The calculation of the theoretical elongation of the prestressed tension is carried out according to the requirements of the code, and the average tensile stress method is used. The specific formula is as follows:
△L=Fp*L\/Ap*Es
Fp - Average tensile force (kN)
When the following conditions are found during tension, the jack should be relaxed immediately, the cause should be identified, and the tension should be resumed after taking corrective measures:
① Broken wire, sliding wire or anchor broken;
② The concrete is broken, and the backing plate is trapped in the concrete;
③ Abnormal sound;
④ After reaching the tensile force, the elongation value is insufficient, or the tensile force is not enough, and the elongation value is out of range;
The clip of the tool anchor should be kept clean and well lubricated. The hole of the anchor plate and the clip should be coated with lubricant.
The exposed length of the prestressed tendon after anchoring should not be less than 30mm.
The exposed prestressed tendon at the tension end should be cut after grouting.
Tensioning principle
1 The prestress adopts the double control method of symmetrical tension at both ends, tension tension and elongation, and the work of lifting pressure, drawing line, measuring elongation and inserting pad at both ends is carried out together.
2 After the jack is in place, first fill the main cylinder with a little oil, push it tight, and straighten the prestressed steel strand. When the steel strand reaches the specified initial stress σ0, stop the machine to measure the original gap and draw a line for marking.
3 Tension application procedure of prestressed steel strand: 0→ initial stress σ0→ control stress σk (load holding for 2 minutes). σk is the control stress (including loss of prestress) during tension.
4 Work sequence of tensioning construction: threading → installation of anchors → installation of jacks and tensioning equipment → tensioning and anchoring → removal of jacks and tensioning equipment → grouting → storage of beams.