The design of many engineering systems can be a complex process, which means many possibilities and factors must be considered during problem formulation. The process of searching for a design that meets both performance and safety standards with reliable impact is the goal of structural optimization. Structural optimization is an approach whereby the structural design is subjected to being optimized in terms of weight while maintaining all design constraints such as stress, strain, and stability. Structural design optimization problems involve searching for the minimum of the stated objective function, usually the weight of the structure and constructability. Trusses are triangular frame works in which the members are subjected to essentially axial forces due to externally applied load at the joints only. Truss structures can be optimized by varying the structure’s size, shape, and topology. Although combining these three prototypes of optimization can ultimately a yield better result, the underlying mathematical model becomes complicated. Over the last decade, new optimization strategies based on metaheuristic algorithms have been devised to obtain the optimal design for structural systems. Harmony search (HS) is a metaheuristic algorithm proposed by Geem et al., inspired by the observation that the aim of music is to search for a perfect state of harmony. In this research, the implementation of HS algorithm has been applied to optimize the size of truss structures that results in the weight reduction of the truss members. The results obtained with HS were compared to those obtained from the original sizes before optimization, to verify the influence on the optimal design of truss structures subjected to stresses, deflections, vertical and lateral displacements, and buckling constrains. / Civil Engineering

No / Abstract: This paper is to apply the species conserving genetic algorithm (SCGA) to search multiple solutions of truss topology optimization problems in a single run. A real-vector is used to represent the corresponding cross-sectional areas and a member is thought to be existent if its area is bigger than a critical area. A finite element analysis model has been developed to deal with more practical considerations in modeling, such as existences of members, kinematic stability analysis and the computation of stresses and displacements. Cross-sectional areas and node connections are taken as decision variables and optimized simultaneously to minimize the total weight of trusses. Numerical results demonstrate that some truss topology optimization examples have many global and local solutions and different topologies can be found by using the proposed algorithm in a single run and some trusses have smaller weight than the solutions in the literature.

Post-earthquake fires can negatively affect the safety and collapse probability of Reinforced Concrete (RC) structures. At present, there has been no systematic effort to assess the performance of RC structures for combined earthquake and fire effects. Developing appropriate guidelines for this scenario requires simulation tools that can accurately capture material behavior during cyclic loading and at elevated temperatures. Ideally, simulation tools must also be conceptually simple and computationally efficient to allow extensive parametric analyses. The goal of the present study is to enhance a previously established modeling approach so that it can describe the performance of RC structures for both cyclic loading and changes in material behavior due to elevated temperatures. The modeling approach is based on the nonlinear truss analogy and has been extensively validated for cyclic loading of RC shear walls and columns. The constitutive models for concrete and reinforcing steel are enhanced with the capability to account for the effect of elevated temperatures. The enhanced material models are validated using experimental data for concrete and steel at elevated temperatures. The capability of the proposed model to analyze structural-level behavior is verified and compared with experimental testing. The method is also endowed with the capability to describe the time-dependent heat conduction in a fire simulation. The use of the enhanced nonlinear truss model is more advantageous than refined finite element models because of its computational efficiency and conceptual simplicity. / Master of Science

The objectives of this research were to determine the required net lateral restraining force to brace j-webs or j-chords braced by one or more continuous lateral braces (CLB's), and to develop a methodology for permanent bracing design using a combination of lateral and diagonal braces. SAP2000 (CSI, 1995), a finite element analysis program, was used to analyze structural analogs for three sets of truss chords braced by n-CLB's and one or two diagonals, one web braced by one and two CLB's, and j-truss chords braced by n-CLB's. System analogs used to model five eight-foot truss chords braced by three CLB's and one diagonal, six twenty-foot truss chords braced by nine CLB's and two diagonals, and eleven twenty-foot truss chords braced by nine CLB's and two diagonals were analyzed. For each of the three cases analyzed, the chord lumber was assumed to be 2x4 No. 2 Southern Pine (S. Pine) braced by 2x4 STUD Spruce-Pine-Fir (SPF). Chord load levels of 10% to 50% of the allowable compression load parallel-to-grain assuming le/d of 16 were studied. All wood-to-wood brace connections were assumed to be made with 2-16d Common nails. A nonlinear load-displacement function was used to model the behavior of the nail connections. Single member analogs were analyzed that represented web members varying in length from four-feet to twelve-feet braced by one and two CLB's. The web and CLB's were assumed to be 2x4 STUD SPF. The web members were also analyzed assuming 2x6 STUD SPF. Single member analogs were analyzed that represented chord members varying in length from four-feet to forty-feet braced by n-CLB's spaced twenty-four inches on-center. The truss chord was assumed to be No. 2 Southern Pine and the CLB's were assumed to be STUD SPF. The chord size was varied from 2x4 to 2x12 and connections were assumed to consist of 2-16d Common nails. The system analog analysis results were compared to the single member chord analysis results based on the number of truss chords and the diagonal brace configuration. For the three cases studied involving multiple 2x4 chords braced as a unit (and believed to be representative of typical truss construction), the bracing force from the single member analog analysis was a conservative estimate for bracing design purposes. It was concluded that the single member analysis analog yields approximate bracing forces for chords larger than 2x4 and for typical constructions beyond the three cases studied in this research. For analysis and design purposes, a ratio R was defined as the net lateral restraining force per web or chord divided by the axial compressive load in the web or chord. For both 2x4 and 2x6 webs braced with one CLB, the R-value was 2.3% for all web lengths studied. For both 2x4 and 2x6 webs braced with two CLB's, the R-value was 2.8% for all web lengths studied. The web and CLB lumber species did not affect the R-values for the braced webs. Calculated R-values for truss chords, 2x4 up to 2x12, braced by n-CLB's assumed to be spaced two feet on-center for chords four to twelve feet in length ranged from 2.2% to 3.0%, respectively. For chords from sixteen to forty feet in length, R ranged from 3.1% to 2.6%, respectively. The lumber species and grade assumed for the chord and CLB did not affect the R-values for the truss chords. Step-by-step design procedure was developed for determining the net lateral restraining force required for bracing j-chords based on the results of the single member analogs studied. The required total lateral restraining force for j-compression members in a row can be calculated based on the R-value for or the number of CLB's installed at 2 feet on-center, the design axial compression load in the chord, and number of trusses to be braced. / Master of Science

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The application of composite action ushered a new era in the use of double-layer spaceframes as efficient floor systems in addition to their competitiveness as roof coveringstructural systems. Earlier research on space frames demonstrated large improvementsin their static behaviour caused by the introduction of composite action. Theseimprovements included an increase in ductility to avoid progressive collapse, a largeincrease in load-carrying capacity and a considerable reduction in materialconsumption.In this work, the effect of introducing composite action in changing the dynamiccharacteristics of space frames, in particular the natural frequencies and damping ratioswas presented. The study was expanded to determine the effect of composite action inchanging the response to dynamic excitations. The measured responses included thelateral displacements and changes in the internal member force distribution undershaking table vibrations.Three aluminium space frame models of the square on square (SOS) configuration weremanufactured. The first model was non-composite, while composite action was appliedto the other two models with a top aluminium deck and a timber deck, respectively.Two common cases of support conditions were used in connecting the models to theloading frame, which was the platform of the shaking table.Initial displacement method (snap test) was used to determine the frequency of vibrationand the damping ratio of test models in the vertical and horizontal directions usinglogarithmic decrement method. All models were then exposed to shaking tablevibrations to determine the changes in dynamic responses between different models.These tests were repeated for the three models after the successive removal of panelsfrom one direction to identify the changes to their characteristics and behaviour withdifferent aspect ratios.The second part of the study was carried out numerically by using the finite elementpackage ABAQUS. It started by selecting a valid finite element model from nineproposed models using experimental test results on physical structures. A parametricstudy was conducted using the validated finite element model to expand the study toinclude two common space frame configurations; the square on large square (SOLS)and square on diagonal (SOD), and two other cases of support configurations, namely,fully edge-supported and supports at corners and middle edges of models.Based on the work done in this study, it can be concluded that composite actionchanged the dynamic characteristics of space frames, which was clear in the increase oftheir vibration frequencies in all directions as a result of the increase in stiffness.Furthermore, the increase in stiffness resulted in a general reduction in the dampingratio of space frames covered with aluminium deck, while the high friction with topjoints and the nature of timber as a good energy absorbent material resulted in a variableeffect on the damping ratio associated with the increase in aspect ratio.The effect of composite action was clear in reducing the lateral displacement ofcomposite models by more than 50% compared to the non-composite case. Moreover,composite action resulted in changing the distribution of internal forces in diagonal andlower chord members such that forces became more concentrated at corners and edgesparallel to the direction of vibrations in both cases of corner and edge-supportedmodels.

The purpose of this study is to discuss the fitness of using PHMGA (Parallel Multilevel Hybrid Genetic Algorithm), which is a fast and efficient method, in the geometrically nonlinear structural optimization. Parallel genetic algorithms can solve the problem of traditional sequential genetic algorithms, such as premature convergence, large number of function evaluations, and a difficulty in setting parameters. By using several concurrent sub-population, parallel genetic algorithms can avoid premature convergence resulting from the single genetic searching environment of sequential genetic algorithms. It is useful to speed up the operation rate of joining timely multilevel optimization with parallel genetic algorithms. Because multilevel optimization can resolve one problem into several smaller subproblems, each subproblem is independent and not interference with one another. Then the subsystem of each level can be connected by sensitivity analysis. So we can solve the entire problem. Because each subproblem contains less variables and constrains, it can achieve the faster converge rate of the entire optimization. PHMGA integrates advantages of two methods including the parallel genetic algorithms and the multilevel optimization. In this study, PHMGA is adopted to solve several design optimization problems for nonlinear geometrically trusses on the parallel computer IBM SP2. The use of PHMGA helps reduce execution time because of integrating a multilevel optimization and a parallel technique. PHMGA helps speed up the searching efficiency in solving structural optimization problems of nonlinear truss. It is hoped that this study will demonstrate PHMGA is an efficient and powerful tool in solving large geometrically nonlinear structural optimization problems.

Undesirable cabin noise has an adverse physiological effect on passengers and crews in an aircraft. In order to reduce the noise level, a passive approach using a truss-core sandwich (TCS) panel as a sound insulator is proposed. Design guidelines and analysis methodologies were developed in order to explore the vibro-acoustic characteristics of TCS structure. Its sound isolation properties can be thereby assessed. Theoretical analyses show that the transmission-loss and sound radiation properties of a TCS structure can be represented by the root-mean-square velocity of its surface, and a beam structure analysis is sufficient to reveal many of the important aspects of TCS panel design. Using finite element analysis, a sensitivity study was performed to create design guidelines for TCS structures. Transmission-loss experiments show that the analytical and numerical analyses correctly predict the trend of TCS structure’s vibro-acoustic performance.

Undesirable cabin noise has an adverse physiological effect on passengers and crews in an aircraft. In order to reduce the noise level, a passive approach using a truss-core sandwich (TCS) panel as a sound insulator is proposed. Design guidelines and analysis methodologies were developed in order to explore the vibro-acoustic characteristics of TCS structure. Its sound isolation properties can be thereby assessed. Theoretical analyses show that the transmission-loss and sound radiation properties of a TCS structure can be represented by the root-mean-square velocity of its surface, and a beam structure analysis is sufficient to reveal many of the important aspects of TCS panel design. Using finite element analysis, a sensitivity study was performed to create design guidelines for TCS structures. Transmission-loss experiments show that the analytical and numerical analyses correctly predict the trend of TCS structure’s vibro-acoustic performance.

This study investigates the possibility of fabricating periodic cellular materials (PCMs) via the lost foam casting (LFC) process using aluminum alloy A356 and magnesium alloy AZ91. This approach combines the structural efficiency of PCM architectures with the processing advantages of near-net-shape LFC. An initial feasibility study fabricated corrugated A356 panels. This was followed by a study of casting variables such as pattern design, vacuum assistance, and alloying additions in order to improve the fillability of the small cross-section struts. Finally, integrated pyramidal sandwich panels having different relative densities were subjected to artificial aging treatments and subsequently tested in uniaxial compression. The A356 PCMs experienced a continuous increase after yielding while the AZ91 PCMs exhibited strut fracture after peak strength. The results showed the compressive yield strengths of this study are comparable with those previously reported PCMs produced by different fabrication methods.