## Determination of Modal Parameters of a Steel Bridge by Finite Element Analysis (FEA)

To predict the dynamic vibration behavior of a steel bridge, a numerical analysis was performed using Finite Element Analysis (FEA). Based on the structural geometry, the specification of material properties and the imposed boundary conditions, the modal parameters of the structure can be calculated.

## Finite Element Model

The dimensions of the bridge volume model are 73 m in span, 22 m in width and 25 m in height.
The typical material properties (young's modulus, density and Poisson's ratio) of steel were used for the model (Table 1). The road surface was omitted.
To simulate the bridge dynamics in the given situation, the boundary conditions at the respective faces of the model were defined as "fixed". This accounts for the connections at the abutments of the bridge.

The resulting finite element (FE) model, which consists of over 3000 elements, is shown in Figure 1.

Table 1: Material properties of the steel bridge
Material parameterValue
Young's modulus210 GPa
Density7850 kg/m3
Poisson's ratio0,3
Table 2: Natural frequencies of the steel bridge resulting from FEA analysis
ModeNatural frequency (Hz)
12,2
24,4
36,4

The FE simulation resulted in the numerically determined values listed in Table 2 for the first three natural frequencies. The simulation of the corresponding mode shapes is shown in Figures 2-1 up to 2-3.

The calculation provides the modal parameters of the steel bridge structure, which are very important in the development and design phase, for example, to prevent failure of the structure under dynamic excitation. This type of structure is constantly exposed to environmental forces (traffic, wind, earthquakes) that can cause undesirable and damaging vibrations of the structure. To simulate the dynamic loading, the expected dynamic excitation forces are applied to the model in order to avoid resonance-like vibrations.

In addition to simulation, our specialists also perform Experimental Modal Analyses, with applications ranging from individual components such as impeller blades to entire structures.