The workhorse. A slab bridge is a solid or voided concrete plate spanning between supports, economical to about 15 m solid and 25 m voided, with nothing to inspect but the underside. Beyond that the slab becomes a deck resting on girders, and the girder does the spanning.

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• Reinforced concrete T-beams cast in place: 12–25 m, common in older highway stock.
• Precast prestressed concrete girders, AASHTO I-sections, bulb-tees, NU and NEXT beams: 20–50 m per span, occasionally 60 m with spliced segments. Factory-made, erected in a night, the dominant highway bridge in most countries.
• Steel plate girders, welded I-sections acting compositely with the deck: 30–120 m, longer when continuous and haunched. Light to erect, need painting or weathering steel.

Girders carry load by bending, so depth is the main lever: typical span-to-depth ratios run from about 15 for simple concrete spans to 30 or more for continuous steel. The site's girder depth tool gives the AASHTO minimums.

Close the section into a hollow box and two things happen: the top and bottom flanges are both wide, so the section is efficient in both positive and negative bending; and the closed cell is enormously stiff in torsion, which lets the bridge be curved in plan or carry eccentric load without twisting. That is why almost every long-span concrete girder bridge and every curved elevated highway is a box.

single-cell boxmulti-cell boxsteel tub

Cast-in-place prestressed concrete boxes built by balanced cantilever reach 200–300 m; the Shibanpo Bridge in Chongqing holds the girder record at 330 m by using a steel box for the middle of its main span. Steel boxes with orthotropic decks are lighter and are used both as girders and as the deck of cable-supported bridges. Single-cell, multi-cell, and trapezoidal "tub" boxes are chosen by width and by how the deck will be built.

The price of the box is complexity: internal diaphragms, thermal gradients through the section, distortion of the cross-section under torsion, and a hollow interior that must be ventilated and inspected.

A truss is a beam with the web replaced by triangles, so every member carries axial force only. Members can be small, connections are the work, and the structure is transparent to wind. The classic forms are named for their diagonals: Pratt (diagonals in tension, the steel favourite), Howe (diagonals in compression, the timber favourite), and Warren (alternating, no verticals).

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Trusses are deck, through or half-through depending on where the traffic runs. They dominated railway bridges from 1850 to 1950 and reached 549 m in cantilever form at Quebec. Today they survive where stiffness matters, long railway spans, the stiffening girders of some suspension bridges, and the Ikitsuki (400 m) and similar continuous truss spans in Japan.

Their weakness is maintenance. Hundreds of connections gather water and dirt; eyebars and pins cannot be inspected without dismantling; and a two-truss bridge has no redundancy, which is why the Silver Bridge collapse of 1967 created the fracture-critical member inspection rules that Chapter 10 describes.

An arch carries load in compression and pushes outward at its springings. If the ground can take that thrust, rock in a gorge, the arch is the natural form; if it cannot, a tie between the springings takes the thrust and the arch becomes a self-contained beam, the tied arch or bowstring.

deck archthrough (tied) archhalf-through arch

• Deck arch: traffic above, spandrel columns down to the rib. Gorges and valleys.
• Through arch: traffic hangs below the rib on hangers, needed where clearance below is limited. Usually tied.
• Half-through: deck cuts through the rib mid-height.

Hinges control how the arch responds to settlement and temperature: three-hinged arches are statically determinate and forgiving; two-hinged are stiffer; fixed arches are stiffest and most sensitive. Materials went from stone to concrete (Krk, 390 m) and steel (New River Gorge, 518 m; Chaotianmen, 552 m) to concrete-filled steel tube, which gives China the current record at the Tian'e Longtan Bridge, 600 m (2024), ahead of Pingnan Third at 575 m. Arches are built by cantilevering the rib out on temporary stays, or by rotating half-arches into place; see the Methods animations.

Straight cables run from a tower directly to points along the deck, so the deck is a beam on many elastic supports, and it is also a compressed column because the horizontal component of every stay pushes it toward the tower. That combination, deck in compression, stays in tension, tower in compression, is very stiff for its weight and builds itself: each new deck segment is hung on its stay and used as the platform for the next.

Stays are arranged as a fan (all converging at the tower top), a harp (parallel), or the semi-fan most large bridges use. Design choices: one plane of stays (needs a torsionally stiff box deck) or two; concrete, steel or composite deck; tower shapes from a single mast to H, A, inverted-Y and diamond. Spans of 200–600 m are routine; the largest (Changtai, 1,208 m; Russky, 1,104 m; Sutong, 1,088 m) needed stays over 500 m long and decks that resist buckling under the accumulated compression. Stays are replaceable strand bundles in sheathing, and their fatigue and vibration behaviour (rain–wind vibration especially) is a specialist subject.

Drape a main cable over two towers and anchor it in the ground; hang the deck from it on vertical hangers. The main cable is a pure tension member, the towers are columns, and the deck is a stiffening girder whose only job is to spread concentrated loads and keep the shape stable. Nothing spans farther for less material, which is why every span over about 1,250 m is a suspension bridge.

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The parts:

• Main cable, thousands of 5 mm galvanised wires, spun in place from anchorage to anchorage (aerial spinning) or lifted as prefabricated parallel-wire strands (PPWS), then compacted and wrapped.
• Anchorages, gravity blocks of concrete, or tunnels into rock, holding a pull measured in hundreds of meganewtons.
• Towers, steel or concrete, 200–330 m tall on the largest bridges.
• Stiffening girder, a truss (Akashi, Golden Gate) or a streamlined steel box (Severn onwards). The box is lighter and aerodynamically stable; the truss is stiffer for railways.

Where there is no room for anchorages the cable can be tied to the deck itself, a self-anchored suspension bridge (San Francisco–Oakland Bay east span, Yeongjong), at the cost of building the deck first. The span record is 2,023 m (1915 Çanakkale); designs beyond 3,000 m exist and are limited by cable weight and aerodynamics rather than by strength.

An extradosed bridge looks like a cable-stayed bridge with the towers cut short: stays leave a low pylon at a shallow angle and act less like supports than like external prestressing tendons draped outside the girder. The deck stays deep and stiff, the stays see small stress ranges and can be designed at higher allowable stress, and the whole thing is built as a balanced cantilever. The form fits spans of 100–275 m where a box girder would be too deep and a cable-stayed bridge too tall; Japan built the first (Odawara, 1994) and many since, and it has become common in Korea, China and India.

cable-stayed: tall tower, steep staysextradosed: short pylon, shallow stays, deep girder

Other hybrids exist because real sites are awkward: cable-stayed with a suspended centre (a "cable-stayed suspension" system used at the Third Bosphorus and in several Chinese Yangtze crossings), arches with stays, and cantilever trusses with cable assistance. Each mixes two load paths, and the designer's job is to make sure the stiffer one does not steal all the load from the other.

When ships need more clearance than a fixed bridge can give, part of the bridge moves. Three families cover almost every case: the bascule tilts one or two leaves upward about a horizontal axis, balanced by a counterweight (Tower Bridge); the swing bridge rotates about a central pier to open two channels; the vertical lift raises a whole span between two towers on counterweighted cables, the choice for long spans and railways.

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Movable bridges are machines as much as structures. The engineering questions are balance, machinery and control, fatigue from thousands of openings, the alignment of the span when it lands, and what happens when the power fails. They cost more to run than to build, and are usually replaced by high-level fixed bridges when traffic allows.

Type selection is where a bridge is really designed. The factors, roughly in the order a designer meets them:

• Span layout, set by the obstacle, the navigation channel, the flood, and where piers can stand. It decides the family: under 50 m girders, 50–250 m boxes and arches, 250–1,000 m cable-stayed, beyond that suspension.
• Ground, rock allows arches and cheap anchorages; soft soil pushes toward light superstructures and fewer, larger foundations.
• Construction access, can cranes reach? can barges? Is the crossing over live traffic, so it must be built from above?
• Environment, seismicity, wind, ship impact, salt, ice, temperature range.
• Life-cycle cost and maintenance, joints and bearings, paint, cable replacement.
• Appearance, which is not last; bridges are the most public structures a society builds.

Two tools on this site encode the first pass: the span-type recommender and the method selector. They will not choose a bridge for you, but they will show you which options are not silly.