The Dynamic Response Of Bridges

Since the bridge’s deck spans a long distance, it must be very heavy in weight by its own, not to mention all the weight of the heavy load of traffic that it must carry. Because of all this weight, this results in two active forces: compression and tension. The cable’s parabolic shape results in order for it to effectively address these forces acting upon the bridge. For instance, the deck sags from all the weight of the traffic because of compression forces, which travels upwards the cables. The cables then transfer those compression forces downwards the vertical towers, down into the foundations buried deep within the earth. However, the cables receive the brunt of the tension forces, as they are supporting the bridge’s weight and its load of traffic, being stretched by the anchors' ends

When bridges are poorly constructed, they may fail due to load they carry, collision by derailed trains or even by vehicles that lose control leading to catastrophic incidences. However, if bridges are well-constructed, the chances or injury are greatly minimized in the event of an accident. Discussed in the subsections are the requirements of AS 5100 standards in relation to cable-stayed bridges and include design requirements for bridges, bridge aesthetics, bridge functional requirements, bridge component design requirements and bridge construction

The first example of an external force is the static (dead) load, this refers to the gravitational forces acting on the bridge itself. Every structure has to be able to support the weight of its own materials in order for it not to collapse, this is before any live load is applied to it. Another example is dynamic (live) load which refers to traffic, from people or vehicles, which move across the bridge and apply additional weight to it increasing the magnitude of vertical forces acting on the bridge. But environmental factors such as changes in temperature, precipitation and winds can also create vertical and horizontal loads on the bridge. (Bagga

Initially, suspension bridges before 1940 were made of piers, towers, wires, anchorages, and roadways. Piers were the main foundation for the suspension bridges. There usually were two of them, which were made out of cement and were entrenched in ground underneath the body of water that the bridge was spanned across. Towers were built on top of the piers to provide a means of connection for the roadways and wires. Wires were connected to the towers, roadways, and anchorages to provide tension support for the weight of the bridge. The anchorages were large cement platforms that were planted into the ground on either side of the land so that the wires could be connected to it. Lastly, the roadways were the main point of the suspension bridge. They usually were wide enough to provide four lanes of traffic and stretched from one side of the bridge to the other. This was the basic design of the suspension bridges

Calculations were performed to determine the effectiveness of the design of the platform. Allowing for a safety factor of 1.5 times the design weight of 10kg and considering the bridge must not be overdesigned; plans were made for the bridge to fail at 25kg, 2.5 times that of the design weight. According to the calculations, the bridge would hold a load of over 15kg and experience failure at 20kg in the members. These calculations were later disproven in the testing, breaking 8kg earlier than expected, due to unforseen errors. An analysis of the bridge design and calculations has been included at the end of this report.

Furthermore, when the millennium bridge opened within the first weekend around 100,000 people had crossed the bridge. Due to such heavy traffic, this lead to something called resonance. Resonance is when the input vibrations frequency coincides with the natural frequency of the structure itself, causing large deflections to develop. The issue caused the bridge to undergo a swaying movement; hence led to its closure. After extensive research and analysis, it was found that the movement was caused by synchronized pedestrian footfall. To prevent this there were two options taken into account the lateral stiffening and the damping; both were used in order to increase the natural frequency of the structure so it did not match the footfall. In addition structures called fluid-viscous dampers and tuned mass dampers were installed to control both horizontal and vertical movements.

Bridges are structures that become very susceptible as time passes. In Oregon there have been many bridges being identified to be seismic vulnerable. In an article by Ed Jahn, he sates, “nearly sixty percent of state-identified lifeline bridges likely to collapse or be potentially taken out of use after a quake” (Jahn). Bridges are a whole different story because they aren’t easy to be re-evaluated when they are used constantly by people to get around. The problem with many of these structures, though is that more than half of these bridges were built before 1970 (Jahn). Because of this they aren’t reinforced with new building codes making them highly vulnerable to any disaster to the point of collapsing. It isn’t an easy thing to fix the problems with a bridge because many seem stable but are still at great risk. It is known that “Today, they're still building fracture critical bridges with the belief that they're not going to break,” (Rosenker). Even when a bridge is being identified to see if it’s stable a lot of the time they are thought to be ok, but are really in a bad condition. Because they aren’t fully evaluated, and if a disaster were to hit in a certain location it could cause the bridge to

The most common types are the beam bridges, arch bridges, suspension bridges, cantilever bridges, cable-style, and the kind I am going to research is truss bridge.

Since most transportation experts say is necessary at least in the short term—raise the federal gas tax, which has been frozen at 18.4 cents per gallon for the past two decades— FHTF-RAIAPS is considerable a viable approach for completing the job of restructuring American bridges. The urgency exists in any way the problem is defined, the FHTF is on the edge of bankruptcy and American bridges are collapsing, but, FHTF-RAIAPS finds the rational way in evaluating both faces of the problem within a systematic and operational applicability to ensure maximum utility.

The initial design of the truss bridge shows trusses that are symmetrical throughout the bridge which allowed the weight of the truck moving across to be spread evenly throughout the bridge, allowing it to be stable when the weight is added resulting in minimal deflection of the overall bridge. Throughout the process, most supporting beams were mainly solid bars as they are good in tension and are more sturdy giving the bridge stronger structure. Other beams that didn’t take much load were changed to hollow tubes to lower the cost as well as hold the tensile load of the bridge, this overall reducing the cost and making the bridge more sturdy.

As one looks across a gap in between two land masses, a thought of how one could cross comes to mind. When one thinks of this, a bridge comes to mind. Although they might now know what kind, a suspension bridge is the first idea. They picture something like the magnificent Golden Gate Bridge, one of the most recognizable structures in all of the world. Although suspension bridges now span over miles, they had very humble beginnings. The evolution and expansion of suspension bridges is a topic worthy of this very essay.

The Tacoma Narrows Bridge was designed for a wind speed of 100 miles per hour and a static wind pressure of 30 pounds-force per square foot but unfortunately the bridge failed at a wind speed of less than half and a static wind pressure of one-sixth (Delatte, 2009). This phenomenon raises a big question mark on how this is possible. At first, resonance was suspected to be the main cause of the bridge failure (The New York Times, 1940). After some proper research and thorough investigations by engineers, they came up with several theories on how did the bridge collapse. The primary cause of the collapse lies in the general proportions of the bridge and the type of stiffening girders and floor. The ratio of the width of the bridge to the length of the main span was so much smaller and the vertical stiffness was so much less than those of previously constructed bridges that

My dependent variable how much weight each bridge can endure before the bridge breaks apart. When you add weight tension is created. (tension is a pulling force that occurs at the bottom of a bridge) when tension is applied the connecting points (glue, pieces that snap together, etc.) will unconnect so everything will fall apart and the bridge collapse to the ground.

Because of advancements in today’s technology in construction field, many types of bridges are being constructed depending on the requirement and their suitability for the situations.

A Standard Penetration Test information (SPT) and a Cone Penetration Test data were available for both bridges. The value of N60 ranges from 5 to 63 and this value depend on the characterization of soil and the depth of tests (Table 3-3 and 3-4). Also, the site pile plan is shown in Figure 3.1 and 3.2 for each the project of bridge.