7 Vibration Assessment and Control in Technical Facilities Using an Integrated Multidisciplinary Approach 61 1.00E-08 1 10 Linear ave: Vertical 1% exceedance: Vertical Max max-hold: Vertical VC-G VC-F VC-E VC-D VC-C VC-B VC-A 1/3rd Octave Centre Frequency (Hz) RMS Velocity (m/s) 1.00E-07 1.00E-06 1.00E-05 1.00E-04 1.00E-03 Fig. 7.5 Measured borehole vibration vs. VC-criteria 7.5 Computational Methods During the design stage of a building project the structural performance attributes need to be evaluated and the design steered to meet the respective criteria. For low vibration environment projects it is quite common for the strength design to be developed further to meet the vibration criteria with particular attention needed to the grid size, suspended slab thickness and the foundation structure. Computational methods such as finite element analysis (FEA) are a useful means of predicting structural vibration performance for the various dynamic loads associated with internal and external sources of vibration. Footfall is often the most critical dynamic loads for suspended slab structures, i.e. laboratory floors, and it is not normally possible to meet criteria such as VC-A with a strength design alone and without careful consideration of grid size and slab thickness. The effects of footfall on the structural design can be evaluated using published industry methods such as [5, 6]. An example of floor plate FEA using the GSA analysis software [7] is shown in Fig. 7.6. In this case the floor plate supports a typical laboratory space function with criteria VC-A. Footfalls in circulation areas, corridors and laboratory spaces need to be considered. As with test data processing, some consideration of the occurrence of peak events is necessary. Footfall induced vibration can produce resonant and impulsive response types. Resonant response involves a build-up of vibration with each footfall and occurs because one or more of the load harmonics align with a structural mode for a given walking speed. This occurs with so-called low frequency slab designs, with fundamental vertical natural frequencies of 10 Hz or less. High frequency floors exhibit impulsive response where each footfall produces a decaying impulse response. When comparing with the VC criteria it is necessary to evaluate the maximum RMS velocity in the 1/3rd octave band. A rough estimate of 1/3rd octave band RMS can be obtained by taking 70% of the whole signal RMS. However, this can be calculated by passing the predicted vibration time-history though the ANSI 1/3rd octave band filters [8] or by modal response based on modes within the 1/3rd octave bands. The predicted response of the floor plate in Fig. 7.6 is within VC-A for more than 90% of the floor plate area. The vibration response is higher than VC-A in some small areas and is lower in others. Floor plate areas around columns are clearly less mobile and in this case have a vibration performance of VC-B to VC-C or better. This makes it possible to realise more vibration performance out of a structure designed to the VC-A criteria and enables equipment that might require VC-B or VC-C to be used on the floor plate subject to feasibility constraints. FEA can be used to assess the vibration performance of foundation structures which is the part of the building structure where ground vibration transmitted from external sources such as railway and highway traffic, first arrives. An example is shown in which a ground bearing or raft foundation design has been proposed which requires a thickness of 350 mm to meet the strength performance requirements. The objective of the FEA study in this case is to determine whether this slab thickness would enable the vibration criteria to be met or whether it would need to be increased. A review of the principles of soil structure interaction in a low vibration environment context is available in [9]. The structural dynamics of the foundation
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