Curved structures

Curved Structures are constructions generated by one or more generatrices (which can be either curves or surfaces) through geometrical operations. They traditionally differentiate from the other most diffused construction technology, namely the post and lintel, which results from the addition of regular and linear architectural elements. The most common and diffused examples of curved structures are the Arch, the Vault, and the Dome.

They have been exploited for their advantageous characteristics since the first civilisations and for different purposes. The materials, the shapes and the assemblage techniques followed the technological and cultural evolution of the societies over time. Curved structures have been preferred to cover large spaces of public buildings. In spite of their sensitivity to earthquakes, they work well from the structural point of view. By the way, in the next sections, a broad view over some of the curved structures characteristics is given.

The Geometry of curved structures

Main article: Descriptive geometry

See also: Algebra of sets

See also: Paraboloid

From the geometrical point of view, curved structures are three-dimensional solids. They are generated starting from genetratrices which undergo the geometrical operations of extrusion or revolution. The three classes of structures stated previously can be explained as follows:

• An arch is generated by the revolution of a point or a surface around a centre (or, from a mechanical standpoint it can be thought as a section of a vault);
• A Vault is generated by the extrusion of an arched surface;
• A Dome is generated by the revolution of an arched surface around an axis.

More complex shapes can be generated by boolean operations on a set of interacting volumes. The simplest examples, resulting from the intersection of two or more vaults and the successive subtraction of the excess volumes, are:

• The Groin vault or Cross Vault, resulting in a set of lunettes (either of circular or pointed vaults);
• The Domical Vault - and the particular case of the Cloister - which is formed by a set of fuses;
• The Umbrella Vault, a set of ribbed fuses, joint at the top and terminating in lunettes at the base;
• The Pendentive or Penditive dome, generated by subtracting volumes from a dome;
• The Saddle Vault, generated by either translating one parabola through a second one or by a ruled surface.

The actions performed to make these solids are the same needed to generate them in a CAD or - to some extent - in a FEM software environment to design objects and analyse them if need be.

Gaussian curvature and shape-resistant structures

See also: Curvature

Example of surfaces to which all the possible surfaces can be reduced to. Starting from the foreground: the Hyperboloid (negative), the Cylinder (null), and the Sphere (positive Gaussian curvature).

Differently from the post and lintel construction, which capacity depends upon the resistance of the single members, curved structures can rely on their shape too. Indeed, in a curved structure subject to a given loading condition, the axial stress flows towards the springers following a definite path, which depends on the boundary conditions, namely the constraints and the loads.

However, single curvature structures (that is, simple vaults) show less capacity than double curvature ones (e.g., domes, domical and cloister and saddle). This is because a simple vault - from a geometric point of view - corresponds to a developable surface, which has null Gaussian curvature, therefore it can be flattened to a planar surface with no distortion. Dome-like and saddle structures have respectively a positive and a negative Gaussian curvature, being shape-resistant structures par excellence.

Architecture and engineering

See also: Ancient Roman architecture

All the typologies of arches, vaults and domes come from the operations stated in the previous section. They are comprehensively collected and explained in each correspondant Wikipedia article. Curved shapes were used in the past for covering large rooms in buildings, as happened for example in the Domus Aurea of Emperor Nero, the Basilica of Maxentius, the Pantheon, Rome, or Hagia Sophia. However, they could be used for infrastructures too. For instance, the Ancient Roman civilization exploited curved structures for bridges, aqueducts, sewage ducts, and arch-dam. The main materials of such constructions were Masonry and Roman concrete.

With the Industrial Revolution, the material chosen were more likely wrought, cast iron or, later, reinforced concrete. In this way, also the shape of the infrastructures started to change. Some example of curved structures were the Palm House, Kew Gardens by Turner and Burton and theThe Crystal Palace by Paxton, or on the infrastructures side, the Garabit viaduct. Later in 20^th century, Pier Luigi Nervi started studying the possibilities of reinforced concrete, building his famous ribbed hangars.

Many other structures have been built by exploiting curved surface, the Philips Pavilion in Brussels by Le Corbusier, L'Oceanogràfic in Valencia by Félix Candela and Alberto Domingo.

Construction process

Main article: Opus spicatum

Because of their nature, curved structures cannot stand alone until their completion, especially vaults and arches. Therefore, the construction of a supporting structure (referred to as centring) is almost always necessary. These are temporary falsework which stay in place until the keystone has been set down and the arch is stabilised. However, there are a few cases in which, by some expedient and careful design of the construction process, some structures have been erected without any centring.

A widely known example is the domical vault of the Florence Cathedral, built by Filippo Brunelleschi in the 15^th century. He achieved such a challenge by building a massive structure, mechanically behaving like a spherical dome, but with large ribs and exploiting the masonry herringbone bond to lean and fix every new layer on the previous one. Each layer of the structure seems to be composed by many small arches. The vault is also double-skin, with an intermediate hollow space hosting the staircases, through which air can flow to avoid humidity concentration. To resist parallel tensile stresses which may separate the fuses of the vault, Brunelleschi arranged sandstone chain along some parallel plane.

Another example of structure built with no formwork is the Global Vipassana Pagoda, located in the North of Mumbai, between the Gorai Creek and the Arabian Sea. It is a meditation hall covered by the largest masonry dome in the world, with an inner diameter at ground level of about 85m. The absence of centring was possible thanks to the double curvature of the dome and the special shape given to the carved sandstone blocks constituting the skin.

Structural behaviour

statics eartquake sensibility

thrust line

limit analysis theo

FEM & DEM

Daylighting

See also: Gothic architecture

Daylighting is usually guaranteed by openings at the end of vaulted bays, as happens in Gloucester Cathedral, Chartres Cathedral, or Sainte-Chapelle (Paris), and specifically in the lunettes (where the vaults end against a wall) like in the Church of Santa Maria del Suffragio in L'Aquila (Italy) and in the Church of San Paolo in Albano Laziale (Italy).

Another structurally relevant place for an opening is the top of the domes, where in many cases an oculus can be found. Sometimes it is bare, as in the Roman Pantheon, while often is covered by another architectural element referred to as Lantern, as happens - for instance - in the Florence Cathedral.

Acoustics

Main article: Room acoustics

See also: whispering-gallery wave

Some double curvature structures are known for the echo or the reverberation phenomena they create. These are due to the size of the spaces and the materials exploited for the structure or the finishing (usually hard and with small pores). The shape does a lot in preventing or enhancing the effect. Cross or cloister vaults do not generate an echo. Pointed domes easily create reverberation more than echo. At the same time, spherical surfaces are highly reflective due to their concavity . Indeed, hemispheres, paraboloid, or similar surfaces are eager to reflect and redirect sound, sometimes constituting a whispering gallery. Examples of whispering galleries can be found in well-known architectures like St Paul's Cathedral in London, where the phenomenon has been studied by Lord Rauyleigh or the Archbasilica of Saint John Lateran in Rome, but also in caves like the Ear of Dionysius in Syracuse, Sicily, which has been treated by Wallace Clement Sabine.

The existing variety of domes is due to the assignation of symbolic meanings related to history and cultures, ranging from funerary to palatine and religious architecture, but also the response to practical problems. A recent study addressed how in the Baroque staircase of the Royal Palace of Caserta (Italy), designed by Luigi Vanvitelli, the double dome could make the listeners feel as if they were enveloped by the music. Thus enhancing the marvel typically researched by baroque architects.

A modern example of architecture thought to respond and participate to sound was the Philips Pavilion designed by Le Corbusier and Iannis Xenakis for the Expo 58 in Brussels.

Further readings

• Allen, Edward (2017). Come funzionano gli edifici [How Buildings Work. The Natural Order of Architecture] (in Italian). Translated by Portoghese, Pasquale; Guarini, Mirta A. M. (2nd ed.). Bari: Edizioni Dedalo. ISBN 978-8822008268.
• Pulvirenti, Emanuela. "Meraviglie dell'architettura: dieci modelli di arco".
• Pulvirenti, Emanuela. "Quando il soffitto è un capolavoro: dieci modelli di volta".
• Nervi, Pier Luigi (1945). Scienza o arte del costruire? Caratteristiche e possibilità del cemento armato (in Italian). CittàStudi. ISBN 978-8825173864.
• Salvadori, Mario (1993). Perché gli edifici stanno in piedi [Why Buildings Stand Up] (in Italian). Translated by Brandolini, Sebastiano; Pace, Carlo (2nd ed.). Milano: Bompiani. ISBN 978-8845215131.
• Levy, Matthys; Salvadori, Mario (1994). Why Buildings Fall Down: How Structures Fail. Milano: W W Norton & Co Inc. ISBN 978-0393311525.{{cite book}}: CS1 maint: date and year (link)
• King, Ross (2000). Brunelleschi's Dome: How a Renaissance Genius Reinvented Architecture. London: Chatto & Windus. ISBN 9781620401941.
• Heyman, Jacques (1997). The Stone Skeleton: Structural Engineering of Masonry Architecture. Cambridge University Press. ISBN 978-0521629638.{{cite book}}: CS1 maint: date and year (link)

External Links

• Geometria descrittiva (in Italian)
• Global Vipassana Pagoda Official Website
• Auroville Earth Institute. UNESCO chair earthen architecture