Accuracy of Measurement

Abstract

The relative uncertainty about dynamic stiffnesses shows the tie-rod stability during testing and also can define the tie rods which will be re-tensioned. The shocks caused by the hammer modify the behavior of the tie rod.

The eight strikes made on the head of the tie rod reveal different behaviors of the tie rods.

In the first case, the tie bar can be stable under the stresses caused by the hammer.

For example, tie rod 115 which contributes to the stability of a wall, the 8 values of stiffnesses calculated.

7,31E+08 by removing the maximum and minimum stiffness

7,19E+08 Average

6,77E+08 6,80E+08 N/m

6,70E+08

6,66E+08 relative incertitude

6,80E+08 1,21%

6,35E+08

6,66E+08

The tie rod is sensitive to the vibrations caused by the hammer.

The internal tension of the tie rod increases with the second stroke, decreases sharply with the third, and decreases again with the sixth stroke, and then decreases again with the 7th and 8th strokes.

• 2,30 E8 by removing the maximum and minimum stiffness

• 2,39 E8

• 8,23 E7 Average 1,53 E8 N/m

• 1,00 E8

• 1,65 E8 Relative incertitude 13,9%

• 1,95 E8

• 1,17 E8

• 1,13 E8

We have chosen the example of a 12-level tensioned wall to carry out the calculation of incertitude on the calculated dynamic stiffnesses, we have chosen the acquisitions in two columns of tie rods.

For each tie rod, 8 acquisitions are made, the calculation of the 8 stiffnesses and will be kept 6 values, removing the smallest and largest value.

The calculation of the mean and the standard deviation was carried out for each of the 24 ties. The relative incertitude is calculated from the standard deviation of the mean value over the 6 values.

The relative incertitude in percent are as follows (Fig. 18.1):

Fig. 18.1

Force and incertitude in percent results. Average: 5,8%. Standard deviation: 0,64

The measurement which gives a relative incertitude of 25% is considered as a particular tie rod. The shocks caused by the hammer modify the behavior of the tie rod which has a low internal force which, in this case, is 3.7 tons.

This type of tie rod is unstable and fragile even under the impact of the hammer that generates the vibration wave. The 10 tie rods whose internal force is greater than 15 tons have a measurement relative incertitude of 5.1%.

Another example of a wall (Fig. 18.2).

Fig. 18.2

Two panels of a wall, effort and relative incertitude

84% of the relative incertitude are lower than 5%, when the values are higher than 5%, these results concern particular tie rods in situations and location, for example, construction joints, water outflow, etc. (Fig. 18.3).

Fig. 18.3

Photography of the same wall