Cantilever form traveler plays a crucial role in modern bridge construction. The design and construction of it directly affect the efficiency, quality and safety of the project. In this article, we will comprehensively discuss various aspects related to the design of the cantilever form traveler.

1. Segmented Length of Cantilever Casting

When designing the cantilever form traveler, the first step is to determine the segmented length of the cantilever casting of the box girder.

The segmentation of the cantilever beam depends on the equipment and construction conditions. If each segment is long, the number of segments will be small, and the construction speed will be relatively fast. However, the amount of concrete poured each time will be large, and the form traveler and its equipment will need to be correspondingly enlarged. On the other hand, if each segment is short, the cantilever form traveler's load-bearing requirement is small, which can avoid overly large forming traveler equipment. However, with a large number of segments, the form traveler has a higher turnover rate and the overall construction progress is relatively slow. In short, the segmented length of the cantilever casting box girder should be comprehensively considered by weighing the pros and cons based on the construction conditions. In recent years, the segmented length of cantilever construction is generally about 3-5m, and the maximum reaches 8m.

2. Cross-section of the Form Traveler

The layout of the cross-section of the cantilever form traveler depends on the width of the bridge and the cross-sectional form of the box girder. Generally, one forming traveler can be used for construction on the entire cross-section. However, when the bridge width is large (over 15m) and the cross-section of the box girder is a double-box or multi-box structure, in order to provide the form traveler with flexibility during construction, two  form travelers can be used for construction respectively on one cross-section.

3. Overall Concept of Form Traveler Design 

In the design of the form traveler, often based on existing data and actual construction requirements, the form of the form traveler, connection method, and rods are selected, and then checked and calculated. This is the most commonly used method. Therefore, the design of the form traveler must consider the form of the traveler structure.

Based on the current level of form traveler construction at home and abroad, the lightweight form traveler should meet the needs of the actual cantilever construction of bridges. The design should focus on the overall function and not pursue lightweight indicators one-sidedly. The overall concept is often reflected in the following aspects:

(1) Reduce the Self-weight of the Form Traveler

① Select a lightweight structure with reasonable stress, safety and reliability, and large stiffness as the main truss of the form traveler for load-bearing;

② The materials used for the form traveler are based on high-strength and lightweight steel produced domestically and are convenient for processing;

③ When pouring concrete in the form traveler, the vertical prestressed tendons of the box girder are fully utilized at the tail to balance the overturning moment to eliminate the counterweight, thereby reducing the self-weight.

(2) Reduce the Construction Cycle of the Form Traveler

① Hydraulic devices or easily operable screw jacks are used for the movement of the balanced form traveler and the lifting and lowering of the formwork, and centralized electrical control is adopted to improve production efficiency and reduce the labor intensity of workers through mechanization and automation;

② Under the condition that the rear anchorage can bear the force, when the form traveler moves, the internal and external forms can be in place simultaneously, minimizing the working procedures and shortening the construction cycle;

③ The suspension system and anchorage system are convenient for assembly and disassembly and simple for adjustment.

(3) Ensure the Quality of the Suspension-cast Concrete

The formwork is designed with stiffness control. New materials can be used to make the formwork, which is both lightweight and durable and can be used for all suspension-cast beam segments without the need for replacement or repair. Formulate detailed construction implementation rules, strictly regulate the measures to maintain balanced construction and limit construction loads, and ensure construction safety and quality.

(4) Improving Construction Conditions and Environment

When designing the form traveler, a spacious workspace should be considered to facilitate the placement of various tools and movement of operators back and forth. A sunshade and rain shelter are placed above the main truss of the form traveler to improve the working environment.

(5) Consider Universality

The appropriate treatment of the form traveler structure can be adapted to the construction of various box girders with different spans and widths.

4. Calculation of the Main Truss of the Form Traveler

As the load-bearing structure, the main truss of the balanced forming traveler is the most crucial design and calculation component. The calculation content mainly includes internal force calculation and cross-section design of each member and anchor rod, deformation and overturning stability calculation of the form traveler. The calculation should be carried out respectively according to the situations such as two form travelers connected as one, pouring beam segments, and moving without load.

(1) Load Coefficient

According to the "Design and Construction Specifications for Highway Bridges and Culverts" issued by the Ministry of Transport, the load coefficient is taken as follows: the overload coefficient, taking into account factors such as formwork expansion during the pouring of box girder concrete is 1.05; the dynamic coefficient during concrete pouring is 1.2; the impact coefficient when the form traveler moves without load is 1.3; the anti-overturning stability coefficient during concrete pouring and form traveler movement is 2.

(2) Construction Load

During the process of pouring beam segments and movement without load, the main truss of the form traveler bears the following loads:

① Maximum segment concrete weight: This is the control weight when designing the form traveler;

② Self-weight of the form traveler: mainly including the weight of the main truss, suspension system, formwork system and tensioning operation platform;

③Construction machinery and equipment, crowd load: calculated based on the number of attached vibrators, tensioning jacks, and oil pumps used in the construction and the number of construction workers. In the absence of measured data, it can be calculated as 2.5kN/m.

④Dynamic additional load: considering the dynamic factors during concrete pouring and the importance of form traveler construction safety, two items are mainly considered for the dynamic additional load here: a. The vibration force generated by the vibration system; b. The dynamic influence of the beam segment itself during vibration. Referring to the dynamic coefficient of 1.2 during concrete pouring, the dynamic influence of the beam segment is 0.2 times the weight of the beam segment concrete.

Impact additional load: mainly considering the impact influence when the form traveler moves without load. Based on the impact coefficient of 1.3 when the form traveler moves without load, the impact additional load=0.3xself-weight of the form traveler.

⑤Wind load: It is difficult to ensure construction quality under the condition of strong winds above level 6. Generally, the form traveler construction is stopped in strong winds. However, since cantilever construction is a high-altitude operation and has high safety requirements, the anchorage, movement, and overall stability of theform traveler under possible strong wind conditions should be checked and calculated for safety. The value can be provided by local meteorological data or obtained from the national wind pressure map.

⑥Concrete eccentric load: It can be calculated according to the maximum deviation of concrete pouring on both webs when pouring beam segments.

(3) Load Combination

Based on the calculation above, the load combination for each construction stage is as follows:

Load combination I: Concrete weight+dynamic additional load+self-weight of the form traveler + construction machinery and equipment and crowd weight;

Load combination II: Concrete weight+self-weight of the form traveler+concrete eccentric load+construction machinery and equipment and crowd weight;

Load combination III: Concrete weight+self-weight of the form traveler+wind load;

Load combination IV: Concrete weight+self-weight of the form traveler+construction machinery and equipment and crowd weight;

Load combination V: Self-weight of the form traveler+impact additional load+wind load.

Load combinations I-III are used for the strength and stability calculation of the main truss load-bearing system; load combination IV is used for deformation calculation; load combination V is used for the movement check calculation of the form traveler.

Calculation of the internal force and deformation of the main truss structure: Simplified calculations often assumes that each truss of the main truss bears the same load and is calculated as a plane structure.

5. Check Calculation of Other Component

The suspension system, formwork system, and walking system of the cantilever form traveler are designed according to the general calculation method of steel structures. However, since the form traveler is a movable support and requires high-altitude operation, sufficient safety must be ensured in the design. Particularly, attention should be paid to the front sling of the suspension system. It suffers the greatest impact vibration during the demolding process and bears repeated actions, and there is a risk of end fractures. Therefore, the front sling of the suspension system must have a safety factor of at least 2. All calculations are based on different calculation diagrams and load combinations to obtain the maximum stress of each component. After multiple adjustments of the structure's geometric dimensions, it can save the most materials and have the lightest weight under the premise of meeting the specifications.

To Wrap Up

In conclusion, the design of the cantilever form traveler is a complex yet vital process. By carefully considering factors such as segmented length, cross-section layout, overall concept, load coefficient, construction load, load combination, and ensuring the safety and reliability of all components including the main truss and other related systems, we can achieve an optimized design that meets the demands of bridge construction projects. This not only helps to improve construction speed and quality but also ensures the safety of the entire construction operation. The proper design and application of the cantilever form traveler contribute significantly to the successful completion of modern bridge engineering works.