A Structural Design Optimization Package Using an Expert System

Transcription

1 A Structural Design Optimization Package Using an Expert System By Haytham Mahmoud Department of Civil and Environmental Engineering Department University of Central Florida, Orlando, Florida, U S A Abstract The objective of this research was to develop an expert system which chooses the optimum two-dimensional steel structural systems, capable of transferring loads from their points of application to the ground level. This tool will choose from such structural systems as beams, trusses, moment-resistant frames. The type of structures studied in this research are one story buildings which include factories, warehouses, and residences. Several types of loading condition will be considered such as gravity loads (dead, live), lateral loads (wind load, and earthquake). The governing criterion in the choice of the of the most suitable structural system will be to minimize the weight of structure. The study combines analysis, and design of steel structures in a package that chooses the structural system and considers it as a an important variable that affects the optimization of the structure's weight. The choice of the most suitable structural system will be based on using an expert's knowledge and methodology, combined with new computer techniques. These

2 computer techniques will include the use of expert system shell, to achieve the accuracy and speed wanted in this analysis and design. 1 Introduction Steel structures are used extensively throughout the world due mainly to their light weight, small sections, and versatility. In spite of these properties, steel structure must be utilized in the best possible way. For example, the use of beams in a structural system can be a waste of material, if a truss that has the same height can be used instead. The designer, therefore, must make the right decision concerning the shape of the steel structure, in order to minimize the cost of the structure. If done manually, the engineer may at times take a long time to decide on the most economical structural system that would suit the existing circumstances. This research develops a computer package that contains all the required knowledge necessary to choose the most suitable structural system for the given conditions. Such a package promises to reduce the amount of time and effort for the decision process in selecting a structural system. Further, the package tries to overcome errors this process may otherwise generate. In addition, this package can deal with both English and SI (Standard International) units. Programs written to achieve this aim include, one to build in the models' data base, another to use finite element method to analyze structures and a design subroutine. A third program "SSS - STRUCTURAL SYSTEMS SELECTOR" [1] was modified to suit specific applications of this expert system. Program "Datainp", is used to create files that contain the structural systems' properties (the structural joint coordinates and the members'

3 connectivity) and can generate input files for some of the and analysis and design packages. Program "Adg", is used to perform the using finite element method analysis. LRFD specifications will be used for design. S. S. S. Inference Engine Black Board Knowledge Base User Interface Materials DBase Loads DBase Mathimatical Model DBase Sections DBase Construction Costs Mathimatical Models Builder Stuructrural Analysis Subrotine or Interface Design Subrotine or interface Construction Costs User Figure 1 The SSS program propose is to choose, out of a knowledge base called "SYSTEMS", the proper model file that represents the desired structural system. Many factors affect the engineer's decision for the choice of the structural system Figure 2. Some of them are from the point of view of the structural engineer; for example, the weight of the building and its ability to carry the loads it is subjected to. Other factors are from the point of view of the architectural engineer, like the allowable space envelope and the overall shape of the structure. Also environmental factors could effect the height and material used in the structure. From an expert's point of view, the larger the

4 span, the heavier the structure will be, as long as the structural system is not changed. As an example, a simply supported beam can not be used for spans more than X feet. D iffe re n t fa c to rs a ffe c tin g th e S tru c tu re S p a n ( ft. ) L o a d s ( K ip s ) beam truss fram e1 beam 3 fram e2 Figure 2 Either the structural system must be changed or the number of supports must be increased. The reason is that the weight of the beam will increase to the point where it will not support itself and external loads acting on it. In this study, the present method to choose the optimum structural system requires a considerable number of trial_and_error solutions to examine all the alternative and to select the structural system which has the lowest weight and which meets all constrains. Also, the designer works between limited alternatives depending on the references available and the designer's experience.

5 2 Review of Optimization Routines Previous studies and programs in structural optimization concentrated mainly on minimizing the cross-sectional area of the members for a given structural system. Constrains were put on the displacement and the stresses while decreasing the members' area. Sheppard and Andrew, who are considered as the founding fathers of the optimization idea, showed in their 1970 paper [2], that they can optimize the weight of a certain structure up to 20% over the conventional way. Later in 1975, Rath and Distefano [3] showed an application to minimize the weight of statically determinate trusses by adding some displacement and stress limitations on the structure. Durrant and Pachia [4] optimized a steel structure by dividing the structure into subdivisions and having a multi-stage decision process that helps achieve optimization. More results, Saka [5] in "Optimum Design of Pin-jointed Steel Structure with Practical Applications" Showed a flexible algorithm to optimize the structures' members according to the dominant constrains acting on the structure. The methods developed by the above authors take a long time in developing solutions, because they mainly depend on redesigning the structure a number of times until optimization is achieved. Furthermore, these methods focus only on a single structural system, neglecting the fact that this structural system may not be the most economical solution for various space envelopes or various kinds of loading. Other researchers in the structural field attempted to build integrated programs that can analyze and design any structure according to a certain code. A major common drawback of these packages is that they all

6 ignore one of the main objectives of the designer; optimization of the weight of the structure. 3 Modeling The approach presented in this study makes the shape of the structural system an important variable that effects the optimization of its weight In the designer's search to find the optimum system to cover a certain area, the designer takes into consideration the factors that control the selection of the optimum structural system, for example the shape of the structural system, its weight, the space envelope, the loads acting on the structure, and the system's boundary conditions. These factors have a nonlinear relationship and some of these relationships are a matter of engineering judgment. Artificial intelligence (A.I.) techniques can be employed to exploit engineering judgment of the expert whose knowledge can be manipulated in simplifying problems, and solving them. This study develops a package that will include the automatic choice of the most suitable two-dimensional structural system that can cover a certain piece of land, size its members, minimize its weight and estimate the cost of the structure. The Modeling procedure in this package depend on representing the structures as of unit length. The Datainput program generates a database of different structures has been assembled. This database is accessible by the inference engine of the expert system from it different structural systems is selected depending on the factors affecting the structure ( area covered, height of the structure, loads acting on the structure.)

7 4 Current Optimization Technique In this research the experts' knowledge and experience which is available in electronic format will be used to decide the structural shape of structure, materials used and the way of constructing it. The optimization procedure will be obtained in two steps Figure 3 : - Optimization by whole structural system ( beam, truss, frame) depending on different factors. 1 - Area covered by the system ( length and breadth of the piece of land covered effecting the span of the structure. ) 2 - Height of the structure ( for the effect of the wind pressure and earthquakes.) 3 - Estimated Gravity loads acting on the structure ( Live load.) 4 - Lateral Loads acting on the structure. - Optimization for the structure s member by conventional way. Intelligent Processor Mathimatical Model Structural Analysis Construction Procedure Is weight optimized? Detailed Design Cost Estimation Stop Figure 3

8 4.1 Optimization by whole structural system This optimization step will be decided upon by the Expert system shell depending upon the area of the land going to be covered, the amount of load acting on the structure (gravity and lateral loads), and height of the structure. The shell will give to the user as a result of the input, a number of choices and run the analysis and design program on the mathematical model of the best choice. Then the design program will design the structure based on the LRFD method and number of time the structure weight has been optimized. 5 Conclusion This expert system chooses the optimum structural system which best meets the design criteria and satisfies all codes constrains. It is an integrated package that can optimize the size of the members and estimate the cost of the chosen structural system depending on the factors affecting the structural system. REFERENCES 1 M. Scneider, M. Friedman, A. Kandel, On Fuzzy Reasoning in Expert Systems, Proceeding of the 1987 International Symposium on Multiplevalued Logic : Palmer A.C, and J. Sheppard, Optimizing the Shape of Pin-Jointed Structures, Proceedings of the Institution of Civil Engineering, 47, (November 1970).

9 3 Roth A. and Distfano N., Dynamic Programming Approach to the optimization of Elastic Trusses, Journal of Optimization Theory and Application, 15(1), (January 1977), Pacia P. and Olani S. Optimum Structural Design by Dynamic Programming, Journal of Structural Division, ASCE, 102, (August 1976): M.P.Saka, Optimum Design of Pin - Jointed Steel Structures with actical Applications, Journal of Structural Engineering, ASCE, 112 (October 1990):