A semi-dome, also called a “half-dome”, is the term in architecture for half a dome (“cut” vertically), used to cover a semi-circular area. Similar structures occur in nature.

Architecture
Semi-domes are a common feature of apses in Ancient Roman and traditional church architecture, and mosques and iwans in Islamic architecture.

A semi-dome, or the whole apse, may also be called a conch after the scallop shell often carved as decoration of the semi-dome (all shells were conches in Ancient Greek), though this is usually used for subsidiary semi-domes, rather than the one over the main apse. Small semi-domes have been often decorated in a shell shape from ancient times, as in Piero della Francesca’s Throned Madonna with saints and Federigo da Montefeltro, and the example in the gallery below. Islamic examples may use muqarnas decorative corbelling, while in Late Antique, Byzantine and medieval church architecture the semi-dome is the classic location for a focal mosaic, or later fresco.

Found in many Ancient Greek exedras, the semi-dome became a common feature of the apse at the end of Ancient Roman secular basilicas, which was adopted in Early Christian architecture as the commonest shape for churches, becoming the focal point for decoration. In buildings like Hagia Sophia in Byzantine architecture, apsidal openings or exhedras from the central nave appear in several directions, not just to the litugical east. The tetraconch, triconch and cross-in-square are other typically Eastern Christian church plans that produce several semi-domes.

When the Byzantine styles were adapted in Ottoman architecture, which was even less concerned with maintaining a central axis, a multiplicity of domes and semi-domes becomes the dominating feature of both the internal space and the external appearance of the building. The buildings of Mimar Sinan and his pupil Sedefkar Mehmed Agha are the masterpieces of this style. Mihrabs are another common location for semi-domes.

In Western Europe the external appearance of a semi-dome is less often exploited than in Byzantine and Ottoman architecture, and is often disguised as a sloping rather than curved semi-circular roof.

The Cincinnati Museum Center at Union Terminal in Cincinnati, OH features the largest semi-dome in the western hemisphere, measuring 180 feet wide and 106 feet high.

Statics
While elongated barrel , rib or cross vaults develop an enormous vaulting that could even lead to their collapse, dome buckling statically rather unproblematic – only in rare cases, cracks.

Design
Many apse beds have been left without decoration, but already in late antiquity the apse beds were decorated with figurative mosaics , later also with frescoes (mostly with Pantocrator – or Majestas Domini representations), especially in important buildings . In rarer cases, they received a supportive structure by unprofiled ribs , later by profiled vault ribs.

Symbolism
While in the lower part of an apse usually figures of saints have been depicted, the apse salve is always provided with images of stars or figures of the heavenly world (mainly Christ as Pantokrator surrounded by the symbolic representations of the 4 evangelists ).

Non-Christian world
Even in parts of the Jewish and Islamic world – even outside of Ottoman architecture – comparable forms, often shell-like ribbed or provided with Muqarnas -Decor etc., but always without figurative decoration (→ iconophobia or image prohibition in Islam ), as covering the Torah or to find mihrab niches.

Source From Wikipedia