About the Pendentive in Architecture and Engineering

A pendentive is a triangular piece beneath a dome that allows the dome to rise high above the floor. Usually ornamented and four to a dome, pendentives make the dome appear as if its hanging in the air, like a pendent. The word is from the Latin pendens meaning hanging.   Pendentives are used for stabilizing a round dome on a square frame, resulting in enormous interior open space beneath the dome. The Dictionary of Architecture and Construction defines a pendentive as One of a set of curved wall surfaces which form a transition between a dome (or its drum) and the supporting masonry. Architectural historian G. E. Kidder Smith has defined the pendentive as A triangular spheroid section used to effect the transition from a square or polygonal base to a dome above. How did early structural engineers design round domes to be supported over square buildings? Beginning in about A.D. 500, builders began using pendentives to create additional height and carry the weight of domes in early Christian architecture of the Byzantine era. Dont worry if you just cant visualize this engineering. It took civilization hundreds of years to figure out the geometry and physics. Pendentives are significant in the history of architecture because they defined a new engineering technique that allowed interior domes to rise to new heights. Pendentives alsocreated a geometrically interesting interior space to be ornamented. Four pendentive areas could tell a visual story. More than anything, however, pendentives tell the real story of architecture. Architecture is about solving problems. For early Christians the problem was how to create soaring interiors that expresses mans adoration of God. Architecture also evolves over time. We say that architects build on each others discoveries, which makes the art and craft an iterative process. Many, many domes fell down into a crumble of ruin before the mathematics of geometry solved the problem. Pendentives allowed domes to soar and gave artists another canvas — the triangular pendentive became a defined, framed space. The Geometry of Pendentives Although Romans experimented with pendentives early on, the structural use of pendentives was an Eastern idea for Western architecture. It was not until the Byzantine period and under the Eastern Empire that the enormous structural possibilities of the pendentive were appreciated, writes Professor Talbot Hamlin, FAIA. To support a dome over the corners of a square room, builders realized that the diameter of the dome had to equal the diagonal of the room and not its width. Professor Hamlin explains: To understand the form of a pendentive, it is only necessary to place half an orange with its flat side down on a plate and cut equal portions vertically off the sides. What is left of the original hemisphere is called a pendentive dome. Each vertical cut will be in the shape of a semicircle. Sometimes these semicircles were built as independent arches to support the upper spherical surface of the dome. If the top of the orange is cut off horizontally at the height of the top of these semicircles, the traingular pieces still left will be exactly the shape of pendentives. This new circle can be made the base for a new complete dome, or a vertical cylinder can be built upon it to support another dome higher up. — Talbot Hamlin Summary: The Pendentive Look Sixth Century, Hagia Sophia in Istanbul, Turkey, Salvator Barki/Moment/Getty Images 18th Century, Paris Pantheon, Chesnot/Getty Images 18th Century, St. Pauls Cathedral Dome, London, Peter Adams/Getty Images 18th Century, Mission Church in Concà ¡, Arroyo Seco, Querà ©taro, Mexico, AlejandroLinaresGarcia via Wikimedia Commons, CC-BY-SA-3.0-2.5-2.0-1.0 Sources Source Book of American Architecture, G. E. Kidder Smith, Princeton Architectural Press, 1996, p. 646Dictionary of Architecture and Construction, Cyril M. Harris, ed., McGraw- Hill, 1975, p. 355Architecture through the Ages by Talbot Hamlin, Putnam, Revised 1953, pp. 229-230