A flat arch, also known as a jack arch, works by transferring the load above it to the supporting walls or piers at its sides, similar to a regular arch but without the curved shape. It achieves this through carefully angled voussoirs (wedge-shaped blocks) that create horizontal thrust, which is then resisted by the abutments.
Here's a breakdown of the mechanics:
- Keystone: At the center is a keystone, which is usually the final and topmost voussoir.
- Vousssoirs: The arch is composed of multiple wedge-shaped bricks or stones called voussoirs. These are carefully cut and positioned to direct the load outwards.
- Thrust: The weight of the masonry above the arch creates a downward force. The angled faces of the voussoirs convert this downward force into a horizontal force called thrust. This thrust pushes outwards against the supporting walls (abutments).
- Abutments: The abutments (walls or piers) must be strong enough to resist the outward thrust created by the arch. If the abutments are not strong enough, the arch will collapse.
- Horizontal Reaction: The abutments resist the thrust, creating an equal and opposite horizontal reaction force, thereby maintaining the arch's structural integrity.
In essence, a flat arch functions as a horizontal beam that leverages the principles of arch construction. The angled voussoirs redirect the vertical load into horizontal thrust, which is contained by strong abutments. Unlike a curved arch, a flat arch relies even more heavily on the strength and stability of the abutments since there is less vertical curvature to naturally resolve some of the forces.
Essentially it is like a shallow arch, only without the curve. Therefore, its thrust forces are greater than those of a regular arch and the abutments need to be stronger.
