What is the name of member, in centering of arches, a thick wooden plank shaped to the curvature of arch and it is supported by props?
🏗️ Understanding Arch Centering
An arch is not self-supporting until the final keystone is placed. Therefore, a temporary support structure, known as Centering, is required during construction. This framework holds the voussoirs (wedge-shaped stones) in their correct position until the arch is complete. The question asks to identify a specific component of this temporary structure.
🧩 Components of Arch Centering
Arch centering can range from simple to complex depending on the size of the arch. Here are the definitions of the terms in the options:
(C) Turning piece: The Simple Solution
A Turning piece is the correct answer. It is a single, thick wooden plank that is cut and shaped to match the exact curvature of the arch's undersurface (soffit). This single piece is then supported by vertical props. This method is typically used for smaller, simpler arches where a single plank is strong enough to support the weight of the voussoirs during construction.
(A) Ribs: The Skeleton of Large Centering
For larger arches, a single plank is not sufficient. Instead, a series of parallel, built-up wooden frames called Ribs are used. These ribs act as the primary structural skeleton, defining the shape and curvature of the arch.
(B) Laggings: The Skin of the Centering
Laggings are narrow strips of timber that are placed on top of the ribs, running parallel to the length of the arch. They form a continuous, smooth surface (the "skin") upon which the voussoirs are actually laid. A turning piece essentially combines the function of ribs and laggings into one member for small arches.
(D) Brace: The Stiffening Member
A Brace is a general term for a diagonal support member used in any framework to provide stiffness and prevent distortion. In arch centering, braces would be used to connect the props and ribs, ensuring the entire temporary structure is rigid and stable.
📝 Key Distinction
The key difference is scale:
- For small arches, a single shaped plank called a Turning Piece is used.
- For large arches, a more complex system of Ribs (the skeleton) and Laggings (the skin) is required.
Since the question describes "a thick wooden plank shaped to the curvature," it is specifically referring to a Turning Piece.