List of bridge types

A list of bridge types catalogs the diverse designs employed to construct crossings over obstacles such as waterways, valleys, and roadways, enabling efficient transportation and infrastructure development. These include natural formations and engineered structural types, primarily classified by their load-bearing mechanisms and configurations, with the most common structural types including beam (or girder) bridges, arch bridges, truss bridges, cantilever bridges, suspension bridges, and cable-stayed bridges, each optimized for specific span lengths, site conditions, and economic factors.^[1][2][3] Beam bridges, the simplest and most prevalent form, consist of horizontal beams or girders supported at each end by piers or abutments, effectively distributing loads through bending and shear; they are ideal for short spans up to 30–37 m in continuous configurations and are widely used in urban overpasses due to their straightforward construction and cost-effectiveness with materials like steel or reinforced concrete.^[2][1][3] Arch bridges rely on curved structures that transfer compressive forces to abutments, providing stability for spans up to 575 meters or more, and have been constructed since ancient times using stone or modern materials like steel, though they require substantial foundations to resist horizontal thrust.^[1][4][5][3] Truss bridges feature a framework of interconnected triangular elements that handle tension and compression axially, allowing for spans typically between 50 and 200 meters with fewer intermediate supports, and are often prefabricated for efficiency in medium-length crossings like railways.^[1][2][4] Cantilever bridges extend projecting arms from central piers, balanced by counterweights, to achieve medium spans of 150-500 meters without temporary supports over water, making them suitable for challenging sites like deep rivers, as exemplified by Scotland's Forth Bridge.^[1][4][3] For longer distances, suspension bridges suspend the deck from main cables draped over towers and anchored at ends, enabling spans exceeding 2,000 meters, such as Turkey's 1915 Çanakkale Bridge with a main span of 2,023 meters, while offering lightweight yet resilient designs against wind and seismic forces.^[1][2][6][3] Cable-stayed bridges, a modern alternative, use diagonal cables directly from towers to support the deck in patterns like fan or harp, economically spanning 200-1,000 meters with simpler construction than suspension types, as seen in Greece's Rio-Antirrio Bridge.^[1][2][4] Selection among these types depends on factors such as anticipated span, environmental loads, material availability, and constructability, with steel predominant in contemporary designs for its strength-to-weight ratio and durability.^[2][3]

Natural Bridges

Arch-Shaped Natural Bridges

Arch-shaped natural bridges are geological formations created through natural erosive processes, where a curved span of resistant rock material bridges a gap or valley, typically resulting from the action of wind, water, freeze-thaw cycles, or tectonic forces without any human intervention.^[7] These structures differ from man-made arches primarily in their organic formation and irregular shapes, often serving as spans over streams, canyons, or coastal areas.^[8] One prominent example is Rainbow Bridge in Utah, USA, recognized as the largest known natural rock span at a span of 84 meters (275 feet) and height of 88 meters (290 feet), formed primarily from Navajo Sandstone through the erosive action of a meandering river over millions of years.^[9] Another notable instance is Natural Bridge in Virginia, USA, a 66-meter (215-foot) high limestone arch spanning 27 meters (90 feet), sculpted by Cedar Creek's persistent stream erosion in a karst landscape since the Paleozoic era.^[10] These examples illustrate how such bridges can achieve impressive scales, with Rainbow Bridge's thin span highlighting the delicate balance of rock strength and erosion.^[11] The formation of arch-shaped natural bridges typically begins with tectonic uplift that exposes layered sedimentary rocks, such as sandstone or limestone, to surface weathering.^[11] Joints or cracks develop in the rock due to stress from uplift, folding, or salt movement beneath, creating parallel fins of rock separated by narrow slots.^[8] Differential erosion then plays a key role: softer, less resistant layers (like mudstone) erode faster than harder caprocks (such as sandstone), widening the slots and forming potholes or caves within the fins through water infiltration, wind abrasion, and freeze-thaw expansion.^[7] Over time—often spanning thousands to millions of years—these openings enlarge until the unsupported rock between them collapses or erodes away, leaving a stable, curved arch where the remaining resistant material spans the gap.^[12] This process is most common in arid or semi-arid regions with horizontal strata, like the Colorado Plateau, but occurs globally in various rock types. However, due to climate change, erosion patterns and stability may be altered in vulnerable areas.^[11] Variations of arch-shaped natural bridges include temporary ice formations in polar regions and more permanent lava structures from volcanic activity. In the Arctic, such as Nares Strait between Greenland and Ellesmere Island, seasonal ice arches form when thick sea ice ridges ground against landmasses, creating curved barriers that span up to several kilometers and temporarily block ice export until warmer conditions cause collapse; however, due to Arctic warming, these arches have been forming less reliably as of 2023.^[13][14] Lava arches, conversely, arise from the partial collapse of ancient lava tubes during volcanic eruptions, leaving arched overhangs of solidified basalt; examples include Sunshine Arch in California's Lava Beds National Monument, where tube roof failures expose tunnel-like spans.^[12] These variants underscore the diverse natural mechanisms—beyond subaerial erosion—that can produce arched spans, adapting to extreme environments like glacial or igneous settings.^[15]

Slab or Beam-Like Natural Bridges

Slab-like natural bridges consist of flat or horizontal natural formations that span gaps, such as streams or chasms, without relying on arching structures, typically created by fallen timber, large boulders, or frozen water surfaces. These features function similarly to simple beam bridges in their load-bearing mechanism, distributing weight evenly across the span. Unlike arched natural bridges formed by erosion, slab-like variants emerge from abrupt biological or climatic events, providing temporary crossings in otherwise impassable terrain.^[16] Formation of these bridges occurs through biological processes, such as trees toppling due to advanced age, storms, or root instability, where the trunk lands across a watercourse or ravine. In forested environments, large trees like redwoods in California often fall naturally, their massive trunks bridging creeks when roots fail from soil saturation or wind. Climatic formation involves the freezing of rivers, lakes, or sea surfaces into solid ice sheets during winter, creating seasonal spans that connect landmasses or islands. These ice formations develop as water temperatures drop below freezing, with thickness building from successive layers of snowfall and refreezing. With ongoing climate change, these ice spans are forming later and melting earlier, reducing their duration and reliability as of 2025.^[17][18] Prominent examples include fallen redwood trees in California's coastal forests, where ancient specimens span streams in areas like Redwood National and State Parks, facilitating passage for hikers and wildlife. In the Arctic, seasonal sea ice in the Chukchi Sea enables polar bears to traverse from mainland Siberia to Alaska during winter hunts. These ice spans, up to tens of kilometers wide, support movements that would otherwise require energy-intensive swimming.^[19] Stability of slab-like natural bridges depends on factors like material integrity, span length, and load distribution; for fallen logs, this includes the tree's diameter, wood density, and rate of decay from fungi or insects, which can reduce load-bearing capacity over months to years. Ice bridges remain viable as long as temperatures stay below freezing, with thickness of at least 1-2 meters needed to support wildlife weights up to several tons, though cracks or tidal forces can compromise them. Environmental duration varies, with log bridges lasting 5-20 years in moist forests before full decomposition, while ice versions persist only 3-6 months annually but are shortening due to warming.^[20][18] These structures play vital roles in ecosystems by serving as migration corridors for mammals, such as ocelots and armadillos using fallen trees in Brazilian Atlantic Forests or polar bears crossing Arctic sea ice, thereby maintaining genetic connectivity across fragmented habitats. They also risk sudden collapse from thawing ice in spring or accelerating decay in logs during heavy rains, posing hazards to traversing animals and limiting their use to lighter loads. Impacts from climate change may further disrupt these roles by reducing ice formation periods.^[16][19]

Fixed Structural Bridges

Arch Bridges

Arch bridges are fixed structural bridges that utilize one or more curved arches as the primary load-bearing elements to span obstacles such as rivers or valleys. The arch shape efficiently transfers vertical loads from the deck downward and outward through compression along its curve, converting them into horizontal thrusts that are resisted by abutments at each end. Key components include voussoirs, the wedge-shaped blocks that form the arch's curve and interlock under compression; the keystone, the central topmost voussoir that locks the structure together; and abutments, the end supports that anchor the arch and counteract the outward thrust to prevent spreading.^[21][22][23] The historical development of arch bridges traces back to ancient civilizations, with the Romans mastering the form for durable, long-lasting infrastructure. A seminal example is the Pont du Gard in southern France, constructed in the 1st century AD as part of an aqueduct, featuring three tiers of arches with the largest single span measuring approximately 25 meters. This design exemplified Roman engineering prowess in using precisely cut stone voussoirs without mortar for stability. In the medieval period, arch construction continued with true arches built using temporary centering (falsework) for support during erection.^[24][25] Modern arch bridges evolved with industrial materials, incorporating steel and reinforced concrete to achieve greater spans and versatility. The Sydney Harbour Bridge in Australia, completed in 1932, is a prominent steel through-arch example with a main span of 503 meters, constructed using riveted steel segments erected from both abutments toward the center. A more recent example is China's Pingnan Third Bridge, opened in 2020, which holds the record for the longest arch span at 575 meters.^[5] Tied-arch variations, where a horizontal tie beam connects the arch ends to internally balance thrust and minimize abutment loads, further reduced foundation demands; notable instances include the Fort Duquesne Bridge in Pittsburgh. These advancements drew partial inspiration from natural rock arches but emphasized engineered compression for urban and harbor crossings.^[26][22] Arch bridges offer significant advantages in compressive strength, enabling efficient material use for spans up to 575 meters or more while providing aesthetic appeal and rigidity against lateral forces. However, they demand solid, unyielding foundations to handle horizontal thrusts, and construction often requires temporary supports like falsework, limiting practicality in soft soils or seismically active areas. Span constraints arise from the arch's geometry, as excessive length increases thrust exponentially, typically capping practical designs at around 500-600 meters for steel variants.^[22][27][28]

Beam Bridges

A beam bridge consists of horizontal beams supported at each end by piers or abutments, with the deck placed atop the beams to carry loads. These structures primarily resist applied forces through bending, where the bottom fibers of the beam experience tension and the top fibers undergo compression, while shear forces act vertically along the beam's length.^[29][30] Beam bridges are classified into several types based on configuration and design. Simple span beams support a single span between two supports, relying on individual beams without intermediate connections. Continuous beams extend over multiple spans, with the beam continuing across intermediate supports to distribute loads more evenly and reduce material use compared to multiple simple spans.^[3][31] Historically, beam bridges originated with simple timber log constructions, where felled trees served as natural beams spanning short distances over streams or ravines, a practice dating back to ancient civilizations and indigenous engineering. In modern applications, steel plate girders form the backbone of many highway overpasses and approach structures, exemplified by the short-span concrete beam segments in the Lake Pontchartrain Causeway, the world's longest continuous over-water beam bridge at 38.4 km, composed of thousands of interconnected precast units.^[32][33] Construction of beam bridges typically employs materials like timber for short, low-load spans; reinforced or prestressed concrete for moderate durability and fire resistance; and steel for high-strength, lightweight applications. Span lengths are generally limited to under 100 meters without additional supports, with steel beams reaching up to 25 meters and deeper girders extending to 50 meters or more, beyond which alternative designs become more economical.^[34][31][30] Load distribution in a simple beam bridge follows basic principles of structural mechanics, with the maximum bending moment occurring at the center for a concentrated load at midspan, given by the equation: $$M = \frac{WL}{4}$$M=4WL​ where $M$M is the maximum bending moment, $W$W is the applied load, and $L$L is the span length. This moment determines the required beam section modulus to limit stresses within material allowable limits. Beam bridges may also combine with trusses to extend effective spans beyond typical limits.^[35]

Truss and Girder Bridges

Truss Bridges

A truss bridge is a structural type that employs a framework of interconnected triangular elements, typically constructed from metal members such as iron or steel, to support loads through axial forces primarily in tension and compression. The triangular configuration provides inherent rigidity, allowing the bridge to efficiently distribute weight from the deck to the supports without significant bending or shear in the members. This design leverages the geometric stability of triangles, where each member carries loads along its length, minimizing material use while maximizing strength.^[36][37][38] Common configurations of truss bridges include the deck truss, where the roadway is positioned atop the upper chords for enhanced structural depth and load capacity; the through truss, with the roadway passing between parallel trusses and overhead bracing for protection and stability; and the pony truss, featuring the roadway below the trusses without top lateral bracing, suitable for shorter spans and lighter loads. Key truss types vary in member arrangement to optimize force distribution: the Pratt truss uses vertical members in compression and diagonals in tension; the Warren truss employs equilateral triangles with alternating tension and compression in diagonals; the Howe truss reverses this with diagonals in compression and verticals in tension, originally developed for wood but adapted to metal; and the K-truss subdivides vertical compression members into shorter segments to reduce buckling risk.^[37][39][40] The development of truss bridges gained prominence in the 19th century with advancements in iron and steel fabrication, enabling longer spans and greater durability compared to wooden predecessors. A seminal example is the Britannia Bridge, completed in 1850 across the Menai Strait in Wales, designed by Robert Stephenson as a wrought-iron tubular box girder bridge that represented an early innovation in continuous rectangular box-section spans for railway use. Another pioneering structure is the Bollman Truss Bridge in Savage, Maryland, patented in 1852 by Wendel Bollman and recognized as the first successful all-iron truss design widely adopted for railroads in the United States. In modern applications, truss bridges remain prevalent in highway overpasses, where prefabricated steel trusses provide economical solutions for moderate spans over roads or railways.^[41][42][22] Structural analysis of truss bridges often relies on the method of joints, which involves isolating each connection point (joint) and applying equilibrium equations—sum of forces in x and y directions equals zero—to calculate the axial force in each connected member, assuming pin joints and loads only at joints. This approach highlights the triangulated framework's efficiency, as it ensures all forces are axial, preventing secondary effects like moments. Hybrid designs may integrate truss elements with beams for specific load conditions, but pure trusses emphasize triangulation for optimal performance.^[38][43]

Girder Bridges

Girder bridges utilize girders as the principal structural members to support the deck, functioning as deep beams that primarily resist flexural bending through their efficient I-shaped or box-shaped cross-sections. These girders efficiently transfer vertical loads from the deck to the bridge supports by developing internal shear forces and bending moments along their length, with the cross-section's geometry optimizing material use to handle compressive and tensile stresses.^[44] The primary types of girders employed in these bridges are plate girders, which consist of welded steel plates assembled into an I-section for straightforward spanning, and box girders, featuring enclosed hollow rectangular or trapezoidal sections that provide superior resistance to torsional forces due to their closed geometry.^[45] Box girders are particularly advantageous in skewed or curved alignments where twisting moments are significant.^[46] Welded girders marked a significant advancement in early 20th-century bridge engineering, with the Maurzyce Bridge in Poland, completed in 1929, serving as the world's first all-welded steel road bridge, a truss design using welded steel members and eliminating the need for rivets or bolts. This innovation reduced fabrication costs and improved structural integrity, paving the way for widespread adoption in subsequent decades. In contemporary practice, prestressed concrete girders dominate highway bridge construction due to their durability and economy, with post-tensioned designs enabling spans exceeding 300 meters in segmental applications.^[47] These girders incorporate high-strength concrete and tendons to counteract tensile stresses, allowing longer clear spans without intermediate supports. Key design considerations for girder bridges encompass limiting deflections to ensure user comfort and structural serviceability, typically to L/800 under live loads per AASHTO LRFD specifications, alongside mitigating fatigue damage from cyclic traffic loading through detail categorization and stress range controls.^[48] For a simply supported girder under uniform loading, the maximum shear force at the supports is calculated as $V = \frac{W}{2}$V=2W​, where $W$W represents the total distributed load, providing a fundamental basis for shear capacity checks.^[45] Girders can be further stiffened with transverse framing for enhanced stability under dynamic loads.

Cable-Supported Bridges

Suspension Bridges

A suspension bridge is a structure in which the deck is suspended below main cables that are draped over tall towers and anchored at each end of the span.^[49] The main cables, under their own weight and the load of the deck, form a catenary curve, a natural hyperbolic shape resulting from uniform tension and gravity.^[50] Vertical suspenders, attached at regular intervals along the main cables, transfer the weight of the roadway to the cables, allowing the deck to hang flexibly while distributing loads to the towers and anchorages.^[51] This design excels in spanning long distances, often exceeding 1,000 meters, due to the efficiency of tension in the cables.^[49] Early suspension bridges relied on chains or wrought iron links rather than wire cables, marking a transition from ancient rope designs to modern engineering. The Menai Suspension Bridge in Wales, completed in 1826 by Thomas Telford, was one of the first major iron-chain suspension bridges, with a main span of 176 meters that set a record for its time.^[52] Advancements in high-strength steel wire enabled longer spans in the 20th century; the Golden Gate Bridge in San Francisco, opened in 1937, featured a 1,280-meter main span and represented a leap in scale using parallel-wire cables.^[53] Key components include the towers, which rise high above the deck to support the main cables via saddles and bear compressive forces from the cable tension; massive anchorages, often embedded in concrete blocks weighing tens of thousands of tons, that secure the cable ends against horizontal pull; main cables composed of thousands of individual galvanized steel wires bundled into strands and wrapped for protection; and stiffening trusses or girders along the deck to minimize vertical deflection and torsional oscillations under load or wind.^[49][54] Engineering challenges include vulnerability to wind-induced vibrations, as demonstrated by the 1940 collapse of the Tacoma Narrows Bridge in Washington state, where aeroelastic flutter—a self-reinforcing interaction between wind and the slender, flat deck—amplified torsional oscillations until the main cables failed after just four months of service.^[55][56] Modern solutions incorporate aerodynamic deck profiles, such as streamlined boxes or trusses with fairings, to disrupt vortex shedding and reduce flutter onset speeds, alongside dampers and tuned mass systems for stability.^[57] The Akashi Kaikyō Bridge in Japan holds the record for the longest suspension bridge main span at 1,991 meters, completed in 1998 as part of the Honshu-Shikoku link and designed to withstand typhoons and earthquakes.^[58] Suspension bridges typically suit ultra-long spans where cable-stayed designs are less efficient for spans over about 1,000 meters.^[54]

Cable-Stayed Bridges

Cable-stayed bridges are structural systems in which the bridge deck is supported directly by inclined cables extending from one or more towers, or pylons, to various points along the deck, allowing for even distribution of loads through tension in the cables. This design contrasts with other cable-supported types by eliminating a main load-bearing cable, instead relying on the stays to transfer compressive forces to the pylons and anchorages. Common cable arrangement patterns include the fan pattern, where all cables radiate from the top of the pylon; the harp pattern, featuring parallel cables spaced evenly along the pylon height; and the semi-fan pattern, a hybrid that balances the two for optimized aesthetics and efficiency. These configurations enable the bridge to handle both vertical dead loads and live loads effectively, with the deck often acting as a stiffened girder to resist bending. The modern development of cable-stayed bridges emerged in the mid-20th century, building on earlier concepts but gaining prominence with the completion of the Strömsund Bridge in Sweden in 1955, recognized as the first contemporary example of the type with a 182-meter central span. Prior experimental designs existed in the 19th and early 20th centuries, but post-World War II advancements in materials and construction techniques, such as prefabrication, facilitated their widespread adoption for medium- to long-span crossings. By the 1970s and 1980s, the type proliferated globally, with notable early implementations like the Severinsbrücke in Germany (1959) and the Maracaibo Bridge in Venezuela (1962), marking a shift toward more economical alternatives for urban and riverine infrastructure. Key components of cable-stayed bridges include the central towers, typically constructed from reinforced concrete or steel to withstand high compressive forces; the stay cables, often made from high-strength steel strands encased in protective sheathing or, in newer designs, advanced composites for enhanced durability and reduced weight; and balanced counterweights or anchorages at the deck ends to maintain stability. Towers may be single for simpler spans or multiple (A-shaped, H-shaped, or portal frames) for greater rigidity, while the cables are arranged in single or double planes depending on the bridge's width and aesthetic goals. These elements work in tandem to create a self-anchored or externally anchored system, where the deck's own weight helps tension the stays. Cable-stayed bridges offer significant advantages for spans ranging from 200 to 1,000 meters, requiring less material and simpler construction than suspension bridges due to the direct load path from deck to towers, which reduces the need for extensive anchorage blocks. This efficiency is exemplified by the Millau Viaduct in France, completed in 2004, which features the tallest bridge pylons in the world at 343 meters and a main span of 342 meters, demonstrating the type's suitability for challenging terrains like deep valleys. Other advantages include faster erection times through incremental launching or balanced cantilever methods and adaptability to aesthetic integration in urban settings. To address environmental challenges, aerodynamic and seismic designs in cable-stayed bridges incorporate dampers, such as viscous or tuned mass dampers, attached to the cables or deck to mitigate wind-induced vibrations and oscillations from traffic or earthquakes. These devices absorb energy and stabilize the structure, ensuring longevity in regions prone to high winds or tectonic activity, as seen in bridges like the Rion-Antirion Bridge in Greece, which uses hydraulic dampers for seismic resilience. Such innovations have extended the practical application of cable-stayed designs to seismically active areas without compromising span capabilities.

Cantilever and Balanced Bridges

Cantilever Bridges

Cantilever bridges are constructed by extending projecting arms, known as cantilevers, from support piers or towers, which meet in the middle to form the span without requiring extensive falsework beneath. These structures achieve balance through moment equilibrium, where the cantilever arms are anchored or counterweighted at the supports to counteract the bending moments induced by loads. The mechanics rely on the cantilever's ability to transfer loads via compression in the lower chords or members and tension in the upper ones, ensuring stability through rigid connections at the fixed end.^[59][1] For a cantilever beam under uniform distributed load, the maximum bending moment at the support is given by $M = \frac{w L^2}{2}$M=2wL2​, where $w$w is the load per unit length and $L$L is the projection length, highlighting the quadratic increase in stress with span length.^[60] One of the earliest and most prominent historical examples is the Forth Bridge in Scotland, completed in 1890, which features cantilever truss arms spanning 521 meters across the Firth of Forth and was engineered using tubular steel sections for enhanced rigidity.^[61][59][62] This design marked a significant advancement in long-span bridge construction, overcoming previous limitations in iron-based structures. Another notable case is the Quebec Bridge in Canada, initiated in 1900, where initial cantilever construction attempts failed due to buckling under compressive loads, underscoring the need for precise stability analysis during erection.^[61][59] These examples demonstrate how cantilever bridges often incorporate truss frameworks to distribute forces efficiently across projecting members. The primary construction technique involves sequential building from the piers, extending cantilever arms outward using temporary ties, braces, or guy wires to maintain equilibrium and prevent collapse before the spans connect. This method minimizes the need for scaffolding over water or difficult terrain, allowing spans up to 550 meters by erecting segments incrementally with cranes or derricks. For steel cantilever bridges like the Forth Bridge, assembly occurs on-site with riveted joints, while concrete variations employ form travelers to cast segments progressively.^[59][1] Variations in cantilever bridges include balanced designs for concrete structures, where opposing arms are built symmetrically to equalize moments, as seen in the Confederation Bridge in Canada, completed in 1997 with 44 piers supporting 250-meter spans using precast prestressed segments. This approach enhances efficiency for marine environments by reducing material waste and construction time. Cantilever bridges may also feature suspended spans between the projecting arms to complete the crossing.^[63][64] Limitations of cantilever bridges stem from their high material demands to achieve the stiffness required for long projections, as the structure must resist significant bending moments without intermediate supports. The risk of instability during construction, such as lateral buckling or uplift, necessitates extensive temporary bracing, increasing costs and complexity; for instance, unbraced lengths can lead to eigenvalue buckling factors below 1.0, requiring redesign. Additionally, the quadratic moment relationship limits economical spans to around 500 meters without supplementary measures.^[59][1]

Balanced Cantilever Bridges

Balanced cantilever bridges are a specialized form of prestressed concrete bridge where symmetric cantilever arms extend equally from each pier toward the mid-span, eventually meeting with a closure pour to create continuous spans without intermediate supports. This construction technique relies on post-tensioning to manage tensile stresses, enabling longer spans and efficient load distribution in the concrete superstructure.^[65] In terms of mechanics, post-tensioned tendons are installed within the concrete segments to generate compressive forces that counteract the bending moments from self-weight and applied loads, allowing for slender box-girder sections with reduced material usage. The balanced approach ensures equilibrium during construction by symmetrically extending arms from piers, using formwork or lifting equipment that maintains minimal unbalanced moments on the substructure and prevents excessive deflection.^[66][65] Notable modern examples include the approach viaducts of the Rio-Niterói Bridge in Brazil, completed in 1974, which utilized balanced cantilever construction for post-tensioned concrete box girders with spans up to 80 meters. More recent designs incorporate seismic-resistant features, such as the balanced cantilever bridge over the Acheloos River in Greece, completed in 2024 with a three-span configuration of 47.5 m–80 m–47.5 m, featuring high-strength concrete and ductile detailing to accommodate earthquake forces in a high-seismicity zone.^[67][68][69] Compared to traditional cantilever methods, balanced cantilever construction requires fewer temporary supports, as the symmetric progression eliminates the need for extensive falsework beneath the span, and it adapts well to curved alignments through incremental segment placement.^[70][71] The construction process typically begins with completing the pier and casting or placing a starter segment atop it, anchored to the pier head. Subsequent segments—either cast-in-place using traveling formwork or precast off-site—are then added alternately to each cantilever arm: one segment is positioned and temporarily stressed on one side, followed by a matching segment on the opposite side to restore balance. Post-tensioning tendons are stressed after each pair of segments to compress the structure and control deflections. This alternating sequence continues outward from the pier until the arms from adjacent piers approach mid-span, at which point a final closure segment is cast or erected, followed by full tensioning and grouting of ducts to complete the span. In some precast applications, segments may be match-cast vertically near the site and rotated into horizontal position before lifting into place with cranes or gantries.^[72][73]

Movable Bridges

Bascule Bridges

A bascule bridge is a movable bridge in which the deck, or span, pivots upward around a fixed horizontal axis to provide vertical clearance for marine traffic beneath it.^[74] The term "bascule" derives from the French word for "seesaw," reflecting the balanced tilting motion achieved through counterweights that offset the weight of the lifting span, allowing it to rotate with minimal energy input. There are two primary types: single-leaf bascule bridges, where one span pivots from a single support point to cover the full navigation channel, and double-leaf bascule bridges, featuring two opposing spans that tilt upward simultaneously to meet in the center when closed.^[75] This design is particularly suited for urban waterways where space is limited, offering efficient clearance similar to other movable bridges without requiring extensive horizontal clearance.^[76] The mechanics of bascule bridges center on the trunnion, a fixed pivot axle typically located near the base of the span to optimize balance, positioned to minimize counterweight size.^[77] Counterweights, usually made of concrete or steel and attached to the rear of the span, provide the balancing force, significantly reducing the torque needed for lifting by balancing the span's weight.^[78] Operation is powered by hydraulic cylinders or electric motors connected to gear systems that drive the rotation, with hydraulic systems preferred for their high torque in modern designs and electric motors used in older or lighter spans.^[79] Trunnion placement and counterweight configuration are critical for smooth operation, ensuring the center of gravity aligns closely with the pivot axis throughout the lift.^[75] One of the earliest and most iconic historical examples is the Tower Bridge in London, completed in 1894, which features a double-leaf bascule design with distinctive Gothic-style towers housing the counterweights and machinery.^[80] Designed by Sir John Wolfe Barry and Horace Jones, it was engineered to lift its 1,210-tonne central spans using steam-powered hydraulics, opening over 6,000 times in its first year to accommodate river traffic.^[81] This bridge marked a significant advancement in bascule engineering, demonstrating the feasibility of large-scale tilting spans, with each leaf approximately 30 meters long.^[81] In modern applications, bascule bridges continue to serve urban waterways, with typical span lengths ranging from 30 to 100 meters to balance navigational needs and structural efficiency.^[82] A notable example is the Pegasus Bridge in Normandy, France, originally constructed in 1934 as a single-leaf rolling bascule over the Caen Canal; it gained historical significance during World War II when British forces captured it intact on D-Day, June 6, 1944, to secure Allied supply lines.^[83] Today, upgraded versions of such bridges incorporate advanced materials and controls for frequent urban use, such as in port cities where they facilitate both vehicular and maritime traffic.^[84] Safety features in bascule bridges include mechanical interlocks that prevent operation unless the span is fully aligned and locked in the closed position, ensuring continuity of the deck surface and rail alignment where applicable.^[76] End locks and tail locks secure the leaf against unintended movement, while wind bracing systems, such as diagonal struts and guy wires, stabilize the structure during lifting to resist lateral forces from wind.^[85] Additionally, automated control systems integrate sensors for position, load, and environmental conditions, halting operations if anomalies like high winds or obstructions are detected.^[77]

Swing Bridges

A swing bridge is a type of movable bridge that rotates horizontally around a vertical pivot axis, typically located on a central pier, to provide clearance for marine traffic. The deck, when closed, aligns parallel to the roadway or rail traffic flow, supported at its ends by fixed piers or abutments; during opening, it swings perpendicular to the flow, powered by mechanical systems such as electric motors, gears, or hydraulic drives that engage a pinion with a rack or bull gear on the pivot.^[86][78] Swing bridges are classified primarily by their bearing systems, which determine how the span's weight is supported. In rim-bearing designs, the dead load is carried by tapered rollers or a slewing bearing along the outer rim or track, providing stability through multiple support points and often used for longer spans where rigidity is needed. Center-bearing types, more common in modern applications, transfer the weight directly to a central pivot thrust bearing, such as a bronze-on-steel or spherical anti-friction unit, requiring fewer end supports but necessitating balance wheels on a circular track to prevent tipping from unbalanced loads like wind or live traffic. A combined-bearing variant distributes the load between the center pivot and rim supports for enhanced stability in demanding conditions.^[87][78][86] Historically, swing bridges emerged in the late 19th century as one of the earliest movable types, particularly for railroad applications where navigation channels intersected rail lines, with development accelerating from the 1890s to the 1920s before being largely supplanted by bascule and vertical lift designs due to their slower operation and pier obstruction issues. Early examples include the Sault Ste. Marie International Railroad Bridge, constructed in 1887 with a swing span added in 1895 to accommodate vessel passage over the St. Marys River. For wider channels requiring greater navigational clearance, double swing configurations—featuring two adjacent or back-to-back swing spans—were employed, as seen in the Coleman Memorial Bridge in Virginia (1952), the largest such structure in the United States at 3,750 feet long.^[87][88] Engineering features emphasize balance and protection to ensure reliable operation. The span is typically designed with symmetrical arms of equal length extending from the pivot to distribute weight evenly and minimize rotational torque, though bobtailed (unequal) arms may incorporate counterweights on the shorter side; this self-balancing acts as a double cantilever during swinging. Fender systems, constructed from heavy timber, steel, or energy-absorbing materials around the central pier, protect against vessel collisions by deflecting or cushioning impacts. The rotation itself can be completed in under two minutes, though full operations including safety checks often extend to several minutes total.^[78][86][89]

Vertical-Lift Bridges

A vertical-lift bridge is a type of movable bridge in which the deck is raised vertically while remaining horizontal, using cables and counterweights attached to towers on either side of the span, to provide clearance for marine traffic. This design is ideal for sites requiring high vertical clearance over deep or wide channels without the horizontal swing space needed for other movable types. The mechanics involve wire ropes or chains that lift the deck via sheaves at the top of the towers, with counterweights typically equal to or slightly heavier than half the deck's weight to facilitate smooth operation and reduce power requirements. Power is supplied by electric motors or hydraulics driving winches, allowing lifts of 50-100 meters in height for spans up to 100 meters.^[86] Historically, vertical-lift bridges gained prominence in the early 20th century for railway and highway crossings, exemplified by the Arthur Kill Vertical Lift Bridge in New York (opened 1959, later replaced), which featured a 210-meter span lifted 41 meters. They are valued for their ability to handle unbalanced loads and seismic conditions better than bascules or swings. Modern examples include the Pegasus Bay Bridge in New Zealand, incorporating advanced sensors for automated operation as of 2020.^[90]

Special-Purpose Bridges

Floating Bridges

Floating bridges, also known as pontoon bridges, are structures that utilize buoyant pontoons to support a continuous deck across bodies of water, providing a viable alternative to traditional fixed-span bridges in locations where deep water, soft seabeds, or high construction costs make piers impractical.^[91] The mechanics rely on the pontoons—typically hollow concrete caissons, steel barges, or wooden floats—displacing water to generate buoyancy, while the deck is anchored laterally with tension cables or chains to resist currents and winds, allowing the entire assembly to flex and rise with wave action or tidal changes without structural failure.^[92] This design distributes loads across multiple pontoons spaced at regular intervals, typically 50 to 100 meters apart, enabling spans up to several kilometers in calm or semi-protected waters.^[93] Floating bridges are categorized into temporary and permanent types, with temporary versions often deployed for military or emergency purposes using modular, prefabricated components that can be rapidly assembled and disassembled. The Bailey pontoon bridge, developed by the British Army in 1940–1941, exemplifies this type; it combines portable truss panels with inflatable or rigid pontoons to create crossings over rivers, supporting loads up to 40 tons and enabling swift advances during World War II operations, such as the crossing of the Rhine in 1945.^[94] Permanent floating bridges, in contrast, employ more robust, fixed pontoons designed for long-term vehicular traffic, such as the Evergreen Point Floating Bridge in Washington State, USA, completed in 1963 with a floating span of 2,310 meters, which held the record as the world's longest until its replacement in 2016.^[95] Construction of floating bridges involves modular prefabrication on shore, where individual pontoons are built, ballasted for stability, and towed to the site before being connected via steel girders and tensioned cables to maintain alignment under load.^[96] Anchoring systems, including deadman piles or helical anchors embedded in the seabed, secure the pontoons against longitudinal drift, while flexible joints accommodate vertical movements from waves or tides up to several meters.^[97] Key challenges include managing tidal fluctuations, which require the bridge to expand or contract via sliding expansion joints without buckling, and ensuring seismic stability in earthquake-prone areas through damping systems and curved alignments that reduce wave-induced vibrations.^[98] A prominent example is the Lake Washington Floating Bridge in Washington, USA—formally the Lacey V. Murrow Memorial Bridge—opened in 1940 as the world's first major concrete pontoon bridge, spanning 2,020 meters across the lake and utilizing 25 watertight concrete pontoons that were sunk during a 1990 storm while under widening construction but later recovered, repaired, and reopened in 1993.^[99] This bridge demonstrated the feasibility of permanent floating structures for urban commuting, continuing to serve as a major route for urban commuting, carrying significant daily traffic as of 2025.^[93] Despite their advantages, floating bridges have limitations, including vulnerability to high winds that can cause excessive swaying beyond design tolerances and the need for integrated openings, such as bascule spans, in active shipping lanes to permit vessel passage.^[100] They are generally restricted to inland lakes, sheltered straits, or low-current environments, as strong tidal flows or open ocean conditions can overwhelm anchoring systems and compromise safety.^[101]

Pedestrian and Footbridges

Pedestrian and footbridges are engineering structures designed primarily for non-motorized traffic, including walkers, cyclists, and occasionally equestrians, to cross obstacles such as roads, railways, rivers, or urban barriers. These bridges emphasize lightweight construction, aesthetic integration with surroundings, and enhanced user comfort, as they handle lower loads than vehicular bridges but require stringent controls on vibrations from human activity. Unlike general-purpose bridges, pedestrian designs prioritize serviceability limits to prevent excessive dynamic responses, with spans typically ranging from 10 to 100 meters depending on the structural system.^[102] Common structural types for pedestrian bridges adapt proven forms to lighter loads and pedestrian-scale dimensions. Beam bridges, often using steel or concrete girders, suit short spans up to 30 meters and offer simple, cost-effective construction with minimal on-site assembly. Truss bridges, featuring triangular frameworks of steel members, excel in spans of 15 to 60 meters by distributing loads efficiently through prefabricated elements, reducing material use and enabling quick installation; examples include Pratt or Warren configurations with depths of at least 1/20th the span for stability. Arch bridges provide elegant support for medium spans up to 90 meters, leveraging compressive forces in curved ribs made of concrete or steel to create visually striking forms while maintaining structural rigidity. For longer crossings, cable-stayed bridges (spans over 40 meters) employ towers and diagonal cables for balanced tension, and suspension bridges (over 70 meters) use main cables draped over towers to hang the deck, allowing flexible, lightweight designs ideal for scenic or landmark applications.^{[102][103][104]} Design standards, such as the AASHTO LRFD Guide Specifications for the Design of Pedestrian Bridges, dictate key parameters including a uniform live load of 90 pounds per square foot (4.3 kN/m²) for pedestrian crowds, minimum railing heights of 42 inches (increasing to 54 inches for cyclist or equestrian use), and deflection limits to ensure vibration frequencies exceed 3 Hz for vertical modes to avoid resonance with walking paces. Accessibility features mandate minimum clear widths of 2 meters (3.5 meters for shared pedestrian-cyclist paths) and ramp gradients no steeper than 1:20 for wheelchair compliance, often extending ramp lengths up to 120 meters to achieve required clearances like 5.7 meters over highways. Materials commonly include structural steel (e.g., hollow sections or plates) for durability and ease of fabrication, with composite decks of concrete on steel beams for added stiffness; weathering steel or galvanization protects against corrosion in exposed environments. In a comparative study of systems for a 32-meter span in Kuwait, steel truss designs proved lightest (561.6 kN total weight) and most economical (4,288.5 Kuwaiti dinars per span), though concrete options offered superior stiffness at higher weights (2,186.7 kN).^{[105][106][102]} These bridges also incorporate safety and sustainability elements, such as non-slip deck surfaces, wind-resistant parapets (1.15 meters high over roads, 1.5 meters over railways), and low-carbon materials to minimize environmental impact; for instance, steel systems emit the least CO₂ (64.2 tons per span in the Kuwait analysis) during production. Representative examples include the truss-based Al-Jawazat roundabout bridge in Kuwait, which spans 32 meters with 6-meter clearance using X-braced steel for optimal weight-to-strength ratio. Overall, pedestrian bridge engineering balances functionality, economy, and visual appeal to enhance urban connectivity while adhering to codes like Eurocode 3 or AASHTO for reliability.^{[107][102][107]}