«A Dissertation Submitted to the Graduate Faculty of the Louisiana State University and Agricultural and Mechanical College in partial fulfillment of ...»
(b) Investigation of vehicle-bridge-wind system (Chapters 4 and 5) With the modal coupling techniques and hybrid aerodynamic analysis approaches developed in the first part, the vehicle-bridge-wind system is systematically studied in the dissertation. This part covers two consecutive topics: interaction analysis of the coupled system and accident assessment of vehicles. A rational prediction of the performance of the vehiclebridge system under strong winds is of utmost importance to the maximum evacuation efficiency and the safety of vehicles and bridges. Most existent works focus on either wind action on vehicles running on a roadway (not on bridges), wind effect on the bridge without considering vehicles on the bridge, or vehicle-bridge interaction analysis without considering wind effect. A comprehensive vehicle-bridge-wind coupled analysis is very rare.
The present study aims at building a framework for the vehicle-bridge-wind aerodynamic analysis, which will lay a very important foundation for vehicle accident analysis, based on dynamic analysis results, and facilitate the aerodynamic analysis of bridges, considering vehiclebridge-wind interaction. The framework starts with building a general dynamic-mechanical model of a vehicle-bridge-wind coupled system. After the framework is established, a series of 2-axle four-wheel high-sided vehicles on long-span bridges under strong winds are chosen as a numerical example to demonstrate the methodology. With the mechanical model of the vehiclebridge system, dynamic performance of vehicles as well as bridges is studied under strong winds.
Based on the dynamic interaction analysis results, an assessment model for vehicle accidents on bridges and on roads under wind action is introduced. All the existent accident analysis models are only for vehicles on roadways, and dynamic vibrations of the vehicles are not considered. The proposed model starts with a full interaction analysis between the bridge and the vehicle, which predicts, in addition to the bridge vibration, the vehicle response in the directions of vertical, rolling and rotation under the wind action and road roughness. Such vehicle and bridge vibration information is carried over to the following accident analysis of the vehicle only. With given accident criteria, the accident driving speed can then be predicted under
any wind speed. The main conclusions include:
• The proposed accident analysis model can be used to predict the accident-related response. With suggested accident criteria and driving behavior model, the accident risks can be assessed.
• Lowering driving speed is effective in lowering the accident risk only if the wind speed is not extremely high. Setting suitable driving speed limits is important to decrease the likeliness of accident occurrence.
• When wind speed reaches a certain high level, the vehicle should not be on the bridge, no matter what its driving speed. Rational critical wind speed limits should be set to decide when to close the bridge.
• Vehicles on the bridge are more vulnerable to accidents than those on the road. Lower driving speed limits should be set for vehicles on bridges than in the road to avoid accidents when strong wind speed exists.
(c) Mitigation of excessive responses of bridges (Chapters 6-8) Excessive responses of the bridge usually exist when wind speed is high. Under strong winds, modal coupling effects usually become stronger for long-span bridges. It may result in a significant additional component to the buffeting response of each individual mode, compared with cases of weak modal coupling. In addition to the traditional resonant suppression control mechanism of Tuned Mass Dampers, a more efficient control approach may exist for the coupled buffeting control of bridges in strong wind. Approximated closed-form solutions of coupled buffeting response are first derived for a multi-mode coupled bridge system attached with an arbitrary number of TMDs. This derivation clearly shows the contributions of all components of the response and indicates how TMDs can be designed to control each part. Finally, the applicability of dual-objective control with a passive TMD system, based on the new control approach, is briefly discussed.
After the new features of TMD, control on vibrations are studied. Optimal variables of TMDs in the coupled vibration control are numerically studied. The finding in this part verifies the common assumption that single-mode-based control strategy can be used for bridges with well-separated modal frequencies. However, for coupling-prone bridges with low frequency ratio, the control strategy should be based on the analysis of coupled vibrations. Many modern long-span bridges may fall into this category.
As an exploratory application of vibration control for long-span bridges in a hurricane-prone area, a movable vehicle-type of control facility is designed and its efficiency is studied. Placement of movable control system on bridge during evacuation will certainly block traffic and is not a perfect solution. However, it is better than otherwise to completely close bridge in evacuation. In extreme cases, to protect the bridge from being damaged, the movable control system can also be placed on the bridge when the traffic is completely closed. Some issues for practical implementation certainly need to be further addressed.
9.2 Future Work
The writer believes that the following issues deserve further research:
• In the dissertation, determinant results are given to assess the risks of vehicle accidents. However, the driving process of vehicles on the road or on the bridge is actually affected by many uncertain factors, so the reliability-based accident analysis is needed and can be carried out in the future work.
• Wind- and vehicle-induced fatigue is a very important topic. All existent works treat vehicle-induced fatigue and wind-induced fatigue separately. Based on the interaction analysis of the vehicle-bridge-wind system, the combined fatigue problem can be rationally predicted, which is a very interesting direction for future research.
• Vehicle performance on roadways under wind loading deserves more study. In some typical highway locations with various curvatures, slopes, and road surface conditions, vehicles may exhibit different accident-related performance. To envisage the transportation scenario of vehicles from several typical locations of the highway to even the whole highway system in one specific area will give the transportation authorities much valuable information. This is another very promising future direction based on the present study.
• The driver-behavior model, which can be adopted in the accident analysis model, should be improved by adopting statistic approaches, since every driver behaves differently. As a very challenging task, it deserves further study.
• In the dissertation, some temporary approaches with Tuned Mass Dampers are proposed to mitigate the vibration of long-span bridges under strong wind. Some more adaptive and economical ways can be studied, based on more advanced materials and mechanical engineering techniques.
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