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Elevated metro viaducts are built by creating foundations and piers, preparing the bridge spans, then erecting, joining and finishing the superstructure above them. For precast concrete segmental viaducts, crews may work outward from piers in balanced cantilever construction or assemble one span at a time with an overhead gantry; longitudinal incremental launching is a different method that pushes an assembled structure forward from behind an abutment. The method depends on the alignment, spans, site access, structural design and construction sequence.
What are the main parts of an elevated metro viaduct?
A viaduct carries the track and trains on a series of spans supported by piers. Its visible deck is only one part of a connected load path: forces pass from the superstructure through bearings or other connections into pier caps, shafts and foundations, then into the supporting ground.
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- Pier: The support between foundation and span. In a general FHWA component breakdown, a pier includes a footing, a shaft—which may be a wall or columns—and a pier cap.
- Superstructure: The span or spans that carry the track. In the methods described here, it may consist of precast concrete segments joined into a box girder. Other bridge forms are possible.
- Bearings or integral connections: The interface between span and support. Bearings transfer forces while allowing specified movement and rotation; some designs instead connect the superstructure directly to the pier.
The right arrangement is project-specific. Ground conditions, span geometry, alignment, movement demands, access for construction and maintenance, and applicable owner and jurisdiction requirements all affect the design.
How are foundations and piers built?
Construction starts with a foundation and support system designed for the site—not with a universal pier detail. The foundation type depends on subsurface conditions and the loads it must carry. A shallow footing may bear directly on soil or rock, while deep foundations such as driven piles or drilled shafts may be used where the design calls for them. FHWA reference material also identifies caissons as an option.
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Above the foundation, the pier shaft carries load to a cap that supports the superstructure. A shaft can take the form of a wall or columns. The cap provides a support area and connection geometry for the span, including a bearing seat where bearings are specified. FHWA’s overview of prefabricated bridge elements and systems describes a range of substructure components, including piles, columns, pier walls, drilled shafts, pile caps and spread footings.
Elements may be cast in place, precast and assembled on site, or built with a combination of these approaches. For prefabricated components, the general construction priorities include planned erection, accurate alignment and dimensions, stable temporary support, clean connection surfaces and controlled grouting. The exact details and acceptance checks are set by project documents and governing standards; these general points are not a substitute for a project-specific erection plan.
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How are viaduct segments made and aligned?
Precast segments are made to fit together in a planned sequence, so geometric control matters both in the casting process and during erection. The Federal Highway Administration calls proper segment geometry and control during precasting key to the design and construction of precast segmental bridges in its Bridge Geometry Manual, Part 2.
In the manual’s example process, survey coordinates for a starter segment are related to the casting cell; the bulkhead is surveyed; reinforcement and forms are installed; and reference elevations and centerline marks are recorded. Later segments are set up relative to the match-cast segment so their mating faces and geometry correspond to the design. Match casting is one way to produce fitting faces and control geometry; it does not mean every project uses an identical casting setup.
The scale varies with the project. FHWA’s manual describes Foothills Parkway Bridge No. 2, a 790-foot, five-span precast concrete segmental box-girder bridge with spans of 125 feet, three at 180 feet, and 125 feet. The example follows an S-shaped alignment with curve radii of 262 and 650 feet. Typical segments were 8 feet 8 inches long, and the cited segment weights ranged from 40 to 45 tons. Those figures describe that bridge—not standard dimensions or weights for metro viaducts.
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How are viaduct segments lifted into place?
For precast segmental construction, two common erection approaches are balanced cantilever and span by span. Both require an engineered sequence: temporary equipment and supports must work with the segment weights, span geometry, access and structural demands at each stage, not just the completed bridge.
Balanced cantilever: building outward from a pier
In balanced-cantilever construction, segments are erected outward from a pier on each side. The two advancing arms help balance one another during construction. Depending on site access and the erection plan, segments may be placed with a ground crane and temporary stability towers or by an overhead launching gantry that carries segments to the work area, including over completed portions of the viaduct.
After the cantilever arms reach the planned closure point, a closure segment or joint connects them. Tendons are then installed and stressed according to the sequence so the connected spans act as designed. The structure’s temporary cantilever condition and its final continuous condition are distinct design stages.
Span by span: supporting and completing a full span
In span-by-span construction, an erection gantry supports a span while its match-cast segments are positioned and stressed together. Once that span is completed, the gantry advances or is repositioned for the next span, and the sequence repeats. A gantry’s capacity and arrangement have to suit the project’s spans, segment weights and access conditions; the name of the equipment alone does not tell you how the structure is supported at every stage.
Both methods can reduce reliance on access from the ground in some settings, but the workable choice depends on the site and design. A comparison should consider where cranes can stand, how materials reach the work area, temporary stability, alignment and span arrangement, closure locations, tendon sequence and the construction-stage forces. There is no universal winner for every metro alignment.
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What does a launching girder do?
“Launching girder” is sometimes used loosely, but two different ideas should be kept separate. An overhead launching gantry is erection equipment that supports or places segments. Longitudinal incremental launching is a construction method in which a longer, assembled superstructure is pushed forward from behind an abutment over successive supports.
In incremental launching, the structure advances in planned stages, and the supports and launching equipment must accommodate the movement and forces involved. FHWA’s international case study describes a curved, continuous 13-span steel-girder highway viaduct with spans of about 60 metres (197 feet). It was assembled in 180-metre (590-foot) lengths and launched in increments of 120 metres (394 feet). Laterally adjustable roller bearings accommodated movement during launching, and jacking guided the nose at piers. This illustrates the concept; it is not a metro specification, and its dimensions should not be treated as typical for metro construction.
The practical distinction is simple: a gantry moves or supports pieces as a span is erected; incremental launching moves an assembled length of superstructure across supports. Which method is feasible depends on the alignment, structure, work area behind an abutment, support arrangement and planned construction stages.
Why are segments post-tensioned and closed together?
Post-tensioning ties segments together and helps the structure resist forces as construction progresses and after spans become continuous. The tendon layout and stressing order are part of the structural design and erection sequence; they are not an afterthought once the segments are in place.
FHWA’s Post-Tensioning Tendon Installation and Grouting Manual explains that cantilever tendons resist dead-load moments during construction and effects from later loads. Continuity tendons connect adjacent cantilevers and resist effects including positive moments from superimposed loads, creep redistribution and live loads.
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In the balanced-cantilever sequence, tendons are stressed as the cantilever segments are erected. When the arms are complete, a mid-span closure is made; continuity tendons are installed and stressed through the closure as specified. The sequence matters because the bridge changes from separate advancing cantilevers to a connected span system. A segmental viaduct is therefore not simply a row of blocks placed in their final positions.
What do bridge bearings do?
Where a bridge uses bearings, they sit between the superstructure and substructure and transfer load while allowing the movement and rotation specified by the design. Movement may be needed as the structure responds to changing conditions; if a bearing restrains movement that the bridge needs, forces can develop and must be accounted for in the design.
Bearing selection has to suit the expected loads and movements. The interface also needs practical attention: FHWA’s prestressed concrete girder example for an elastomeric bearing notes that dirt and moisture can contribute to corrosion and deterioration in bearing areas. Protection and access for inspection are therefore relevant durability considerations. The example’s assumed properties and design values apply to that specific design case, not to every bearing or viaduct.
Bearings are common, but not universal. FHWA’s guidance on connections for prefabricated bridge elements also describes integral pier connections and closure pours. These alternatives can require close control of grade and tolerances. Project details govern whether a bearing or another connection is appropriate, including how movement, load transfer and adjustment are handled.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What determines the construction method for a particular metro line?
The method follows the project’s geometry, ground, structure and construction constraints. A decision about balanced cantilever, span-by-span erection or incremental launching is connected to the full sequence, including how segments arrive, where equipment can operate, how temporary stability is maintained and how closures and tendons are completed.
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| Design or construction factor | Why it matters |
|---|---|
| Ground and foundations | Subsurface conditions influence the foundation and the support system that must carry the pier and span loads. |
| Alignment and span arrangement | Curves, span lengths and closure locations affect segment geometry, erection access and the construction sequence. |
| Work access | Ground-crane access differs from delivering segments over completed structure or using overhead equipment. |
| Temporary and construction-stage forces | Cantilevers, gantries, launching operations, closures and tendon stressing create conditions that must be considered before the bridge is complete. |
| Support interface and maintenance | Load and movement requirements, tolerances, adjustment and inspection access affect bearings or alternative connections. |
The cited FHWA publications are general bridge-engineering references, mainly focused on highway bridges, plus an international launching case study. They explain relevant construction principles but do not establish the transit loads, metro clearances, seismic rules, fire and life-safety provisions or contract requirements for an unidentified metro line. Those requirements depend on the project and its jurisdiction. The source material also does not provide a general metro-wide construction cost, duration or productivity figure, so individual bridge examples cannot establish one.
How we picked
This is an explanation of construction methods, not a ranking of them. The comparison focuses on the factors the engineering references identify as material: work access, erection sequence and temporary stability, geometry and site constraints, construction-stage demands, and the foundation and bearing interface. A project team must evaluate those factors against its own design and governing criteria.
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Are elevated metro viaducts always made from concrete segments?
No. The construction methods discussed here include precast concrete segmental bridges, but the FHWA incremental-launching example is a steel-girder viaduct. The material and structural form depend on the specific project design.
Can a highway bridge manual set the design rules for a metro viaduct?
Not by itself. The FHWA sources explain general bridge principles, while a metro project also has to follow the applicable transit, seismic, clearance, safety and owner requirements for its location.
Why does a curved viaduct need careful survey control?
Segments must mate and follow the intended alignment, so their geometry and reference points need to be controlled during casting and erection. The FHWA geometry manual’s Foothills Parkway example illustrates that a segmental bridge can follow an S-shaped alignment, but its dimensions are specific to that bridge.
