I. Project Introduction

The main bridge type of a certain large bridge is84+200+84 (m) three-span continuous steel-concretecomposite girder rigid frame bridge. The mid-span80 m adopts a steel box girder, while the remaining main girder adopts a concrete structure, with a single-box single-cell inclined web section.The theoretical length of the mid-span steel box girder80 = 2.2 + 75.6 + 2.2 (m) in three segments; the 2.2 m long steel box girder is the steel-concrete joint section (variable stiffness structure), serving as the transition between the concrete box girder and the steel box girder, and the 75.6 m mid-span steel box girder is integrally hoisted. External tendons are installed between the two 0# blocks of the main bridge.

Figure 1 External tendon threading

Figure 2 External tendon tensioning

II.Principles, Characteristics, and Scope of Application of the "Four News"

1. Principle

A certain prestressing force is applied to the member outside the constructed wall plate to achieve the purpose of improving the structural bearing capacity and to facilitate replacement of the prestressing system after use.

2. Characteristics

1) The stress of the steel strand bundle can be controlled and adjusted, facilitating inspection and replacement of the tendon body.

2) There are no prestressing ducts within the box girder wall, which can minimize the wall thickness of the box girder.

3) Prestress loss is small.

4) The arrangement of prestressing tendons is simple, and duct grouting can be omitted or partially omitted.

3. Scope of Application

Applicable to structures where the prestressing system needs to be replaced after use, especially suitable for temporary works and projects for repairing and strengthening existing structures.

III. Application Effect

1. The steel strands are cut to length as a whole and threaded one by one.

2. Before threading, the strand number should be marked at each passing hole to prevent strands from being threaded into the wrong backing plate hole.

3. When there are multiple rows in the transverse or vertical direction of a single end face, horizontal layering is preferred, threading row by row from top to bottom.

4. Each horizontal layer of steel strands threaded needs to be properly pre-tensioned to ensure they do not sag and affect subsequent threading.

5. The tensioning of each steel strand bundle should be carried out in stages.