12.1 Example

A railway company has implemented a tension monitoring of 4 tie rods stabilizing an embankment. This monitoring has been done with load cells for more than 25 years.

One cell shows that the force decreases and the other three show that the tension force remains constant.

The concessionaire requested a non-destructive test, which we performed. The question concerned cell 1, which has a questionable behavior (Figs. 12.1 and 12.2).

Fig. 12.1
figure 1

Source Rincent BTP—France

Load cells.

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Fig. 12.2
figure 2

Forces measured by the load cells

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From the dynamic tests, we calculated the dynamic stiffness of the tie rods.

Tie Rod 1 7,31E + 8 N/mTie Rod 2 7,03E + 8 N/m.

Tie Rod 3 6,58E + 8 N/m Tie Rod 4 6,19E + 8 N/m.

This leads to the following graph (Fig. 12.3).

Fig. 12.3
figure 3

Square root of stiffness versus force (kN)

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Using the equation from the diagram in the previous figure it is possible to calculate the real tension force of tie rod 1 which is 473 kN (Fig. 12.4).

Fig. 12.4
figure 4

Real value of the force—gauge 1

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The results of the non-destructive tests show the reliability of the method and the different uses that can be made of it.

Fig. 12.5
figure 5

Source Glötz Company

Ring cells.

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Fig. 12.6
figure 6

Source Rincent BTP France

Vibrating string.

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12.2 Load Cells

The cells used in the example in the previous chapter are ring cells that have been in service for over 25 years (Fig. 12.5).

The vibrating strings are used to know the value of the tension in different points of the same tie to know its operation.

To obtain the results it is necessary to excite the vibrating string with an electric current so that the vibrating string gives a vibratory response in relation to the tension force (Fig. 12.6).