Marine Technology - January 2008 - Page#82

<BODY> <span id="lblContents">loaded with six cars parked on the right half of the deck; (III) deck loaded with one car at the center of the deck above the centerline (CL) girder; (IV) deck loaded with one car on the right side of the deck close to the side shell; (V) deck loaded with a rigid cargo load on the right half of the deck. The value of the cargo load in case (V) is equal to the load of six cars parked on the deck. It is found that the cars act as a dynamic absorber of the vibration. As such, the cars reduce the structure response when the excitation frequency is below 11 Hz, while above 11 Hz the carloads increase the structure response. Therefore, it is suggested that carloads have a similar mechanism as that of mass dampers. The above observations are specific to the deck structure; similar analyses may give different results for another structure. However, it is reasonable to point out that parked vehicles can reduce at least one mode shape response. It is also shown that the locations of the cars parked on the deck have an influence on the dynamic response of the deck. When a car is parked at the center of the deck, the maximum structure response is less significant than for a case with the car parked on the panel close to the side shell. 5.2. Modal testing A dynamic analysis focuses on eigenfrequencies and mode shapes. From a physical viewpoint, an initial excitation of an undamped system will cause it to vibrate, and the system response is a combination of eigenmodes, where each eigenmode oscillates at its associated eigenfrequency. Modal testing has become a widely accepted experimental tool for characterizing the dynamic characteristics of structures, such as eigenpairs (eigenfrequencies and the corresponding mode shapes) and dampings. Typical modal testing approaches apply point forces, such as impact hammers or shakers, as actuation forces and use accelerometers as sensing devices. Through the processing of input and output signals, the frequency response function (FRF) can be determined. By analyzing the FRF data, the dynamic characteristics of the testing structure are identified (Wang 1998). A modal testing corresponding to the finite element model presented in Jia and Ulfvarson (2004) was carried out. Figure 11 shows the measurement positions of the modal testing on a lightweight aluminum panel deck. The comparison between the results of the finite element calculation and the modal testing for the unloaded deck is shown in Table 4. The small relative error of 1.58% as well as the good correspondence of the first eigenmode shape indicate that the FE model presented in Jia and Ulfvarson (2004) is suitable for Fig. 9 Finite element mesh (top) and time history of rear suspension resultant force (bottom) when the truck is running on a deck bridge at a speed of 80 km/h well as pneumatic and rotating wheels, and appropriate contact algorithms are used in the contacts between the tires and the bridge deck. Figure 9 shows the FE mesh (top) and parts of the time history of the rear suspension resultant force (bottom) when the truck is running on the bridge. Jia and Ulfvarson (2005a) modeled the vehicles as springdamper systems and the lightweight decks with thin shell elements. The results in Fig. 10 show the vertical displacement of a lightweight high-tensile steel deck structure under different vehicle load cases. The load cases are: (II) deck Fig. 10 Response spectra of the vertical displacement at the center of the deck for four different load cases; the horizontal axis represents the excitation frequency. See the text for an explanation of the different load cases. Fig. 11 Measurement positions on the deck are illustrated with markers 38 JANUARY 2008 MARINE TECHNOLOGY</span> <br /> <br /> <a href="http://www.digitalwavepublishing.com/" title="Digital Magazine">Digital Wave Publishing</a> </BODY>