Definition A segmental arch is a type of arch whose curve describes a segment of a circle, meaning its curvature is less than a full semicircle. The center point from which the arch's curve is struck lies below the springing line (the line connecting the two points where the arch begins to curve upwards from its supports).
Overview The segmental arch is a common structural element found in various forms of architecture and civil engineering, ranging from historical buildings to modern constructions. Its design allows for a relatively shallow rise compared to a semicircular arch of the same span, which can be advantageous in situations requiring reduced height or a flatter aesthetic profile. This type of arch efficiently transfers vertical loads to its supporting abutments primarily through compressive forces. Its widespread use is attributed to its structural efficiency, ease of construction, and adaptability to different architectural styles and functional requirements, particularly for window and door openings, fireplaces, and bridges.
Etymology/Origin The term "segmental" refers to a segment of a circle, while "arch" derives from the Latin arcus, meaning "bow" or "curve." The concept of the arch itself dates back to ancient civilizations, notably perfected by the Romans. While the semicircular arch was a hallmark of Roman engineering, the understanding and application of arches with flatter profiles, including segmental arches, evolved through subsequent periods as engineers and builders explored more diverse structural solutions for various spans and load conditions. Its practical advantages for specific applications, such as bridge construction where a lower profile was often desirable to allow for greater clearance or reduce approach gradients, became particularly evident during the medieval and early modern periods.
Characteristics
- Shape: The distinctive feature is that its curve forms only a portion of a circle, specifically less than 180 degrees. The imposts (springing points) are connected by a chord that is longer than the radius of the circle from which the arch is struck.
- Rise: For a given span, a segmental arch generally has a smaller rise (vertical distance from the springing line to the intrados at the crown) compared to a semicircular arch. This makes it suitable for openings where headroom is a constraint.
- Load Distribution: It effectively converts vertical forces into outward horizontal thrusts at its springing points, which must be counteracted by strong abutments or tie rods.
- Structural Efficiency: The flatter curve can sometimes result in less material usage compared to a full-centered arch for certain spans, though it may require stronger abutments to resist increased horizontal thrust.
- Versatility: Used extensively in various applications, including:
- Windows and Doors: To create openings with a more subdued arch profile.
- Fireplaces: Providing a robust lintel for the firebox opening.
- Bridges: Many masonry arch bridges, particularly those built from the Renaissance onwards, utilize segmental arches to achieve longer spans with less vertical obstruction and shallower approaches.
- Construction: Typically constructed from voussoirs (wedge-shaped stones or bricks) that radiate from the center of the circle defining the arch's segment. A temporary support, known as centering, is used during construction until the keystone is set and the mortar has cured.
Related Topics
- Semicircular Arch: An arch whose curve is a full half-circle, often contrasted with the segmental arch for its greater rise and different structural behavior.
- Flat Arch (Jack Arch): An arch with a very shallow rise, appearing almost flat, but still constructed with voussoirs and performing as an arch.
- Gothic Arch (Pointed Arch): An arch characterized by its pointed apex, common in medieval European architecture.
- Voussoir: The wedge-shaped blocks (of stone, brick, or concrete) that make up an arch.
- Keystone: The central, topmost voussoir of an arch, which locks the other voussoirs in place.
- Springing Line (Impost): The horizontal line where the arch begins to curve upwards from its supports.
- Span: The horizontal distance between the two abutments of an arch.
- Rise: The vertical distance from the springing line to the highest point of the arch's intrados (soffit).
- Arch Bridge: A type of bridge that relies on the arch form for its structural integrity.