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The JCSS probabilistic model code

The JCSS is developing a model code for full probabilistic design. This note gives an overview of the set up and contents of this code.

Assessment Criteria for Existing Structures

Within short time, the Eurocodes will be accepted as the official rules for design of new structures in most European countries. Also for the existing stock Eurocodes will be a suitable starting point for the assessment as these codes are expected to be based on the best and most recent knowledge available. However, it would be uneconomical to require all existing buildings and civil engineering works like bridges to comply fully with these new codes. Already without the introduction of a set of new codes, the assessment of existing structures differs from the design situation. This paper describes the main differences with respect to the relevant reliability requirements and develops a set of partial factors that could be used in those cases.

An integrated reliability-based design optimization of offshore towers

After recognizing the uncertainty in the parameters such as material, loading, geometry and so on in contrast with the conventional optimization, the reliability-based design optimization (RBDO) concept has become more meaningful to perform an economical design implementation, which includes a reliability analysis and an optimization algorithm. RBDO procedures include structural analysis, reliability analysis and sensitivity analysis both for optimization and for reliability. The efficiency of the RBDO system depends on the mentioned numerical algorithms. In this work, an integrated algorithms system is proposed to implement the RBDO of the offshore towers, which are subjected to the extreme wave loading. The numerical strategies interacting with each other to fulfill the RBDO of towers are as follows: (a) a structural analysis program, SAPOS, (b) an optimization program, SQP and (c) a reliability analysis program based on FORM. A demonstration of an example tripod tower under the reliability constraints based on limit states of the critical stress, buckling and the natural frequency is presented.

Robustness and the Eurocodes

The topic of robustness is essentially covered by two Eurocodes, EN 1990: Eurocode: Basis of Structural Design [5] which provides the high level principles for achieving robustness and EN 1991-1-7 Eurocode 1: Part 1-7 Accidental Actions [6] which provides strategies and methods to obtain robustness and the actions to consider. The EN 1991-1-7 [6] has been completed in 2004 and recently received a positive vote by the member states. The code describes the principles and application rules for the assessment of accidental actions on buildings and bridges. The leading design principle is that local damage is acceptable, provided that it will not endanger the structure and that the overall load-bearing capacity is maintained during an appropriate length of time to allow necessary emergency measures to be taken. As measures to mitigate the risk, various strategies are proposed like prevention of actions, evacuation of persons, physical protection of the structure and sufficient structural redundancy and ductility. The code makes a clear distinction between identified and unidentified accidental actions. For the identified accidental actions (impact, explosions) a structural analysis is proposed, the level of which may depend on the envisaged consequences of failure. It may vary from an analysis on the basis of static equivalent forces to a quantitative risk analysis including nonlinear dynamic structural analysis. Also for unidentified accidental actions, the measures depend on the consequence class. In these cases, more measures that are general are proposed to ensure a sufficient robustness of the structure. Enhanced redundancy, design of special key elements and three-dimensional tying for additional integrity are recommended by EN 1991-1-7 [6]. This paper summarizes the requirements of EN 1990 and discusses the choice of events to be considered for a particular situation. The paper describes EN 1991-1-7 and provides some background information as well as design examples.

Optimization of Steel Monopod-Offshore-Towers Under Probabilistic Constraints

In this work, economical design implementation of a circular steel monopod-offshore-tower, which is subjected to the extreme wave loading, is presented. The mass of the tower is considered as the objective function. The thickness and radius of the cross section of the tower are adopted as design variables of the optimization. Moreover, stress or buckling is specified as probabilistic constraints. The numerical strategy employed for performing the optimization uses the International Mathematics and Statistics Library (IMSL) routine that is based on the sequential quadratic programming. The first-order reliability method (FORM) is used for the reliability calculation from a specified limit state function based on the stress or buckling. A demonstration of an example monopod tower is presented.

Reliability-based optimisation of offshore jacket-type structures with an integrated-algorithms system

Reliability-based optimisation (RBO) is a powerful tool for including uncertainties in the optimisation process, in which structural and reliability analyses and optimisation algorithms based on mathematical or evolutionary computation concepts have to be combined effectively. This process is rather complicated and difficult to carry out for large structural systems such as steel offshore structures. In this paper, a calculation system of integrated algorithms for the RBO of the offshore towers is presented. The calculation process is composed of a structural analysis package (SAPOS) based on the finite element method, a reliability analysis program based on the first-order reliability method and an optimisation program based on sequential quadratic programming using the International Mathematics and Statistics Library. In the RBO analysis, multiple limit states based on different criteria are used to check a probable failure condition and to identify the limit state criterion. An offshore jacket-type structure is considered as an example to demonstrate the applicability of the implemented algorithm to realistic structural systems.

Optimization of Steel Monopod Offshore-Towers Under Probabilistic Constraints

In this work, economical design implementation of a circular steel monopod-offshore-tower, which is subjected to the extreme wave loading, is presented. The mass of the tower is considered as the objective function. The thickness and radius of the cross-section of the tower are adopted as design variables of the optimization. Moreover, stress or buckling is specified as probabilistic constraints. The numerical strategy employed for performing the optimization uses the IMSLLibraries routine that is based on the Sequential Quadratic Programming (SQP). The FORM is used for the reliability calculation from a specified limit state function based on the stress or buckling. A demonstration of an example monopod tower is presented.

The fundamentals of structural building codes

Partial Factor Design is nowadays a generally accepted design method for building and civil engineering structures. For most engineers the general philosophy that the safety factors depend on the type of the load and on the limit state under consideration makes sense. However, the background, in particular the reliability aspect, seems to be vague and is only understood by a handful of specialists. Even a general notion and feeling has not been developed in the minds of most designers. This is a pity: reliability aspects are in the same way essential for the design as the mechanical aspects are, but the knowledge and appreciation of most engineers is quite unbalanced. The reliability aspect plays also a key role in the development of a prescriptive code to a performance based code. The paper intends to address and explain these fundamental aspects of the codification processes and will also consider them in an historic perspective. On the one side, the present day codes will be compared with the allowable stress and load factor methods of the past. On the other a look into future developments like full probabilistic assessment, system approach, risk analysis and the inclusion of durability and maintenance strategies will be given

Eurocodes and Their Implications for Bridge Design: Background, Implementation, and Comparison to North American Practice

Codes and standards used for guiding structural design of buildings and bridges traditionally have been prescriptive and quantitative. In an era of gradual change in design and construction technologies, this traditional approach to structural design generally served the public and the profession well. However, prescriptive standards do not accommodate technological advances and innovations easily, and in the last three decades, such occurrences have changed the nature of building design and construction rapidly. At the same time, the performance of buildings and other structures during extreme manufactured events and natural phenomena hazards, such as hurricanes and earthquakes and flooding, has led to intense public and professional scrutiny and criticism of current engineering and construction practices. Finally, uncertainties are invariably present in structural engineering, and standards that do not take these uncertainties into account consistently (or, worse, do not account for them at all) are an obstacle to advancing structural engineering practice. With the advent of structural reliability as a tool for the treatment and analysis of uncertainty, the decades of the 1970s and 1980s brought the realization that although absolute safety is an unattainable goal, uncertainties in structural performance could be quantified (in terms of uncertainties in structural actions and response and in material strength and stiffness characteristics) and risk-informed structural design criteria could be developed that were consistent with a desired level of performance. With this realization, practical structural design standards that reflected these reliability principles evolved quite rapidly. Not only did this transition in thinking regarding structural safety and serviceability evolve rapidly, it evolved in most modern, postindustrial societies at about the same time. Modern building and bridge codes used in structural engineering practice are based on the notions of probability-based limit states design (PBLSD). In the bridge arena, these include the AASHTO LRFD Bridge Design Specifications (AASHTO 2012), the Canadian Highway Bridge Design Code (Canadian Standards Association 2006), EN 1990 [European Commission for Standardization (CEN) 1990] and EN 1991-2 (CEN 1991), and EN 1992-2 (Eurocode 2) (CEN 1992a). There are some differences in the way that PBLSD has been implemented in the countries that have adopted it, but its fundamentals are similar in all countries. In addition to the quantitative modeling of uncertainties using probabilistic models and statistical data, the developers of PBLSD bridge standards have strived to base such standards on present day principles of structural load modeling and to use models of structural behavior that are founded on sound principles of structural mechanics in order for the structural response to be modeled as accurately as possible (within the constraints of practical design). Not surprisingly, PBLSD has opened the door to new research opportunities and challenges. For one, there are differences in code format; in theUnited States, the LRFD format is practically universal, whereas in Europe, a format that involves partial material factors and companion action factors has been adopted. From the viewpoint of the practicing structural engineer, these differences are superficial rather than substantive and stem from country-dependent traditional design practices that predate the introduction of PBLSD. The collection of papers presented in the December 2013 special section on “Eurocodes and Their Implications for Bridge Design: Background, Implementation, and Comparison to North American Practice” reflects a broad spectrum of the commonalities and differences that have arisen in Europe, North America, and elsewhere as part of the move toward implementing PBLSD in practical bridge engineering. Marti-Vargas and Hale (2013) compare the treatment inNorthAmerican andEurocode standards of strand transfer length in prestressed concrete construction. The American Concrete Institute model is based only on the strand parameters, whereas the Eurocode 2 approach considers concrete properties as well, leading to a model that is more conservative in its predictions of required transfer lengths. Walbridge et al. (2013) focus their attention on United States, Canadian, Eurocode, and Swiss approaches to assess fatigue in metallic bridge structures using a simulation approach to show that simultaneous vehicle crossings, which currently are not considered in fatigue assessment, might increase fatigue damage substantially. Granata et al. (2013) consider the effects of creep and shrinkage on prestressed concrete girders usingNorth American and European approaches, concluding that the Eurocode predictions underestimate final deflections and the extent of stress redistribution among girders. Kappos et al. (2013) present a methodology for evaluating response modifications factors for earthquake-resistant design of concrete bridges in Europe and find that the available force-reduction factors for seven typical bridges are higher than those used for design, indicating that the strength reserves are typically larger than those provided by Eurocode 8 (CEN 1992b) or the AASHTO Bridge Design Specifications (AASHTO 2012). A broad comparison by Maiorana and Pellegrino (2013) of design provisions for steel bridge connections in Eurocodes and North American, Australian, and Japanese bridge standards reveals vast differences in assumptions regarding bearing/shear, slip, and minimum edge and end distances and indicated that the Eurocode provisions are the most conservative for typical steel connections in bearing and friction shear and in tension. Anitori et al. (2013) review current and proposed methods for assessing robustness and redundancy of bridge structures in Europe andNorth America; although noting that the North American provisions are more specific in this regard, they recommend that future bridge codes should be based on quantifiable measures of risk. The paper by Gara et al. (2013) on slab cracking in continuous bridge decks advocates the modular ration approach in

Reliability Based Optimization of Steel Monopod Offshore-Towers

In this work, the implementation of reliability-based optimization (RBO) of a circular steel monopod-offshore-tower with constant and variable diameters (represented by segmentations) and thicknesses is presented. The tower is subjected to the extreme wave loading. For this purpose, the deterministic optimization of the tower is performed with constraints including stress, buckling, and the lowest natural frequency firstly. Then, a reliability-based optimization of the tower is performed. The reliability index is calculated from FORM using a limit state function based on the lowest natural frequency. The mass of the tower is considered as being the objective function; the thickness and diameter of the cross-section of the tower are taken as being design variables of the optimization. The numerical strategy employed for performing the optimization uses the IMSL-Libraries routine that is based on the Sequential Quadratic Programming (SQP). In addition, to check the results obtained from aforementioned procedure, the RBO of the tower is also performed using the genetic algorithms (GA) tool of the MATLAB. Finally, a demonstration of an example monopod tower is presented.Copyright