An overview of bridge and viaduct construction techniques
Bridge construction techniques are the building methods that determine how a deck (the superstructure) is carried over the piers and how it is assembled. Choosing the right technique is not driven by a single formula; span length, the topography of the terrain, the nature of the obstacle below (river, valley, highway, railway), ground conditions, schedule and budget are weighed together. Two different methods may be technically feasible for the same crossing; the difference usually shows up in cost, duration and risk.
In practice, the distinction between a bridge and a viaduct also influences the method. A bridge typically describes a structure spanning a watercourse or a single obstacle, while a viaduct describes a long structure crossing a valley or low ground on tall piers with many consecutive spans. Because the spans of a viaduct repeat, serial and repetitive (mechanised) construction methods become attractive; for a single long-span bridge, by contrast, each span becomes almost a bespoke engineering problem.
This article covers the three main families most common in the sector: balanced cantilever, incremental launching and the prestressed concrete bridge methods cast on falsework. We then touch on steel and composite alternatives, the criteria for method selection, and quality and safety. The aim is for a project owner or a junior civil engineer to clearly grasp which method makes sense under which conditions.
The balanced cantilever method
The balanced cantilever method builds the deck segment by segment, advancing symmetrically in two directions from each pier. The two arms of the pier are extended simultaneously so that the moment stays balanced and no falsework is needed underneath. This makes the method ideal for deep valleys, wide rivers and crossings where traffic must continue uninterrupted. It is typically economical for spans between 60 and 250 metres; for larger spans it is combined with cable-stayed systems.
There are two main variants in practice. The first is cast-in-place: form travellers are hung at the two ends of the pier and, in each cycle, a 3 to 5 metre segment is cast in place, the prestressing tendons are stressed, and the traveller then advances to the next segment. The second is precast segmental: segments are produced in advance in a factory or site yard, lifted by crane or launching gantry into position and joined with epoxy joints and prestressing. The precast approach is superior in quality control and speed, while cast-in-place is more flexible for singular, unusual geometries.
The critical engineering concern of the method is balance and stability. The weight difference at the cantilever tips, wind, the self-weight of the form traveller and any imbalance during casting impose a bending moment on the pier in real time; for this reason the static analysis of the construction (erection) stage is as important as that of the finished structure. The most common mistakes are under-designing the temporary anchorages or supports, miscalculating the camber predictions so that the two cantilevers fail to meet at the midpoint, and applying a prestressing sequence inconsistent with the static model.
The incremental launching method
The incremental launching method produces the deck piece by piece in a fixed casting bed set up at one abutment (behind it) and positions it by pushing it forward over the piers with hydraulic jacks. In each cycle a new deck segment is cast at the abutment and joined to the previous part by prestressing; then the whole deck is slid forward by one span length. The process repeats until the bridge reaches the far bank. The method is very efficient for long viaducts with equal spans on straight or constant-curvature alignments, especially in the 30 to 60 metre range.
The greatest advantage of incremental launching is that the production area is concentrated at a single point. Labour, concreting and quality control all take place in the same location, at ground level, in a safe and controlled environment; no falsework or form traveller is needed underneath. It is therefore preferred in deep valleys, environmentally sensitive terrain, flood-prone rivers and crossings where traffic keeps flowing below. During launching, a lightweight steel nose (launching nose) is attached to the forward-projecting tip of the deck; this nose reduces the moment at the tip of the not-yet-supported cantilever so it can reach the next pier.
The challenges of the method also arise from this continuous movement. Each cross section of the deck is subjected to alternating positive and negative moments during launching; for this reason the prestressing is usually designed as concentric and uniform along the structure, with additional (continuity) prestressing applied after launching is complete if necessary. The geometric tolerance is very tight: the bridge must be a straight line or a constant-radius arc, because the deck moves like a rigid body over sliding surfaces. To control friction, low-friction sliding bearings (PTFE on stainless steel) are placed at the pier heads, and lateral guide devices are used.
Prestressed concrete and casting on falsework
Prestressed concrete bridge methods form the foundation of all modern bridge engineering. Prestressing is the act of imparting compression to concrete in advance by stressing high-strength steel strands (tendons) placed within the reinforcement. This closes the tension zones where concrete is weak, making thinner decks with longer spans possible. There are two main approaches: pre-tensioning - typically used for precast factory-made beams where the strands are stressed first and the concrete cast afterwards; and post-tensioning - where tendons are left inside ducts, stressed after the concrete hardens, and protected by grouting.
Many short and medium-span bridges and viaducts are built by lifting factory-made precast prestressed beams (I, T or box sections) onto the piers with a crane and casting an in-situ deck slab on top. This method is fast, economical and extremely common for spans of 20 to 45 metres. A more monolithic alternative is cast-in-situ on falsework: the deck is cast in one go or span by span on a temporary scaffold and formwork system erected below. Where the ground is suitable and the height reasonable, this is the simplest and most flexible method; however, it requires continuous access and firm ground underneath.
Common mistakes in these methods stem from a lack of technical discipline. Incomplete grouting of tendon ducts leaves internal voids that lead to corrosion and tendon failure over the long term; this is a significant cause of bridge collapses worldwide. Insufficient reinforcement of anchorage zones causes local cracking during stressing. In the falsework method, the settlement of the scaffold and the de-shoring sequence are critical; if the falsework is struck in the wrong order, undesirable internal forces can develop in a deck that has not yet reached its full strength.
Steel and composite bridges: when are they preferred?
Alongside concrete methods, steel and steel-concrete composite bridges form a major branch of viaduct and bridge construction. Steel decks are light, which reduces the load on piers and foundations and offers an advantage on weak soils and in seismic zones. Steel girders can also be fabricated in a factory, delivered to site in pieces and erected quickly, minimising traffic disruption on urban crossings.
The most common modern solution is the composite section: steel box or I girders below, connected by shear studs to a reinforced concrete deck slab cast in place on top. Compression is thus carried by the concrete and tension by the steel; each material works where it is strongest. Steel arch bridges, cable-stayed and suspension bridges are almost unrivalled at very large spans (above 300 metres). Even so, steel brings periodic painting and corrosion maintenance and usually a higher initial investment cost.
A practical comparison guides the decision: reinforced and prestressed concrete favour low maintenance and long life, while steel favours speed, lightness and large spans. Most long viaducts end up as hybrid solutions; for example, prestressed concrete beams on the approach spans and a steel or composite main span at the principal crossing. The right mix is optimised span by span according to each one's conditions.
Method selection: span, terrain and traffic criteria
Choosing the right method means balancing several competing variables. The most decisive criterion is span length. Roughly: precast prestressed beams or casting on falsework for spans of 20 to 45 metres; incremental launching for repetitive viaducts of 30 to 60 metres; balanced cantilever for large spans of 60 to 250 metres; and cable-stayed or suspension systems above 300 metres. These ranges are starting points, not hard boundaries.
The second criterion is the terrain and the obstacle below. If falsework can be erected underneath (shallow water, flat and firm ground, reasonable height), casting on falsework is the most economical option. Where there is no access below - a deep valley, a wide river, live traffic, environmental sensitivity - falsework-free methods such as balanced cantilever or incremental launching come into play. Alignment geometry is also decisive: incremental launching demands constant curvature, while balanced cantilever adapts to variable spans and geometries.
The third group of criteria is schedule, repetition and mechanisation. Where there are many equal spans, the cost of a casting bed or launching gantry set up once is spread over dozens of spans and the unit cost falls; this makes incremental launching and precast segmental methods economical. For a single special span, that setup cost does not pay off. Finally, local labour, equipment availability, site logistics and climate shape the final decision. An experienced contractor models these variables early on, lowering both cost and risk.
Quality, safety and durability
A bridge is designed for a service life of decades, often 100 years and more; therefore the quality of construction is as decisive as the choice of method. Concrete quality is the starting point here: a low water/cement ratio, proper curing, adequate cover to reinforcement and impermeability protect the steel from corrosion. In prestressed structures, the full and complete grouting of the tendons together with the protection of the anchorage zones is indispensable for long life.
Construction-stage safety is a separate discipline. In methods such as balanced cantilever and incremental launching, the structure passes through loading conditions entirely different from its completed state during erection; for this reason a separate static check for every erection step, the design of temporary supports and anchorages, the certification of lifting equipment, and step-by-step monitoring are mandatory. Load tests on jacks, sliding bearings and form travellers cannot be neglected. Weather (especially wind) and temperature effects are incorporated into the design at the cantilever stage.
The factors that extend durability are proper drainage and waterproofing, quality expansion joints, accessible and replaceable bearings, and regular inspection programmes. Many bridges in our country and worldwide are damaged not by the structural system but by the neglect of secondary yet critical details such as waterproofing and joints. Quality management systems such as ISO 9001 ensure that these details are recorded and that a repeatable level of quality is guaranteed.
The value of experience and the right partner
Bridge and viaduct construction is, by its nature, a high-risk engineering activity that is hard to reverse; a mistake cannot be corrected on site and must be redesigned from scratch. For this reason, at every stage from method selection to erection engineering, an experienced contractor directly affects both the cost and the risk of the project. Experience means foreseeing problems encountered before in similar geometries, choosing the right equipment and method, and disciplined quality control.
KMB Metro Altyapı, born from the partnership of Troy (1996) from Türkiye and Kyivmetrobud (1949) from Ukraine with more than 75 years of accumulated know-how, specialises in such multi-stage projects in metro, railway, highway and especially bridge and viaduct construction. The firm's work across different countries and ground conditions - from tunnels to dams, from railways to airports - complements the multidisciplinary approach that bridge and viaduct projects require. The ISO 9001 quality certificate is a documented reflection of this approach.
In conclusion, balanced cantilever, incremental launching and prestressed concrete methods are each the right solution for specific conditions. What matters is analysing the project's span, terrain, traffic and schedule constraints early and honestly, and selecting the method that fits. When the right method, the right detail and the right partner come together, the result is a crossing structure that will serve safely for decades.