DESIGN METHODS
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strategy in PS, 2) the design requirements generated, and 3) the information access [Restrepo et al.
2004]. All three elements are crucial and need a better formalism. Dorst and Cross (2001) in extensive
creative design practice studies have come out with two notional design “spaces” – problem space and
solution space. These spaces co-evolve with interchange of information between them. PS is a process
of drawing upon knowledge (or external information) to compensate for missing information and
using it to construct the problem space. Requirements are used to specify the design assignment
(defining the problem space) and to constraint the desired solution (exploring the solution space) and
are therefore an important aspect of PS [Restrepo et al. 2004]. They are often given in narrative format
as design problems (narrative requirements). The design problem can be characterized as not being
subjected to systematization, incomplete, vague and there is a lack of information in each of the three
components stated. NR is dynamically generated during the design process from a design requirement
into a design specification (DS). The requirement formalism is triggered by prior knowledge or by
knowledge through interaction with design object or with external sources of information.
Requirements are discriminated to functional requirements and non-functional requirements or
constraints.
Several approaches that use decomposition to enhance dynamic generation of requirements include the
use of RC and function models (scenario creation) [Cross, 2000, Ulman 2002, Otto et al. 2001, Pahl et
al. 2007]. Both methods are effective for design specifications and analysis, i.e. problem space
structuring. Engineering design is an activity that requires both logic and creativity. Systematic
methods and tools have continuously been created to better conduct the logical analysis in order to
unleash the designer to engage in the creative aspects of problem solving. Research in different design
disciplines has produced models concentrated on different aspects of design process [Cross, 2000,
Restrepo et al. 2004]. For instance, architecture models propose that solution concepts go before
problem structuring. Models from software design propose the designer negotiating the structure of the
design problem. Engineering design provide models, with the basis that problem analysis precedes
synthesis of solution [Restrepo et al. 2004, Pahl et al. 2007]. In a complex system design, the design
strategy is not discipline specific and a combination of these strategies is crucial. The RC is generic
based on design goals, constraints and all relevant system parameters and information needed for a
successful system design. RC and function models are used not only by Pahl and Beitz (2007); various
engineering textbooks make use of the idea of the checklist with different names and categories. RC
such as the one shown in table 1 provides a decomposition strategy useful for logical analysis and
intelligent system application for a combination of the different model strategies in requirement
formalism. The RC is used to facilitate the shift from a document-centric to a model-centric problem
structuring. Rational design methods unlike creative methods encourage a systematic approach to
design. Checklist is the simplest kind of rational design method. It externalizes the requirement
process so that important design issues are not overlooked, and formalizes the process by making a
record of items which can be analysed and checked-off as they are completed. It also allows sub-
division of task such as allocation task to different team members [Cross, 2000].
Table 1. Part of a requirements checklist [Pahl et al., 2007]
Category Example
Geometry Size, height, length, diameter, space, connection, arrangement
Forces
Direction, magnitude, frequencies, Weight, load, deformation, stiffness, elasticity,
inertia, resonance
Energy
Output, efficiency, loss, friction, ventilation, state, pressure, temperature, heating,
cooling, supply,
Material
Flow and transport, physical and chemical properties, design for manufacturing
(DFM)
Costs Maximum permissible manufacturing cost, cost of tooling, material cost, time,
Schedules
Time constraints, end date of development, project planning and control, delivery
date