The present issue contains a selection of edited papers presented at the Conference on High Performance and Optimum Design of Structures and Methods held at the University of Siena and co-organised by the Wessex Institute, UK, the Free University of Brussels and the University of A Coruña in Spain.

The issue contains papers on advanced types of structures, based on new design concepts. Modern structural design requires the development of new methods that can lead to systems able to resist a range of external stimuli. Particular emphasis is being placed on intelligent structures and materials.

Modern materials used in engineering components of structures are required to withstand a wide range of external stimuli. These range from high temperatures to special materials for restoration of heritage structures. Textile structures are usually highly stressed but they must result in minimal creep or relaxation for instance. Current research is also focused on their durability, which allows them to operate properly during their required lifetime.

Structural engineers must find adequate answers to the challenges of contemporary civil architecture, very often being a combination of lightweight structures with large spans. This requires sophisticated calculation techniques, including non-linear and vibration behaviour.

These new challenges require analysis, not only in terms of ultimate strength, serviceability and limit states but also in terms of their reliability and integrity. The engineer is also faced with the need to design ecological friendly structures to reduce their environmental impacts and incorporate reasonable resources.

The development and application of modern computational methods and powerful computers for structural modelling, control and management has increased the probabilities of using graphic interfaces and the incorporation of optimisation in the design process.

Some of the contributions in this issue are devoted to theoretical advances and practical applications of optimum design methodologies to several engineering disciplines. They demonstrate the current maturity of this design technique that has evolved with time from academic research to become a tool, useful to practising engineers. In fact, papers included in this issue originate not only from universities and research institutions but also from engineering companies. The papers are related to optimization of concrete and steel bridges, special structures and mechanical engineering. The problems formulated are very diverse and include size, shape and topology optimization, composite materials and a variety of nonlinear analysis.

Re-use and recyclability of materials and structural components is becoming increasingly important, i.e. supporting the “cradle to cradle” approach. Re-use is also found nowadays in two levels, not only from the re-use of structural components and materials but also the transformation of complete buildings, such as offices into schools or residential accommodation.

The papers included in this issue reflect these advances and provide a state of the art view of some of the most recent advances in high performance structures and materials. They are published by WIT Press and available Open Access in the eLibrary of the Institute (witpress. com/elibrary) where they can be downloaded for free by the international community.

The Editors are grateful to the authors for their papers and to the reviewers for their help in ensuring the quality of the contents of this issue.

The Editors

Siena, 2016

Research into structural reliability for tensile structures is needed. The semi-probabilistic format for verification of so-called form-passive structures is well-established in the Eurocodes. Partial factors are the main features of this semi-probabilistic or design value method. Whereas for conventional structures these partial factors are calibrated to previous experience [1], appropriate partial factors have to be proposed and evaluated for tensile structures. A cable net structure built in 1958 was used as a case study to gain insight into the effect of partial factors according to Eurocode 3 (steel structures). Prestress contributes to the stiffness in the non-linear structural behaviour of membrane structures and thus increasing the prestress with factor 1.35 according to the Eurocodes might be non-conservative. The article investigates the effect of the partial factor for prestress (1.0 or 1.35) on a membrane structure. A similar geometry as the steel cable net structure is designed and analysed for comparison with the cable net structure. For the primary steel structure the partial factor for prestress 1.35 has to be applied. An in-depth study of the effect of the partial factor for prestress on the stress distribution in the membrane in warp and weft direction is performed. The stress distribution clearly depends on the boundary conditions. A sound conclusion though requires a thorough in-depth study for different shapes and membrane types. In a first step towards a reliability approach, the structural reliability of a three segments cable net structure is currently being analysed, taking into account the uncertainties associated with the pre-tensioned system.

Crack formations in concrete may cause major damages in concrete structures. These damages require extensive maintenance work and thus have high costs. This paper addresses issues such as what causes cracks in concrete structures and how does the appearance of cracks look like with respect to an applied load? Can the appearance, distance, and size of the crack tell us something about crack initiation and propagation, or is it just by pure coincidence that cracks occur in structures as they do?

This research work investigated the effect of external factors such as load variables, time, the dimensions of the beam and the relative humidity on crack formation. Internal factors that have been investigated are the various constituents of the concrete, and how various levels of these constituents have an impact on cracking. In addition, the influence of concrete quality, tensile reinforcement, shear reinforcement, and anchoring reinforcement was investigated.

The paper presents technical calculations, where both the bending moment and shear forces are included in the analysis to determine how crack formations will propagate in the beam as a function of the applied loads. The first part of the paper deals with the theoretical factors that influence cracking in concrete. The second part deals with the calculations of crack formation in concrete. The results show how the cracks propagate in the x and y directions as a function of the load being applied.

Nowadays, in different industrial fields as transport or aerospace, a research effort is conducted to reduce the structural weight. One of the most promising solutions is the use of composite structures due to their high stiffness, their low mass density and their low damping factor. At the same time, there is an intensification of the operational dynamic environment and an increase of durability requirements. These different expectations seem to be contradictory. One solution to manage these points is to design and manufacture smart composite structures with a fully distributed set of integrated piezoelectric transducers. These structures are able to modify their mechanical properties with respect to their environment (e.g. active vibration control), to interact with other structures (e.g. mechatronic) or with human beings (e.g. Human–Machine Interaction).

To meet the technical specifications of smart composite structures, in particular for complex geometries, it is necessary to master the manufacturing process and consequently the material parameters of the manufactured composite. Indeed, during the design phase, these parameters have to be absolutely known. A design approach based on engineering system theory and uncertainty calculation is applied to our manufacturing process of smart composite structures. In this paper, two different material identification methods (the Resonalyser technique and the Time-of-Flight technique) were selected and are applied to several test plates and, finally, on a large smart spherical cap. The Resonalyser technique is a good method to extract overall material parameters. Its major drawback in terms of cost and difficulty of implementation is the use of contactless devices for the measurements. The Time-of-Flight technique is based on the duration measurements of pulse propagation with a simple and low cost experimental setup. Integrated piezoelectric transducers are used for this purpose in the present analysis. The results obtained are quite local (mean values along the propagation path) and need a strong physical interpretation. The different material parameters obtained are compared and discussed.

The present paper describes experimental measurements of wood stiffness and analytical homogenization to provide estimates of the Micro-fibril angle (MFA). It is known that the orientation of fiber-like aggregates of crystalline cellulose in S2 layer of the wood cell with respect to the alignment of lumens considerably influences the overall stiffness of wood. Recently an inverse approach exploiting the results of nanoindentation at the level of wood cell and analytical homogenization has been proposed as a suitable tool for the MFA determination. A simpler methodology based on the results of indentation at the structural level using the Pilodyn 6J testing device has also been advocated as an alternative appealing particularly to engineering practice. Comparison of the two approaches suggesting their advantages as well as drawbacks is the principal objective of this contribution. As an example, an application to spruce as the most common type of wood used in building structures is considered.

The effects of ceramic powder, a waste material, on the properties of lime-cement plasters were investigated in this article. The influence of the addition of the pozzolana as a supplementary cementitious material on mechanical and thermal properties of the studied materials was assessed in relation to its basic physical properties and pore structure characterization. Investigated parameters were bulk density, matrix density, open porosity, pore-size distribution, compressive strength, tensile strength, thermal conductivity and specific heat capacity. The results revealed the densifying effect of the pozzolana on the plaster microstructure as the open porosity decreased and bulk density rose to binder replacement level. Although the mean diameter of pores for plasters with higher amount of pozzolana was slightly higher, the volume of pores was lower. The presence of ceramic powder also showed a positive effect on the mechanical properties of plasters. Both compressive and tensile strength rose with increasing replacement ratio. Varying porosities were reflected in the increasing trend of thermal conductivity with rising binder replacement level. On the contrary, specific heat capacity showed the lower values the higher the amount of pozzolana.

We propose a new framework for topology optimization based on the boundary element discretization and kernel-independent fast multipole method (KIFMM). The boundary value problem for the considered partial differential equation is reformulated as a surface integral equation and is solved on the domain boundary. Volume solution at selected points is found via surface integrals. At every iteration of the optimization process, the new boundary is extracted as a level set of a topological derivative. Both surface and volume solutions are accelerated using KIFMM. The obtained technique is highly universal, fully parallelized, it allows achieving asymptotically the best performance with the optimization iteration complexity proportional to a number of surface discretization elements. More-over, our approach is free of the artifacts that are inherent for finite element optimization techniques, such as “checkerboard” instability. The performance of the approach is showcased on few illustrative examples.

This research reports a vehicle occupant restraint system design by using evolutionary multi-objective optimization with response surface model. The vehicle occupant restraint systems are composed of restraint equipment, such as an airbag, a seat belt and a knee bolster. The optimization aims to improve the safety of the system by evaluating some indexes based on some safety regulations. Estimation mod- els of the safety indexes are introduced for accelerating the optimization. The estimation models, which are called the response surface models, are constructed by using Gaussian Process, which is a kind of machine learning method. The Gaussian Process constructs the estimation model from sampling results, which are calculated by using multi-body dynamics simulation. Some helpful information for designing the restraint systems, such as trade-off information of safety performance and contribution of design variables for the safety performance, is obtained by analysing the Pareto optimal solutions.

In bridge design, many variables like material grades, cross-sectional dimensions, passive and prestressing steel need to be modeled to evaluate structural performance. Efficiency gains are intended while satisfying the serviceability and ultimate limit states imposed by the structural code. In this paper, a computer-support tool is presented to analyze continuous post-tensioned concrete (PSC) box-girder road bridges, to minimize the cost as well as to provide optimum design variables. The program encompasses six modules to perform the optimization process, the finite-element analysis, and the limit states verification. The methodology is defined and applied to a case study. A harmony search (HS) algorithm optimizes 33 variables that define a three-span PSC box-girder bridge located in a coastal region. However, the same procedure could be implemented to optimize any structure. This tool enables one to define the fixed parameters and the variables that are optimized by the heuristic algorithm. Moreover, the output provides useful rules to guide engineers in designing PSC box-girder road bridges.

In this paper, the influence of steel fiber-reinforcement when designing precast-prestressed concrete (PPC) road bridges with a double U-shape cross-section is studied through heuristic optimization. A hybrid evolutionary algorithm (EA) combining a genetic algorithm (GA) with variable-depth neighborhood search (VDNS) is formulated to minimize the economic cost and CO2 emissions, while imposing constraints on all the relevant limit states. The case study proposed is a 30-m span-length with a deck width of 12 m. The problem involved 41 discrete design variables. The algorithm requires the initial calibration. Moreover, the heuristic is run nine times so as to obtain statistical information about the minimum, average and deviation of the results. The evolution of the objective function during the opti- mization procedure is highlighted. Findings show that heuristic optimization is a forthcoming option for the design of real-life prestressed structures. This paper provides useful knowledge that could offer a better understanding of the steel fiber-reinforcement in U-beam road bridges.

Composite structures with the shell-like geometry must provide the sufficient mechanical stiffness in order to eliminate the unwanted deformations caused by the action of airflow. Because the pressure field on the design surface caused by airflow is generally uniform, the ensuring of the necessary stiffness can be achieved by creating a nonuniform thickness along the shell surface. In the present study the CFD finite element analysis of the virtual wind-tunnel test for the studied composite shell is performed assuming its absolute stiffness. The problem is further parameterized by the introduction of the auxiliary sphere, which causes a smooth distribution function of the shell thickness. The optimum seeking is performed by means of the four parameter variation of this function, providing a minimum total energy of the shell deformation under the given restrictions on its weight.

Producing a light structure with affordable cost without sacrificing strength has always been a challenging task for designers. Using a hybrid material approach provides an expanded methodology to combine materials having different costs and properties (for example, combining fibers with high cost and high stiffness such as carbon with low cost, less stiffness fibers such as glass). Hence, a comparative approach is useful for the evaluation of design solutions in terms of weight and cost. In this study, a methodology for a combined weight and cost optimization for sandwich plates with hybrid composite facesheets and foam core is presented. The weight and cost of the hybrid sandwich plates considered are the objective functions subject to required equality constraints based on the bending and torsional stiffnesses. The hybrid sandwich plates considered consisted of thin hybrid composite facesheets, symmetric with respect to the mid-plane of the sandwich plates. The facesheets considered consisted of carbon/epoxy and E-glass/epoxy fiber-reinforced polymer. The layup of the fibers of the facesheets were restricted to some discrete sets of plies having orientation angles of 0, ±45 and 90. A multi-objective optimization technique was applied to minimize simultaneously the weight and the cost of the hybrid sandwich plate. The normalized normal constraint method with Pareto filter was used to generate the Pareto frontier trade-off curve. The Pareto trade-off curve was constructed by optimizing a sequence of combining weight and cost objective functions, while every function was minimized using the Active Set Algorithm.