«A Dissertation Submitted to the Graduate Faculty of the Louisiana State University and Agricultural and Mechanical College in partial fulfillment of ...»
In additional to huge loss of property, loss of life is even more stunning. Compared to the U. S., developing countries which lack predicting and warning systems are suffering even more from hurricane-associated hazards. The cyclone in October 1999 killed tens of thousands in India, and Hurricane Mitch killed thousands in Honduras in 1998. Even as storm prediction and tracking technologies improve, providing greater warning times, the U. S. is still becoming ever more susceptible to the effects of hurricanes, due to the massive population growth in the South and Southeast along the hurricane coast from Texas to Florida to the Carolinas. This growth has 2 spurred tremendous investments in areas of greatest risk. The transportation infrastructure has not increased capacity at anything like a similar pace, necessitating longer lead times for evacuations and forcing some communities to adopt a shelter-in-place concept. This concept recognizes that it will not be possible for everyone to evacuate, so only those in areas of greatest risk from storm-surge are given evacuation orders.
New Orleans is a typical example of the hurricane-prone cities in the United States. Due to the fact that most of the city is at or below sea level, protected only by levees, it has been estimated that a direct hit by a Category 3 or larger hurricane will “fill the bowl”, submerging most of the city in 20 feet or more of water (Fischetti 2001). In extreme cases, evacuations are essential to minimize the loss of lives and properties. In New Orleans, four of the five major evacuation routes out of the city include highway bridges over open water. The Louisiana Office of Emergency Preparedness estimates that under current conditions, there will be time to evacuate only 60-65% of the 1.3 million Metro area populations in the best-case scenario, with a 10% casualty rate for those remaining in the city.
To ensure a successful evacuation, smooth transportation is the key to the whole evacuation process. There are two categories of problems to be dealt with: the safety and efficient service of the transportation infrastructures, such as bridges and highways; the safe operation of vehicles on those transportation infrastructures (Baker 1994; Baker and Reynolds 1992). It is very obvious that maximizing the opening time of the evacuation routes as the storm approaches is very important. The present study investigates these two kinds of problems.
1.2 Bridge Aerodynamics The record of span length for flexible structures, such as suspension and cable-stayed bridges, has been broken with the development of modern materials and construction techniques.
The susceptibility to wind actions of these large bridges is increasing accordingly. The wellknown failure of the Tacoma Narrows Bridge due to the wind shocked and intrigued bridge engineers to conduct various scientific investigations on bridge aerodynamics (Davenport et al.
1971, Scanlan and Tomko 1977, Simiu and Scanlan 1996, Bucher and Lin 1988). In addition to the Tacoma Narrows Bridge, some existing bridges, such as the Golden Gate Bridge, have also experienced large, wind-induced oscillations and were stiffened against aerodynamic actions (Cai 1993). Basically, three approaches are currently used in the investigation of bridge aerodynamics: the wind tunnel experiment approach, the analytical approach and the computational fluid dynamics approach.
Wind Tunnel Experiment Approach: The wind tunnel experiment approach tests the scaled model of the structure in the wind tunnel laboratory to simulate and reproduce the real world.
Wind tunnel tests can either be used to predict the performance of structures in the wind or be used to verify the results from other approaches. The wind tunnel experiment approach is designed to obtain all the dynamic information of the structure with wind tunnel experiments.
Bluff body aerodynamics emphasizes on flows around sharp corners, or separate flows.
Simulating the atmospheric flows with characteristics in the wind tunnel similar to those of natural wind is usually required in order to investigate the wind effect on the structures. For such purposes, the wind environment should be reproduced in a similar manner, and the structures should be modeled with similarity criteria (Simiu and Scanlan 1986). To achieve similarity between the model and the prototype, it is desirable to reproduce at the requisite scale the 3 characteristics of atmospheric flows expected to affect the structure of concern. These characteristics include: (1) the variation of the mean wind speed with height; (2) the variation of turbulence intensities and integral scales with height; and (3) the spectra and cross-spectra of turbulence in the along-wind, across-wind, and vertical directions.
Wind tunnels used for civil engineering are referred to as long tunnels, short wind tunnels and tunnels with active devices. The long wind tunnels, a boundary layer with a typical depth of
0.5 m to 1 m, develop naturally over a rough floor of the order of 20 m to 30 m in length. The depth of the boundary layer can be increased by placing passive devices at the test section entrance. Atmospheric turbulence simulations in long wind tunnels are probably the best that can be achieved currently. The short wind tunnel has the short test section, and is ideal for tests under smooth flow, as in aeronautical engineering. To be used in civil engineering applications, passive devices, such as grids, barriers, fences and spires usually should be added in the test section entrance to generate a thick boundary layer (Simiu and Scanlan 1986). The wind tunnel approach totally relies on the experiments in the laboratory and may be very expensive and timeconsuming.
Analytical Approach: Another way is to build up analytical models based on the insight of aerodynamic aspects of the structure obtained from the wind tunnel tests, as well as knowledge of structural dynamics and fluid mechanics. With the models, the dynamic performance of the structure can be predicted numerically. However, although the science of theoretical fluid mechanics is well developed and computational methods are experiencing rapid growth in the area, it still remains necessary to perform physical wind tunnel experiments to gain necessary insights into many aspects associated with fluid. So the analytical approach is actually a hybrid approach of numerical analysis and wind tunnel tests. Due to its convenient and inexpensive nature, the analytical approach is adopted in most cases. The dissertation also uses the analytical approach to carry out all the research.
Computational Fluid Dynamics (CFD): Computational fluid dynamics (CFD) techniques have been under development in wind engineering for several years. Since this topic is out of the scope for the dissertation, no comprehensive review is intended here.
Long cable-stayed and suspension bridges must be designed to withstand the drag forces induced by the mean wind. In addition, such bridges are susceptible to aeroelastic effects, which include torsion divergence (or lateral buckling), vortex-induced oscillation, flutter, galloping, and buffeting in the presence of self-excited forces (Simiu and Scanlan 1986). The aeroelastic effects between the bridge deck and the moving air are deformation dependent, while the aerodynamic effects are induced by the forced vibration from the turbulence of the air. Usually divergence, galloping and flutter are classified as aerodynamic instability problems, while vortex shedding and buffeting are classified as wind-induced vibration problems. All these phenomena may occur alone or in combination. For example, both galloping and flutter only happen under certain conditions. At the mean time, the wind-induced vibrations, like vortex shedding and buffeting may exist. The main categories of wind effects on bridges with boundary layer flow theory are flutter and buffeting. While flutter may result in dynamic instability and the collapse of the whole structures, large buffeting amplitude may cause serious fatigue damage to structural members or noticeable serviceability problems.
4 Typically, to deal with these wind-induced problems, a scaled-down bridge model is tested in a wind tunnel for two purposes. First is to observe the aerodynamic behavior and then to develop some experimentally-based countermeasures (Huston 1986). Second is to measure some aerodynamic coefficients, such as flutter derivatives and static force coefficients, in order to establish reasonable analytical prediction models (Tsiatas and Sarkar 1988, Scanlan and Jones 1990, Namini et al. 1992).
Long-span bridges are often the backbones of the transportation lines in coastal areas and are vulnerable to wind loads. Maintaining the highest transportation capacity of these long-span bridges, such as the Luling Bridge near New Orleans, Louisiana, and the Sunshine Skyway Bridge near Tampa, Florida, is vital to supporting hurricane evacuations. Due to the aeroelastic and aerodynamic effects from high winds on long-span bridges, strong dynamic vibrations will be expected. Excessive vibrations will cause the service and safety problems of bridges (Conti et al. 1996; Gu et al. 1998, 2001, 2002). Stress induced from dynamic response may also cause fatigue accumulation on some local members and damages to some connections. With the increase of wind speed, the aerodynamic stability of the bridge may also become a problem. In extreme high wind speed, the aerodynamic instability phenomenon, flutter, may happen. As a result of flutter, the bridge may collapse catastrophically (Amann et al. 1941).
1.3 Vehicle Dynamic Performance on the Bridge under Wind Economic and social developments increase tremendously the traffic volume over bridges and roads. Heavy vehicles on bridges may significantly change the local dynamic behavior and affect the fatigue life of the bridge. On the other hand, the vibrations of the bridge under wind loads also in turn affect the safety of the vehicles. For vehicles running on highway roads, the wind loading on the vehicle, as well as grade and curvature of the road, may cause safety and comfort problems (Baker 1991a-c, 1994).
Interaction analysis between moving vehicles and continuum structures originated in the middle of 20th century. From an initial moving load simplification (Timishenko et al. 1974), to a moving-mass model (Blejwas et al. 1979) to full-interaction analysis (Yang and Yau 1997; Pan and Li 2002; Guo and Xu 2001), the interaction analysis of vehicles and continuum structures (e.g. bridges) has been investigated by many people for a long time. In these studies, road roughness was treated as the sole excitation source of the coupled system.
Recently, the dynamic response of suspension bridges to high wind and a moving train has been investigated (Xu et al. 2003), while no wind loading on the train was considered since the train was moving inside the suspension bridge deck. It was found that the suspension bridge response is dominated by wind force in high wind speed. The bridge motions due to high winds considerably affected the safety of the train and the comfort of passengers (Xu et al. 2003). The coupled dynamic analysis of vehicle and cable-stayed bridge system under turbulent wind was also conducted recently (Xu and Guo 2003). However, only vehicles under low wind speed were explored, and the study did not consider many important factors, such as vehicle number, and driving speeds.
Studied on the wind effects on ground vehicles were mainly focused on cross wind. The cross wind effect can be broadly considered as two types: (1) low wind speed effects, such as an increase of drag coefficient and vehicle aerodynamic stability considerations; and (2) high wind 5 speed effects (Baker 1991a). The latter is, of most concern to researchers, is composed of variety of forms. For example, the suspension modes of vehicles may be excited by the strong crosswind if the wind energy is enough around the modal frequency of the suspension system. It is believed that there are three major types of accidents for wind-induced vehicle accidents in high wind: overturning accidents, sideslip accidents and rotation accidents. It is possible for highsided vehicles to be overturned, especially where some grade on section and road curvature exists (Coleman and Baker 1990; Sigbjornsson and Snajornsson 1998; Baker 1991a-c, 1994).
Even for small vehicles, like vans and cars, severe course deviation at gusty sites may occur (Baker 1991 a-b). For vehicles traveling on bridges, the problems are even more complicated because the bridge itself is a kind of dynamic-sensitive structure in a strong wind (Bucher and Lin 1988; Cai and Albrecht 2000; Cai et al. 1999a). The interactive effect between the bridge and vehicles makes the assessment of the vehicle performance on bridges more difficult.
In the 1990’s, many vehicle accidents were reported around the world (Baker and Reynolds 1992; Coleman and Baker 1990; Sigbjornsson and Snajornsson 1998). From the statistics of a great many accidents which occurred during the severe storm on Jan 25 1990 in British, it is reported that overturning accidents were the most common type of wind-induced accidents, accounting for 47% of all accidents. Course deviation accidents made up 19% of the total accidents, and accidents involving trees made up 16%. Among all accidents, 66% involved high-sided lorries and vans, and only 27% involved cars (Baker and Reynolds 1992).
Safety study of vehicles under wind on highways began in the 1980s. In his representative work, Baker (Baker 1991 a-c) proposed the fundamental equations for wind action on vehicles.