148 B. Pridham et al. This paper first investigates the source of vehicle forces and common control strategies for parking garage vibrations. This is followed by a case study on the vibration design of an ambulatory surgical clinic located below a multi-level parking garage. The case study includes a discussion on recommended design criteria, preliminary numerical assessments of the issue, and the results from measurements on the built structure and detailed modelling for development of design solutions. 19.2 Vehicle Vibrations in Parking Garages Vehicle-induced vibrations in parking garages are caused by the dynamic interaction that occurs between the tires, suspension system and sprung mass, and irregularities in the roadway surface. The magnitude of the forces are a function of the stiffness and damping of the suspension system, the speed of travel, and the roughness of the roadway surface. A stiffer, more heavily damped vehicle suspension will generally create higher dynamic forces than a ‘soft’ suspension on an equivalent surface. Additionally, fast moving vehicles on rough surfaces will generally create higher dynamic forces than slow-moving vehicles on smooth surfaces. The dynamic loads are associated with two classes of vibration modes: ‘body bounce’ or cabin motion typically located at frequencies between 0–2 Hz, and ‘axle hop’ or wheel motions typically located at frequencies between 10–15 Hz. The axle hop modes are the primary concern for vibration-sensitive building structures (both ground-borne from roadways and structure-borne from parking garages and connected ramps), since floor modal frequencies are normally well above 2 Hz in these types of structures. Figure 19.1 is an example of dynamic load spectra for a car and heavy truck computed using the quarter car model (QCM). The loads are normalized by the amplitude of surface displacement/roughness, h, and tire stiffness kt, based on the analytical procedure presented in [5]. The peaks in the spectrum are the location of the vehicle system resonances, i.e., body and axle modes. This model is useful for screening models of vehicle-induced vibrations on parking garage structures. The axle hop forces are characterized by a transient temporal signature, having a peak at the first point of tire impact followed by a train of reduced amplitude forces (wheel hops) that decay quickly. In parking garages these transient forces are generated by vehicles traversing irregularities such as speed bumps, expansion joints and drainage grates. Fig. 19.1 Example of dynamic load spectra computed with the QCM [5]
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