The cantilever construction method refers to the construction method of balancing and pouring or assembling beam sections section by section towards the middle of the bridge span on a working platform set on both sides of the bridge piers, until the bridge span structure is connected. The form traveller cantilever construction method is currently one of the most widely used methods for large-span pre-stressed concrete cantilever beams, continuous beams, and rigid frame bridges in the industry. This article will introduce the form traveller cantilever construction method.

Advantages of Form Traveller construction

The advantages of form traveller cantilever construction method mainly include the following three aspects:

1) It does not require large support structures or temporary equipment;

2) During the construction period, it does not affect the traffic flow under or over the bridge;

3) It fully utilizes the strong ability of prestressed concrete structures to withstand negative bending moments, transferring the positive bending moment at the mid-span to negative bending moments at the supports, thus improving the bridge's ability to span.

Cantilever Construction Steps

The general steps of the cantilever construction method are as follows: firstly, pour block 0 at the top of the pier; secondly, cast the cantilevered segments with forming traveller; and then, construct the combined segments and convert the structural system.

When cast-in-suspension girder body is divided into segments, the length of block 0 is generally 5~10 m, and the other beams are generally 2.5-5 m. The construction period of a beam section usually takes 6-10 days.

The construction period for a beam section usually takes 6-10 days. Based on prior design and construction experience, the number of beam segments should be increased without affecting the construction period, so that the results of reinforcement calculation are more accurate and the overall structure is more evenly stressed. Additionally, the closure section is mostly 2-3 meters in length.

1. Construction of Block 0

Among the various sections of the beam, section 0# has the most number of longitudinal prestressed reinforcement bars, resulting in a larger prestressed reinforcement pipe. Additionally, ordinary steel bars are densely packed, and the structure is complex, making construction extremely difficult. This stage of construction is critical and essential for the next phase. Generally, the construction of block 0# on the pier top is carried out by standing formwork casting on the pier top bracket. During the construction process, temporary beam pier fixtures or support measures are required to enable block 0# to withstand the unbalanced bending moments generated during the construction on both sides.

2. Installation of Cantilever Form Traveler

Following the completion of construction in Block 0, cantilever form travelers should be installed. The balanced form travelers is the key equipment used for cantilever pouring construction. It is less affected by factors such as water depth, pier height, and weather during construction, and it can be repeatedly used. Furthermore, it is easy to control the construction process and ensure construction quality. During construction, adjustments can be constantly made to correct segmental construction errors and ensure the quality of construction.

Install the form traveler in the 0# section, make the main anchor and bottom anchor → support the main beam → conduct the static load test of the hanging basket → adjust the formwork to pour the concrete in the l# section → longitudinal prestressed tensioning → unload the front sling and bottom anchor rod → remove the formwork → lay the steel tie → drop the main beam support → lift the front and rear joists with the walking sling → lift the main anchor → check the running wheels, anti-locking wheels and hydraulic traction system, clear the obstacles to the forward movement of the hanging basket, and be ready to move forward → start the oil cylinder, and the two form travelers at both ends of the T structure move forward symmetrically. The form traveler move in place → first make the main anchor and bottom anchor → set up the main beam support → adjust the formwork in place, tie the steel bars and pipes, and pour concrete , then moving on to the next step cycle.

3. Cantilever End Construction

The pouring of concrete into beams is typically done through pumping, and the slump should be controlled between 14-18 cm. When there are significant changes in temperature or other factors, the pouring speed should be adjusted accordingly.

During the pouring of concrete, construction control must also be conducted, specifically deformation control and stress control. Construction control involves thoroughly inspecting the cantilever form traveler centerline, bottom mold elevation, and prestressed tendon ducts before pouring concrete. Only after checking for errors can concrete be poured. The elevation of each stage of the box girder formwork is equal to the design elevation + pre-camber + deformation of the cantilever form travelers after being filled with concrete. After pouring a section of concrete, the measured results should be observed, and the elevation of the concrete to be continued to be poured should be adjusted to gradually eliminate errors and make the structural line uniform. Stress monitoring involves controlling the stress state of the structure during construction and after bridging to ensure its safety in any conditions. Specifically, simulation analysis is required to identify the stress and deformation characteristics of each stage under ideal conditions and make targeted adjustments to ensure that the bridge structure meets the usage requirements.

4.Construction of Closure Section

Cantilever bridges require complete closure at the mid-span. Closures are typically executed in the order of the edge span, secondary edge span, and middle span. During the closure segment construction, the two cantilever form travelers transition to one cantilever form traveler, so one cantilever form traveler is dismantled first and the other is used to traverse the closure section onto the other end of the cantilever construction beam segment to form a closure segment construction support. Closures are preferred during low temperature periods. Temporary locking measures should be implemented, partial prestressing rods should be tensioned, and the closure section concrete should be poured. After the concrete in the closure section reaches design strength, the remaining prestressing rods should be tensioned, and then the locking device should be dismantled.

The closure segment construction process is a transformation process that enables the bridge to complete its system transformation through the closure segment construction.

To Wrap up

The application of form cantilever construction technology in bridge engineering is becoming increasingly widespread. This technology can reduce the cost of bridge engineering, solve the problem of large spans, shorten construction period, greatly reduce construction costs, and bring more economic benefits to the construction side. If you are interested in the balanced form cantilever construction technology, please feel free to contact us at any time.