July 25th, 2019

Test load on composite anchors in case of insufficient anchorage ground

Figure 1: Test setup schematic

In spring 2019, a steel staircase anchored to the shaft wall, anchored to the shaft wall, was installed in the shaft of the Malter Dam.

The stairways were easily dislocated into the platforms. The platforms had to be attached individually to the shaft wall, as the support up to the sliding cellar was not possible geometrically. The platforms were placed on three consoles anchored in the shaft wall.

The rock eruption was equalized in the construction period (1913/1914) by a concrete shell (t > 50 cm), which has strengths between 3 N/mm2  and 30 N/mm2 and has widespread gravel nests. In 1996, the shaft wall was clad by a 10 cm thick pre-suspended sprayed concrete shell to counter the leachate water. The sprayed concrete shell is statically unsuitable for fixing the consoles. The consoles were therefore to be attached to the tamping concrete and anchored in it.

Common and relevant composite anchor systems have general building supervision approvals by ETA or DIBT (abZ) for concrete strengths above 20 N/mm2. Their use was therefore not readily permissible. It was not certain whether the expected failure mechanisms would still be recorded with the evidence according to ETAG or ENSO. However,the evidence was kept and could not be provided with an approach of 3 N/mm2.  Initially, therefore, it was planned to remunerate the old concrete by injection. However, due to the risk that the performance of the drainage layer between the concrete and the sprayed concrete shell will be impaired, it was decided to anchor in the uncoated concrete. The sufficient load-bearing capacity had to be ensured by means of test loads.

Pure tensile tests were out of the question, because the failure of the perforated body under transverse force was also possible as a failure mechanism. The stress in later operation should be mapped as accurately as possible in the experiment.

One of the two test setups is symbolically represented in Figure 1. The load was applied to the console by tightening the clamping nut screwed onto the anchoring rope. A dowel tester (HILTIDPG) was intermediated between the clamping nut and the console for force measurement. Thedeformation measurement was carried out directly on the console using a measuring clock. The test regime was specially developed on the basis of pile sample loads in accordance with DIN 14199, as no adequate methods could be researched. The definition of border or failure criteria proved to be particularly difficult.

Three criteria were formulated, the ones described below being particularly useful. Thus, the increase in deformation had to converge over the load holding time to zero. In addition, the deformation had to remain constant in the last 120 seconds of the load holding time. Starting from a preload, the characteristic load was adjusted on the first load stage, and the rated load was set in the second. Figure 2 (see page 3) shows typical load and deformation curves. The load drop at the beginning of the holding phase of the initial load is typical and can be attributed to the slip to be overcome in the test setup. Four of the 54 consoles to be tested did not meet the two-minute criterion in load level 2. As a result, it was decided to reduce the permissible payload of the stairs from 5 kN/ m2  (category T2 according to DIN 1991-1-1/NA) to 3 kN/m2  (T1). This traffic load is covered by the first load stage and meets the requirement for stairs in normal building without heavy equipment.

Dominik Fiedler – Dresden

Figure 2: Load and Deformation Curves