19 Addressing Parking Garage Vibrations for the Design of Research and Healthcare Facilities 153 19.4.3 Screening Level Assessment: Design Phase During design of the base building a screening level review was conducted to determine compatibility of the site with surgical use. Finite element models (FEM) were constructed for a review of system dynamics. The models were used to establish preferred locations for placement of operating rooms on level one, and to confirm the potential for motions of the parking slab to exceed design criteria during use. A numerical study was conducted to compare the dynamic forces from a typical heavy truck travelling at a speed of 15 mph in the garage (the speed limit), with the vehicle forces required to excite the structure at a response equal to the design criteria. Vehicle loads were estimated using the quarter car model, with parameters as specified in Fig. 19.1. The amplitude of surface irregularities (h), which are required to compute the vehicle forces, were specified based on the roughness profiles defined in ISO 8608 [16]. Three degrees of surface roughness were examined: ‘very good’, ‘average’ and ‘very poor’. These were considered representative of varying levels of obstructions such as expansion joints, drainage grates etc. The height of irregularities for these profiles ranges approximately between 2 mm (very good) and 35 mm (very poor). The estimated vehicle loads were compared to the force amplitudes required to excite each mode of vibration at a response level equal to the design criteria. These response amplitude for the ithmode was computed as follows: Fi DVcrit 2 fi p2 Mi (19.1) Where Fi is the modal force amplitude; Vcrit is the root-mean-square vibration velocity; fi is the frequency of the ithmode of the FEM; and, Mi is the generalized (modal) mass of the ith mode of the FEM. Note that the estimate computed in Eq. (19.1) is conservative as it neglects any dynamic interaction between the vehicle and parking surface, and it is assumed that the vehicle forces are applied at the maximum mode shape coordinate. In reality, responses are reduced by the interaction between the vehicle and parking slab systems and the load varies in space and time as the vehicle traverses the garage. An example of the results from the screening level assessment is shown in Fig. 19.3. The modal force amplitudes for the first 100 modes of the FEM of the north (left) half of level two of the garage are shown together with the truck forces estimated using the QCM. The modal force amplitudes were estimated using Eq. (19.1), assuming a critical velocity of 1000 micro-in./s. Modal force amplitudes that fall below the vehicle force curves indicate modes of vibration that may be excited by vehicle forces. The results illustrate that even for very good roadway conditions, forces from trucks traversing the garage may result in vibration levels above 1000 micro-in./s. Fig. 19.3 Vehicle force curves and modal force amplitudes associated with the 1000 micro-in./s criterion
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