A brick arch works by efficiently redirecting vertical downward forces into outward and downward thrusts that are safely transferred to sturdy side supports called abutments.
The Fundamental Principle of Load Transfer
The inherent strength and structural efficiency of a brick arch are primarily attributed to its curvature. Unlike a flat beam that bends under vertical loads, an arch converts these loads into compressive forces that are transferred laterally along its curved path.
As the provided reference states: "The brick masonry arch has been used to span openings of considerable length in many different applications. Structural efficiency is attributed to the curvature of the arch, which transfers vertical loads laterally along the arch to the abutments at each end."
Here's how this lateral load transfer mechanism functions:
- Compression: The bricks within the arch, known as voussoirs, are designed to be primarily in compression. This means they are being squeezed together, which is a state that brick and mortar handle exceptionally well.
- Thrust: As the vertical weight pushes down on the arch, the curved geometry causes these downward forces to be resolved into outward and downward forces, known as thrust. This thrust travels along the arch to its base.
- Abutments: The outward thrust generated by the arch must be resisted by strong, unyielding supports at each end. These supports, called abutments, are critical to prevent the arch from spreading or collapsing.
Key Components of a Brick Arch
Understanding the parts of a brick arch helps illustrate its working mechanism:
| Component | Description | Role |
|---|---|---|
| Voussoirs | Individual, wedge-shaped bricks or stones. | Transmit compressive forces from one to the next. |
| Keystone | The central, topmost voussoir. | Locks the arch in place and evenly distributes forces to other voussoirs. |
| Abutments | The solid supports at each end of the arch. | Resist the lateral thrust, preventing the arch from spreading outwards. |
| Span | The horizontal distance between the two abutments. | Defines the width of the opening the arch covers. |
| Rise | The vertical distance from the spring line to the keystone (soffit). | Influences the angle of the thrust; a higher rise generally means less outward thrust. |
| Spring Line | The point where the arch begins to curve upwards from the abutment. | The base line of the arch. |
| Intrados (Soffit) | The inner curve of the arch. | The underside of the arch. |
| Extrados (Haunch) | The outer curve of the arch. | The upper surface of the arch. |
How Forces are Managed
The brilliance of a brick arch lies in its ability to manage forces by converting them into compression, a stress state that bricks are highly resistant to.
- Vertical Load Application: Any weight placed on top of or pushing down on the arch, such as the weight of the wall above or a roof, applies vertical pressure.
- Conversion to Compression: Due to its curved shape, the arch effectively converts these vertical forces into internal compressive forces that run along the curve of the arch.
- Transfer through Voussoirs: Each voussoir (brick) in the arch is compressed by its neighbors. This compression is transferred from one brick to the next, from the top of the arch (the keystone) downwards and outwards towards the ends.
- Generation of Thrust: As the compressive forces travel down the arch's curve, they are redirected as an outward and downward push – the lateral thrust.
- Resistance by Abutments: The powerful abutments at each end of the arch absorb and counteract this lateral thrust, preventing the arch from collapsing or spreading apart. Without adequately strong abutments, the arch would simply push its supports out and fail.
Why Brick Arches are Efficient and Used
Brick arches have been utilized for centuries in various applications due to their inherent strength and structural efficiency. They allow for the spanning of considerable openings without requiring internal supports.
Practical examples of their use include:
- Doorways and Window Openings: They effectively support the masonry above these openings in buildings.
- Bridges: Historically, brick and stone arches were primary components in bridge construction, allowing them to span rivers and valleys.
- Tunnels and Culverts: Their curved form is ideal for resisting earth pressure in underground structures.
- Vaults and Domes: Extended forms of the arch principle are used to create large, self-supporting roof structures.
The continuous line of compression within the arch means that, theoretically, an arch can stand without mortar if the bricks are perfectly cut and the abutments are strong enough, though mortar provides crucial stability and distributes stress more evenly.
