Arches & Openings
Arch types and terminology, keystone placement, centering and formwork for arch construction, and structural principles of masonry arches.
- Define an arch in masonry and its structural function
- Identify the keystone and its location
- Explain the purpose of centering or formwork during arch construction
- Describe common arch types and their applications
Lesson 1
Arch Types, Keystones & Centering
What Is an Arch?
An arch in masonry is a curved structure spanning an opening. Unlike a lintel, which is a straight horizontal member, an arch uses a curved arrangement of masonry units to transfer loads around an opening and down into the supporting walls (abutments) on each side. Arches work primarily in compression, making them ideally suited to masonry's natural strength.
Arch Terminology
| Term | Definition |
|---|---|
| Keystone | The central brick at the crown of the arch |
| Springer | The first brick of the arch at each abutment |
| Voussoir | Any of the wedge-shaped units forming the arch |
| Crown | The highest point of the arch curve |
| Intrados | The inner (concave) curve of the arch |
| Extrados | The outer (convex) curve of the arch |
| Abutment | The wall or pier supporting the arch ends |
| Span | The horizontal distance between abutments |
| Rise | The vertical distance from spring line to crown |
The Keystone
The keystone of an arch is the central brick at the crown. It is the last unit placed during construction and locks all the other voussoirs into position. Once the keystone is set, the arch becomes self-supporting and can carry load. The keystone is often slightly larger or more ornate than the other voussoirs for visual emphasis.
Exam Tip
The keystone is always at the crown (top center) of the arch. It is the last brick placed and the one that locks the arch together. The arch cannot support itself until the keystone is installed.
Common Arch Types
Semicircular Arch
Rise equals half the span
True half-circle shape
Strongest arch form
Classic Roman architecture
Segmental Arch
Rise is less than half the span
Shallow curve - most common in modern work
Less height needed above opening
Higher lateral thrust than semicircular
Jack (Flat) Arch
Appears flat but has slight camber
Units are wedge-shaped
Decorative over windows and doors
Requires steel reinforcement for structural loads
- Gothic (pointed) arch - two curves meeting at a point at the crown
- Horseshoe arch - curve extends beyond the semicircle
- Tudor arch - flattened pointed arch
An arch is a curved structure spanning an opening that works in compression. The keystone is the central brick at the crown that locks the arch together. The keystone is the last unit installed and transforms the arch from a collection of loose units into a self-supporting structure.