The natural bedding plane of stones and the direction of pressure in stone masonry is–
Try it first: pick an option, then read the explanation.
Understanding the Natural Bedding Plane
Many stones used in masonry, particularly sedimentary rocks like sandstone and limestone, are formed in layers over millions of years. These layers are called bedding planes. The stone is inherently weaker along these planes and can split or delaminate if stress is applied incorrectly.
The Principle of Stone Masonry: To achieve maximum strength and durability, the stones in a wall must be oriented so that the pressure (load) from the structure above is applied perpendicular to these weak bedding planes. This ensures the load is carried by the strong, solid mass of the stone rather than trying to split the layers apart.
Detailed Analysis of the Options
Let's evaluate the relationship between the bedding plane and the direction of pressure.
(d) normal
Why it's correct: "Normal" is a geometric term meaning perpendicular, or at a 90° angle. Placing the stones so that the pressure is normal to the bedding planes is the fundamental rule of good stone masonry. This orientation provides the highest possible compressive strength and prevents the stone from splitting under load.
(b) parallel
Why it's incorrect: This is the worst possible orientation. If the pressure is applied parallel to the bedding planes (e.g., placing the stone on its side in the wall), the layers can easily shear and split apart, leading to failure. This is especially dangerous in arches and cornices.
(a) at 45° & (c) at 30°
Why they're incorrect: Applying the load at an angle would create a combination of compressive and shear stresses along the weak bedding planes. This is still a very weak configuration and would likely cause the stone to fail by shearing along its layers.
Other Important Rules for Stone Masonry
In addition to the correct orientation of bedding planes, several other rules are crucial for strong and durable stone masonry:
- Broken Vertical Joints: The vertical joints in any course should never align with the vertical joints in the course below it. They must be staggered or "broken" to properly distribute the load.
- Mortar Thickness: For a neat and strong finish, the thickness of the mortar joints on the face of the wall should generally not exceed 2 cm.
- Raking Joints: As the work progresses, the mortar in the joints on the face of the wall should be "raked out" to a depth of about 25 mm. This creates a key for the final plaster or pointing to adhere to securely.