What is an Arch?
An arch is a curved structural element that spans an opening and supports loads primarily through compression. In construction, arches are used above doors, windows, bridges, and decorative features. The three most common types are semicircular (Roman), segmental (less than a half circle), and pointed (Gothic) arches.
Proper arch design requires calculating the radius, rise, arc length, and segment area to ensure structural integrity and aesthetic proportion. The radius determines the curvature, while the rise defines the height of the arch above the spring line (where the arch begins its curve).
Arch Formulas
For a segmental arch (most common in construction):
For a semicircular arch, the radius equals half the span, and the rise equals the radius.
Arch Types
| Type | Rise Ratio | Characteristics |
|---|---|---|
| Semicircular | Rise = Span/2 | Perfect half-circle, strongest form |
| Segmental | Rise < Span/2 | Flatter curve, most common in buildings |
| Pointed (Gothic) | Rise > Span/2 | Two arcs meeting at a point, medieval style |
| Flat (Jack) | Minimal rise | Nearly flat, decorative lintels |
| Parabolic | Variable | Follows parabola, efficient for uniform loads |
Construction Considerations
- Arches require temporary support (centering) during construction until the keystone is set.
- Brick arches need proper mortar joint tapering for aesthetic appearance.
- The abutments must be strong enough to resist the horizontal thrust forces.
- For structural arches, consult an engineer for load calculations.
Frequently Asked Questions
What is the ideal rise-to-span ratio for a segmental arch?
A common ratio is 1:4 to 1:6 (rise to span). For a 48-inch span, a rise of 8-12 inches creates an aesthetically pleasing segmental arch. The minimum structural rise is typically 1/8 of the span.
How do I build a brick arch?
Build a plywood template (centering) matching the calculated arch curve, set it on temporary supports, lay bricks along the curve with tapered mortar joints, install the keystone last, then remove the centering after the mortar has cured (typically 7 days).
Can arches support structural loads?
Yes, arches are inherently strong structural elements that transfer loads through compression to the abutments. Masonry arches have been supporting massive loads for thousands of years. However, structural arches in modern construction should be engineered for specific load requirements.