Choice between split and continuous bridges begins at the design stage, when engineers analyze span lengths, soil characteristics, and expected loads. The main difference lies in the static scheme of the structure: in the first case, the beams rest on each support independently, and in the second, they are rigidly connected to each other, which changes the distribution of bending moments. An incorrect choice of system can lead to a critical waste of materials or, conversely, to insufficient strength of the structure during thermal expansion.
These two types of structures dominate modern bridge construction, occupying more than 90% of all constructed objects. Cutting system more often used in areas with heterogeneous geology, where the risk of subsidence of one of the supports is high, whereas continuous allows you to cover large distances with fewer expansion joints. Understanding their mechanics is necessary to assess the durability and cost of a future bridge.
Operating principle of split beam systems
basis split bridge is a sequence of independently operating beams, each of which rests on only two adjacent supports. This scheme makes the structure statically determinate, which greatly simplifies the calculation of forces. If one of the supports experiences vertical settlement, this does not cause additional stresses in the superstructure of adjacent spans, since the beams are not rigidly connected to each other.
In places where they rest on intermediate supports, gaps are formed, which are closed by expansion joints. It is these elements that are the most vulnerable places, as they are subject to constant dynamic impact from passing traffic and require regular maintenance. But the absence of a rigid connection makes it easy to replace individual spans without stopping traffic along the entire bridge.
⚠️ Attention: An excessive number of expansion joints on long bridges impairs traffic comfort and increases noise, which is critical in urban areas.
The static diagram of a split beam assumes that the maximum bending moment occurs in the middle of the span, and at the supports it is equal to zero. This dictates the form of reinforcement: the main working reinforcement is located in the lower zone of the section, where tensile stresses act. The upper part of the beam in the middle of the span is in compression and often requires minimal reinforcement, which saves metal.
For bridges up to 30 meters long, the split design is often more economical due to the ease of manufacturing and installation of the structures.
Features of continuous bridge structures
Continuous bridge is a single beam spanned across several supports, making it a statically indeterminate system. This design makes it possible to redistribute loads: when a temporary load (for example, a truck) is applied in one span, adjacent spans are included in the work, which reduces the overall deflection. This is the main advantage, allowing for higher and lighter spans.
Unlike split analogues, negative support moments arise here. This means that above the supports, the upper part of the beam is in tension, and the lower part is in compression. Consequently, working reinforcement should be placed not only at the bottom of the span, but also in the upper zone above the supports. This double reinforcement complicates production but increases overall rigidity.
Temperature changes in the length of the span in continuous systems cause complex movements. To compensate for the expansion of concrete or metal, the outer supports are often made movable, or special compensators are used. The absence of seams above the intermediate supports ensures a smooth ride, which is highly valued on expressways and railways.
- 🏗️ Reducing the height of the span by 20-30% compared to split analogues of the same length.
- 🚗 Improved performance due to the absence of transverse shocks at the joints.
- 💰 Saving concrete and steel due to more rational use of section material.
- 🔧 The complexity of calculations and the need to take into account foundation settlement during design.
Comparative analysis of static circuits
The key difference between the types of bridges considered is the bending moment diagrams. B split system the diagram looks like a parabola in each span separately, with zero values on the supports. B continuous system, the diagram becomes a continuous curve enveloping the support, which indicates the presence of moments “pressing” the beam to the support.
This fundamental difference determines the approach to strength design. Continuous systems require more complex mathematical models that take into account the interaction of all spans. However, the gain in material often outweighs the design costs. This is especially important for reinforced concrete, as it makes it possible to more effectively use the compressive strength characteristics of concrete.
Dynamic loads are also perceived differently. A continuous beam has greater inertia and dampens vibrations better than a series of short split beams. However, in the event of an emergency destruction of one of the supports, a continuous bridge can be damaged on adjacent spans (“domino effect”), while a split bridge localizes the accident within one span.
| Comparison parameter | Split bridges | Continuous bridges |
|---|---|---|
| Static circuit | Statically definable | Statically indeterminate |
| Reaction to settlement of supports | Doesn't require extra effort | Causes additional stress |
| Number of seams | Many (above each support) | Few (only at ends) |
| Structure height | More (1/12 - 1/15 span length) | Less (1/15 - 1/20 span length) |
| Installation | Simple, independent elements | Complex, requires consistency |
Continuous systems benefit from material savings and ride comfort, but lose in sensitivity to uneven ground settlements.
The influence of temperature deformations and settlement
Thermal expansion is one of the main enemies of bridge structures. B split bridges each beam is lengthened or shortened independently. The total movement at the ends of the short beam is small, so the requirements for expansion joints are moderate. Problems begin when spans become very long and the range of motion requires complex and expensive expansion joints.
B continuous bridges the situation is different. A long beam mounted on many supports tends to change its length when the temperature changes, but encounters resistance from the supports. This creates significant horizontal forces. To extinguish them, one of the supports is often made motionless (embedded), and the rest are movable, or flexible supports are used that can bend, taking on temperature movements.
Foundation settlement is a critical factor for continuous systems. If the soil under one of the supports “floats” and the support goes down, colossal additional stresses arise in the body of the beam, which can cause cracks. In this regard, split bridges are more “tenacious”: the settlement of the support simply changes the angle of inclination of the end of the beam without breaking the structure itself.
⚠️ Attention: When designing continuous bridges in seismically active zones, special calculations of hinges and moving supporting parts are required.
Installation and construction technologies
Construction methods directly depend on the chosen scheme. Split beams ideal for factory production. They are brought to the finished site and simply installed on the supports by crane. This ensures high speed of work and independence from weather conditions, since the main part of the processes takes place in the workshop.
Construction continuous bridges often requires the use of suspended concreting or sequential sliding. Concreting occurs in areas directly above a river or ravine. After concreting the span, it is tensioned (tension of the reinforcement), which creates the compression force necessary to work on negative moments.
☑️ Stages of installation of a continuous span
There is also a combined method, when the span is assembled on the shore and then pushed onto the supports using hydraulic jacks. In this case, the bow part of the beam experiences alternating loads, which requires temporary reinforcement of the structure with special steel “beaks”.
Historical background
The first large continuous bridges appeared in the middle of the 19th century, when the development of mathematics made it possible to accurately calculate complex static diagrams that were inaccessible to engineers of the Roman era.
Operation and repair of spans
During operation split bridges require constant attention to expansion joints. They are the first to fail, allowing water and reagents to pass through to the supporting parts and body of the supports. Corrosion of the support pads can lead to jamming of temperature movements, which will cause destruction of the ends of the beams.
Continuous structures Drivers are pleased with the absence of the characteristic “poke-poke” knock when passing joints. However, diagnosing their condition is more difficult. Cracks in concrete are often hidden in negative moment areas (above supports) and may not be visible when visually inspected from below. Requires the use of ultrasonic methods or radar scanning.
Replacing supports in continuous bridges is a daunting engineering task. To remove the load from the support, it is necessary to use powerful jacks to forcibly change the geometry of the span, which requires precise calculations so as not to destroy the beam. In split bridges, the support is replaced by lifting a separate beam, which is technically much simpler.
What is the main economic difference between types of bridges?
Continuous bridges are cheaper to produce materials (less concrete and steel per linear meter), but more expensive to design and install. Slitting bridges have a higher material consumption, but are simpler and faster to construct, which can be more profitable in the short term.
Is it possible to turn a split bridge into a continuous bridge?
Yes, there is a method of “splitting” or a continuous cutting device. The ends of adjacent beams are connected with reinforcement and the joint is concreted, creating a rigid connection. This eliminates expansion joints and improves comfort, but requires careful checking of the load-bearing capacity of existing supports.
What type of bridge is best for seismically active areas?
Often preference is given to split schemes or special continuous systems with seismic isolating supporting parts. The split design is simpler in terms of seismicity, since the behavior of each beam is independent, but modern continuous bridges with regular hinges are also successfully built in such zones.
Why does the asphalt crack over the supports of a continuous bridge?
This is due to the negative bending moment. The upper part of the beam stretches, and if the reinforcement is insufficient or the concrete is of poor quality, transverse cracks appear, which are reflected in the road surface. This is a sign that repairs are needed.