halfarch

A halfarch, also known as a segmental arch or a cribriform arch, is a type of arch that spans less than a semicircle. Unlike a true semicircular arch, its intrados, or inner curve, is a segment of a circle with a radius greater than half the span. This means the center of the circle from which the arch is struck lies above the springing line, the point where the arch begins to curve upwards.

Halfarches are commonly found in construction where a full semicircular arch might be too tall for the available space or where a shallower profile is desired. They distribute the weight of the structure above them outwards and downwards to the supporting piers or abutments, just as a full arch does. The use of a halfarch can sometimes lead to greater outward thrust compared to a semicircular arch of the same span, requiring stronger supports.

Historically, halfarches have been employed in various architectural styles, including Romanesque and Gothic architecture, as well as in more utilitarian structures like bridges and viaducts. Their flexibility in shape allows them to be adapted to different aesthetic and structural requirements. The precise curvature of a halfarch is determined by the chosen radius and the angle of the segment.