How to design a bridge
How to design a bridge
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At this point in the design process, you have optimized your design for one particular site configuration and load case--one particular combination of height, length, support configuration, deck material, and truck loading. But you won't know if your design is truly optimal until you have considered other site configurations and load cases as well.
The West Point Bridge Designer 2010 allows for 98 possible site configurations, consisting of various combinations of deck elevation, support type, and support height; and four possible load cases, consisting of various combinations of deck material (i.e., deck weight) and truck loading. The total cost of the bridge equals the plus the truss cost. Each site configuration supports the bridge in a different way, and thus each one has a different site cost. Each load case has a different effect on the steel truss, and thus each one is likely to result in a different truss cost.
Even though the site cost makes up a substantial portion of the total cost of the bridge, picking the configuration with the lowest site cost will not necessarily result in the lowest total cost. In general, site configurations that have a low site cost tend to have a relatively high truss cost and vice versa.
A site configuration with a high deck elevation will generally have a relatively low site cost, because a higher deck requires little or no excavation. But a configuration with a high deck elevation also has a greater span length. A longer span requires a larger, heavier truss, which results in a higher truss cost.
Arch cost more than , and tall cost more than short ones. Thus site configurations that use arches tend to have higher site cost. But because of the V-shape of the river valley, arch abutments also reduce the span length (for a given deck height)--the taller the , the shorter the span. Arch abutments also provide more lateral restraint than standard abutments. Both of these factors tend to cause the truss cost to be less for arches.
Building a in the middle of a river can be quite expensive. Thus configurations with have significantly higher site costs than those without piers. But the pier also divides one long span into two short ones, and two short trusses are usually much less expensive than a single long one.
Cable are also expensive, but they provide for additional support (e.g., the cable supports of a cable-stayed bridge) and thus can reduce the truss cost significantly.
The choice of deck material affects both the site cost and the applied during the Load Test. Medium-strength is less expensive than high-strength concrete but results in a thicker deck, which is heavier. High-strength concrete is more expensive but results in a thinner deck, which is lighter. Thus the less expensive deck material tends to result in a higher truss cost, while the more expensive deck material results in a lower truss cost.
Your choice of truck loading has no effect on the site cost but will have a significant effect on the truss cost.
Engineering design always involves tradeoffs, and the tradeoff between the cost of a structure and the cost of its supporting is a critically important aspect of most real-world bridge designs.
So which site configuration and load case will result in the lowest total cost? The only way you can answer this question is by trial and error, combined with careful logical reasoning.