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Numerical modeling and in situ vibration measurements during the design and construction of low vibration floors at the nanotechnology laboratory corelab 1b

At KU Leuven, the nanotechnology laboratory Corelab 1B has recently been constructed, consisting of low vibration floors, clean rooms and offices. The top 8 m of soil on the site consist of very soft clay and peat, necessitating the use of a piled foundation. Nearby road traffic imposes a challenge to achieve low vibration levels required for sensitive equipment, especially during rush hours. Therefore, vibration control has been of prime concern from an early design stage.

The dynamic response of several foundation designs have been analysed using state of the art 3D dynamic soil-structure interaction models. A finite element (FE) model of a single module of the structure supported by 4 foundation piles has been coupled to a boundary element (BE) model of the layered soil, accounting for dynamic soil-structure interaction. The transfer function between a unit harmonic vertical point load on a nearby road and the foundation has been computed. It was concluded that a sufficiently high bending stiffness of the foundation piles is needed to reduce vibration levels.

Predicted vibrations are subject to a large level of uncertainty. Therefore, a hybrid methodology has been elaborated where structural models are combined with in situ vibration measurements. The measured pile impedance has been coupled to a finite element model of the low vibration floor, resulting in an updated estimate of the transfer function between the nearby road and the structure. This intermediate verification of the design confirmed the performance of the constructed pile foundation, avoiding late structural adjustments.

Numerical predictions obtained with the coupled FE-BE model and the hybrid methodology are finally compared to vibration measurements in the finalized structure, demonstrating that the actual performance of the low vibration floor. This is due to the fact that only a single module of the structure has been considered in the coupled FE-BE and hybrid analysis. Rigid connections between different modules result in a stiffer and heavier ensemble that is less vibration sensitive than a single module. The presence of a large group of piles also causes a screening effect that reduces vibration levels.

Author(s):

Stijn François    
KU Leuven, Department of Civil Engineering
Belgium

Geert Lombaert    
KU Leuven, Department of Civil Engineering
Belgium

Geert Degrande    
KU Leuven, Department of Civil Engineering
Belgium