Arch

Lore & Background

A true arch is a load-bearing arch with elements held together by compression. In much of the world introduction of the true arch was a result of European influence. The term false arch has a few meanings, usually designating an arch with no structural purpose, like a proscenium arch in theaters, but also applied to corbelled and triangular arches not based on compression. A typical true masonry arch consists of elements such as the keystone, voussoirs, extrados, impost, intrados, rise, clear span, and abutment. The roughly triangular portion of the wall between the extrados and the horizontal division above is called a spandrel.

Reader's Guide

The arch had several advantages over the lintel, especially in masonry construction: with the same amount of material an arch can have larger span, carry more weight, and can be made from smaller and thus more manageable pieces. Its role in construction was diminished in the middle of the 19th century with introduction of wrought iron (and later steel): the high tensile strength of these new materials made long lintels possible. A true arch, due to its rise, resolves vertical loads into horizontal and vertical reactions at the ends, a so-called arch action. The vertical load produces a positive bending moment in the arch, while the inward-directed horizontal reaction from the spandrel/abutment provides a counterbalancing negative moment. As a result, the bending moment in any segment of the arch is much smaller than in a beam with the equivalent load and span. In the past, when arches were made of masonry pieces, the horizontal forces at the ends of an arch (thrust) caused the need for heavy abutments. The other way to counteract the forces was to use counter-arches, as in an arcade arrangement, where the horizontal thrust of each arch is counterbalanced by its neighbors. With new construction materials (steel, concrete, engineered wood), not only the arches themselves got lighter, but the horizontal thrust can be further relieved by a tie connecting the ends of an arch (bowstring arch).

Did You Know?

• The arch dates back to the 4th millennium BC as a decorative element.
• Structural load-bearing arches became popular after adoption by the Ancient Romans in the 4th century BC.
• A true arch resolves vertical loads into horizontal and vertical reactions at the ends, called arch action.

A History Spanning Millennia

The arch as a decorative motif stretches back to the fourth millennium BC, long before it ever bore a structural load. It was the Ancient Romans, working in the fourth century BC, who transformed this ornamental curve into a genuine load-bearing element, and from that point forward the arch became a defining feature of Western construction. For centuries it dominated masonry building, enabling spans and loads that simple post-and-beam systems could never achieve. That dominance began to erode in the mid-nineteenth century, when wrought iron and subsequently steel entered the builder's toolkit. The extraordinary tensile strength of these metals made long, straight lintels practical for the first time, and the arch's once-essential role in spanning open spaces gradually faded from everyday construction. Yet the principle never disappeared; it simply migrated into new materials and new geometries, from concrete shells to the bowstring arches of modern engineering.

The Mechanics of Compression

A true arch works by converting vertical loads into a combination of vertical and horizontal reactions at its supports. The rise of the curve generates a positive bending moment, while the inward horizontal thrust from the abutment or spandrel produces an opposing negative moment. The net result is that the bending moment in any given segment of the arch is far smaller than in an equivalent beam carrying the same load over the same span. Because the constituent blocks are primarily in compression rather than bending, the structure can be built from relatively small, manageable masonry pieces. This principle was formalized in the language of funicular curves: Robert Hooke observed in 1676 that a flexible hanging line, when inverted, describes the ideal rigid arch. In practical masonry design, where tensile strength was effectively zero, engineers simply verified that the pressure polygon stayed within the voussoir boundaries—a criterion known as the safe theorem.

Anatomy of a True Arch

Every load-bearing masonry arch is assembled from a vocabulary of precisely named parts. At the apex sits the keystone, the top wedge that locks the assembly together; the surrounding region is called the crown. The individual wedge-shaped blocks are voussoirs, and when several concentric rings of them are stacked the result is a compound arch, with the rowlock arch being a special case where brick headers form the faces. The outer surface is the extrados, the inner curve the intrados or soffit. At each end, the impost block marks the springing level, and the voussoir immediately above it is the springer. The zone between the springing level and the crown, centered around the forty-five-degree angle, is the haunch. The roughly triangular wall area between the extrados and the horizontal line above is the spandrel, and the supporting structure at the base is the abutment. If the voussoir sides incorporate angles and curves for interlocking, the arch is termed joggled.

Arches in the Broader Construction Landscape

The arch occupies a specific niche within the great divide between arcuated and trabeated construction. In the trabeated system—dominant in ancient Greek, Chinese, and Japanese architecture, and echoed in modern steel framing—posts and beams carry the load. The arcuated approach, by contrast, relies on arches and the vaults they support, with the barrel vault being essentially a continuous arch extended in one direction. Compared with a simple lintel, an arch of the same material volume can span a wider opening, carry a heavier load, and be assembled from smaller, more workable pieces. The horizontal thrust that an arch generates at its ends demanded massive abutments in Roman triumphal arches, or alternatively a series of counter-arches in an arcade, where each arch's lateral force is neutralized by its neighbors and only the terminal arches require buttressing. In modern practice, a tie connecting the two ends of the arch—producing a bowstring arch—relieves the thrust entirely, permitting slender supports.

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