Keywords: Bridge, Ground-Anchors, Stress, Ultrasonic Test, Working Safety
This paper was presented at the International Symposium Non-DestructiveTesting in Civil Engineering (NDT-CE) 26.-28.09.1995 in Berlin. NDT-CE, Full Program or the Ultrasound Part
In additon to the tensile forces induced in the anchoring bolts, alternating dynamic stresses resulting form traffic and climatic conditions also affect the tensioning points and the anchoring bolts. Moreover, after a period of useful life that often extends over decades, corrosion is to be expected.
The knowledge of the trouble-free service-ability of this tension-subjected elements is of safety and economic interest for administrative authorities and owners. The DMT-lnstitut fur Forderung und Transport was given the order in serveral cases by the bridge controlling authorities to make tests for the safe evalutation of the good working order structures. For this purpose testing concepts were developed.
Figure 1: Reflector comparison diagram
With decreasing anchoring bolt diameter we recorded assemblies of amplitudes at radial reflectors and especially at the back end because of the constrained guide of the sound ray and the side-swiped incidence, figure 2.
Figure 2: Measurement of real diameter of ground anchoring bolts
d = anchoring bolt diameter
s = sound path
c1 = sound velocity-longitudional
c2 = sound velocity-transversal"
a = sound angle
For the evaluation of the reflector distance to the sound ray entrance these effects are to be considered among others when making the approximative echo height comparison.
|The first testing aim was reached for known thread lengths and diameters with a high resolution measurement. We may suppose that damages on the upper tensioning points will be primed in the force induction zone nearly as radial flaws and not profile surrounding. The reference bolts (here 30 mm and 100 mm in diameter) were provided with artificial flaws of 1 mm in depth. From the failures clear signals were produced on the ultra sonic screen, figure 3.|
Fig 3: Ultrasonic diagram of artificial reflector
(reflector 1mm in depth, bolt diameter 30 mm)
In the middle of the shaft radial flaws can start after cross section losses and pitting by corrosion. The cross sections losses can here start around the f ull profile or over segments of a circle. Such corrosion points were realiably detected with cross section losses greater than 10 % . Corrosion attacks In ground anchoring bolts can also be primed in form of pittings or needle shaped. The detection of such damages was provided on reference bolts on radial bore holes. The bore hole diameters and the radial depth were determined according to the required resolution and of course with respect to the detection possibilities In greater sound path distances.
The most demanding task for the available testing systems is to spot damages on the ground anchoring bolts in the lower length and the bottom end. figure 4
Fig 4: Ultrasonic diagram of reflector in 5020 mm distance
Attention must be paid to the comparability of the material properties of the reference bolts and those at the tested ground anchoring bolts, because they influence the ultra sonic signal. Tests were made with two reference bolts of the same geometry, the same reflectors, but different material characteristics.
The reference bolts test delivered as result pictures, that were worked out before the reference piece method for the testing of ground anchoring bolts was used in several bridge constructions. For different reflector sizes and sound path distances reflector reference diagrams could be referred to for the evaluation of findings in the field.
The anchoring of the bridge is assured by what we call ,,pylon anchoring bolts" (32 mm In diamter, cat 5 m long) for the vertical forces and by what we call ,,cross-wind anchoring bolts" (also 32 mm diameter, cat 3 m long) for the horizontal forces. The ultra-sonic tests identified no defects.
Example 2 - River crossing, federal highway
The anchorig of the bridge is assured, for its four pylons (of an older design), by four ground anchoring bolts ( 140 mm diameter, ca 6 m long) per pylon. Two additional pylons (of a later design) are held by 16 anchoring bolts of 35 mm diameter per pylon. The testing of the 140 mm thick anchoring bolts on the pylons of the older design identified by ultra-sonic defects over the smooth length. The order of magnitude of the defects was below the statutory threshold. All tensioning points and abutments were found to be without damage. What we call ,,Preservice inspections" were carried out on the anchoring bolts of the two pylons of newer designs.
Example 3 - Canal crossing, federal railway bridge
The ground anchoring bolts' dimensions were of 65 mm to 12.0 mm in diameter and cat 3,5 and 8,0 m of length. The echo sizes of ultra-sonic reflector found in different distances should be compared with known reflector echo sizes of the reference bolt. The association was made dependent on the diameter. Damages on the ground anchoring bolts directly affecting the safety of the bridge were not found in any case. Indications of local corrosion damages were given. Possibly occurring further development of damages may be monitored by recurrent tests.
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