The Guile of Structure

Unlike his contemporaries, Louis I Kahn did not leave a legacy of stylistic principles or theoretical manifestos to the architectural world.  He made his mark by creating compelling buildings that could be labeled neither Beaux-Arts nor Modernist, but rather assumed a category of their own – a category that is based on the existential and spiritual quality of space. In Kahn’s own words, this is “what the building wants to be.” It represents the truth, the perfect form of a building that cannot take any other shape, and it is to the search for this perfection that Kahn devoted his life.

Kahn’s search for authenticity had led him away from his Beaux-Arts training, Like many Modernists, Kahn instead believed in disclosing the functional aspects of a building by making apparent the mechanical and structural components.  Differing from Modernists who were only marginally concerned with material, however, Kahn placed great emphasis on expressing the natural quality of material – never allowing a surface to be painted or patched even for damage done during construction.  This obstinacy in material expression caused some to perceive Kahn as a purist.

However, it would be a mistake to call Kahn a purist if the term means someone who is dogmatically honest about the integrity of structure and someone who strips away everything non-essential.  For Kahn, structure meant more than simply the supports that hold up a building; it was an ordering element, a pedagogical tool, an integral part of the building that affects its aesthetic reading. Thus, in Kahn’s view, structure is like a theatrical element that could be manipulated deliberately in order to fit the overall message of the building.

This mode of thinking differs greatly from Kahn’s peers, most of whom viewed structure as merely a technological manifestation rather than as an expression of art.  A popular practice in the 1950s and 60s was to use the lightest and fewest steel columns possible in framing expansive open spaces that were enclosed with large panes of glass. The goal was to demonstrate the lightness and the large spanning capacity of structural members made possible by technological advancement. Although Kahn appreciated and utilized the opportunities provided by these new construction technologies, he was not content with this type of Modernist aesthetic.  For him, structural members ought to have mass to counteract the natural force of gravity. They gave weight to the building and anchored it psychologically. Rather than choosing the smallest structural members, Kahn sometimes might even exaggerate the mass of these components to please the human eye.

Kahn’s admiration for mass could be traced back to his Beaux-Arts education under Paul Cret at the University of Pennsylvania. Graduating in 1924, Kahn was trained to design hierarchical spaces that were symmetrically and centrally organized with great emphasis on the plan.  Influenced by these principles, Kahn’s buildings were spatially differentiated with a clear sense of order. Instead of an open plan, Kahn’s buildings were often broken up to underline the sequence of space. Yet Kahn refused to adhere strictly to the Beaux-Arts ideology. What guided him in his designs was not any particular school of thought; it was rather his intuition.

By intuition, Kahn meant the feeling, the sensitivity and the “intangible side of his mental processes.”[1] In both his writings and speeches, Kahn frequently used the words “I sense.”[2] The sensuous quality of a space was of immense importance to him. Not only did he pay great attention to the texture and color of materials, he also devoted much time to studying the properties of light and the proportions of his structures. In theater, the colors of the backdrop, the dramatic use of light, and the form of the set enable a stage to extend beyond its physical limitations, to separate the audience from the familiar and house them in a world of fantasy. Similarly, Kahn used texture, light and structure to construct his stage for human action. This theatrical approach to design can be seen in several of Kahn’s buildings.

In the Yale Art Gallery, Kahn developed a tetrahedral ceiling system that formed a dramatic setting for the artwork. In 1951, Kahn was asked by Yale University to design an addition to its Art Gallery. The program called for a loft building with maximum flexibility that would allow the museum to rearrange spaces according to different needs. It also stressed the importance of creating an efficient and “relatively inexpensively constructed” design.[3] George Howe, then the chair of the architecture department, drafted a preliminary plan composed of three floors that were four bays wide by five bays long, with one and one-half bays devoted to circulation and mechanical ducts.

While it would have been easy for Kahn to meet the criteria of the program by simply carrying out Howe’s plan in typical column-and-floor-slab construction, Kahn felt that a compelling architectural feature was necessary to enliven the space and to frame the exhibited artwork. Working closely with his employee Anne Tyng, who had a fascination for Buckminster Fuller’s “complex geometries and the structural efficiency of space-frames,” Kahn proposed a tetrahedral ceiling system, a concrete version of the space-frame [figure A].[4] Kahn believed that the strength of the tetrahedral ceiling emerged not only from its great spanning capacity, but also from its ability to “harbor” the mechanical duct works. The triangular openings at the top of the tetrahedral unit would provide flexible locations for lighting fixtures to accommodate changing exhibitions [figure B].

The structural and mechanical capacity of the tetrahedral ceiling system was convincing enough that the Yale building committee accepted it enthusiastically, but it was far from what it appeared to be. Structurally, it did not function as a space-frame. While Fuller’s space-frame structures and geodesic domes focused on achieving great strength with minimal mass, Kahn’s tetrahedral system was entirely the opposite – showcasing the mass of the concrete.  As a result, the system failed to generate satisfactory results after a mock-up test. After the New Haven building department’s scrutiny, Kahn and his structural engineer, Henry A. Pfisterer, altered the design to eventually become merely a complicated version of a typical beam-and-girder system. In his description of the ceiling system, Pfisterer described it as “concrete T-beams with deep inclined stems spanning 40 feet between centers of supporting girders, combined with triangular inclined bridging elements arranged to simulate the original concept.”[5]  Therefore, the structural capability that was envisioned with the system at the beginning was no longer a justification for its use.

However, what intrigued Kahn in the tetrahedral ceiling system was only marginally its spanning ability. Kahn was captivated by the dominating visual order offered by the structure and the “dramatic play of light and shadow across the sculptural surface of reinforced concrete” [figure C].[6]  Furthermore, the tetrahedral ceiling system facilitated a better integration of the ventilation and lighting systems with the structural system, despite the fact that it created more difficulties and thus higher costs for maintenance. A more efficient method would have been to run the duct work above a dropped ceiling.  Moreover, the complicated steel formworks and additional reinforced steel need for the tetrahedral ceiling would increase the construction cost substantially [figure D].  Efficiency, however, was not what Kahn was aiming for.

What Kahn sought in the design of the Yale Art Gallery was beauty – beauty that comes from an integrated system. Just as in a set design, if one took away the lighting, or the music or the actors’ costumes, the script could no longer deliver its message and the play would fall apart. All components must work coherently for a successful performance. The tetrahedral ceiling system of the Yale Art Gallery was the binding element that tied everything together. It facilitated the dramatic play of light and shadow, enabled the integration of mechanical system, and imposed an aesthetic order that gave the space a defined character. While the tetrahedral ceiling system is misleading in its appearance as a space-frame, to Kahn’s mind, this “dishonesty” was acceptable because it not only animated the artworks, but also strengthened the emotional reading of the space [figure E].

The Yale Art Gallery was only the first place where Kahn utilized the illusionary appearance of structural members to achieve a more expressive impression of a building. In the Richards Medical Research Laboratories at the University of Pennsylvania, Kahn created a sharp contrast between the massive concrete towers and the seemingly floating floor plates [figure F]. While the concrete towers gave the impression of holding up the floor slabs, in fact, they were self-supporting hollow members. The floor plates were actually held up by a system of prefabricated beams and columns [figure G].

The plan of the Richards was organized in a pinwheel fashion.  One nine-story centralized tower containing mechanical systems, animal quarters, elevators and stairs was surrounded on three sides by identical laboratory buildings. These laboratory buildings were each eight-stories high and had their own stair and subsidiary service towers [figure H]. These non-structural hollow concrete towers, while lacking any penetrations on the outside, gave the false impression of being solid and bearing. To further accentuate their massiveness, vertical striations on the exterior surface were created intentionally to make the towers appeared taller than they were. Thus the seemingly load-bearing utilitarian towers dominated the reading of the project, while the actual laboratory space became the backdrop.

Reinforcing this contrast, Kahn depicted the typical laboratory floor slab as less heavy than it was by diminishing the depth of the spandrel beams as they stepped toward the corners [figure I]. This method, while truly reflecting the decreasing load resistance of the beams, also minimized the visual impact of the horizontal slab on the façade.  In addition to downplaying the weight of the floor, Kahn also succeeded in creating the illusion of lightness by flattening the façade. Kahn achieved this by pushing the infill brick wall and glass cladding to the perimeter of the building. By aligning them with the exterior edge of the structural members, Kahn reduced the amount of shadow line and hence composed a tightly stitched fabric that read as one continuous surface [figure J].

Another element that contributed to the graphic reading of the façade was the razor-thin window mullion. Conventional curtain wall mullions were made of rolled steel; however, they often had a stocky appearance. A more refined alternative would have been bent steel, but its manufacturing process often left the corners of these profiles rounded. To sharpen these corners, labor-intensive grinding was necessary and would have destroyed the fine finish Kahn desired for the delicate façade. As a result, Kahn developed a stainless steel mullion system that was folded out of thin-gauge steel sheets.  Since this folding procedure can achieve a very tight radius, it was able to generate precise and thin profiles that met Kahn’s demands for this building. Kahn’s close attention to such small details as the window mullion demonstrated his commitment to carry out his design intent in every aspect of the design.

At the Richards, this intent was to establish a clear and hierarchical order that spoke of solid and void, verticality and horizontality, and what Kahn called “servant” and “served” space. For Kahn, the structural, mechanical, plumbing and circulation systems were the crucial elements for the operation of a laboratory. They were the “servant” of the building, supplying the necessary framework, air, water and access. By encasing them in massive subsidiary towers and exposing them in the ceiling, Kahn gave primary visual importance to these “systems” both on the exterior and in the interior. On the other hand, the laboratory spaces were given a secondary role. They were portrayed as light, applied, and acting as the “served,” whose survival rested upon the support provided by the “servant.”

Despite these distinct characterizations on the exterior, the actual system division between the towers and the laboratories was less clearly defined.  In reality, the majority of the mechanical works actually occupied the central core tower rather than the subsidiary towers, and all the load-bearing structural members were situated in the laboratory space. Hence, the massive concrete towers were much more of a gestural move for creating a visual order than an accurate representation of its function.  Disagreeing with Reyner Banham’s criticism about giving too much of a monumental form to the subsidiary towers, Kahn believed the dramatic forms were necessary in sculpting an apparent personality to the space. Kahn was not interested in one-to-one representation of the building system on the exterior; he was striving to express the story behind the workings of a laboratory.

While the Richards was similar to the Yale Art Gallery in its use of structural illusion as an ordering tool for the space, what differentiated the two buildings was the increased sophistication in Kahn’s use of structure. Compared to the crude ceiling structure at the Yale Art Galley, the intricate and complex modular structural system at the Richards showed how Kahn was beginning to balance his intuitive feeling for design with rational thinking. Undoubtedly, the structural success of the Richards was largely due to the expertise of the structural engineer, August Komendant, and to the precision taken by the concrete contractors. Yet one could not deny Kahn’s credit for his ingenious conception of a kit-of-parts structural system that permitted full integration of mechanical ductworks while fulfilling the aesthetic quality of the space.  As Kahn continued to develop the rational side of his mind with his sensibility, he gained new insight into the possibility of structure and became even more skillful in its application.

This was demonstrated with great effect in his manipulation of materials and structural members in the Exeter Library. At Exeter, Kahn plotted a climactic procession of space. As one entered the building under the low and shaded brick arcade, one is then led up a curving marble staircase to an expansive space lighted warmly from above [figure K].  The openness and brightness of this main room creates a surprising and sharp contrast with the compressed and dark entryway.  As one enters the space, one’s eyes are inevitably drawn to the ceiling where the light creeps in mysteriously between a sixteen-foot-deep concrete “X” beam. 

Weighting eighty-seven ton, the “X” beam seems to be providing shear bracing for the building and supporting the weight of the roof.  In fact, it serves only minimally as a lateral support and is detached from the roof plane. Since most of the lateral loads of the building are resisted by the piers at the perimeter, the central “X” beam needed not to be as massive as it was. However, Kahn did not treat the “X” beam as purely a structural member. He utilized it as the climax for his carefully orchestrated entry sequence and as a sculptural element that left an imprint on people’s memory. Rather than leaving the “X” beam to its structural dimensions, Kahn exaggerated its depth intentionally to give it weight and presence in the large open space. This not only enhanced the dramatic quality of the main room, but also strengthened the perception of stability – anchoring the building to the ground and conveying a sense of safety and protection to the users.

Kahn also manipulated the depth of the “X” beam in order to make it an effective light reflector.  By increasing the depth of the beam, he provided sufficient surface area to bounce off the direct sunlight entering from the clerestory windows. This lighting strategy generated soft diffuse light that was not only gentle to the eyes of the patrons, but also allowed the sharp daylight to be distributed evenly throughout the main room [figure L]. Although perspective would deceive one into believing that the “X” beam was supporting the roof plane, a closer study of the light on the ceiling would reveal a gap between the beam and the ceiling.  The roof plane did not sit on the beam directly; it was actually held up by small square concrete stud columns attached to the top of the “X” beam [figure M]. Since these stud columns measured only about six inches by six inches, they were unnoticeable from the view on the main floor.  Kahn purposely pulled the roof plane away from the beam to allow light from all four clerestories to penetrate the entire width of the building.  Thus, the “X” beam not only functioned as a structural component, but had also, through Kahn’s intelligent manipulation, transformed into both an aesthetic element and a lighting device.

The “guile” with which Kahn manipulated structure played out not only in the interior of the Exeter Library, but could also be seen on the exterior. The façade of the building is composed of a series of tapered brick columns with widening jack arches between them.  While the upward narrowing of the columns seemed to “express” the decreasing loads as the building rose, these columns were in fact not supporting any weight other than their own. They were free-standing elements, linked to a concrete core. The weight of the floors was actually supported by a separate column system right behind the tapered exterior brick columns [figure N]. These load-bearing columns, by contrast, had the same dimension throughout their entire length.  It became obvious that Kahn was playing the same trick as he had at the Richards, creating a false image of the exterior columns as load bearing.

However, Kahn was more playful with his illusion at the Exeter than at the Richards. On one hand, he wanted to convey the rigidity of the façade by rendering the column as load bearing and using a solid material such as brick. On the other hand, he gave away the fact that the façade was only an applied element by revealing its true thickness at the chamfered corners of the building [figure O].  These moves seemed to contradict each other. Yet Kahn was fully aware of his intention and every detail in his building was chosen for a purpose. Kahn peeled away the corner of the building to break up the solid reading of the “brick box” and bring light into the circulation space behind the corners.  This design enriched the building façade with a level of complexity that compelled the patrons to try to understand the structure of the building. It facilitated an engagement between the patrons and the building that went far beyond the physical, but that penetrated deep into the mind and the heart.

Kahn’s sophistication in structural manipulation continued to show refinement in the Kimbell Art Museum. The greatest feature of the museum was the curved ceiling with its suspended light reflector in the middle [figure P]. This ceiling system created an effective lighting strategy that permitted diffused light to illuminate the space without damaging the artwork. Although the ceiling read like a vault, it did not carry the structural property of a true vault. At the beginning, Kahn perceived the curved roof at the Kimbell to have the shape of a semi-circle. However, as it did not provide the desired lighting effect, the curve was modified to take the form of a cycloid. This curve is the line that results from tracing a point on a moving wheel. Even though the cycloid shell resolved the lighting problem for the ceiling, it had an entirely different structural capacity than the cylindrical vault conceived in the earlier scheme. While the cylindrical vault could span across a large distance because loads were distributed across the entire surface area of the vault, the cycloid shell acted like a series of curved beams carrying loads along its edge. [figure Q]. As the architectural scholar Thomas Leslie has written, its structural capacity was “no better or worse than a singly curved arc of the same height.”[7]

Furthermore, the cycloid shell ceiling was pierced with a slit at its apex to bring light into the building. The fact that the shell was separated into two pieces defied the definition of a vault or a shell which described a continuous surface. Hence, the ceiling at Kimbell was far from being a collection of vaults from both a structural and architectural point of view. A correct description of it should really be a series of gullwing-shaped beams supported by marginal beams on top of the columns [figure R].

Despite the true structural property of the ceiling, Kahn tried in every attempt to portray it as a series of cylindrical vaults. At the two ends of the cycloid shell, a one foot deep curved beam was used to stiffen the shells and prevent them from flatting out because of the post-tension reinforcement embedded in the shell.  Wanting to distinguish the roof from the enclosing walls on the façade, Kahn inserted a tapered glass lunette between the curved beam and the concrete wall below [figure S]. This glass lunette, while reinforcing the misreading of the roof as a vault, its tapering shape also falsely represented the amount of load resistance performed by a true vault.

To strengthen this illusion, Kahn designed two open porches at the ends of the building. These porches did not have any functional purpose other than serving as a pedagogical tool to illustrate how the building was constructed. In describing the porches, Kahn said,

“Because of the open porches, how the building is made is completely clear before you go into it. It is the same realization behind Renaissance buildings, which gave the arcade to the street, though the buildings themselves did not need the arcade for their own purposes. So the porch sits there, made as the interior is made, without any obligation of painting on its walls, a realization of what is architecture. When you look at the building and porch, it is as an offering. You know it wasn’t programmed; it is something that emerged….You know what’s so wonderful about those porches? They’re so unnecessary.”[8]

 

Although the porches were meant to provide a truthful representation of the construction of the interior, they not only did not give hints about the assembly of the roof, they even strengthened the illusion of the ceiling as vault. By purposely over-sizing the gutter, Kahn wanted it to read as having the same dimensions as the interior marginal beams; thus, making the marginal beams seem a mere cap to the end of the vault rather than being the connecting element between the gullwing-shaped beams [figure T].

Kahn’s deliberate move to camouflage the real structural property of the ceiling at Kimbell was not deceitful. His priority was to maintain the autonomous reading of each gallery and create an ambient lighting quality that would serve the needs of the museum. Even though the perforated light reflector in the interior further exacerbated the reading of the ceiling as a vault by hiding the slit in the ceiling, its primary purpose was not meant to be a disguise; rather it is a brilliant lighting device that gives the Kimbell its unique illumination character [figure U].

Compared to the crude tetrahedral ceiling at theYale Art Gallery, the elegant light-reflecting ceiling at the Kimbell Art Museum demonstrated the maturity in Kahn’s fusion of sensibility with rational thinking. When designing the Yale Art Gallery, Kahn viewed feeling and thinking as separate. Feeling was the superior source of creative ideas, and thinking was “an academic process useful for the disciplining of his creative drive.”[9] However, as Kahn progressed in his career, he began to realize that both modes of design were equally necessary in the creative process. While feeling expressed the aspiration to create, thinking provided the means to create.[10] This balance between feeling and thinking enabled Kahn to produce buildings that were true to themselves and without reference to any style. 

In Kahn’s view, structure was one of the instruments in achieving this authenticity. Kahn did not consider structure as merely a load carrier. He treated it as a design element with many possibilities in broadening one’s creative instinct. It made up part of the stage set and needed to be integrated with other building components in order for the performance to be successful. This meant manipulating the size, the shape and the appearance of structural members to accommodate the mechanical, lighting and aesthetics concerns of the building. For Kahn, these modifications were not being “dishonest” to the integrity of the building; rather they made possible the achievement of “what the building wants to be.”  

[1] Alexandra Tyng, Beginnings: Louis I. Kahn’s Philosophy of Architecture (New York: John Wiley & Son, 1984), 27.
[2] Ibid., 27.
[3] Patricia Cummings Loud, The Museums of Louis I. Kahn (London: Duke University Press, 1989), 61.
[4] Robert McCarter, Louis I Kahn (New York: Phaidon Press, 2005), 70.
[5] David B. Brownlee and David G. DeLong, Louis I Kahn: In the Realm of Architecture (New York: Rizzoli, 1991), 418.
[6] Loud, The Museums of Louis I. Kahn, 70.
[7] Thomas Leslie, Louis I. Kahn: Building Art, Building Science (New York: George Braziller, 2005), 193.
[8] Nell E. Johnson, Light is the Theme (Fort Worth: Kimbell Art Foundation, 1975), 28.
[9] Alexandra Tyng, Beginnings: Louis I. Kahn’s Philosophy of Architecture, 27.
[10] Ibid., 28.