Showing papers on "Minimum weight published in 1984"

Optimum cyclic redundancy codes for noisy channels (Corresp.)

Abstract: Binary cyclic redundancy codes for feedback communication over noisy digital links are considered. The standard 16-bit ADCCPt polynomial is designed for digital links that already have a low input bit error probability. For file transfer between personal computers over telephone circuits, the quality of the resulting digital circuit may be much lower. This leads to the consideration of 3-byte (24-bit) and 4-byte (32-bit) polynomials. Generator polynomials of a certain class are found that have minimum weight and yet achieve the bound on minimum distance for arbitrary codes. Particular polynomials for 24-bit and 32-bit redundancies are exhibited, of weight and distance 6 in the 24-bit case and weight 10 and distance 8 in the 32-bit case.

On the minimum distance of some quadratic residue codes (Corresp.)

Abstract: We find the minimum distances of the binary (113, 57) , and ternary (37, 19), (61, 31), (71, 36) , and (73, 37) quadratic residue codes and the corresponding extended codes. These distances are 15, 10, 11, 17 , and 17 , respectively, for the nonextended codes and are increased by one for the respective extended codes. We also characterize the minimum weight codewords for the (113, 57) binary code and its extended counterpart.

Optimum design sensitivity based on approximation concepts and dual methods

Abstract: An optimum design sensitivity analysis capability is reported which exploits the approximation concepts-dual method formulation of the minimum weight structural sizing problem. An efficient iterative solution technique is used to facilitate determination of sensitivity derivatives for both primal and dual variables. Estimates on the useful range of parameter perturbations, over which the optimum design sensitivity projections can be expected to yield satisfactory revised optimum designs, are also obtained. Numerical results for several example problems will be presented to illustrate the effectiveness of the capability reported.

A proof of the odd-symmetry of the phases for minimum weight perturbation phase-only null synthesis

Abstract: It is proved that if phase-only weight control is used to impose nulls in a real pattern of a linear array of isotropic equispaced elements while minimizing the sum of the squares of the absolute weight perturbations then the phase perturbations are odd-symmetric with respect to a phase reference at the array center. Equivalently, the perturbed pattern is real.

Optimal Layout Theory: Allowance for selfweight

Abstract: Structural layout theory is based on two underlying concepts, i.e., the Prager-Shield theory of plastic design and the notion of "structural universe." In this paper, optimal layout theory is extended to allow for the effect of selfweight (dead load). The proposed extension is of practical importance because in the design of long-span structures selfweight is a significant load component and the total weight is strongly dependent on the choice of layout. The application of the modified optimality criteria is illustrated with examples of minimum weight grillages (truss grids) as well as plane frames. The degree of economy achieved is then demonstrated through comparisons with nonoptimal solutions. It is also shown that in the preceding examples both primal and dual formulations give the same minimum weight. This comparison is a convenient check on the optimality of the proposed solutions.

A two-phase optimization method for minimum-weight design of trusses

Abstract: In this paper, a new twp-stage procedure for the minimum weight design of trusses is proposed. In the first stage (behavior phase) of optimization the behavior variables (joint displacements) are chosen as basic unknowns and the optimum displacement state of the truss or its searched region is found by using a maximum total strain energy criterion with linear programming. In the second stage (structural phase) the design variables (cross-section areas) are chosen as basic unknowns and the minimum weight design of the truss is obtained by using another linear program or a direct search technique. This method is effective for the minimum weight design of truss type structures with stress, displacement and geometric constraints. In most cases it can avoid the need for repeated iterations and structural reanalyses. Moreover, a minimum-maximum total strain energy criterion to select the optimal layout of structures is presented in this paper.

Influence of analysis and design models on minimum weight design

Abstract: The results of numerical experiments designed to illustrate how the minimum weight design, accuracy, and cost can be influenced by: (1) refinement of the finite element analysis model and associated load path problems, and (2) refinement of the design variable linking model are examined. The numerical experiments range from simple structures where the modelling decisions are relatively obvious and less costly to the more complex structures where such decisions are less obvious and more costly. All numerical experiments used employ the dual formulation in ACCESS-3 computer program. Guidelines are suggested for creating analysis and design models that predict a minimum weight structure with greater accuracy and less cost. These guidelines can be useful in an interactive optimization environment and in the design of heuristic rules for the development of knowledge-based expert optimization systems.

Computer aided design of timber space frames

Abstract: This paper illustrates how the sequential linear programming technique has been used to design three-dimensional rigidly jointed timber framed structures in accordance with the British Code of Practice. The objective was to obtain a minimum weight design by varying the member cross-sectional dimensions and the coordinate positions of the joints of the structure employing stress constraints formulated using the Code. To arrive at a design algorithm that yielded practical designs, several features were introduced into the algorithm which included provision for uniformly distributed loading and the grouping of member cross-sectional properties or joint coordinates as single design variables. Example structures are given and the design reviewed with respect to the shape optimization of the structure and the resulting saving in weight.

Dynamic programming in minimum-weight design of axisymmetric plates

Abstract: The paper deals with minimum-weight design of axisymmetric plates. The design is subject to restrictions imposed by static equilibrium conditions and various yield criteria governing the failure of the material. Herein, the optimization problem is formulated as a dynamic programming problem and is solved. Such a formulation unifies the design procedure. The paper also discusses the dynamic programming approach.

Towards minimum weight and cost-effective designs of flat and pitched structures

Abstract: •The main objective of this paper for weight and cost consideration designs: :is to give the designer a better tool to compare the relative merits of various design alternatives and to be able to select the corresponding most minimum weight or cost-effective design. An example of a typical problem in shipbuilding is studied in order to explore some general aspects of -design. Two different structural forms are considered (flat and pitched). • In order to obtain realistic comparisons, the production costs considered in this research are according to the current practice in Egyptian shipyards. •A key finding from the results is that if the optimisation is based on a minimum weight or minimum cost criterion, the pitched structures are superior over the flat ones. In other words, significant savings in weight and :cost can be achieved by replacing flat structures by pitched structures. • The labour cost should include the pre-fabrication, sub-assembly, fitting and other relevant activities costs in addition to the welding costs. In .addition to the material and labour costs, the total costs should include the overhead costs in order to make a realistic decision for best design. I. OPTIMUM DESIGN CRITERIA OF SHIP STRUCTURES Jr) these competitive days, it has become important for the designer to find_ structural solutions that satisfy design requirements, and at the same time: have low weight and low production costs. To obtain reliable results, however, the designer needs accurate tools for analyses which take the complexity and coupling effects of the various structures into account. _In qualitative terms, a structure can be judged by its reliability, cost, and efficiency, which are the main attributes for measuring the quality of a structure. The most convenient definition used for the latter attribute,: efficiency, is strength/weight ratio as a criterion, which is more relevant' to ships as the weight savings can be translated into improved cargocarrying capacity. Thus, maximising structureal efficiency is of great *Lecturer, Dept. of Ship Engineering, Faculty of Engineering at Port Said, Suez Canal University, Egypt. I MD-15 170 1 FIRST A.M.E. CONFERENCE 29-31 May 1984, Cairo :interest to the designer which must be compromised with the builder's prime objective in the search for 'better' design of minimum construction costs. ; Regarding ship structures, the interest of the approving authority and the :operator is greater reliability or more safety of such structure design, further, the operator's interest is lower maintenance costs. ; Details of design, especially with reference to the qualitative aspect of design and construction of structural connections, is the most important factor affecting the structural reliability. To have such a structure !design of great reliability, an increased initial cost is required. Therefore, there may be a point at which further increase in reliability : mes uneconomic in terms of its effect on the life-cycle cost of a ship. This means that the life-cycle cost of a structure might be minimised by designing it with an optimum level of reliability which gives the best co: mpromise between initial cost (which increases as reliability increases) • and repair cost (which decreases as reliability increases). For a structure having a specific demand and reliability, either total :structural weight or total cost, or a combination of both, may be the prime criterion involved in establishing the superiority of one design over another. Ship structures usually fall between aircraft structures, with :their predominant interest in weight saving, and land-based civil engineering structures where their prime objective is cost saving. Although the criterion differs considerably for each type of ship, structure weight and :cost acknowledged to the important design considerations in shipbuilding. It either weight or cost was considered independently of the other as the prime objective, different designs would be obtained. Therefore, the true: :optimum would undoubtedly involve some compromise between the two, depending upon the analysis of the ship's operation. Hence, Caldwell and Hewitt [1] have proposed a generalised objective function in terms of a dimensio-: :nless quantity,U, as : U ti F(W/W0) + (1-F)(C/C0) where Wo and Co are the weight and initial cost of some basis or standard design. W and C are the weight and cost of a proposed design, and F is a number which varies between zero (where least cost is the objective) and 'unity (where weight saving is of paramount importance). II. WEIGHT OPTIMISATION :he latter reference, it is shown that .if the main objective of a is minimising the weight of a structure having a specific demand and : reliability, there are basically three strategies a designer may investigate, singly or in combination : I. For a given material, and specified constraints (geometry and behaviour), find the more efficient combination of the free scantling variables, 2. Use different material to suit, 3. Change the constraints for a given material. It might be possible to consider eliminating the redundant material as one more strategy. This needs to consider adding plates where greater moment

A bi-factor algorithm for adjusting structural parameters

Abstract: The bi-factor algorithm presented in this paper is a development of the single factor optimally criteria algorithm.3-4 The present algorithm does not execute a trial and error procedure, but converges more quickly and stably. Satisfactory results have been attained over a series of minimum weight design problems (including layout optimization) subjected to such constraints as frequency-prohibited-band. structural global buckling load, displacement amplitudes caused by sinusoidal loads, etc.

Optimal design of heat conducting structural elements

Abstract: The problem of optimal design of heat conducting structural elements under steady-state temperature distribution is formulated and analyzed. Minimum weight is taken as the design objective and the shape of the structural component is design. Constraints are placed on temperature or heat flux due to thermal loading conditions encountered by the structural element. Finite element method is used to obtain the temperature distribution at each stage of optimization process. The material derivative concept of continuum mechanics and the adjoint variable method are used to obtain design sensitivity of cost and constraints with respect to boundary movement. A nonlinear programming technique is then used to numerically construct optimal designs. Optimal design of a thermal diffuser is presented as a numerical example.

On finding constrained minimum weight spanning subgraphs

Abstract: We consider problems of finding a minimum weight spanning subgraph which has to satisfy some additional constraint defined in terms of distances between vertices. Three distinct problems of this type are defined and all are shown to be NP-hard. Heuristic algorithms for their solution are discussed.

Modelling considerations in optimum design of reinforced

Abstract: The paper provides some potentially useful modelling schemes for reducing computational cost in designing reinforced structures. The schemes are based on the concepts of element and cross-section idealizations while maintaining the characteristics of the load path. An example of a double layer reinforced panel is given and it is shown how these schemes can lead to increased flexibility and reduced cost in obtaining minimum weight designs.

Compression helical springs, a method of design for a minimum weight

Abstract: This paper discusses an approach for the design of compression closely c:oild helical springs subjected to axial load. With the presented proposed method, one can specify the significant characteristics of the spring that contains the least possible volume of material while still providing sufficient strength and stiffness. To do so, the classic design equations are developed so that they can be graphically represented in a (weight-diameter)-plane from which the global optimum design point can be determined. Also, a computation flow chart is given, as an alternative way to the graphical method. Based on the computation flow chart, a digital computer program can be written for the optimum design problem. INTRODUCTION The priamary purpose of a spring is to satisfy certain functional requirements of the mechanism. These functional requirements are determined before designing the spring; from either a static or a dynamic force analysis of the entire mechanism, and usually are expressed by the specification of load versus defltction characteristics. A typical compression spring is shown in Fig. (1). For a given material, its significant characteristics are uniquely defined by the wire diameter (d), the mean coil diameter (D), the free (unloaded) height (hf ), and the active number of coils (Na). The design of helical springs by the classical deterministic approach can be found in many references [1 33 . Conventional methods for determining the spring dimensions, that use the least possible amount of material, reiuire a cumbersome itrative calculations before a satisfactory solution is obtained. When load, deflection, mean coil diameter and stress are prespecified, the following is a new proposed method that determines the significant characteristics of spring (d ,hf,and Na) that contains the least possible volume of material while still providing sufficient strength and stiffness. This method is based on the graphically representation of the relation between the weight of the spring and its wire diameter. This representation must be done for each step in the spring design, i.e for static loading and/or * Assistant Professor ,Production Engineering & Machine Design Department, Faculty of Engineering & Technology, Menoufia University, Shebin El-Kom, Egypt.

Optimum kinematic design of machine tool gear boxes

Abstract: Published work on computer aided kinematic design of machine tool gear boxes has been limited to normal & overlapping structures only, using a single criterion, such as minimum number of gears, minimum weight etc. for selecting the optimum layout. This paper presnets a comprehensive methodology for optimum kinematic design of machine tool gear boxes, using a multiple criterion optimising approach. A computer program in FORTRAN-IV has been developed to provide the optimum layout version for normal and all types of special structures. The CPU time is 5 sec. The program has been checked by case study of 3 existing machines.

Optimal design of a radiating fin for communications satellites

Abstract: This paper is motivated by the design of a minimum weight thermal radiator or radiating fin for communication satellites. As a first step towards the solution of this problem we present an efficient finite element method to solve the non-linear boundary-value problem describing the temperature distribution. It leads to a non-linear programming problem. Steepest descent, conjugate gradient and Newton's method are compared. Numerical experiments are presented. This basic tool will be used many times in the iteration process to find the optimal shape.