Keywords: dynamic analysis of structures, finite element analysis, structures dynamic diagnosis, full scale test.

The water tanks belong to structures that should be verified with regard to their wind and earthquake response and resistance. The progress in development of new National and International Standards activates verifications of analytical models, design and execution of new and existing structures. The design procedure includes also the dynamic analysis of the respective structure. In order to create the appropriate amount of data and limit statements necessary for simplified or more sophisticated analytical models, any knowledge of actual existing structure dynamic behaviour is welcome.

The analysed steel water tank tower was designed and constructed at Drahnov in Eastern Slovakia (Figure 1) and now serves for field watering. The tower is 97 m high constructed like lattice tower with three corner tubes that create the basic carrying system. Two of tubes have diameter 1.34 m and the third one has 2.54 m. The cylindrical water tank is situated on the top of the tower and has diameter 15.5 m, height 6.8 m and its content can reach 500 m. Beside the measurements of ambient vibrations the mechanical exciter was used like the basic source of excitation of investigated water tank tower. The position of exciter during the tests was inside of the largest corner tube at the mid-height of the first brace. The set of accelerometers was used for measurement of global and local vibrations in the most representing sections and points. For the recording both analog and digital methods were used including A/D converters, filters, on/off line evaluation and reasonable data acquisition, Bencat et al. [1].

The damping of tower determined from free tail vibration and that from steady harmonic vibration varied through damping ratio = 0.024 for empty tank and = 0.021 for the full tank. Local vibration damping ratio of the largest tube was determined from impact tests and reached the value up to 0.05.

The recorded signals were evaluated in laboratory of the Dept. of Structural Mechanic (CEF - University of Zilina) or preferably in situ using amplitude, frequency and amplitude - phase analysis, by method of spectral or correlation analysis using two channel frequency analyser (Bruel-Kjaer) connected via GPIB with relevant software (e.g.DISYS).The main experimental natural frequencies of the tested water tank tower via spectral analysis are in Figure 2.

The original calculations were performed before any test started. After the results of the tests were known, the sensitivity study included few analytical model modifications, Juhásová et al. [2].

Calculated modes and frequencies of tower with empty tank when using FEM model that considered boundary springs and beam elements and measured frequencies are in Figure 2.

The structure of the water tower tank was subjected to the verification of its dynamic properties and dynamic resistance capacity before its full service operation. Experiences obtained from this case study give also the indications of existing uncertainties and assessment variance in the design period comparing to actual dynamic properties and the expected structure performance in the case of "design seismic effects". It was confirmed, the experimental verification of dynamic properties and expected response to any accidental dynamic action, contributes to the appropriate determination of the reliability and safety of civil engineering structures including towers and water tanks.

1

Bencat J., Juhásová E., Podhorský P., Pubal J., "The Results of Dynamic Test of Water Tank Drahnov", Report No. 67-SvF-2000 FCE Zilina, University of Zilina, 2000.

2

Juhásová E., Seismic Effects on Structures, Amsterdam, Elsevier, 1991.

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