Short-line match casting is a construction method in which a full-span continuous girder is divided into several small segments, which are prefabricated in a precasting yard and then erected piece by piece using a launching gantry. This method imposes high requirements on geometric alignment control. The quality of alignment control during the precasting stage is key to whether the as-built bridge alignment meets the design requirements. This paper discusses the alignment control methods for the segment erection stage in short-line match casting.

1. Survey Control

For construction monitoring of short-line match cast segmental precasting and erection, the 6-point coordinate method is generally used for alignment control. Six survey control points are set on the top surface of each segment. Among these, the two axis control points are used for axis alignment control, while the four control points on the webs are used for elevation control.

The spatial position of a segment during erection is determined by the coordinate values of these six control points. A local coordinate system is established based on the pier top block or the pier shaft top, or the overall coordinate system specified in the design may be used directly. The coordinate values are converted from the six-point measurement coordinates obtained during the precasting stage.

2. Pier Top Block Positioning

The positioning accuracy of the pier top block significantly affects the alignment of the entire span after erection. Inaccurate positioning can lead to errors at the interfaces between the two closure segments, resulting in issues such as axis misalignment and vertical discrepancies, thereby affecting alignment smoothness.

When using the span-by-span erection method, both sides of each pier top block are connected by cast-in-place joints. After the mid-span segments are erected, the cast-in-place joints are constructed to complete the closure between the full-span box girder and the pier top block. Positioning errors of the pier top block can be accommodated through the cast-in-place joints of the two adjacent spans, provided that the errors do not exceed the design tolerance.

The balanced cantilever erection method imposes extremely high requirements on the positioning accuracy of the pier top block. From the installation of the pier top block to the completion of the entire T-structure erection, no cast-in-place joints are used between segments. Moreover, the alignment of all T-structure segments is fixed after precasting in the yard. Therefore, during the erection of the T-structure, positioning errors of the pier top block increase linearly with the erection length. Consequently, the positioning accuracy of the pier top block is particularly critical in balanced cantilever erection. During construction, adequate temporary support and anchorage measures must be provided for the pier top block to prevent any change in its spatial position during T-structure cantilever erection. During installation and positioning, the accuracy must be controlled within ±1 mm, and errors of the same sign are required. Specifically, the offset errors for the two axis survey points (front and rear) should both be less than 1 mm and should be consistently positive or consistently negative. Practice has shown that controlling the positioning accuracy of the pier top block according to this method effectively manages the erection alignment.

During pier top block positioning, the preset camber must also be considered. The preset camber mainly includes the compression settlement of the substructure, the compression of the bearings, and the compressive deformation of the pier top block itself under the dead load of the full span. The preset camber value is approximated through finite element numerical simulation analysis. For the first pier top block of the initial span, the calculated approximate value is used for the preset camber. After completing the erection of the initial span, the actual elevation change ΔZ of the pier top block (relative to its position at the time of initial positioning) must be measured on-site. This measured ΔZ serves as the preset camber value for subsequent pier top blocks.

3. Starting Segment Positioning

During span-by-span cantilever erection, two cast-in-place joints are arranged near the pier top for each span. The starting segment is the first segment erected in each span. After the starting segment is positioned, the remaining segments are erected sequentially. The starting segment in the span-by-span erection method is equivalent to the pier top block in balanced cantilever erection and requires the same high positioning accuracy. The positioning of the six survey control points must meet the same requirements as for the pier top block in balanced cantilever erection. After positioning, the starting segment must be reliably connected to the pier top block to prevent any change in its spatial position during the box girder erection. Therefore, embedded components for the cast-in-place joint between the pier top block and the starting segment must be properly arranged during the precasting stage.

4. Positioning of Remaining Segments

Due to the alignment being fixed during the precasting stage, there is limited flexibility for adjusting the erection alignment during the field assembly. The erection alignment essentially follows the precast alignment. However, measurement errors exist in both the precasting and erection processes, and zero-error positioning of the starting segment is unattainable. Therefore, the error adjustment process during the sequential erection of the remaining segments is as follows: dry match test fitting of the segment → measurement of the six survey point data → error analysis. If the requirements are met, the segment is directly epoxy-glued and erected. If not, adjustments are made until the errors are within tolerance before proceeding with epoxy gluing and erection.

Conclusion

In short-line match cast segmental bridge construction, alignment control is a systematic process spanning the entire sequence of precasting, positioning, and erection. This paper begins with the establishment of the survey control system and systematically elaborates the key positioning techniques and control points for pier top blocks, starting segments, and general segments during the erection stage. Practice demonstrates that strictly adhering to the six-point coordinate method, rationally setting the preset camber for pier top blocks, and adopting differentiated precision management strategies based on the specific erection method are essential prerequisites for ensuring the final bridge alignment meets the design requirements. However, given that unavoidable error accumulation occurs during both the precasting and erection stages, relying solely on positioning control is insufficient to eliminate alignment deviations. A systematic and scientifically based error adjustment method is therefore crucial to achieving the final erection accuracy.

The next article will focus on error adjustment methods during the segment erection stage, providing a complete technical framework for achieving high-precision assembly in short-line match cast segmental bridges.