Arches: Technical Terms, Types and Centring
An arch is a curved structural element used in construction to span openings and support loads from above. Arches have been used since ancient times in bridges, buildings, and monuments due to their strength and aesthetic appearance. In the ITI Draughtsman Civil trade, understanding arches is essential for preparing architectural drawings and structural designs.
Arches transfer loads to the supports at both ends, known as abutments, and are widely used in doors, windows, bridges, and decorative structures.
Technical Terms of Arches
The following are important technical terms related to arches:
- Intrados: Inner curved surface of the arch.
- Extrados: Outer curved surface of the arch.
- Voussoirs: Wedge-shaped stones or bricks forming the arch.
- Keystone: The topmost central voussoir that locks the arch.
- Abutment: The support at the ends of the arch.
- Span: Horizontal distance between supports.
- Rise: Vertical distance from the base to the highest point of the arch.
- Crown: Highest point of the arch.
- Springing Line: Level at which the arch starts.
Understanding these terms is important for drawing and construction of arches.
Types of Arches
Arches are classified based on their shape and construction:
1. Flat Arch
Appears flat but constructed with slight curvature to carry load.
2. Segmental Arch
Formed by a segment of a circle.
3. Semicircular Arch
Forms a half-circle shape.
4. Pointed Arch
Consists of two arcs meeting at a point, commonly used in Gothic architecture.
5. Elliptical Arch
Has an elliptical shape, used for aesthetic purposes.
6. Horseshoe Arch
Extends beyond a semicircle, commonly used in decorative works.
7. Relieving Arch
Provided above lintels to reduce load.
Each type of arch is selected based on structural and architectural requirements.
Centring of Arches
Centring is a temporary framework used to support an arch during construction until it becomes self-supporting. It is usually made of timber or steel.
Purpose of Centring:
- Supports arch during construction
- Maintains proper shape and alignment
- Prevents collapse before completion
Types of Centring
- Timber Centring: Made of wooden frames, commonly used for small works.
- Steel Centring: Used for large structures due to strength and durability.
Requirements of Good Centring:
- Strong enough to support loads
- Easy to assemble and remove
- Accurate in shape
Centring is removed after the mortar has gained sufficient strength.
Advantages of Arches
- High strength and stability
- Efficient load distribution
- Aesthetic appearance
- Suitable for large spans
Limitations of Arches
- Requires skilled workmanship
- Time-consuming construction
- Needs proper centring
Applications of Arches
Arches are widely used in:
- Bridges and culverts
- Doors and windows
- Historical monuments
- Decorative structures
Importance in Draughtsman Civil Trade
Knowledge of arches helps draughtsmen in preparing architectural drawings and understanding structural behavior. It also helps in selecting suitable arch types for different constructions.
Conclusion
Arches are important structural elements used for spanning openings and supporting loads. Understanding their technical terms, types, and centring is essential for proper design and construction.
For students of the ITI Draughtsman Civil trade, knowledge of arches is crucial for creating accurate drawings and contributing to safe and aesthetically pleasing structures.