Netherlands: high-tech fastening for high-speed track ... - Hilti

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Engineering Page 22 To allow application of the specified loads and achievement of the necessary displacement, the Hilti test engineers anchored the substructure rigidly to the supporting floor of the test area. A hydraulic cylinder attached to a steel frame set up around the test specimen was used to apply loads to the superstructure. In this way, loads of up to 250 kN could be applied vertically and the specified horizontal displacement also achieved (image 4). A separate test rig was constructed by the engineers for the purpose of applying the static loads that occur until failure (image 5). These loads were allowed to act only in the direction of the application of the force, as any longitudinal displacement in the system is of no further significance due to the considerable plastic deformation to be expected under these loading conditions. The test specimen, however, had to be supported adequately to prevent torsion of the superstructure. Evaluation of the system a) Tests with alternating loads at service load level and until failure by fracture The alternating dynamic loads specified by the Infraspeed Project were applied up to 10 million times to specimens equipped with inner and outer dowels. After testing at service load level, the engineers measured the relative displacement between the substructure and superstructure occurring during alternation between minimum and maximum load. This amounted to 0.4 or, respectively, 0.6 mm and Image 3: Test specimen before casting the superstructure. Image 4: Test setups for Woehler tests. Image 5: Static loading test setup. was thus within the specified tolerance. Testing of the system then continued at increased load levels and with fewer cycles, the loads being increased until failure by fracture. Image 6 shows the load-displacement curves obtained before these tests and after the individual steps in the application of loads in the fatigue strength tests. This made it possible to determine the system’s rigidity or, in other words, the increase in force per millimeter of displacement of the inner and outer dowels. System rigidity provides a basis for the definitive calculation of load distribution between the inner and outer dowels. b) Determination of fatigue strength In order to be able to draw a Woehler curve (stress-number curve) for the system, the engineers, in a second step, applied various levels of alternating dynamic loads and determined the number of cycles achieved before fracture in each case. When two concrete slabs connected in this way are subjected to shear loads, various amounts of concrete break away, depending on the load applied, before the dowels fail. This results in the dowels being subjected to greatly varying combinations of shear and bending forces. Accordingly, the tests were carried out with the greatest possible range of stress amplitudes. The mean failure load was determined by regression analysis as a function of the number of load cycles. It was possible to calculate the 5% fractile from scatter in the deviation of the test values from the mean value. This model, used to determine the function of the mean values and the desired fractile from the test results,
Magazine Fall/Winter 2005 Engineering Page 23 is suitable for use even when only few test results are available. Accordingly, fatigue strength could be estimated relatively reliably even after a series of only nine tests, thus helping the engineers to meet the tight schedule. Test results All of the dowels failed at their full, round cross section where set in the concrete of the substructure. Fatigue fracture occurred at 40 to 120 mm below the surface of the concrete as this is where the point of maximum bending moment and thus maximum stress amplitude is located due to elasticity of the concrete in which the dowel is set. With higher loads, the point of fatigue fracture was located deeper in the concrete than with lower loads. This is because a higher load also causes more concrete to break away around the dowel, with the result that the point of maximum stress then lies deeper in the concrete. Evaluation Image 7 shows the test results and the mean value function derived from them, as well as the function for the 5% fractile. 52 million loading cycles were taken to represent 30 years of operational service. Although the tests were only run to maximum of 15 million cycles, the calculated function for resistance to fatigue can be extrapolated to the total of 52 million load cycles as stainless steel has been found to posses a form of long-term fatigue resistance at over 10 million load cycles. A mean fatigue strength of 86.8 kN could thus be calculated, the characteristic value being 75.4kN. Image 6: Load-displacement curves before and after load application. Image 7: Woehler curve. Image 8: The shear connector setting cart. A material safety factor of S=1.15 was specified for the steel. The tests, however, were carried out in the laboratory, whereas the actual structure is exposed to a corrosive environment. According to information provided by the manufacturer, the steel’s resistance to fatigue may thus be reduced by up to 20%. As a result, the characteristic resistance to fatigue was further divided by a safety factor of corr=1.2 to allow for environmental influences. This produced a design value of R d,fat=54.6 kN for fatigue strength. The data determined thus fulfilled all specifications for the inner and outer dowels. Efficient installation The connectors were required to meet the specifications while, on the other hand, they also had to be easy to handle and install so that the planned rate of progress of 300 meters per day could be achieved. This meant that about 600 dowels had to be set each day. Accordingly, an automated drilling and setting procedure was developed for this purpose (image 8). * Jakob Kunz, based at Hilti headquarters in Liechtenstein, is a consulting engineer for fastening systems. He specializes in solutions to complex problems. Please contact him at jakob.kunz@hilti.com for further information. The article on page 12 continues the story, taking you all the way from the test laboratory to the construction site.
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Netherlands: high-tech fastening for high-speed track ... - Hilti

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