An arch is one of the oldest and most important structural elements in architecture and civil engineering. It serves as a curved structure that spans an opening and supports loads from above by transferring them to the supports (abutments). Arches are not only functional but also add aesthetic beauty to buildings, bridges, and monuments. Over centuries, engineers and architects have developed various types of arches based on shape, material, number of centers, and construction methods. Understanding these types is essential for structural design and heritage architecture.
What Is an Arch?
An arch is a curved load-bearing structure that transfers the weight of superimposed loads primarily through compression to its supports, known as abutments or piers. Unlike a beam, which resists bending, an arch resists loads by developing compressive stresses along its curve. This makes arches extremely efficient in spanning large openings, such as doorways, bridges, and aqueducts.
Key Components of an Arch:
- Voussoirs: The wedge-shaped stones or bricks forming the arch.
- Crown: The highest point or keystone of the arch.
- Keystone: The central voussoir that locks the arch in place.
- Springing Line: The horizontal line from which the curve of the arch begins.
- Springing Points: The points where the arch starts to curve from the supports.
- Abutment: The support that carries the thrust from the arch.
- Intrados: The inner curve or underside of the arch.
- Extrados: The outer curve or upper surface of the arch.
- Span: The clear distance between the two supports of the arch.
- Rise: The vertical height between the springing line and the highest point (crown).
Classification of Arches
Arches can be classified based on various factors such as shape, number of centers, material used, and method of construction.
1. Classification Based on Shape
The shape of the intrados (inner curve) determines the overall geometry and type of an arch.
1.1 Flat Arch
A flat arch appears almost horizontal but has a slight rise to transfer loads efficiently. The intrados forms a straight line, while the extrados has a gentle camber for load distribution.
Rise: About 10–15 mm per 300 mm of span.
Use: Door and window openings with small spans.
1.2 Segmental Arch
A segmental arch forms a segment of a circle, with the center located below the springing line. It is one of the strongest and most common types used in bridges and buildings.
Use: Residential openings, bridges, aqueducts.
Advantage: Can resist significant lateral thrust.
1.3 Semi-Circular Arch
A semi-circular arch forms a perfect half-circle. The center of the circle lies on the springing line. It was widely used in Roman architecture.
Use: Culverts, ancient monuments, and stone bridges.
Advantage: Excellent load transfer through compression.
1.4 Horseshoe Arch
The horseshoe arch extends beyond a semi-circle, giving it a distinct elongated shape resembling a horseshoe.
Use: Islamic and Moorish architecture.
Advantage: Aesthetic and symbolic appeal.
1.5 Three-Centered Arch
This type has three different centers, giving it a slightly pointed but smooth curve. It provides a flatter rise than a semicircular arch.
Use: Railway bridges, tunnels, and underpasses.
1.6 Pointed or Gothic Arch
The pointed arch consists of two arcs of circles meeting at a sharp apex. It was a hallmark of Gothic architecture in medieval Europe.
Use: Cathedrals, churches, and ornate structures.
Advantage: Reduces lateral thrust and allows greater height.
1.7 Elliptical Arch
The elliptical arch has the shape of a half-ellipse with three or more centers. It provides a wide span and low rise, making it ideal for large openings.
Use: Modern bridges, auditoriums, and large entranceways.
1.8 Parabolic Arch
A parabolic arch has a curve following a parabola. It efficiently distributes uniform loads, making it structurally superior for long spans.
Use: Bridges, modern roofs, and railway underpasses.
Example: Gateway Arch in St. Louis, USA.
1.9 Stilted Arch
In a stilted arch, the curve starts above the impost level, supported on vertical portions called stilts.
Use: Doorways and arcades in churches and palaces.
1.10 Florentine Arch
A Florentine arch combines a flat arch at the crown with two segmental curves on the sides, giving a decorative look.
Use: Classical buildings and architectural features.
2. Classification Based on Number of Centers
| Type | Number of Centers | Description | Common Use |
|---|---|---|---|
| One-Centered Arch | One | Simple curve such as semi-circular or segmental. | Residential and commercial structures. |
| Two-Centered Arch | Two | Includes pointed or Gothic arches. | Cathedrals, monuments. |
| Three-Centered Arch | Three | Flatter curvature, good for long spans. | Bridges and tunnels. |
| Four-Centered Arch | Four | Elliptical shape with smooth curvature. | Palaces, auditoriums. |
| Five-Centered Arch | Five | Used for complex, decorative shapes. | Ornamental facades. |
3. Classification Based on Materials Used
3.1 Stone Arch
Constructed using wedge-shaped stone voussoirs. Common in historical monuments and bridges.
Advantages: Durable, aesthetic, strong in compression.
Disadvantages: Labor-intensive and expensive.
3.2 Brick Arch
Made of bricks laid in radial courses. It can be plain, gauged, or axed depending on brick cutting precision.
Advantages: Economical and easy to build.
Disadvantages: Limited to small spans.
3.3 Concrete Arch
Formed using reinforced or plain concrete. Widely used in modern bridges.
Advantages: Strong, economical, and moldable into any shape.
Disadvantages: Requires formwork and curing.
3.4 Steel Arch
Made of steel ribs or plates, capable of spanning very large distances.
Advantages: High strength, ductile, suitable for long-span bridges.
Example: Sydney Harbour Bridge, Australia.
3.5 Timber Arch
Used in temporary or small-span structures such as rural bridges.
Advantages: Light and easy to handle.
Disadvantages: Prone to decay and less durable.
3.6 Blockwork or Precast Arch
Made from precast concrete or masonry blocks. Common in modern modular construction.
4. Classification Based on Construction Method
4.1 Monolithic Arch
Constructed as a single unit using cast-in-situ concrete. Provides excellent stability and load transfer.
4.2 Voussoir Arch
Built using individual wedge-shaped units (voussoirs) that fit tightly together. Most historical stone arches fall in this category.
4.3 Relieving Arch
A relieving arch is built above a lintel or flat arch to reduce the load on it. Often used in brick masonry construction.
4.4 Centering Arch
Built with the help of wooden supports or centering until the mortar sets. Common in brick and stone arches.
5. Classification Based on Function
| Type | Purpose | Example |
|---|---|---|
| Fixed Arch | Both ends are fixed, used where movement is minimal. | Arched bridges, tunnels. |
| Two-Hinged Arch | Hinges at abutments allow slight rotation. | Bridge decks and roofs. |
| Three-Hinged Arch | Hinges at both abutments and crown; statically determinate. | Long-span bridges. |
| Tied Arch (Bowstring Arch) | Has a horizontal tie to resist horizontal thrust. | Modern steel bridges. |
Applications of Arches in Construction
- Bridges: Distributes loads efficiently across spans.
- Door and Window Openings: Adds beauty and supports walls above openings.
- Aqueducts and Culverts: Allows water to pass under roads or structures.
- Domes and Vaults: Formed by a series of arches for large enclosed spaces.
- Decorative Architecture: Enhances the visual appeal of facades and interiors.
Advantages of Using Arches
- Excellent load distribution through compression.
- Long lifespan with minimal maintenance.
- Aesthetic appeal suitable for classical and modern architecture.
- Strong resistance against bending stresses.
- Can cover wide spans without intermediate supports.
Disadvantages of Arches
- Requires skilled labor and precise workmanship.
- Generates horizontal thrust on abutments, needing strong supports.
- Construction is time-consuming and material-intensive.
- Limited adaptability to modern rectangular architectural forms.
Conclusion
Arches have been an integral part of human architecture since ancient times, blending engineering efficiency with artistic design. From the semi-circular arches of Roman bridges to the pointed arches of Gothic cathedrals and modern concrete parabolic arches, each type serves a unique structural and aesthetic purpose. Understanding the types of arches, their geometry, and construction methods helps engineers and architects choose the right form for both strength and beauty — ensuring that this ancient element continues to inspire modern design.
