Re-tensioning

Abstract

Analysis of dynamic stiffness and electrical insulation quality is used to select the tie rods that can be re-tensioned. a procedure is presented. The reintroduced forces were limited to several dozens of tie rods. A procedure of re-tensioning is presented.

The analysis of the dynamic tests and static tests allows us to identify the tie rods that can be re-tensioned.

The static tests allow to know the deformations of the tie rods under the selected re-tensioning loads. The deformation generated to reach the new load will be stabilized by the introduction of metal plates of different thicknesses.

The procedure described below consists in inserting metal plates of different thicknesses between the wall and the head of the tie bar to tighten the tie bar to the value defined by the engineering department.

Example of procedure (Fig. 15.1).

Fig. 15.1

Re-tensioning procedure

In order to perform this re-tensioning, it is necessary to design and implement a traction device that allows this operation.

All re-tensioned tie rods were subjected to electrical isolation measurements, which was a first requirement for this operation (Fig. 15.2).

Fig. 15.2

Source Rincent BTP—Recife

Re-tensioning.

The analysis of the results focuses on the 24 10-strand ties, for which there are more results. The chosen tensioning force is 26 tons.

We consider as correct the tension force provided by the thickness of the plates inserted between the support plate and the wall (Fig. 15.3).

Fig. 15.3

Number of tie rods and calculated load values in tons

The figure below shows the difference between the tensile force values from the static tests and those deduced from re-tensioning the tie rods to 26 tons (Fig. 15.4).

Fig. 15.4

Difference in force values from static and re-tensioning tests

Average + 0,84.

Standard deviation 2,70.

The force values above 1, 2, 3 and 4 tons may be related to corrosion and the additional force required to lift the tie rods off the plates and wall.

For the 7, 8 and 9 tons, the results are very high and the values are not in conformity.

The figure below shows the difference between the values of the tension forces from the dynamic tests and those deduced from the re-tensioning of the tie rods for 26 tons (Fig. 15.5).

Fig. 15.5

Force difference between dynamic and re-tensioning tests

Four values are abnormally low and can be attributed to a non-conformity of the acquisition, for example the bad fixing of the sensor on the head of the tie rod.

Excluding these values:

Average -0,85 t

Standard deviation 2,57

The results are close to the static tests in absolute values.

In order to reintroduce the value of 26 tons in the tie rod, it is necessary to reach a higher level of force to be able to introduce the metal plates.

This operation may disturb the behavior of the tie rod. Finally, it should be noted that the calculated forces from the static test are slightly higher than the actual force internal to the tie rod and for the dynamic tests the calculated force is slightly lower.

Re-tensioning and balance force

The following example shows the evolution of the force in a re-tensioned tie. The initial force was 30 tons, after 27 years, the internal force in the tie bar has stabilized at 25 tons (Fig. 15.6).

Fig. 15.6

Values of the tension force (kN) according to the years

Then a re-tensioning of the tie rod to 40 tons was decided. After three months, the force decreases to 27 tons and after five months, the force returns to the initial values. The equilibrium situation before the re-tensioning is a reality of the site (Fig. 15.7).

Fig. 15.7

Force values (kN) versus time in months

Protection of the head of the tie rod

Examples of tie rod head protection are given in TA 2020, published year 2020 (21).

These devices are completely filled with anti-corrosion agent, attached to the support plate.

This protection allows easy access to the tie rod head for re-tensioning (Fig. 15.8).

Fig. 15.8

Source Rincent BTP—France

Protection of the head of the tie rod.

Note that non-destructive testing can be performed with protection. Corrosion protection products, waxes and petroleum-based greases are commonly used.