here, too. Removal of the concrete girder and installation of the new steel girder required shoring of the original terrace to prevent overloading of the windows below. Load transfer from shoring to the new girder was carefully monitored, and the installation was accomplished without damage to existing construction.
The new steel girder was wider than the thickness of the curtainwall above it. It was clad in wood and became another perimeter bench, a popular window seat in the new addition with a view to downtown San Francisco not experienced before the addition was built.
An Architectural Triptych—1 Kearny
The original Mutual Savings Bank was a prominent building constructed in 1902 at “Newspaper Angle,” an important convergence of downtown San Francisco streets originally home to three daily newspapers. The opulent French Renaissance Revival-style edifice was designed by William Curlett and anchors the end of Third Street, a major transit artery into San Francisco’s downtown urban core.
The 12-story building, also known as 1 Kearny, is a sandstone- and terra cotta-clad structure supported by a steel frame designed and erected by the Roebling Construction Company of Brooklyn Bridge fame. The Mutual Savings Bank was one of a very few buildings not significantly damaged during the famous 1906 San Francisco Earthquake. The ensuing fire was not as kind to the facade, but the ductile steel frame performed well. The building was repaired and continued to function as a bank well into the twentieth century.
In the early 1960s, 1 Kearny was purchased by Citizens’ Federal Savings and Loan Association. Architects Clark and Buettler were retained to design an addition to the original 1902 building, replacing a small building at the triangular site’s corner. Charles Moore was the lead designer, and the addition shows early signs of the postmodern style Moore helped usher into the architectural mainstream. The addition, labeled the “annex,” included elevators, fire stairs, and restrooms following a “served-servant” design paradigm, also popular in the 1960s. The annex functioned as the servant, functionally subordinate to the main bank building.
The annex is a brick-clad concrete structure with stiff concrete shear walls around the stairs and elevator shaft.
Eventually the bank moved out, and 1 Kearny became a multi-tenant office building (including another bank on the ground floor). The annex presented two complications. The “servant” was too big for the “master” it served, and the narrow joint between the two made it impossible to divide a floor into separate tenant spaces. And more significantly, a stiff concrete structure attached to a flexible steel frame resulted in a “stiff tail wagging a flexible dog” which would lead to a torsional imbalance resulting in substantial damage during a major earthquake. In fact, the annex made 1 Kearny more vulnerable to earthquake damage than it had been in 1906.
1 Kearny was sold again in the 1990s, and a few years later, the new owners were able to acquire the adjacent property on the side opposite the annex. The author’s firm was retained to design a second addition to replace a small building on the newly acquired site —the “missing tooth” on a block comprised of mid-rise buildings. Filling in the missing tooth enabled the design team to correct the functional and seismic complications.
The property line separating 1 Kearny and the adjacent site was the boundary between two zoning districts with different allowable development densities and height limits. Furthermore, 1 Kearny was a Category I historic building, just below landmark status. The completed project merged the two sites, including rezoning of the adjacent site to allow the addition to be 10 stories. Rezoning involved a complex entitlements process including review by historic preservation advocacy groups, several public hearings, and approval by the mayor.
The seismic design of the addition was the primary contributor to garnering the entitlements for the project. To mitigate the torsional effects caused by the stiff concrete annex, the structure of the new addition was designed as a kind of “seismic bookend” grabbing onto the original 1902 structure from one side while the existing annex bookend grabbed the other side. The two primary facades of the addition contain steel moment frames with enough stiffness to work in concert with the annex shear walls to resist seismic forces in the long direction. Although moment frames are more flexible than shear walls, the footprint of the addition is much larger than the footprint of the annex. A third line of lateral resistance was introduced in the middle of the “dogleg” of the addition which takes 50% of the lateral load. It is a braced frame.
The design team argued that to be effective, the new seismic bookend needed to be as tall as the books supported (in this case the original historic building). Consequently, the rezoning included a height limit increase allowing the new addition to be 10 stories. The bookend approach also permitted the original Mutual Savings Bank building to be seismically strengthened without introducing new elements into it or tearing it apart. Preservationists applauded the approach, and 1 Kearny became the poster child for how a new structure could benefit a historic one.
The steel moment frames were clad with a terra cotta rain screen, an archaic material rendered in a modern vocabulary to fit comfortably next to the original facade and terra cotta Mansard roof. The terra cotta clad moment frames engaged in an architectural dialog with the brick-clad annex shear walls, expressing their function as the bookends. The resulting composition tied together architectural styles from three eras, dubbed an architectural triptych by San Francisco Chronicle’s urban design critic.
These two examples illustrate how structural design can benefit architecture technically and politically. The linkage between firmness and delight can be expressed in modern design albeit in a more subtle manner than in the past when available materials and analytic methods demanded a tighter bond between architecture and structure. ■
About the Author
Charles F. Bloszies, SE, FAIA, LEED AP, is founder and principal of The Office of Charles F. Bloszies, an award-winning San Francisco-based structural engineering and architecture practice founded in 1985. Bloszies is also an educator and author of essays and the 2011 book, Old Buildings, New Designs: Architectural Transformations (Princeton Architectural Press). (chuck@archengine.com)
References
King, John, San Francisco Chronicle, https://www.sfgate.com/entertainment/article/shared-spirit-in-1-kearny-s-styles-from-3-eras-3281388.php