Keywords: natural frequency, natural mode, bridge structure, spectral density.

The high increase in traffic intensity is starting to become a problem for bridge structures. All buildings, mostly bridges that are close to roads or railways are affected by paraseismic loads as a result of intensive traffic. During a structures lifetime, moving loads cause fatigue and cracks and every additional dynamic load may decrease the serviceability of the bridge. One of the most useful methods of bridge assessment is theoretical and experimental evaluation of the spectral characteristics of all the subsystems involved (i.e traffic, soil, and structure) [1,2].

The principal dynamic parameters of the bridges were obtained from the computing system IDA NEXIS using the finite element method. This system enables the creation of a framed and shell dynamic model, which conforms to geometry, mass distribution and the boundary conditions. The real dynamic response of the structure was recorded using appropriate measurement equipment. Themeasurements and instrumentation [3] were performed by the Laboratory of Structural Mechanics at the Faculty of Civil Engineering of the University of Zilina.

This paper identifies the spectral characteristics of the dynamic load of selected bridge structures and the response of the microtremor effect arising from the traffic. It is necessary to describe some of the theoretical fundamentals mainly in the field of statistics, probability, theory of random vibrations and finite element methods used in structural dynamics. The experimental measurements were performed for two bridge structures. The first was the bridge over the Kysucká road excited by the roadway microtremor. The second bridge, was the D201-00-CI/11 Zilina - Scaffold Bridge, was excited by a railway. For both bridges the numerical calculation of the basic dynamic parameters and the experimental investigation using spectral characteristics were completed. The microtremors caused by the railway and roadway were also measured. Impact tests were also performed in the vicinity of the bridges. For all these case studies the frequency function (using spectral characteristics) and the vibration level decay depending on the distance from the vibration source were observed. These case studies showed certain dominant frequency ranges for railways and for roadways. Probable reasons of for the dominant frequency ranges are given in the paper. The procedure for the identification the basic dynamic parameters of the bridge structures using the spectral analysis for the microtremors is also given [4].

1

D. Papán, "The bridges spectral analysis due to traffic paraseismicity Dynamics of civil engineering and transport structures and wind engineering", DYN-WIND 2011, 221-224, Zilina, 2011.

2

D. Papán, Z. Bergerová, "Spectral Characteristics of the Bridge Structures due to Paraseismic Load", Engineering Mechanics 2009, 194-195, Svratka, Czech Republic, 2009.

3

J. Bencat, D. Papán, "Dynamic Response of Structures due to Microtremors", in B.H.V. Topping, (Editor), "Proceedings of the Eleventh International Conference on Civil, Structural and Environmental Engineering Computing", Civil-Comp Press, Stirlingshire, UK, Paper 142, 2007. doi:10.4203/ccp.86.142

4

M. Polak, T. Plachý, "Long-Time Monitoring of Thermal Actions on a Prestressed Concrete Bridge Structure", 48th International Scientific Conference Experimental Stress Analysis, 369-376, Velke Losiny, Czech republic, 2010.

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