Loss of Tension

Abstract

In the main cases studied, load loss in the tie rods ranges from 1 to 5.5% per year, leading to stability problems after several decades. Analysis of two groups of tie rods—one group of tie rods with reinforcing bar and another group with strands.

Prestressed tie rods lose a percentage of their tensile force with time.

This is a general phenomenon, even for tie rods in dams where the tie rods are anchored in concrete on rock. The organizations responsible for the management of dams provide important information for many years.

These tension losses lead to a decrease in the safety factor with respect to the stability of retaining walls.

Diagnosis and maintenance of tie rods are necessary to guarantee the durability of the structures. The managers of retaining structures are aware of the phenomenon of loss of load in the tie rods and are beginning to request periodic dynamic tests to check the evolution of the tensile strength of the tie rods.

Examples

27 Tie rods

Initial tension 25 tons

After 24 years

Average value 19,9 tons

Loss of the tension force by 0.96% per year

15 tie rods

Initial tension 15 tons

After 24 years

Average value 7,78 tons

Loss of tension de 4,28% per year

6 tie rods

Initial tension 27 tons

After 5 years 18,5 tons

Loss of tension 7,28% per year

5 tie rods

Initial tension 32 tons

After 5 years 16,2 tons

Loss of tension 12,73% per year

We carried out tests on 7 retaining walls on 725 tie rods with steel bars as reinforcement. The initial tension value was 35 tons. Carried out 30 years ago, the average tension value is now 7 tons, i.e. an average loss of 5.5 % per year (Fig. 14.1).

Fig. 14.1

Loss of tension

For the wall n°7 the residual force for each tie rod line is (Fig. 14.2):

Fig. 14.2

Residual force per line of tie rods

The loss of tension in the tie rods leads to approaching the limit equilibrium of the wall's stability with impermissible displacements (Fig. 14.3).

Fig. 14.3

Source Rincent BTP—Recife

Non admissible displacement.

Another work, on 19 retaining walls and with 1173 nondestructive tests, have a single retaining wall where 590 tie rods were tested, stress loss analyses were performed for each of the 12 tie lines (Fig. 14.4).

Fig. 14.4

Loss of tension per line de tie rods

Lines 1 à 4 lose 75% à 77%.

Lines 5 et 6 lose more than 60% and less than 70%.

Lines 8 à 10 lose 51% to 53%.

The last two lines lost 33%.

The French nuclear safety organization refers to the problem of load loss in the tie rods.

Regarding losses due to relaxation of the steel, Electricité de France has maintained a relaxation rate of 4% over 1000 h.

This value depends on the nature of the steel used and its heat treatment. For steels that do not undergo any special treatment to reduce relaxation, a relaxation at 1000 h of 8 to 9% is generally retained.