The West Point Bridge Designer 2010 automatically calculates the cost of your bridge design as you create it. This cost is continuously updated and displayed on the Status Toolbar.
The cost calculated by West Point Bridge Designer 2010 does not accurately represent the total cost of an actual bridge project. Rather, the cost calculation is intended to give you a general appreciation for the competing factors that influence the cost of a typical engineering project, while also facilitating the use of WPBD for educational design competitions.
As it is calculated by West Point Bridge Designer 2010, the total cost of your bridge design consists of two major components--the and the truss cost. The site cost, in turn, is the sum of three components: (1) excavation cost, (2) cost, and (3) support cost The truss cost also includes three components: (1) material cost, (2) connection cost, and (3) product cost. All of these cost components are described below. The specific numerical cost factors for each are listed in the Design Specifications.
The site cost consists of all costs associated with your selection of the site configuration--the deck height, length, and support configuration of your bridge.
Excavation Cost
When you select the deck height, you determine the amount of soil that must be excavated in order to achieve the correct highway elevation. The lower the deck, the more excavation is required. As in real-world construction, excavation is priced by the cubic yard or cubic meter. The West Point Bridge Designer 2010 determines the required volume of soil excavation, based on the selected deck elevation.
Deck Cost
Your selection of the deck height also determines the overall span length of the bridge. A higher deck results in a longer span, which increases not only the truss cost, but the cost of the reinforced deck.
In the West Point Bridge Designer 2010, the you can also select the material that the deck is made of--either medium-strength or high-strength concrete. Medium-strength concrete costs less than high-strength concrete; however, its lower strength requires the use of a thicker deck. A thicker medium-strength concrete deck weighs more than a thinner high-strength concrete deck and thus will increase the loading on the truss. Increased loading will cause the truss cost to increase. Thus a cheaper deck will tend to result in a more expensive truss, and vice versa.
In either case, the cost of the deck is specified as a lump-sum cost for each 4-meter deck panel.
Support Cost
When you select the type of , , and cable used in your bridge, the West Point Bridge Designer 2010 determines the costs associated with constructing these supports. Each support configuration has its own unique cost. For a given type of (standard or arch), the cost tends to increase with span length, because longer spans weigh more than shorter spans and thus transmit greater to the supports. In general, cost less than , for a given span length. The costs of arch abutments and piers also vary significantly with height. Higher abutments and piers use more material than shorter ones; thus the higher supports cost more. Cable anchorages have a single lump-sum cost.
The truss cost consists of all costs associated with the structural steel and that make up the two main trusses--the principal load-carrying elements of the bridge.
Material cost
Structural steel is normally priced by weight (or mass); e.g., dollars per pound or dollars per kilogram. Thus the cost of a structure depends, in part, on the total weight (or mass) of material used to build it. The West Point Bridge Designer 2010 calculates the material cost by (1) determining the total mass of the three available materials--carbon steel, high-strength steel, and quenched and tempered steel--in your , (2) multiplying the mass of each material type by the corresponding unit cost, in dollars per kilogram, and (3) adding these together to get the total material cost. As noted in the Design Specifications, each of the three different types of steel has a different unit cost. Carbon steel is least expensive; quenched and tempered steel is most expensive. For a given material, hollow tubes are more expensive (per kilogram) than solid bars.
Connection cost
In real structures, the cost of fabricating and building the connections that join the members together can be very significant. Thus the West Point Bridge Designer 2010 includes a cost per joint as part of the total cost of the structure. Because the actual three-dimensional bridge has two main trusses, the number of connections used as the basis for this calculation is double the number of in your two-dimensional structural model.
Product cost
In structural design and construction, the most economical design is often not the one that simply minimizes the material cost. Often the total cost of the structure can be reduced by standardizing materials and member sizes. If all the members in a structure are different materials and sizes, then the cost of ordering, fabricating, and constructing those members will be relatively high. If many members are the same, fabrication and construction costs will be relatively lower. For this reason, the West Point Bridge Designer 2010 includes a cost per product as part of the total cost of the truss. A product is defined as any unique combination of material, cross-section, and in your structural model.
To see the cost factors (cost per kilogram, cost per joint, cost per product, and site cost) and the actual cost calculations for your current design, click the The Site Design Wizard also displays detailed calculations for each component of the site cost.
As you attempt to minimize the total cost of your bridge design, you will find that you can never minimize the site cost, material cost, connection cost, and product cost simultaneously. Minimizing the total cost is always a compromise between these four competing cost factors.
To minimize the site cost, you would simply select the site configuration that costs the least. But the least expensive site configuration requires a simply supported truss spanning a full 44 meters. This configuration will require a relatively heavy truss--one with a high material cost.
To minimize the material cost, you must make each member as light as it can possibly be without failing. For most truss configurations, achieving this condition requires the use of both solid bars and hollow tubes in a wide variety of different sizes. Minimizing the material cost requires you to use a lot of different products and, as a result, your product cost will be quite high.
To minimize the connection cost, you must use the least possible number of joints in your structural model. But if you minimize the number of joints, you will inevitably have a few very long members in your structural model. If a long member is subjected to compressive loading, it will require a very large member size to keep it from failing. (As a member gets longer, its compressive strength decreases significantly.) Thus minimizing the connection cost usually results in a high material cost.
To minimize the product cost, you would need to use a single material, cross-section, and size for every member in your structural model. But your single member size would have to be large enough to ensure that the most heavily loaded member in the structure does not fail. As a result, many of the other members would be much stronger (and therefore much heavier) than they really need to be, and your material cost would be extremely high.
Clearly there is a tradeoff between the site cost, material cost, connection cost, and product cost. Minimizing one always increases one or more of the others. Your task as the designer is to find the best compromise between the four.