Intrados

Rules that use the word

An arch is named by its curve. Classify arches by the geometry of the intrados: the number and position of the centres (one for round and segmental, two for pointed, three or four for basket and Tudor arches), or a single continuous curve (ellipse, parabola, catenary), and distinguish true arches, with voussoirs, from corbelled “arches” of oversailing courses.

The Derand–Blondel rule for abutments. To find how thick the pier of an arch must be, divide the intrados into three equal parts; draw a line from the springing through the first division, and produce it beyond the springing by its own length. A vertical through the end of that line marks the outer face of the pier.

Every arch is first built on a frame. An arch cannot support itself until it is complete. Build it on a timber centring shaped to the intrados, strike the centring only when the keystone is set and the mortar has taken, and reuse it from bay to bay, which is one reason to keep the spans of an arcade equal.

Where it appears in the book

In the chapters The Arch.

Nearby in the glossary

Insula · Intercolumniation · Interior street · International Style · Ionic · Irimoya · Isodomum · Iwan