In arches, stratified stones are placed so that their planes are
🪨 Recapping the General Rule
As a reminder, the fundamental rule of stone masonry for stratified rocks is that the load should be applied perpendicular (at a right angle) to the natural bedding planes. In a normal wall, this means the bedding planes are kept horizontal to resist the vertical load from above.
🔬 The Special Case: Stone Arches
Arches are unique structures because the pressure does not act vertically downwards. Instead, the load is transmitted through the arch in a curved path, known as the line of thrust. To follow the fundamental rule, the bedding planes must be perpendicular to this curved line of thrust.
C. Radial
This is the correct answer. For the bedding planes to be perpendicular to the curved line of thrust at every point, they must be oriented radially. This means that the planes of each stone (voussoir) in the arch should point towards the center point of the arch's curve. This orientation ensures that the compressive force is always pushing the layers together, maximizing the strength of the arch.
A. Parallel
Placing the bedding planes parallel to the line of thrust would be the weakest possible configuration, causing the stone layers to "peel" apart and the arch to fail.
B. Perpendicular
While the goal is to be perpendicular *to the line of thrust*, simply placing all stones with horizontal bedding planes (perpendicular to gravity) would be incorrect. In the angled parts of the arch, this would create a shearing force along the weak planes, leading to failure.
📊 Walls vs. Arches: A Quick Comparison
| Structure | Direction of Pressure | Correct Bedding Plane Orientation |
|---|---|---|
| Wall | Vertical | Horizontal (Perpendicular to pressure) |
| Arch | Curved (Line of Thrust) | Radial (Perpendicular to pressure at every point) |
💡 Study Tips
- The Rule Never Changes: The fundamental rule is always "Pressure must be perpendicular to the bedding plane." The only thing that changes is the direction of the pressure.
- Arches = Radial: This is a key exception to remember. For walls, the answer is perpendicular/horizontal. For arches, the answer is radial.
- Visualize Spokes on a Wheel: Think of the bedding planes in an arch as the spokes on a wheel. They all point towards the center.