During last week's lab, the three bridge designs which had been created using the specifications of the A1 assignment were analyzed. After agreeing upon what elements of each design were most advantageous, a new design was created with cost being the most important influence in the design process. The final design had a cost of $226,559.12 which was the lowest cost the group was able to attain while still allowing the truck to make its way across. Before last week's lab, none of the group members were aware of the ability to change the material or the thickness of the members of the bridge which was used extensively to reduce the cost in the final design. It was found that if a hollow tube was used, the member needed to be slightly thicker than if it were a solid bar. The group also realized that the cost of manufacturing uncommon length and width combinations was higher than making slightly thicker members. Ultimately, last week was used to experiment with ways to produce the most cost effective bridge as possible. In the coming weeks experimentation on this subject will continue as well as the initial stages of creating a prototype using K'nex pieces.
The West Point Bridge Designer (WPBD) software incorporates many aspects of the bridge design process which are fairly realistic; however, like any other computer simulation, it is not without some aspect of fantasy or a stretch of reality. In terms of what the program does well, it creates a true design situation. Users are afforded many freedoms when it comes to the configuration of their own design. Even with so many freedoms, the program still considers conditions that would be present in a real-world situation. WPBD demonstrates the reality of design trade-offs. In any design, when one aspect of the design is optimized, problems arise in other areas. WPBD stays true to this fact by exhibiting how changing one member's size will affect the force exerted on a surrounding member. Something that is immeasurably important in the world of a structural engineer is building codes. WPBD uses codes and regulations used by the AASHTO Bridge Design Specifications which allows the program to be as true to life as possible.
In reality, the structural integrity of a bridge is the single most important factor in any design followed closely by cost reduction. WPBD will deem a design invalid if the design fails the load test. If the designer chooses to cut costs and thus cuts into the structural integrity of the bridge, the design will fail.
As stated above, the WPBD program is not a flawless program. Practicing engineers are responsible for designing all aspects of the bridge and evaluating their environmental impacts. These aspects include but are not limited to, abutments, piers, roadways, decks, the complete three-dimensional structural system, connections, as well as many secondary members. In WPBD, designers are only asked to develop the main trusses in a two-dimensional design window and make very basic decisions pertaining to the roadway and supports. The program also does not consider fatigue, which is the tendency of a material to fail due to repetitive loading such as vehicular traffic. WPBD only considers two types of vehicular loading and the weight of the bridge itself, but it does not account for things such as wind, snow, collision forces, and natural occurrences. Although WPBD does demonstrate the deflection of the proposed bridge design, what it does not do is use this deflection data as a design criterion.
The West Point Bridge Designer software uses AASHTO truck loading to carry out the load test, but because the program only moves the truck from left to right this causes problems. The AASHTO trucks have heavier rear axles than they do front axles and because the program only moves them across the bridge in one direction, an ideal design in the software may be asymmetrical. In the real world, these trucks would potentially be traveling in both directions across the bridge which would cause an "ideal" WPBD design to fail.
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