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Research in the Structural Mechanics Section of K.U.Leuven is concentrating on the dynamic analysis of structures excited by wind, earthquakes, traffic, building activities, and machinery. The Structural Mechanics Section develops numerical simulation models based on first principles, with deterministic or probabilistic formulations. Numerical modeling is supported and validated by means of laboratory and in situ experiments. Research in the Structural Mechanics Section is concentrating on two main lines, where the section takes a leading international role: (1) vibrations in the built environment and (2) vibration based structural identification and evaluation. In the following, both lines are briefly outlined and it is shown how expertise within these areas has allowed broadening the research scope.

Vibrations in the built environment

Dynamic soil-structure interaction

Vibrations originating from earthquakes, traffic, construction activities, and industrial machinery propagate through the soil and excite foundations of buildings where they may cause disturbance of sensitive equipment, annoyance to people, and structural damage. In the frequency range between 1 and 80 Hz, humans experience these vibrations as mechanical vibrations of the body, while at higher frequencies between 16 and 250 Hz, the vibrations are perceived as low frequent noise radiated by building parts. The Structural Mechanics Section develops and validates numerical models for the prediction of ground-borne noise and vibrations in buildings, as induced by road traffic, rail traffic at grade and in tunnels, pile driving, and blast loading.

These models are based on coupled finite element-boundary element formulations, accounting for dynamic soil-structure interaction and exploiting geometrical invariance or periodicity of the problem geometry if possible. They are used to design vibration mitigation measures, to design facilities with low vibration requirements, and to perform environmental impact studies. Software developed by the section is used by many research partners and industrial partners.

Experimental validation by in situ measurements highlights the crucial role of dynamic soil characteristics in the accuracy of the numerical predictions. Dynamic soil characteristics are typically determined by means of non-intrusive (Spectral Analysis of Surface Waves (SASW), seismic reflection and refraction) and intrusive (Seismic Cone Penetration Test, down-hole, cross-hole) geophysical tests; the section is doing research on the practical use of surface wave methods to determine small strain soil characteristics (shear modulus and material damping ratio). Even with extensive in situ testing, however, it is difficult to obtain good prediction accuracy. This has stimulated the development of hybrid predictions, combining measured transfer functions with numerical predictions.

Research on vibrations in the built environment has recently been extended towards problems of earthquake engineering in areas of low seismicity such as Belgium. In these areas, seismic action is traditionally not taken into account in the design of most structures, although historic data show that earthquakes of moderate intensity may occur. The focus will therefore go to the assessment of the vulnerability of existing buildings and the development of guidelines for the design of new buildings. This research will be supported by the extensive expertise in vibration based methods for structural evaluation as developed in the second line of research.

Finally, vibrations of buildings due to wind loading have been studied for flexible structures where dynamic wind loads may trigger fluid-structure interaction phenomena. Numerical methods of computational fluid dynamics are developed to assess wind-structure interaction and possible aero-elastic instabilities in an early design phase. These methods are complementary to more elaborate wind tunnel testing. This study is performed in close collaboration with UGent where extensive know-how on strong fluid-structure coupling is available.

 

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Collaboration

Measurement data

Key publications

  • Lombaert, G., Clouteau, D. (2009); Elastodynamic wave scattering by finite-sized resonant scatterers at the surface of a horizontally layered halfspace; Journal of The Acoustical Society of America, 125(4): 2041-2052.
  • Schevenels, M., Lombaert, G., Degrande, G., François, S. (2008); A probabilistic assessment of resolution in the SASW test and its impact on the prediction of ground vibrations; Geophysical Journal International, 172(1): 262-275.
  • Lombaert, G., Degrande, G., Kogut, J., François, S. (2006); The experimental validation of a numerical model for the prediction of railway induced vibrations; Journal of Sound and Vibration, 297(3-5): 512-535.
  • Clouteau, D., Arnst, M., Al-Hussaini, T.M., Degrande, G. (2005); Freefield vibrations due to dynamic loading on a tunnel embedded in a stratified medium; Journal of Sound and Vibration, 283(1-2): 173-199.
  • Lombaert, G., Degrande, G., Clouteau, D. (2000); Numerical modelling of free field traffic-induced vibrations; Soil Dynamics and Earthquake Engineering, 19(7): 473-488.
  • Degrande, G., De Roeck, G., Van den Broeck, P., Smeulders, D. (1998); Wave propagation in layered dry, saturated and unsaturated poroelastic media; International Journal of Solids and Structures, 35(34-35): 4753-4778.

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Vibration based structural identification and evaluation

Numerical Simulation

Vibration monitoring of civil engineering structures (e.g. bridges, buildings, dams, wind turbines) has gained a lot of interest over the past decade, due to the relative ease of instrumentation and the development of new powerful system identification techniques, able to extract modal properties (natural frequencies, mode shapes, modal strains) from accelerations, displacements and/or strains.

Special attention is paid to techniques making use of operational data (output-only or operational modal analysis). The availability of models based on operational modal analysis opens the way for in situ model based diagnosis and damage detection (structural health monitoring). Damage assessment can be based on observed changes of modal parameters extracted from vibration measurements. The information content of these modal parameters largely depends on the proper design of the measurement campaign. Therefore, research is performed in the following domains:

  • Enlargement of the frequency domain by adding measured artificial forces, which can have a small amplitude compared to ambient (i.e. operational) forces. System identification algorithms are developed that account for both excitation sources.
  • Application of new sensor types: the resolution of damage identification is enhanced by measuring also dynamic strains. As these tend to be very small in operational conditions, the use of highly sensitive optical fiber strain sensors and 3D camera or laser based optical systems is explored.
  • Optimal sensor location.
  • Automation of the modal parameter extraction process: changes in observed modal parameters can be used for damage assessment. Damage is simulated with a numerical (finite element) model of the structure. Applying optimization techniques, the differences of calculated and measured modal parameters are minimized. Updating, inverse modeling or parameter identification can also be used to tune model based on measured responses.
  • Inverse methods have also been studied to identify dynamic wind loads from in situ vibration data obtained from existing structures. The identified wind loads can be used to evaluate the design and performance of the structure, as well as to verify commonly made assumptions in design codes for wind pressure distributions on structures with a simple geometry.
  • This research has lead to the development of powerful methods for input and state estimation that can also be used to estimate unmeasured response quantities from a limited number of measured vibration data and a numerical model of the structure. This is very useful in the context of fatigue analysis to derive real load spectra, which often can only be estimated in an approximate way by the existing codes.
  • Measuring vibrations is a common way to validate complex models, such as interaction models. One application of particular interest is dynamic vehicle-bridge analysis. Models that include vehicles as well as the supporting bridge structure are validated by in situ measurements. From these measurements, the influence of road or rail irregularities and the stiffness and damping of the ballast in case of railway bridges can be estimated.

Based on the prior experience on optimization in the context of parameter identification, a new research line is recently opened aiming to develop new methodologies for shape and topology optimization. The integration of optimization in computer aided engineering enables design optimization, aimed at finding the best compromise between cost and performance. Data uncertainties have to be taken into account by robust design optimization or reliability based design optimization.

Within the frame of the described research lines, the Structural Mechanics Section is employing expert knowledge in applying, developing and integrating a suite of numerical modeling techniques: static and dynamic finite element and boundary element methods, linear and non-linear finite element analysis, computational fluid dynamics and stochastic structural analysis.

Contact person

Collaboration

Key publications

  • Reynders, E., De Roeck, G. (2008); Reference-based combined deterministic-stochastic subspace identification for experimental and operational modal analysis; Mechanical Systems and Signal Processing, 22(3): 617-637.
  • Reynders, E., Pintelon, R., De Roeck, G. (2008); Uncertainty bounds on modal parameters obtained from Stochastic Subspace Identification; Mechanical Systems and Signal Processing, 22(4): 948-969.
  • Teughels, A., Maeck, J., De Roeck, G. (2002); Damage assessment by FE model updating using damage functions; Computers and Structures, 80(25): 1869-1879.
  • Peeters, B., De Roeck, G. (2001); Stochastic system identification for operational modal analysis: A review; ASME Journal of Dynamic Systems, Measurement, and Control, 123(4): 659-667.
  • Peeters, B., De Roeck, G. (1999); Reference-based stochastic subspace identification for output-only modal analysis; Mechanical Systems and Signal Processing, 13(6): 855-878.

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