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Jack arch
Jack arch
Jack arch
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A jack arch, and a jack arch with a relieving arch above, as illustrated in the Lexikon der gesamten Technik
The Mardin jack arch, view from the crypt underneath

A jack arch is a structural element in masonry construction that provides support at openings in the masonry. Alternate names are lintel arch,[1] flat arch and straight arch.

Unlike regular arches, jack arches are not curved in form. Instead, they are flat in profile and are used under the same circumstances as lintels. Unlike lintels, which are subject to bending stress, jack arches are composed of individual masonry elements cut or formed into a wedge shape that efficiently uses the compressive strength of the masonry in the same manner as a regular arch. Like regular arches, jack arches require a mass of masonry to either side to absorb the considerable lateral thrust created by the jack arch.

Jack arches have the advantage of being constructed from smaller pieces of material that can be handled by individuals, as opposed to lintels which must necessarily be monolithic and which must be oversized unless reinforced by other means.

In small-scale brick masonry projects, jack arches are typically sawn from an appropriately sized fired-clay lintel, giving a more precise and consistent joint width than field-sawn shapes.

There is considerable scope for incorporation of decorative patterns and elements into jack arches. Keystones, stepped or arched top profiles, and polychrome or contrasting colors and materials may all be used to create the desired effect.

One of the earliest jack arches is in the Deyrü'z-Zafaran Monastery in eastern Turkey, near Mardin. The floor of the chapel (and the roof of the crypt) is formed from a jack arch and is said to have been constructed in the centuries BC.[2]

References

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Sources

[edit]
  • Beall, Christine (1987). Masonry Design and Detailing (2nd ed.). McGraw Hill Book Company. ISBN 0-07-004223-3.
  • DeLaine, Janet (1990). "Structural experimentation: The lintel arch, corbel and tie in western Roman architecture". World Archaeology. 21 (3): 407–424. doi:10.1080/00438243.1990.9980116. ISSN 0043-8243.
  • Hourihane, C. (2012). "Arch". The Grove Encyclopedia of Medieval Art and Architecture. Vol. 1. Oxford University Press. pp. 129–134. ISBN 978-0-19-539536-5. Retrieved 2024-12-24.
  • Ramsey, Charles; Sleeper, Harold (1951). Architectural Graphics Standards (4th ed.). John Wiley & Sons, Inc.
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Jack arch

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A jack arch is a flat or nearly flat structural element in , consisting of bricks or stones arranged in a straight or slightly cambered line to span openings such as doors and windows, thereby supporting loads above without the pronounced curve of traditional arches. Unlike semicircular or segmental arches, it features a horizontal intrados and is often built with a subtle camber to accommodate , using tapered mortar joints or voussoirs for stability. This design dates back to ancient Roman techniques, where flat arches were employed, and gained prominence in 19th-century industrial and for its fire-resistant and economical properties. In , jack arches require temporary centering during assembly, with a minimum depth of 4 inches (100 mm) plus 1 inch (25 mm) per foot of span, or 8 inches (200 mm), whichever is greater, and are commonly integrated with lintels in modern applications to enhance load-bearing capacity. They serve key roles in bridging openings in walls, forming fireproof floors between I-beams in , and even constructing bridge decks, as seen in early 20th-century infrastructure like New Hampshire's historic spans. Notable for their simplicity and durability, jack arches allow easier installation of flashing and weep holes compared to curved forms, making them a practical choice in both and contemporary projects.

Overview

Definition

A jack arch is a flat arch employed in masonry construction to span openings such as doors, windows, or floors, offering structural support while eschewing the full semicircular shape of conventional arches. Unlike true arches with pronounced curves, it appears nearly level, relying on its subtle geometry to distribute weight effectively over shorter distances. Structurally, a jack arch functions by transferring vertical loads laterally to its abutments through compression along the masonry units forming its slightly curved profile, effectively behaving as a shallow vault between parallel supports. This compressive action enables it to bear superimposed loads without significant tensile stress, making it suitable for integrating into walls or floors where space constraints limit deeper curvatures. The key geometric characteristic of a jack arch is its minimal rise relative to the span, typically ranging from 1/8 to 1/12 of the span length, which contributes to its flat appearance and distinguishes it from more rounded arch forms. This low rise-to-span ratio ensures the arch remains shallow while still providing the necessary for load distribution.

Characteristics and Variants

A jack arch consists of wedge-shaped voussoirs, typically bricks or stones, arranged in a shallow, nearly flat with minimal or zero rise, enabling it to span openings while aligning with horizontal mortar joints in surrounding . This configuration provides a depth of at least 4 inches (100 mm) plus 1 inch (25 mm) per foot of span, or a minimum of 8 inches (200 mm), whichever is greater, to ensure structural integrity under load. The arch relies on compressive forces to distribute vertical loads laterally to its abutments via arching action, without significant dependence on tensile resistance, making it suitable for supporting moderate spans in compression-dominant systems. Common variants of jack arches are classified by the number of courses in their depth, typically spanning 3, 4, or 5 courses to match the surrounding , with single-course versions used for shallower applications where minimal rise is acceptable. These can be fabricated as one-piece or multi-piece assemblies, often incorporating a tapered keystone for aesthetic and functional closure. A modern adaptation is the concrete jack arch, formed from poured or elements creating shallow vaults between supports like joists, offering enhanced durability and adaptability for larger spans in industrial or infrastructural contexts. Unlike semicircular or segmental arches, which feature pronounced curvature for efficient load resolution, the jack arch maintains a flat profile with negligible camber, sometimes requiring auxiliary support such as a angle beneath for spans exceeding typical limits. It is interchangeably termed a flat arch or arch due to its linear appearance but must be distinguished from a relieving arch, which is superimposed over a jack or to redirect superimposed loads away from the primary spanning element.

History

Ancient and Early Uses

The earliest known applications of jack arch principles date back to the , with evidence from the third century BCE in . In the Sanctuary of the Great Gods on the island of , flat arches—synonymous with jack arches—were employed in the Doric of a stoa constructed around 275–250 BCE, demonstrating an innovative use of stone elements to span openings while maintaining a trabeated aesthetic. These structures represented a transition from earlier methods to more efficient arching techniques, allowing builders to test longer spans in monumental settings without compromising visual harmony. The technique was adopted and further developed by the Romans, who employed flat arches to distribute weight over stone beams in various structures, including the subfloor supports of the built in the CE. In early European contexts, jack arch techniques diffused into medieval and , particularly in timber-framed systems across the . By the late 15th century, timber jack arch floors appeared in the of , where they supported vaulted ceilings in significant buildings; for instance, the House of the Provincial Council in featured an early variant described as "cuberta de cabiros ab volta" in 1481 records. This evolved further in the early , as seen in the restored Palacio Real's Sala Dorada (1501) with its painted jack arch floor and the Palace of the Centelles Family in Oliva, adorned with decorative friezes, highlighting their role in creating stable, aesthetically integrated spans over wide areas. These implementations marked a practical adaptation in regions facing timber shortages, using segmental vaults of flat tiles or plaster to enhance structural integrity. Pre-industrial jack arches primarily served to provide basic spanning capabilities in masonry buildings while protecting underlying wooden elements from environmental exposure. This approach facilitated the construction of multi-story edifices with reliable floor systems, predating later refinements for fire resistance in industrial eras.

Modern Development

The jack arch system emerged in Britain during the late 18th century, with early examples dating to 1792–1793, primarily as a fireproof flooring solution for textile factories where combustible materials posed significant risks. By the mid-1800s, the system had become integral to fireproof mill construction in northern England, using cast-iron beams with shallow brick jack arches to span wide areas while minimizing timber use. The technology spread through British colonial networks, gaining widespread adoption in for its ability to support large spans in public buildings amid challenging tropical climates. In structures like the British Residency in Hyderabad and various administrative offices, jack arches facilitated expansive, uninterrupted interiors without heavy reliance on imported materials, blending with local lime mortars for durability. Similarly, in , , from the late 1800s, jack arches provided an economical flooring alternative for public and commercial buildings, such as the (constructed 1866–1891) and the Lands Department Building (1876–1881), where lightweight or vaults on iron joists offered strength and fire resistance at lower cost than traditional methods. In the , jack arches adapted to infrastructure needs, particularly , where they were applied to bridge construction for short-span durability. In , over 100 such bridges were built between 1912 and 1940, standardized by the state highway department using concrete-encased steel I-beams with jack arch decks, as seen in examples like the New Durham Bridge (1926) over the Merry Meeting River and the Tamworth Bridge (1925) over the Chocorua River. Post-colonial contexts in saw further hybridization, integrating jack arches with indigenous techniques like lime-surkhi mortars and beams to create sustainable roofing systems suited to local materials and climates, as evidenced in adapted residential and institutional buildings continuing into the mid-20th century.

Design and Construction

Materials

Jack arches traditionally utilize fired clay bricks as the primary voussoirs, arranged in a flat or slightly cambered line to form the arch structure. These bricks, typically solid or hollow units conforming to standards such as ASTM C 216 for solid clay bricks, provide the essential for load-bearing while allowing for the flat or low-rise profile characteristic of jack arches. In historical and applications, bricks are laid on edge in a single course, often using for bonding, which offers flexibility and breathability suitable for construction. Lime mortar, composed of slaked lime, sand, and sometimes pozzolanic additives, ensures strong adhesion while accommodating minor movements in the structure. For added support, particularly in spans exceeding typical unreinforced limits, jack arches are sometimes reinforced with iron or joists, such as I-beams that serve as the primary spanning elements with the infilling between them. These metal components, often encased in mortar or for protection, enhance tensile capacity and were commonly employed in early 20th-century industrial and bridge constructions to achieve fireproofing and structural rigidity. Secondary elements include lime concrete toppings applied over the arches to create flat surfaces for roofs or floors, typically mixed in ratios like 1:2:4 (slaked lime, broken aggregate, and fine aggregate) to provide a durable, level finish. During , timber centering—temporary wooden —is used to support the wet mortar and until the arch sets, usually removed after 7-10 days depending on the mortar type. In modern projects, alternatives such as blocks or precast elements have been adopted for jack arches, offering faster installation and improved uniformity; these include load-bearing units per ASTM C 90, bonded with Type M, S, or N mortar for spans up to 6 feet. Considerations for durability often involve selecting fire-resistant or units with low water absorption to enhance longevity in exposed or high-risk environments.

Building Techniques

The construction of a jack arch begins with careful preparation to ensure structural integrity and aesthetic alignment. The span is measured as the clear horizontal distance between the abutments or supports, while the camber (slight rise)—the vertical distance from the spring line to the highest point—is typically about 1/8 inch (3 mm) per foot (0.3 m) of span to maintain a flat profile while accommodating settling. The minimum arch depth is 8 inches (200 mm) or 1 inch (25 mm) per foot of span, whichever is greater. Once dimensions are determined, lines for the extrados (the outer line) and intrados (the inner line) are drawn on a template, often using plywood, to guide the cutting and placement of bricks; this involves marking the skewback angle from the center strike point and accounting for brick thickness and mortar joints to achieve symmetry. The primary construction process relies on temporary support and precise bricklaying. A wooden centering, or , is erected beneath the arch to provide support during assembly, shaped to match the flat or slight camber and secured between abutments or joists. Bricks, cut as voussoirs according to the template, are then laid radially from the springers (the endpoints at the abutments) toward in successive courses, with tapered mortar joints (typically 1/8 to 3/4 inch thick) filled using cement-lime mortar for ; the process culminates at the keystone or crown, ensuring even joint compression. After completion, the centering remains in place for at least seven days to allow the mortar to set fully, after which it is carefully removed to avoid disturbing the structure. For multi-course jack arches, particularly in floor or ceiling applications, bricks are layered in multiple rings, often two to five courses deep, using a running or English bond pattern that alternates headers (bricks laid with ends facing the joint) and (bricks laid with long faces exposed) to enhance stability and distribute loads evenly across the arch depth. Integration with joists is common for vaulted floors, where the arches span between timber or joists spaced 1 to 1.4 meters apart; the is laid on edge between the joists, with the joists providing lateral support and often encased in for added , ensuring the system forms a continuous without sagging.

Applications

Architectural Uses

In architectural design, jack arches serve as a key for spanning openings such as doors and windows in walls, distributing loads efficiently through their without requiring a traditional keystone or curved form. This application is particularly common in load-bearing walls of historic and older buildings, where the shallow, flat profile allows for seamless integration while supporting upper stories or decorative elements above the openings. Jack arches are widely used to construct fireproof floors and ceilings in both residential and industrial buildings, providing non-combustible surfaces that enhance safety and durability. In factories and early industrial structures, they form shallow vaults between iron or joists, creating robust platforms that were initially developed in the late for fire resistance, as seen in public buildings like the General Post Office (1866–1891). For residences, these arches offer economical, long-lasting flooring and ceiling systems, often concealed behind but valued for their strength in multi-story homes. In roofing and flooring, jack arch systems employ elongated brick vaults supported on parallel beams, typically spanning widths of 12 to 15 feet while enabling unlimited lengths along the room's axis, limited only by the design's transverse constraints. This configuration suits room-scale applications, with beams spaced at intervals of up to 3 feet (0.91 meters) to support the arches, making it ideal for creating expansive, uninterrupted interior spaces. In modern residential contexts, exposed jack arches contribute to aesthetic ceilings, as exemplified by The Jack Arch House in Ahmedabad, India (2019), where they form rhythmic, geometric patterns that emphasize craftsmanship and connect interior spaces to outdoor elements like courtyards and gardens. During the British colonial period in India, jack arches were extensively adopted in residential architecture for their durable, fire-resistant qualities and use of local brick, providing cost-effective roofing in homes and quarters, such as those at the Lahore Railway Station.

Engineering and Infrastructure Uses

In , jack arches have been employed in bridge construction, particularly for deck systems supported on beams. Early 20th-century designs in the United States frequently utilized jack arch decks on stringers, offering a cost-effective and durable solution for short-span rural bridges. A notable inventory in documented 37 existing jack arch bridges, with construction spanning from 1912 to 1939, including examples such as the 1926 New Durham Bridge over the Merry Meeting River and the 1922 Franklin Bridge over the Winnipesaukee River, which featured three spans. These structures were standardized by departments in the , providing efficient load distribution while minimizing material use in projects. Beyond bridges, jack arches serve as relieving arches over lintels in load-bearing structures, diverting vertical loads laterally to abutments and reducing stress on underlying elements. This application enhances structural integrity in heavy-load scenarios, with the arch depth being the greater of 8 inches (200 mm) or 4 inches (100 mm) plus 1 inch (25 mm) per foot of span to ensure stability. In tunnel infrastructure, jack arches form vaults in cut-and-cover methods, where closely spaced steel frames (approximately 5 feet on centers) are encased in base slabs, and flat-arch metal forms support the pour, as historically applied by the . Such systems provide lightweight alternatives to full boxes, though they require additional to mitigate cracking risks from thin sections. In modern , jack arches undergo seismic in industrial buildings to address vulnerabilities in existing slabs, which originated in 19th-century British designs and were widely adopted for floors and roofs. Common methods include adding a layer over the slab or installing transverse sections to connect main beams, improving integrity and without excessive weight addition. Tie-bracing alone often proves insufficient for seismic demands, while hybrid approaches combining these techniques enhance overall performance in earthquake-prone regions. Additionally, hybrid jack arch systems appear in post-colonial infrastructure, blending colonial-era roofing with indigenous practices, as seen in early 20th-century Indian constructions where British-introduced shallow vaults integrated local materials for adaptive, economical spans in .

Advantages and Limitations

Benefits

Jack arches provide significant structural benefits through their arching action, which efficiently transfers vertical loads to abutments, resulting in primarily compressive stresses that align with the superior of masonry compared to its tensile capacity. This design makes them particularly effective for spanning openings in load-bearing walls, minimizing the risk of tensile failure and enhancing overall stability under uniform loading. The non-combustible properties of and associated materials render jack arches highly -resistant, protecting structural integrity during events and making them a preferred choice for fireproofing in industrial buildings where rapid spread posed historical risks. Economically, jack arches are cost-effective for short spans, typically up to 6 feet (1.8 m), utilizing readily available rectangular and tapered mortar joints without requiring extensive specialized forming or heavy machinery. Their inherent , derived from robust , leads to low needs, as they resist and degradation over extended periods with proper initial installation. Additionally, the exposed surfaces of jack arches allow for attractive, unfinished that integrate seamlessly with various architectural styles, providing visual interest without additional cladding. In modern applications, they contribute to through improved . In terms of performance, retrofitted jack arch systems exhibit resilience to seismic activity, offering fair and the capacity to endure substantial out-of-plane deformations during earthquakes, thereby improving building safety in vulnerable regions. When constructed with , they also deliver benefits, reducing solar heat gain through self-shading effects and contributing to better indoor in traditional and modern applications.

Challenges and Maintenance

Jack arches, due to their flat or low-rise profile, are unsuitable for very large spans without additional supports, as their structural efficiency diminishes with increasing length, necessitating deeper masonry depths—typically a minimum of 4 inches (100 mm) plus 1 inch (25 mm) per foot of span, or 8 inches (200 mm), whichever is greater—to maintain stability. Premature removal of temporary centering during construction can lead to settlement and potential collapse, as the arch requires at least 7 days for mortar to cure and achieve self-supporting integrity. Construction challenges include higher labor demands for precise placement of voussoirs, particularly when using tapered bricks to achieve the slight camber, compared to simpler flat lintels that require less alignment and jointing precision. In concrete jack arch variants, such as those used in bridge decking, heavy loads can induce cracking and de-bonding of the fill from underlying plates, exacerbating deterioration over time. Maintenance involves regular inspections for mortar degradation in brick versions, which can weaken joints and allow water ingress, and for joist corrosion or rot in timber-supported systems, often caused by moisture levels exceeding 20% or biotic agents like that reduce cross-sections at ends. In seismic zones, retrofitting with modern techniques—such as overlaying thin layers or installing grids and tie-bracing—enhances out-of-plane resistance and diaphragm integrity, addressing vulnerabilities observed in historical earthquakes like the 2003 Bam event. Recent advancements as of 2025 include with fiber-reinforced polymers (FRP) to enhance strength and in existing jack arch systems. These interventions must account for added weight to avoid overloading adjacent elements, with on-site assessments using tools like resistographs recommended for ongoing monitoring.

References

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  2. https://jecrcfoundation.com/wp-content/uploads/notes/btech/Civil%2520Engineering/3rd%2520Semester/Building%2520Metrial%2520Construction/UNIT-9%2520%28Arches%2520&%2520Lintel%29.pdf
  3. https://online.ucpress.edu/jsah/article/82/3/275/197309/Concealing-Structural-Innovation-in-Greek
  4. https://mm.nh.gov/files/uploads/dot/remote-docs/historic-jack-arch-bridges-of-nh-inventory-significance-study.pdf
  5. https://www.archdictionary.com/jackarch
  6. https://oldstructures.com/2017/09/29/jack-arches-are-arching-action/
  7. https://civinnovate.com/2025/08/06/the-rise-of-a-jack-arch-is-kept-at-about/
  8. https://www.iitk.ac.in/nicee/wcee/article/13_3028.pdf
  9. https://brick.com/wp-content/uploads/2025/01/20240729_Special-Shapes-web.pdf
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  15. https://hurstpm.net/FCKfiles/File/An_Iron_Lineage.pdf
  16. https://pjosr.com/index.php/pjs/article/view/690
  17. https://www.researchgate.net/publication/396791041_The_Missing_Link_between_Colonial_Jack_Arch_Roofing_and_Indigenous_Construction_Practices_in_India
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  19. https://www.cracindia.in/admin/uploads/IS-2118.pdf
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  21. https://masonrymagazine.com/blog/2021/03/31/jack-arches/
  22. https://cmqconsulting.com/glossary/jack-arch/
  23. https://www.sciencedirect.com/science/article/abs/pii/S0141029611004561
  24. https://www.thehindu.com/features/homes-and-gardens/jack-arch-roofing/article2399821.ece
  25. https://www.pvdrs.co.in/thejackarchhouse
  26. https://www.sciencedirect.com/science/article/pii/S2095263520300406
  27. https://upload.wikimedia.org/wikipedia/commons/e/ef/Cut-and-cover_tunneling_techniques_%28IA_cutandcovertunne00sver%29.pdf
  28. https://www.researchgate.net/publication/271522046_SEISMIC_RETROFITTING_OF_JACK_ARCH_MASONRY_FLOORS
  29. https://www.sciencedirect.com/science/article/abs/pii/S0950061813006028
  30. https://www.cnrtk.com/blog/benefits-of-jack-arch-in-modern-construction-practices/
  31. https://www.sciencedirect.com/science/article/pii/S221260901730078X
  32. https://revistaalconpat.org/index.php/RA/article/view/476/1316
  33. https://www.researchgate.net/publication/256968010_Seismic_retrofitting_methods_for_the_jack_arch_masonry_slabs
  34. https://www.researchgate.net/publication/368166856_New_technique_used_for_strengthening_jack_arch_slabs
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