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This paper contributes to the ongoing discussion on the tentative target reliability levels for the design of structures and calibration of codes. The proposed extension of the existing approaches for code optimisation allows a consistent calibration of codes through all levels of design. The basis is taken from the normative decision theory as the rational strategy for taking decisions under risk. The method proposed supports code-making by optimizing reliability elements accounting for i) the safety format adopted in the code, ii) the design level that is regulated, and iii) the differences among the design cases regulated by the same code. Some of the decision theoretical aspects that are object of discussions in the research community are addressed. The work aims at providing a more robust and transparent background to target reliabilities in design standards.

To date, our built environment is broadly developed and maintained on the basis of structural design standards. Most design standards contain simplified semi-probabilistic safety concepts that help daily structural engineering decision making using simple calculus. For few projects, the semi-probabilistic safety concept is replaced by advanced probabilistic methods that allow for reliability-based design. Neither include explicit consideration of optimality criteria. However, for the development and the calibration of the semi-probabilistic and probabilistic safety concepts it is important that societal preferences in regard to risk acceptance and the optimal use of natural resources are explicitly considered. In the present contribution, the development and calibration of semi-probabilistic and probabilistic structural design standards is represented as a formal decision problem allowing an optimal combination of reliability elements to be identified.

A framework for calibrating the reliability elements in simplified semi-probabilistic design safety formats is presented. The objective of calibration is to minimize the increase of construction costs, compared to the non-simplified safety format, without reducing the level of structural safety. The framework is utilized for calibrating two simplified safety formats which aim at reducing the number of load combinations relevant in structural timber design. In fact, the load-duration effect makes the design of timber structures more demanding since a larger number of load combinations need to be considered compared with other construction materials. This is the authors' manuscript to the article (preprint).

Submerged floating tunnels (SFTs), also known as Archimedes’ bridges, allow crossing waterways where common bridges, underground tunnels or immersed tunnels are not feasible. In spite of this, no SFTs have been built yet in the world except for a prototype in China. The reasons are numerous and widely discussed in literature. The lack of past experience and ad hoc design codes or guidelines represents a great challenge in design. In fact, the direct use of target reliabilities and design codes, which are specifically developed for common structures and adapted through the years based on the gained experience, is questionable. This is because STFs might be characterized by different failure consequences and marginal safety costs. Optimal target reliabilities can be estimated through a full-risk assessment following the ISO standard 2394:2015 guidelines. Successively simpler design approaches and assessment of decisions such as reliability-based and semi-probabilistic design can be calibrated.

The article presents a contribution to the current debate on the probabilistic representation of the wind speed extremes for calibration of the partial safety factor covering wind action. The requirements for the probabilistic model are formulated. The Gumbel distribution is shown to represents best the 10-min mean wind velocity yearly maxima based on theoretical considerations and analyses of real data with different statistical techniques. Data from locations across a large geographical region indicate that the coefficient of variation of the distribution varies over the territory. A method is proposed for accounting this variation in order to calibrate a single partial safety factor for the whole territory. The distribution location is indirectly given in design standards through the georeferenced characteristic wind speed values. A solution for including the uncertainty affecting these values is suggested. The findings are implemented in an illustrative calibration exercise. The proposed methods and concepts might be applied to other environmental actions such as the snow loads.