TAMBUR. – A wall structure that connects the pendentives to the dome, generally cylindrical or prismatic in shape, and in the latter case usually with an octagonal plan.
It is absent in the early Byzantine churches, where the dome is vaulted directly onto the pendentives (e.g., Hagia Sophia, SS. Sergius and Bacchus in Constantinople), and in Romanesque baptisteries with a central plan, given their tiburio-like conformation: the base structure—mostly octagonal—continues externally for the full height of the dome, concealing the extrados, to form the support for a conical roof covering. Similarly, in the basilica of St Mark in Venice and that of St Anthony in Padua, the tambur is merely apparent, without any structural function, since the external dome serves only as protection, while the true masonry dome is set directly on the pendentives.
An early attempt, clearly inspired by classical models, is seen in the cathedral of Pisa, where, however, the transition from the rectangular base zone to the oval dome is achieved by means of hood pendentives rather than trumpet-shaped ones; later, in the cathedral of Siena, a dodecagonal structure, analogous to the tambur, is inserted between the hexagonal plan of the crossing and the circular dome, but the incongruous irregularities result in structural disorganization and a lack of compositional unity. Only in Santa Maria del Fiore is a unified arrangement realized, beginning the octagonal plan from the load-bearing base structure itself, thus eliminating the connecting pendentives and simplifying all constructional problems. The basilica of Loreto and the cathedral of Pavia follow the same structural scheme. In other 15th-century churches of smaller scale, a further advance is made by setting the tambur—still with an octagonal plan—above a square base by means of simplified connecting pendentives, given the junction with a flat wall (e.g., Santa Maria del Popolo in Rome, Santa Maria delle Grazie or del Calcinaio in Cortona, the cathedral of Turin, the Colleoni Chapel in Bergamo). Externally, these tamburs present smooth plastered walls or brick facings, in which rectangular or arched windows are opened; only at the beginning of the following century do architectural orders appear almost timidly in Santa Maria di Loreto in Rome.
But the fundamental innovation is introduced by Bramante, first in the Tempietto of San Pietro in Montorio, abandoning polygonal forms—a survival of Gothic tradition—and setting the dome above a cylindrical body; and then, developing this classical conception, in the design for the Vatican Basilica, where the architectural order becomes the dominant element in the entablature loggia, transforming the tambur from solid to perforated. Bramante’s conception, admirable for its clarity of proportions, has been justly defined as a “magnificent illogicality” with regard to constructional and static considerations.
The model for subsequent centuries, however, is found in the form given to the tambur by Michelangelo, in which static requirements are brilliantly balanced with compositional unity, emphasizing the load-bearing structure—pilasters and fully projecting columns—thus concentrating the thrusts and conveying a sense of articulation and an expression of plastic power. The Michelangelesque model is followed more or less faithfully by architects of the next two centuries, either repeating the colonnade motif (e.g., Sant’Andrea della Valle and San Carlo al Corso in Rome, the basilica of Superga, the new cathedral of Brescia, the cathedral of Catania) or adopting pilasters in place of columns (e.g., Santa Maria di Provenzano in Siena, San Carlo ai Catinari in Rome, the cathedrals of Palermo and Como), while retaining the broken entablature that imparts a sense of movement and a lively pictorial effect. An exception is the tambur of Sant’Andrea delle Fratte in Rome, whose conformation, no longer cylindrical but on a mixtilinear plan, manifests Borromini’s dynamism.
The stability of domes of considerable size, especially those with a depressed arch, sometimes determines interesting constructional arrangements that also contribute aesthetically to tightening the connection between the different parts of the architectural organism: thus in Santa Maria della Salute in Venice the tambur is surrounded by the characteristic large volutes (the “ears”) that transmit the thrusts to the perimeter walls, while in the Sanctuary of Vicoforte these are absorbed by enormous tower-like buttresses placed in correspondence with the ribs of the dome.
The 19th century coldly continues the preceding schemes, in the form of a loggia (e.g., San Carlo in Milan) or in the form, more consonant with neoclassical taste, of a completely closed cylindrical surface (e.g., San Francesco di Paola in Naples). In the second half of the century, historicist tendencies sometimes lead to the imitation of Renaissance forms (e.g., the cathedral of Cerignola, Santa Maria Ausiliatrice in Turin); a wholly new conception, however, is pursued with bold ingenuity by Antonelli in San Gaudenzio in Novara, subdividing the tambur into a series of superimposed loggias, the upper ones concealing the springing of the dome. – See pl. CXXI.