The rise (also called the height or sagitta) is the vertical distance between the springing line of an arch and the highest point of the arch (the keystone). The rise determines how tall or flat an arch is: a large rise gives a high, round arch, while a small rise gives a flat, depressed arch.
The rise in context
| Term | Description |
|---|---|
| Rise | Height of the arch above the springing line |
| Springing line | Horizontal line between the two springing points |
| Span | Horizontal distance between the springing points |
| Intrados | The curved inner surface of the arch |
Rise in different arch types
| Arch type | Rise relative to span |
|---|---|
| Semicircular arch | Rise = half the span |
| Segmental arch | Rise < half the span (flatter) |
| Horseshoe arch | Rise > half the span (taller) |
| Elliptical arch | Rise varies, elliptical profile |
| Flat arch | Very small rise, nearly straight |
Importance of the rise
- Determines the clear headroom under a bridge or gateway
- Affects the force distribution in the arch: a flatter arch generates more horizontal thrust at the supports
- Determines the appearance of the arch: stately and tall versus broad and depressed
- Is needed for making the centering (the temporary timber formwork)
Tips for the DIY enthusiast
- Always measure the rise vertically from the springing line to the highest point
- When making a centering: draw the arch at full size on a sheet of plywood
- A segmental arch with a rise of 1/8 to 1/6 of the span is typical above windows
- The smaller the rise, the greater the horizontal forces on the walls
Related terms
- Springing point
- Keystone
- Span
- Segmental arch
- Centering