Analysis of vibrations in high-tech facility

University essay from Lunds universitet/Byggnadsmekanik

Abstract: MAX-lab is a national synchrotron radiation facility in Lund. Nowadays, the MAX project consist of three facilities (three storage rings). A new storage ring is needed to improve material science, such as nanotechnology. MAX IV, also in Lund, will be 100 times more efficient than already existing synchrotron radiation facilities. The storage ring is controlled by a large number of magnets that are distributed along the ring. Since the quality of the measurement results from the MAX IV ring is dependent on the precision of the synchrotron light, a very strict requirement regarding the vibration levels of the magnets are defined. Vibration levels must be less than 26 nm during 1 s in the frequency span of 5-100 Hz. The site of MAX IV is located in an area in northeastern Lund called Brunnshög. At the site there is sedimentary bedrock and the soil mostly consists of boulder clay. The floor of the MAX IV building will mainly be constituted of a concrete structure. The inner and the outer radius of the structure are approximately 70 m and 110 m respectively and the storage ring has a circumference of approximately 500 m. The roof reaches the height of approximately 13 m. The aim is to establish realistic finite element models that predict vibrations on the floor at the magnet foundation with high accuracy. The ultimate goal is to show how the structure can be constructed to reduce the vibration levels and to check the fulfilment of the requirements. Vibrations are analysed by the finite element method. Steady-state analyses are performed to investigate vibrations at the magnet foundations for varying parameters. Transient analyses are performed to compare the results with the requirements by using realistic walking loads. The geometry of the FE-model was chosen to include the main laboratory concrete floor, the storage ring tunnel and the soil. Interfaces between building elements are assumed to have full interaction and since the structure is exposed to loads with low magnitude both the concrete and the soil were modeled as linear elastic isotropic materials. A parameter study was performed to investigate the dynamic behavior of the structure. The load was applied as a harmonic concentrated force positioned on the floor, 10 m from the outer boundary. A frequency sweep in the range of 0-40 Hz was made to investigate the behavior of the structure at different load frequencies. It was concluded that the low stiffness of the soil was the main cause of the vibration levels of the magnet foundations in the storage ring tunnel. To simulate the walking load as realistic as possible, it was applied as a transient moving load. Analyses were made for two load patterns on the concrete floor corresponding to tangential and radial walking patterns. It was concluded that the vibration levels of the magnet foundations generated by the walking load of one person exceeds the requirements when walking next to the storage ring tunnel. Even if the walking load is located several meters away from the tunnel walking loads, especially from groups of people, must be considered in the design process.

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