Introduction
Thin-shell concrete is a structural form using reinforced concrete poured (or placed) in thin slabs, and curved in a manner to increase its strength and allowing its use for both structure and cover. With a thin-shell concrete roof, separate support structure (trusses, rafters, etc.) and separate covering (plywood, roofing, etc.) are both replaced by the concrete shell. The thin-shell concrete roof may sit atop normal-appearing walls, or it may extend to ground level.
Folded-plate concrete is a structural form using reinforced concrete plates, joined rigidly at angles in a pleated manner to increase its strength and allow it to cover large spans. Folded plates may also be used to create a vertical wall surface.
The distinction between shells and plates can get blurry when plates joined in a V-shaped manner also curve as they span over a structure. In rare cases folded plates and thin shells may both be present on the same building! Some authors treat concrete folded plates as just another variety of thin-shell concrete, others treat them as separate but related forms; I prefer the former approach.
The use of concrete shells and plates may be strictly functional, to provide a fire-resistant, sturdy method of covering large areas of interior space (warehouses, factories, hangars, etc.). The use may also be a matter of aesthetics, to present an eye-catching appearance (churches, theaters, banks, etc.). Or it may be intended to do both at once.
The use of reinforced concrete thin shells and folded plates dates to the early part of the 20th century in Europe. Shells in the US gained traction in the 1930s and were popular in the 1940s through 1960s. Folded plates in the US caught on later, and were popular in the 1950s and 1960s. Both structural forms faded from common use by the 1970s.
By happy coincidence, these structural engineering methods overlapped with the mid-century modern architectural movement. Terms like "Googie" and "Populuxe" are often used to describe buildings constructed with concrete thin-shells and folded-plates in the 1950s and 1960s. In this architectural context the rising corner of a hyperbolic paraboloid may be described as "rising" or "soaring"; the repeating pattern of a folded-plate roofline may be described as "sawtooth" or "zig zag."
Shapes and Forms
Barrel vault. This is the simplest shape of shell since it has a single curvature. The shape of the curve may be circular, ellipsoidal, parabolic, or funicular (the shape of a piece of rope suspended at each end, also known as a catenary). To the naked eye it is difficult to tell a parabolic curve from a funicular curve, but to engineers the difference is notable. Barrel vault shells will normally have reinforcing ribs--curved beams that support much of the weight of the roof. These concrete ribs may be on the top side of the roof, giving a smooth interior surface, or on the underside of the roof, giving a smooth rooftop surface.
Dome. Engineers call this a surface of revolution. The cross section of a dome may show a circular, ellipsoidal, parabolic, or funicular curve. The dome may or may not have thickened supporting ribs.
Hyperbolic paraboloid. Engineers have complicated definitions of this term, but the simplest way to think of it is a shell surface that curves in two directions, like a saddle or a Pringles® crisp. One single hyperbolic paraboloid (commonly abbreviated as hypar or HP) may cover an entire roof. Frequently, four hypars are joined together creating an umbrella or inverted umbrella shape.
Conoid. Picture a rectangular surface in which three of the edges are flat on the same plane, with the fourth edge describing a curve. The result looks somewhat like an air scoop on the hood of a car. These were commonly used in Europe, particularly to provide north light into buildings, but were rarely built in the US.
This Website
This website will offer a listing of thin-shell and folded-plate references, descriptions of commonly encountered shapes and forms, an inventory of known examples, and eventually discussions of some of the key engineers, architects, and builders in the US. (Thin-shell exists in many other parts of the world, but practical considerations force me to limit the scope of my personal adventure.) If the information here gives you an awareness of the structural methods, an understanding of how shells and plates function, and the ability to recognize these structures in the field, then this web site was worth the effort. My research timeline begins with Anton Tedesko's first US thin-shell structures in the 1930s, and ends with Jack Christiansen's 1975 Seattle King Dome. Since then a second generation of thin-shell concrete has flourished in the form of Monolithic Domes, but I will leave those for others to document. I will limit myself to the first generation, or golden age, of thin shells in the US.
Right now, I have individual pages for more than 200 buildings. I plan to add many more in the next few years, starting with those I've already photographed.
Eventually I'll add an annotated bibliography including engineering textbooks, professional journal articles, and other sources useful for researching this topic. I started this website in early 2021, and it's a work in progress.
When reading about thin-shell and folded-plate concrete structures, you are as likely to encounter the engineer's name as the architect's. Practitioners found that close collaboration among the architect, engineer, and builder was more important with shells and plates than with more typical structures. I plan to add brief biographical sketches of some of these key contributors.
If you are an architect or structural engineer, architectural historian or cultural resources manager, scholar, student, mid-century modern enthusiast, or you just enjoy the idea or the look of thin-shell concrete, feel free to drop me a line. I'd like to learn about your interest in the subject. You can reach me at scott [at sign] thin-shell [dot] net. My research into this topic is fueled by a master's degree in history, work experience as an architectural historian, and a passion to understand, visit, document, and share examples of this fascinating structural form.
This website is a window into one man's hobby. I have no staff of web gurus or research assistants, and my only outside funding is an occasional small PayPal donation (never expected, but always appreciated). If you find an inaccurate statement or even a minor typo I would be pleased, and grateful, to hear from you. I am always glad to learn something new and to improve this site.
Updated March 2, 2024