Design considerations.

While the conceptual design process may be formal or informal, it can be characterized by a series of actions: formulation, analysis, search, decision, specification, and modification. However, at the early stage in the development of a new project, these actions are highly interactive as illustrated in Figure 1 Many iterations of redesign are expected to refine the functional requirements, design concepts and financial constraints, even though the analytic tools applied to the solution of the problem at this stage may be very crude.

Figure 1: Conceptual Design Process
(Adapted with permission from R.W. Jensen and C.C. Tonies, Software Engineering,
Prentice Hall, Englewood Cliffs, NJ, 1979, p.22)

The series of actions taken in the conceptual design process may be described as follows:

• Formulation refers to the definition or description of a design problem in broad terms through the synthesis of ideas describing alternative facilities.
• Analysis refines the problem definition or description by separating important from peripheral information and by pulling together the essential detail. Interpretation and prediction are usually required as part of the analysis.
• Search involves gathering a set of potential solutions for performing the specified functions and satisfying the user requirements.
• Decision means that each of the potential solutions is evaluated and compared to the alternatives until the best solution is obtained.
• Specification is to describe the chosen solution in a form which contains enough detail for implementation.
• Modification refers to the change in the solution or re-design if the solution is found to be wanting or if new information is discovered in the process of design.

As the project moves from conceptual planning to detailed design, the design process becomes more formal. In general, the actions of formulation, analysis, search, decision, specification and modification still hold, but they represent specific steps with less random interactions in detailed design. The design methodology thus formalized can be applied to a variety of design problems. For example, the analogy of the schematic diagrams of the structural design process and of the computer program development process is shown in Figure 2

Figure 2: An Analogy Between Structural Design and Computer Program Development Process
(Reprinted with permission from E.H. Gaylord and C. N. Gaylord, eds., Structural Engineering Handbook,
2nd Ed., McGraw-Hill Book Company, New York, 1979.)

The basic approach to design relies on decomposition and integration. Since design problems are large and complex, they have to be decomposed to yield subproblems that are small enough to solve. There are numerous alternative ways to decompose design problems, such as decomposition by functions of the facility, by spatial locations of its parts, or by links of various functions or parts. Solutions to subproblems must be integrated into an overall solution. The integration often creates conceptual conflicts which must be identified and corrected. A hierarchical structure with an appropriate number of levels may be used for the decomposition of a design problem to subproblems. For example, in the structural design of a multistory building, the building may be decomposed into floors, and each floor may in turn be decomposed into separate areas. Thus, a hierarchy representing the levels of building, floor and area is formed.

Different design styles may be used. The adoption of a particular style often depends on factors such as time pressure or available design tools, as well as the nature of the design problem. Examples of different styles are:

• Top-down design. Begin with a behavior description of the facility and work towards descriptions of its components and their interconnections.
• Bottom-up design. Begin with a set of components, and see if they can be arranged to meet the behavior description of the facility.

The design of a new facility often begins with the search of the files for a design that comes as close as possible to the one needed. The design process is guided by accumulated experience and intuition in the form of heuristic rules to find acceptable solutions. As more experience is gained for this particular type of facility, it often becomes evident that parts of the design problem are amenable to rigorous definition and algorithmic solution. Even formal optimization methods may be applied to some parts of the problem.