Flat arches are used for spans upto:
🧱 Understanding the Flat Arch (Jack Arch)
A Flat Arch, also commonly known as a Jack Arch, is a unique type of arch that appears almost flat, resembling a lintel. However, it is a true arch that functions through compression, using wedge-shaped bricks or stones (voussoirs) to span an opening.
Key Characteristics
- Geometry: The skewbacks (end stones) are typically cut at a 60° angle to the horizontal. This creates an equilateral triangle with the arch's soffit (underside) acting as the base.
- Slight Rise (Camber): Although it looks flat, the intrados (the inner curve) is given a very slight upward curve called a rise or camber. This is crucial for structural stability and is usually about 10 to 15 mm for every 1 meter of span. This small rise ensures the voussoirs are tightly compressed.
- Load Capacity: Due to their minimal rise, flat arches are not as structurally efficient as curved arches. They are therefore only suitable for supporting light loads.
📏 Why is There a Span Limit for Flat Arches?
The effectiveness of an arch is directly related to its rise-to-span ratio. A higher rise allows the arch to transfer vertical loads into horizontal thrust more efficiently.
The Rule of Thumb for Span
Because a flat arch has a very minimal rise, its ability to handle large horizontal thrusts over long distances is limited. As the span increases, the internal stresses and the outward thrust on the supports grow significantly. To ensure safety and prevent failure, flat arches are generally used for relatively small openings.
The standard rule of thumb in construction is that flat arches should only be used for spans up to 1.5 meters (approximately 5 feet).
What Happens Beyond 1.5 Meters?
For spans greater than 1.5 meters, the structural integrity of a flat arch becomes unreliable. The horizontal thrust would be too great for the nearly flat structure to handle, requiring excessively deep voussoirs and extremely strong abutments. In such cases, a more suitable structural element would be:
- A Segmental or Semi-circular Arch, which has a greater rise and is more efficient.
- A reinforced concrete or steel Lintel, which works in bending and is not reliant on arch action.