Overview of the CODA System

Software Implementation of Multicriterion Optimization with Genetic Algorithms

5.2 Overview of the CODA System

The design process in CODA begins with a preliminary design and then involves an iterative procedure of analysis, evaluation, and revision. In CODA, there is a software module devoted to each of these tasks. Figure 5.1 shows the overall architecture of CODA.

The three main modules in CODA, which have been described in Section 2.3, are:

• The ANALYZER module uses finite element analysis to compute performance parameter values based on a building configuration specified by the user and on the current values of the design parameters.

• The EVALUATOR, a module based on multicriterion decision theory, fuzzy logic and structural reliability concepts, determines an overall design evalua- tion measure, or level of acceptability, of the current design based on multiple performance criteria and a treatment of load uncertainties. This is done by ag- gregating preference values for the current design based on each of the individual design criteria, as described in Chapter 2.

• The REVISER performs revisions of the design to find an optimal design based on maximization of the overall design evaluation measure. Several optimization algorithms, both deterministic and stochastic, can be chosen, including the vGA and hGA algorithms presented in Chapter 4.

Finite Element

Agregalion of the design criteria preference

and weights

Deterministic and Stochastic 0 ptimization

Figure 5.1: Overall system architecture of CODA

In addition, there is an EXECUTIVE module, as shown in Figure 5.1, which has a supervisory role with respect to the other modules (the ANALYZER, the EVALU- ATOR, and the REVISER). The EXECUTIVE module acts as an interface between these three modules and the user, assisting in the initialization of the modules, con- trolling the execution of the different processes, and storing the information associated with the analysis, evaluation and optimization so that it is accessible by each of the other modules. The EXECUTIVE also allows the user to view the structure under consideration in graphical form (see Figure 5.2) and to view tabular listings of the structural parameters and analysis results. This centralization of initialization, con- trol and result presentation in the EXECUTIVE makes CODA more modular, since additional features and modifications may be made to the user interface without restructuring the entire software system.

The centralization also facilitates control and monitoring of the numerous pro- cesses involved in the execution of the program; in particular, error-checking and error-recovery can be made at each step of the analysis, evaluation or optimization, so that messages can be displayed to the user by the EXECUTNE when problems arise and recovery from an error can be made without fatal crashing of the program.

The EXECUTNE allows initialization of the ANALYZER by prompting the user to input the physical configuration of the initial preliminary design, including geomet- ric information and individual member and connection information. In addition, the user must select, from a menu of possibilities, the design and performance parameters important for the design decision-making process. These design and performance pa- rameters are combined with preference functions and weights to express the design criteria in a quantitative form. The design parameters, designated by a vector (}, are those parameters of the initial design which are selected to be varied during the search for an optimal design. In CODA, the design parameters control the geometry of the structural members (e.g., flange width or web depth). On the other hand, performance parameters, designated by a vector q, represent quantities related to the "performance" of the design, and can take the form of conventional structural parameters (e.g., stress, deflection, etc.) or other parameters (e.g., material cost of

../~tatus Bar

9

5

8

7

2 Hode..f

Parameters

Figure 5.2: Screen dump of CODA with FEM view

the structural system).

The principal role of the ANALYZER is to calculate the performance parameters q( 8) as a function of the prescribed design parameters, 8. Several types of analyses are available for computing these performance parameters (see Figure 5.3). How- ever, in the case of uncertain loads, the probability density function p( ^qiO) for the corresponding uncertain performance parameters is calculated.

To evaluate the current design, the EVALUATOR requires a user-supplied pref- erence function, J.li, for each design criterion (i

=

1, ... , Nc), which defines the pref- erence for the various values of each design parameter or performance parameter involved in the criterion. The preference function may simply express a minimum and/or maximum (fuzzy) bound on a design quantity, or it may express a more complex design criterion. A value J.Li(q(O)) = 1 indicates perfect acceptability of the design prescribed by

e,

as judged by the ith design criterion alone; whereas, J.li(q(O)) ⁼ 0 indicates absolute unacceptability of the design. Values between 0 and 1 indicate degrees of acceptability or preference between these extreme cases. In ad- dition, the user supplies importance factors or weights, ^Wi, which indicate the relative importance of the ^ith design criterion. A large importance factor for a design crite- rion gives it more influence in the trade-off which occurs between the various criteria during optimization of the design, that is, it indicates that the design should be such that the corresponding preference function value is close to unity. Alternatively, if a design criterion is given a low importance factor, its associated preference function value may be close to zero without greatly affecting the overall design evaluation. All these can be specified in CODA using the dialog box shown in Figure 5.4.

The REVISER takes the overall design evaluation measure, J.L, computed by the EVALUATOR from the individual preference function values, J.Li(i

=

1, ... , Nc), and revises the design to improve it. In the optimization mode of CODA, the ANALYZER, EVALUATOR and REVISER are repeatedly called by the EXECUTIVE in order to find an optimal design. During the optimization process, a close to real time display of the progress is shown via a dialog box as shown in Figure 5.5.

Define .Loads

Qefine Design Parameters Bun An is

·.

liii@ii@M

Figure 5.3: Screen dump of ANALYZER menu

Figure 5.4: Screen dump of EVALUATOR preference function dialog

Figure 5.5: Screen dump of REVISER with optimization progress view