Civil Engineering Reference
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4.3.3.2 Measurements in steel arches
Strain measurement using vibrating wires.
The sensors work on the principle already
described in Section 4.3.3 (Fig. 4-31). In order to be able to calculate the stresses at a
location from the strains, a device must be bolted or welded onto the arch externally and
internally. The external device is covered by a cap and sprayed with shotcrete and thus
cannot be reused.
Such systems have become less popular in recent years.
4.3.4 Measurements of the loading and function of anchors
4.3.4.1 Checking of anchor forces in unbonded anchors
To check the force originally applied to an unbonded anchor and its change depending on
events at the installation location and with time, sensors are installed at the anchor head.
The following systems can be used:
- Mechanical measurement plates.
- Vibrating wire devices.
- Hydraulic measurement process.
- Load cells with strain strips and inductive sensors.
Measuring plate.
This method of measurement is based on the squeezing together of
two load-distribution plates integrated into the anchor head, which have calibrated plate
springs between them. The effective anchor force can be determined from the change of
length of the plate springs through the strain
ε
and the stress
σ
[195]. Fig. 4-33 shows a
diagram of the function.
Figure 4-33
Checking anchor forces with a mechanical
measuring plate; Interfels GmbH.
Vibrating wire sensor.
In a vibrating wire sensor, the axial anchor force to be measured
is introduced into a ring-shaped pick-up, where it produces an elastic deformation of the
measurement body and thus changes the resonance frequency of the installed steel wire.
In the ring-shaped measurement body, which is fixed to the anchor head, the introduced
axial force is distributed to three, or for higher measurement ranges six sectors, each of
which has a vibrating wire measurement system. The sum of the individual values gives
the total force (Fig. 4-34).
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