Asian journal of civil engineering
Volume:12 Issue: 4, Aug 2011

This paper includes description of reliability based design optimization of elasto-plasticskeletal structures under multi-parameter static loading. Presented approach is based on the assumption that the complementary strain energy of the residual forces is considered as an overall measure of the plastic performance for plastic shakedown analysis and optimaldesign of the structure, and this measure is an uncertain quantity responsible for resistance of the structure by assuming Gaussian distribution. The problems yield to nonlinear mathematical programming which are solved by the use of bi-level sequential quadratic algorithm. Simple portal frames are optimized to illustrate the proposed approaches.

An efficient methodology is proposed to optimal design of structures for earthquake loading.In this methodology to reduce the optimization overall time, a serial integration of wavelettransforms, neural networks and evolutionary algorithms are employed. In order to reducethe computational work of the structural time history analysis, a discrete wavelet transform is used by means of which the number of points in the earthquake record is decreased. Also, an advanced metamodel, called self-organizing generalized regression is employed to predict the time history responses. The optimization task is achieved by an evolutionary algorithm called virtual sub population method. A 6-storey space steel frame structure isdesigned for optimal weight for the El Centro earthquake induced loads. The numerical results demonstrate the efficiency and computational advantages of the proposed methodology.

This paper reports a comparative study on design weight efficiency of single span steel trussbridge topologies subjected to gravity loads. The bridges configured according to ninedistinct topological forms (namely, Pratt, Parker, Baltimore, Petit, K-Truss, Warren,Subdivided Warren, Quadrangular Warren and Whipple) are designed for minimum weightunder various span length requirements of single span truss bridges, and the results obtainedwith these topological forms are compared. The optimization process for each bridgetopological form requires achieving optimal sizing of members as well as determiningcoordinates of the top chord nodes such that the least design weight is attained for thebridge. The design constraints and limitations are imposed according to serviceability andstrength provisions of ASD-AISC (Allowable Stress Design Code of American Institute ofSteel Institution) specification. The optimization algorithm employed is based on simulatedannealing method.

Reliability-based optimization of two and three dimensional frame structures is the subjectof this study. For this purpose, a computer program was developed and tested over a numberof examples for validation. Since similar studies have been made previously for trusses andreliably documented in the literature, optimization of such structures based on reliabilityanalysis could therefore be confidently relied on, and thus, designing of such structurescould be considered with less value for safety factors.This probabilistic optimization technique can well substitute that of the deterministicone where a considerable factor of safety and therefore, a heavy structure as always is amust. For this purpose, one may take into account the probabilistic behavior for load,yield stress, young modulus, etc, using parameters such as standard deviation andvariance, through which safety remarks can be embedded into the design procedure bysome mathematical relations, resulting to a probabilistic optimization technique. In thistechnique, one must first define the failure criterion, followed by the computation ofsafety zone (Z), reliability index () and lastly, the failure probability (Pf).In this paper, the applied load and the yield stress are considered probabilistic, whilethe violation of interior forces from the member ultimate strength is the failure criterion.For each of the interior axial, shear, bending and torsion reactions, the failure probabilityis calculated and the maximum value is constrained through optimization process.During the optimization process using Genetic Algorithm (GA), the failure probabilitiesare some boundary constraints and minimizing the weight of structure is the objective of theproblem. The profiles of I-shaped cross-sections are selected from a data file.Finally, the probabilistic technique and deterministic one are investigated andcompared applied to some structural problems.

Particle swarm optimization is an efficient population based algorithm used to solve realvalued and nonlinear continuous optimization problems. To resolve binary optimizationproblems with PSO, binary particle swarm optimization (BPSO) has been developed. In thispaper, optimum design of plates using BPSO is presented. The objective function aims atfinding the optimum weight of plates with the nodal displacements selected as constraints.Numerical examples show that BPSO can be a suitable algorithm to solve optimizationproblems in binary search space.

Natural frequencies are relatively easy parameters to obtain and they represent usefulinformation about the dynamic behavior of structures. Controlling these parameters can help the designer to minimize destructive effect of dynamic loading on the structure.Apart from the aforementioned practical application, weight optimization of thestructures with frequency constraints is a notorious problem because of its highly non-linea behavior. Thus form a challenging field to apply the optimization techniques.In this paper, the charged system search algorithm and its enhanced version are utilized tooptimize various truss structures with multiple frequency constraints. The examplesinvestigated here, are well-known benchmark problems. The results show that the presented algorithms perform better than other optimization techniques for most of the benchmark examples.

Traditional time-cost trade-off (TCTO) analysis in construction management problemsdisregards time value of money. In fact, the value of money decreases with time and,therefore, discounted cash flows should be considered when solving TCTO optimizationproblem. As a combinatorial optimization problem one may apply heuristics and/oroptimization techniques to solve time–cost trade-off problems. The combinatorial nature of the discrete TCTP, in which the solution space of the problem increases exponentially with the increment in the number of activities and/or the number of potential implementation modes, demands special solution algorithm. A multi-objective ant colony optimization (ACO) based model for project TCTO problem is developed, which minimizes project direct cost taking into account discounted cash flows. The model locates the near optimum Pareto front with a set of non-dominated solutions in which precise discrete activity time-cost function may be used. No simplifying assumptions are needed to implement the discount factor into the modeling structure and solution procedure, as required with mathematical optimization techniques. Details of model formulation are illustrated by an example project.The results show that inclusion of discounted cash flow results in different modes ofconstruction as well as activities’ durations and costs and consequently optimal projectduration. The proposed approach can help the practitioners in considering net present value in time-cost decisions leading to identification of the best option.

Calculation of system failure probability in large redundant structures is a time consumingprocess. Therefore in many researches it has been approximated in a conservative mannerdirectly from probability of the failure of the members. This paper proposes three strategies to speed up the calculation of Pf of indeterminate trusses. In the first strategy based on the principles of probability, a criterion is established to discard some correlated paths. In the second strategy the force method is developed and applied due to its efficiency and speed in analyzing large redundant trusses. In the force method, the number of equations to be solved is the same as the degree of static indeterminacy which is usually smaller than the total degree of freedom used in conventional displacement method. Another advantage of the force method is the immediate access to member forces, which is required to be used in reliability analysis. In this research the force method formulation is improved and made it possible for the analysis of trusses of different topologies. The third strategy corresponds to employing an Artificial Intelligent agent to identify and control the repeated failure paths to avoid the use of extra computational time.