Compound arch

A compound arch is an arch built using multiple independent sub-arches stacked vertically, with their arcs of voussoirs placed one on the top of the other. The goal of using a compound arch is usually to increase the overall strength and reliability. Each of these sub-arches, or "rings", of which the whole compound arch is composed, is called an arch order. In some compound orders their faces are in the same plane. But as a rule the orders are successively recessed, i.e. the innermost sub-arch, or order, is narrow, the next above it broader, the next is broader still, and so on.

History

This system of concentric arches was employed by the Romans early in the 6th century BC, in the Cloaca Maxima at Rome; three orders were used where the Cloaca enters the Tiber. In the Anglo-Saxon architecture the use dates at least to the late 8th century. The older
compound arches were built from semicircular rings, but later the pointed arches were also compounded, for example, to form the west door portal of the Assumption of Blessed Mary and St Nicholas church in Etchingham. The early French example is the church of Saint-Philbert-de-Grand-Lieu.

Recessed orders

The recessed orders were also known in Ancient Rome. They can be found in Hagia Sophia, Basilica of Sant'Apollinare in Classe. Romanesque and Gothic architects clearly preferred the recessed orders; this can be partially explained by the desire to save on expensive ashlar (the core of thick walls and of the widest outer order could then be filled with cheap rubble.