Structural Design Bridge Project
See project file here.
With public funds for infrastructure development at a low in government budgets, the “client” asked me to design a bridge that is expected to cost the least amount of money. Much of America’s bridges are in need of repair and replacement and a good design could be used on a large scale to build up America’s infrastructure. Using West Point Bridge Designer software and the engineering design process, I create the lowest cost possible bridge design that meets all design constraints
For this project, the application provides three different types of steels to utilize in our bridge making: carbon steel, high-strength low-alloy steel, and quenched & tempered steel. The American Iron and Steel Institute defines carbon steel as “Steel is considered to be carbon steel when no minimum content is specified or required for chromium, cobalt, columbium [niobium], molybdenum, nickel, titanium, tungsten, vanadium or zirconium, or any other element to be added to obtain a desired alloying effect.” Therefore, carbon steel is a purer steel that may prove useful in the design process. In fact, more than 85% of the steel needed by the United States is composed of carbon steel, making it by far the most commonly used type. High-strength low-allow steels, on the other hand, are alloyed with other materials to provide “…better mechanical properties and/or greater resistance to atmospheric corrosion than conventional carbon steels…” These alloys make this type of steel much more resistant to the elements, which would prove useful in the bearing conditions of a bridge over a river. Last but not least, quenched and tempered steels are said to “…combine high yield strength… and high tensile strength with good notch toughness, ductility, corrosion resistance, or weldability.” With strength and resistance in mind, quenched and tempered steels come first in our list combining both a very good strength value with a formidable resistance to the elements. While quenching and tempering does take time in the manufacturing process, it does help applications such as bridges, as quenching and tempering “…increases the yield point by 30–35% and the resistance to rupture 25–30%, but decreases the plasticity, particularly when the carbon concentration is low.” The decreased and uniform grain size brought about by the process allows this type of steel to have much more strength when compared to regular carbon steel.
There are several types of bridges in existence, largely consisting of: truss, beam, arch, suspension, and cable-stayed bridges. Besides the deck, which is the concrete on which vehicles traverse over, a bridge is composed of a superstructure, substructure, and bearings. The type of bridge built usually depends on the length, cost, and time constraints associated with the build. Truss bridges, for example, take comparatively less time to build and are strong and light-weight, however they cannot span the same length as an arch bridge. Arch bridges, while sturdy, take massive amounts of time to build and even more materials to construct. One of the most famous types of bridges, suspension bridges can span some of the longest distances. Some famous suspension bridges include: The Golden Gate Bridge, the Brooklyn Bridge, and the Akashi Kaikyo Bridge. One of the main downsides of the suspension bridge design is its exorbitant cost and susceptibility to swaying. Cable-stayed bridges, when compared to suspension, are cheaper, but only generally cover shorter distances. One of the downsides of cable-stayed bridges is its difficulty of maintenance, with many of its critical components being centralized in a few key design areas.
Per my research, I chose to use the quenched and tempered steel along with the arch design for my design. Due to the material properties associated with the QTS when compared to carbon steel alone and alloy as well, I deemed tubed QTS to be the most efficient use of my money. Coming in at just about $176 thousand dollars, my design fits the constraints and parameters while fulfilling the problem statement. I deemed it necessary to utilize a 4 m abutment in order to add needed strength at the sides of my bridge after noticing the needed support when compared to the middle. Overall, tubed seemed better than solid due to its near similar strength at a greatly reduced cost. Majority of bridges today are not made from solid steel alone.3 The arch design, while effective, provided difficult to make symmetrical due to its design. If I had to do one thing differently, it would to somehow create the design without utilizing an abutment for a similar if not reduced price point.
