What is Realistic about the West Point Bridge Designer 2010
One of the purposes of the West Point
Bridge Designer 2010 is to provide a realistic, hands-on
experience that will help you to understand how civil engineers design real structures. Many
aspects of the software accurately reflect the structural design process; however, a number
of significant compromises have been made to keep the program from getting too complex. WPBD is intended as an introduction to
engineering design, with emphasis on the design process, rather than
the detailed technical aspects of structural design.
The bottom line is that some aspects of the West Point Bridge Designer
2010 are realistic, and some are not. It is important that you understand the
difference.
The following aspects of the WPBD reflect, with reasonable
accuracy, the nature of engineering design and the process that practicing civil engineers
use to design real bridges:
Design is an open-ended process. Real-world design problems always have many possible
solutions. WPBD demonstrates this aspect of the design process
by allowing you much freedom in developing the configuration of your bridge.
Though design is open-ended, the process is always constrained by real-world conditions and
restrictions. WPBD demonstrates this aspect of the design
process by limiting your design to specific lengths and support configurations that conform to the conditions of the project
site; by limiting your choices of available materials and member types; and by imposing the
requirement to minimize cost.
Design is inherently an iterative process. Because engineers usually work with incomplete
information, they must often make assumptions, then subsequently check and revise those
assumptions as the design process progresses. Developing a high-quality solution always
requires the engineer to consider many different design alternatives, and ultimately select
the best one. WPBD clearly demonstrates this aspect of the
design process. It is impossible to achieve a truly optimal bridge design without
considering many different alternative truss configurations, materials, cross-sections, and
member sizes.
Design always involves trade-offs. It is usually not possible to find a single design
solution that best satisfies all design criteria. Making improvements in one area often
causes unexpected problems somewhere else. You will experience many of these same
trade-offs when you use WPBD. For example, as you attempt to
optimize your design, you will discover that reducing the depth of a
truss causes the cost of the and to decrease (because they get shorter); but it also causes
the cost of the top and bottom to increase (because their increases, and a larger member sizes are needed to preserve structural ). As in real-world structural design, you will need to find the optimum balance
between the two competing criteria.
Structural engineering design is regulated through the use of codes.
Codes ensure that engineering is practiced in a consistent, manner throughout the country, region, or municipality. There are separate,
industry-standard codes governing the design of steel, , and wood structures. There are regional and local building codes that specify
design , fire protection standards, and many other requirements for designing buildings.
Design of highway bridges in the U.S. is governed by the Bridge Design Specification. In WPBD, the uses a standard (but slightly modified) AASHTO truck loading, and the compressive
and tensile strengths of are computed exactly as specified in the AASHTO Specification.
Structures are generally designed such that they can safely carry one or more
code-specified loadings. Minimizing cost is also often an important objective--but never as important as structural safety. Cost reductions can never be made
if those reductions compromise structural safety. The formulation of WPBD is based on this same relationship between safety and cost. In WPBD your design objective is to minimize cost, but a design is never valid if it fails the load test.
Structural design is often characterized by trade-offs between material cost, fabrication
cost, and construction cost. When a structure is designed to minimize material cost, the
design will often include many different member types and sizes. But a variety of member
sizes makes it harder (and therefore more expensive) to and fit the members (fabrication) and to actually assemble them on a job site
(construction). Structural designers usually attempt to achieve a degree of standardization in their selection of structural elements, even if that means
over-designing some of them. The resulting increase in material cost is usually offset by
savings in fabrication and construction costs. The WPBD cost
calculation simulates this trade-off with reasonable authenticity.
In modern structural engineering practice, is generally performed using a computer -based method called matrix structural analysis--more specifically the Direct
Stiffness Method. WPBD uses this same method to compute
member during the load test.