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Evaluating Design Alternatives The Role Of Simple Engineering Analysis And Estimation

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2001 Annual Conference


Albuquerque, New Mexico

Publication Date

June 24, 2001

Start Date

June 24, 2001

End Date

June 27, 2001



Page Count


Page Numbers

6.469.1 - 6.469.8

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Derek Dunn-Rankin

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NOTE: The first page of text has been automatically extracted and included below in lieu of an abstract

Session 2525

Evaluating Design Alternatives – The Role of Simple Engineering Analysis and Estimation

Derek Dunn-Rankin University of California, Irvine


The best design is achieved, in general, after exploring the widest possible design option space. Because there is a finite time in which to choose from these options, it is common practice to restrict the design space a priori or during the concept generation phase of the process. This on-the-fly exclusionary practice is acute among student engineers because in traditional coursework there is rarely a benefit to increasing the design space before solving a homework problem. In fact, the reverse is true since by narrowing the possible options for solution, an example problem pattern match becomes easier to find. Because design problems are open-ended, however, there is a benefit to a wide solution space. In many ways, design experience manifests itself as a filter that rapidly eliminates unpromising design alternatives. Estimation, order-of-magnitude analysis, and back of the envelope reasoning are also strategies that can be employed to eliminate design alternatives efficiently and methodically. Approximate methods are used to uncover the fundamentals of processes,1 and can also be used to explore a larger design space, which increases the opportunities for finding a premium design solution.

Approximate and order-of-magnitude methods are a branch of qualitative reasoning that has been established as an important component in the engineering analysis of physical systems. The importance of approximate reasoning has led to many attempts to quantify outcomes from a fundamentally qualitative understanding so that commonsense information can be formalized as part of a decision process.2-4 These decision-making research efforts notwithstanding, it is important to develop approximate reasoning skills among engineering students for the express purpose of increasing the efficiency with which they navigate design options.

The first task in applying engineering analysis to a real problem is to make the physical system similar to one that has a known solution. This process is generally referred to as modeling. Modeling involves the invocation of assumptions, and with enough assumptions, it is always possible to extract an analyzable model from the real system. Unfortunately, the assumptions may be so drastic as to make the model and its analysis useless or irrelevant. Consequently, the model must always be validated in some way. Often the model is validated through a test, or it can be validated by relaxing the assumptions (i.e., using a more complex model). There is a hierarchy of model complexity that begins with rough estimates and ends with detailed (sometimes exact)

Proceedings of the 2001 American Society for Engineering Education Annual Conference and Exposition Copyright © 2001, American Society for Engineering Education

Dunn-Rankin, D. (2001, June), Evaluating Design Alternatives The Role Of Simple Engineering Analysis And Estimation Paper presented at 2001 Annual Conference, Albuquerque, New Mexico.

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