Pioneering the Structural Terracotta Mullion Façade

Using Post-Tensioned Steel Tendons to Reinforce TerraCotta Extrusions



Architects today must explore alternative enclosure materials to meet evolving energy codes and embodied carbon regulations. Terra cotta has been mostly utilized as a non-structural masonry veneer or rainscreen but there have been precedents for using TC structurally, such as Elladio Dieste’s shells and Guastavino’s vaults. Louis Sullivan used terra cotta to animate the large facades of his tall buildings, and there is a resurgence of TC’s popularity trending now as well, visible in major new buildings such as 1 Vanderbilt, The Fitzroy, and Morris Adjmi’s 7 West 21st St.

A team including architects and an engineer from HOK, terra cotta company Boston Valley, the structural fabricator TriPyramid and façade manufacturer Gartner collaborated in exploring the structural capabilities of terra cotta by putting this brittle material into compression. Compression was achieved with post-tensioned stainless steel tendons running through the interior cavities of extruded terra cotta segments to connect them into a rigid beam. Preliminary destructive testing of the “terra cotta beam” by TriPyramid engineers has proven the concept successful enough to develop further into a lobby wall system. The team has fabricated and erected a one-story mockup of the system. Further developments could include more efficient external tendons, or the TC frames as an interlocking, unitized curtain wall system. By replacing aluminum in glazing systems, the Terra Cotta Mullion system has potential advantages in operational energy savings by decreased heat transfer, ease of recyclability, less embodied carbon, in addition to Terra Cotta’s sculptural aesthetic qualities which have engaged architects for millennia.


Photo of Victoria Ereskina

Victoria Ereskina

Façade Design Professional


Photo of John Neary

John Neary

Sr. Façade Specialist



Background – Architectural Terra Cotta

Terra Cotta has been used for centuries on the exterior of buildings primarily as a veneer over structural backup. More recently, it has gained popularity as a rainscreen finish material. Before delving into the historical background, the material properties will be discussed.

Material Qualities

Terra cotta, or “baked earth,” is a mixture of clay and water, sometimes with varying levels of flux and recycled post-fired terra cotta, that is formed and then fired to solidify the material. Its pliable nature before firing makes it an ideal candidate for casting, carving, and even extruding, or any combination thereof. It may continue to be machined in its solidified state post-firing.

Glaze and pigments may be applied before firing to create the most reliable bond between the coat and the ceramic, however finishes may be applied post-firing as well. Terra cotta products may be found in any color, gradient of colors, glazed or un-glazed, sometimes with pooling of pigments at valleys, and with various texture finishes such as crackling. The Fitzroy building in NYC, for instance, is clad with green glazed extruded planks and formed panels.

Terra cotta may a have higher embodied carbon per unit mass than concrete—which does not need firing but requires the chemical energy to be pre-stored in cement for bondmaking. Yet, the architectural applications of terra cotta compared to concrete are innumerably greater due to the various possible colors, glazes, and textures, the durability of fine details in ornamental pieces, and increased resistance to erosion. While concrete has a tendency to absorb pollutants and stain from water runoff, terra cotta has a significantly lower water absorption, especially when glazed, and therefore has an increased ability to resist staining and superficial weathering.

As framing for glazing, terra cotta is compared to the base case: aluminum frame for commercial applications. Aluminum is one of the most high-embodied carbon building materials. There aren’t any examples of concrete glass framing members the team is aware of to reference, but later in this paper there is a study comparing theoretical values of embodied CO2 in mullions of various materials.

Ceramics have a significantly better thermal conduction resistance than aluminum, making terra cotta a good candidate for having better u-factor performance than conventional mullions and lower condensation risk, thereby eliminating some energy costs and perimeter heating.

Terra cotta is regarded as a fireproof building material, which contributed to its popularity in urban architecture in the late 19th century and early 20th century, especially to clad tall cast-iron-framed buildings. Louis Sullivan, who is regarded as the Father of the Modern Skyscraper, is known for using terra cotta for richly ornamented cladding, as seen in the Guaranty Building (1896).

As most brittle materials, terra cotta is strong in compression and weak in tension. Using terra cotta as a beam would require reinforcement. A structural terra cotta mullion must behave like a beam to resist loads applied perpendicular to the frame; the means of keeping it in compression are the same principle as with port-tensioned concrete, but everything else—from the architectural possibilities to the fabrication—is unchartered territory. This is the focus of the Structural Terra Cotta Mullion Project discussed in this paper—historical background, concept and testing, full-scale mockup, analysis, and future versions to explore and one day implement on a building.

Architectural History and Contemporary Use

Terra Cotta is most familiar in contemporary facades as rainscreen cladding—plank and baguette, some cast shapes—which are essentially like stone veneer or brick except the joints in a rain screen are not closed off with mortar and each piece is supported independently on clips or brackets. (1, 2). There is growing evidence of interest in the sculptural qualities, including color, of Terra Cotta as an alternative to the ubiquitous glass grid, as illustrated by a handful of recent facades such as SOM’s 28 & 7 (3) – which the team was not aware of when starting the structural terra cotta mullion project.

1.; 2.;3.;
1.; 2.;3.; {1} Common TC Rainscreen cladding.; {2} Terra Cotta Rain Screen, sk by John Neary; {3} 28&7 SOM, sk by John Neary

Prior to the advent of the metal and glass curtain wall, which produced the lightweight rainscreen wall assembly with various kinds of non-load-bearing backup, Terra Cotta was employed in early modern architecture as a surrogate for stone cladding. Molded into shapes mimicking the carved tracery and mouldings of the neo-classical and neo-gothic styles, terra cotta afforded a less expensive and lighter-weight surface material than carved stone with glazes and surface finishing techniques that could hardly be distinguished from the real thing. This terra cotta cladding was mortared into place like masonry over backup of un-glazed terra cotta block or on iron framing, attached with wire ties and metal clips which were susceptible to corrosion that caused many terra cotta facades from the early 20th century to require extensive repair after about 50 years, similar to the brick facades of the same period in New York and other cities. (4, 5)

Early modern examples of faux-stone terra cotta include the 1910 Woolworth Building (6,7) and Louis Sullivan’s famous facades such as the Guaranty Building in Buffalo (8). On Sullivan’s buildings the plastic essence of clay became a medium for the animating force of imagination, represented by organic patterns based on the dynamic growth of plants and the structural expression of Classical and Gothic architecture. It was an abstract expression of structure.

With its plastic ability to achieve intricate molding and carving, as in Sullivan’s ornament, Terra Cotta has been used to create surfaces of a richness never matched except in carved stone or cast bronze.

4.; 5.; 6.;
{4} TC clad Mullions simulating stone;[4] {5} This type of cladding was wired and clamped and pinned to structural backup steel. It created the illusion of structural surface. Sketch by J.N.; {6} Woolworth Building Terra Cotta [6]

The emulation of stone in terra cotta created some aesthetically powerful architecture which can have a sculptural effect, and a richness, rarely achieved by the flat panels of recent curtain wall.

Incorporating textured or profiled tiles into the spandrel of the aluminum and glass unitized curtain wall is a strategy that affords some measure of the material substance missing in the average metal and glass grid of panels. This was done, for instance, in 1 Vanderbilt: a unitized curtain wall system which decorated the spandrel zones of the units with extruded trimmed terra cotta planks. (9).

But the aluminum mullion curtain wall is a system of enclosure that is coming under scrutiny as energy codes and other regulations aimed at reducing the operational and embodied carbon produced by buildings become more demanding. The panelization of the curtain wall creates a network of joints which typically interrupt any insulation included in the opaque portion of the façade, resulting in an effective U value higher that current prescriptive code requirements. New approaches to detail of unitized curtain wall are addressing these issues, but the fundamental reality of aluminum’s high conductivity and high embodied energy make it problematic in this context, despite there being many practical reasons that still make it the default material for framing glazing in exterior walls: aluminum’s strength-to-weight ratio, ease of extrusion affording incorporation of reglets for receiving gaskets and fasteners, and resistance to corrosion.

7.; 8.; 9.;
{7} Woolworth Building, Representations of Stone architecture. Sk by JN.; {8} Guaranty Building, Buffalo, Sk by JN; {9} 1 Vanderbilt, KPF [9]

Conception of the Structural Terra Cotta Mullion

Employing Terra Cotta as the body of a mullion framing system, rather than as veneer cladding, could offer advantages relative to aluminum in terms of both conductivity and embodied energy

Access Restricted

Members get unlimited access to all of our resources. Join now for the best value.

Development of the Terra Cotta Mullion

Hypothetical Structural Concept

We investigated a pre-stressed straight mullion essentially replacing the conventional aluminum box beam (23). The first developments began as the attempt to take the stone mullion profile

Access Restricted

Members get unlimited access to all of our resources. Join now for the best value.

Physical Realization: The Full-Sized Visual Mockup

With curtain wall fabrication engineers on board, the mock-up would require a few modifications to the initial assumptions. They had secured three IGU’s to be used in the mockup, such

Access Restricted

Members get unlimited access to all of our resources. Join now for the best value.


Thermal Performance

With the structural mullion developed sufficiently for preliminary shop drawings, it’s possible to compare the thermal performance of the TCM to a base case monumental lobby mullion, made

Access Restricted

Members get unlimited access to all of our resources. Join now for the best value.

Conclusions & Future Works

The Structural Terra Cotta mullion was constructed and has demonstrated thermal advantages and embodied carbon advantages over the conventional metal mullions, especially aluminum.

This concept can be applied to a

Access Restricted

Members get unlimited access to all of our resources. Join now for the best value.


The team includes members of HOK Facades, Structural Engineering and Architecture; TriPyramid Structures designers and mechanics; and Gartner-Permasteelisa managers, engineers and fabricators all working with the Boston Valley team. Credited Photography and Video were created by Patrick and Marzena Bernard.


Facades: John Neary; Victoria Ereskina; Blake Kurasek

Structural Engineering: Francesca Meola

Architecture: John MacCallum; Marie Achalabun; Francisco Moreno; Ritika Kapoor; Sha Li

Josef Gartner - Permasteelisa Team

Roberto Bicchiarelli; Bernhard Rudolf

TriPyramid Team

Michael Mulhern; Nate White; Jeff Anderson; Matt Bull; Dylan Schwallie

Boston Valley Team

John Krouse; Andrew Pries; Andy Brayman; Mitch Bring (Architectural Ceramic Assemblies Workshop)

Rights and Permissions

References, Rights, & Permissions included in Image Captions in Body of Work; else, here:

[1] Accessed 2020-11-01

[4] Accessed 2020-11-01.

[6] Accessed 2020-11-001

[9] Photo Credit Marani, Matthew. Feb 19, 2020. Accessed 2021-11-27

[14] Accessed 2020-11-01

[15] Accessed 2020-11-10.

[17] Accessed 2020-11-10.

[18] Accessed 2020-11-01

[19] Nuevas posibilidades acerca del dise ˜no conceptual de estructuras antifuniculares eficientes Leonardo Todisco ∗ , Hugo Corres Technical University of Madrid, School of Civil Engineering, Lab. of Structural Engineering, Prof. Aranguren sn, 28040 Madrid, Spain Received 6 April 2017; accepted 18 April 2017 Available online 17 June 2017 Accessed 2020-11-01

[21] Accessed 2020-11-01

[75] Accessed 2020-11-01

[76] Accessed 2020-11-01

* All construction photos courtesy of Patrick Bernard Photo Studios,