Session III - Codification¶
Moderator: Ton Vrouwenvelder
Aim of the Session¶
The practical engineer is not familiar with notions like updating, value of information or adopted safety requirements. There is a need for operational codes and guidelines closing the gap between the daily problems on the one hand and the theory on the other. These documents should not only be helpful in explaining the actual step by step procedures but also play a role in the communication with owners and authorities. Giving the wide variety of existing practice as well as local legislation this is a cumbersome and long lasting process. The session shows information about the state of affairs in Europe and USA.
Standardisation for Assessment of Existing Structures¶
by Paul Lüchinger, Former Convenor of Working Group CEN/TC 250/WG2, Switzerland
The initial attempt to develop technical rules for the assessment of existing structures is closely linked to the fundamental change of design concepts for new structures. Furthermore, the initiative to prepare such rules was motivated by the expanding activities in this construction sector. CEN/TC250 decided to proceed in three steps starting with a pre-normative report, development of CEN/TS (Technical Specification) and finally conversion into a part of the Eurocodes. The pre-normative report JRC science and policy report has demonstrated the wide range of different philosophies, strategies and procedures throughout Europe and underlined the need for harmonised European technical rules. The political response was extremely positive. As consequence the work item was included in Mandate M/515 of EU and EFTA. The Project Team WG2/T1 of CEN/TC250 prepared a draft in interaction with CEN/TC250/WG2. This draft differentiates for verification between (a) the semi-probabilistic method (partial factor method, assessment value method), (b) probabilistic method, and (c) risk assessment method. Important to emphasize: these CEN-guidelines include technical rules. However, they do not include recommendations on when assessments should be undertaken.
Standardisation for Assessment of Existing Structures – Evolution from Technical Specification to Eurocode¶
by Thomas Lang, Convenor of Working Group CEN/TC 250/WG2, Switzerland and Fabrizio Palmisano, Leader of Project Team CEN/TC 250/WG 2/WG2.T2, Italy
Extending the life of existing structural assets is a key challenge for structure owners worldwide. This is why, in the last years, the assessment of existing structures has become a topic of huge interest all over the world due to environmental, economic and socio-political assets. However, the approach to this issue is in many aspects different from that for designing new structures. The direct application of design-orientated methods to the assessment of existing structures often leads to a high degree of conservatism. In this context, one of the main goals of the second-generation Eurocodes is to tackle the problem by introducing additional specific rules for the assessment, re-use and retrofitting of existing structures. This presentation will focus on the work of CEN TC 250 Working Group 2 (WG 2) and of its Project Team WG2.T2. The first part of the presentation will illustrate the work done in WG 2 to finalize the Technical Specification (TS) on Assessment of existing structures. It will give a short overview of the content of the TS which is the basis for the ongoing conversion into the corresponding Eurocode. Some information about the procedural steps until the publication as CEN TS 17440 “Assessment and retrofitting of existing structures” in July 2020 will be also included. The second part will be devoted to the details of the work, currently in progress, of WG2.T2 relevant to the development of a new Eurocode that will be the one including general rules for existing structures. A general overview of the last draft of this new Eurocode will be given together with some details on specific topics (e.g. basic assumptions, methods of assessment, assessment process, verification methods, interventions). In some cases, the path followed by the project team will be illustrated with the aim to highlight the peculiar complexity connected to the development of a code on existing structures: since different countries had developed in the past different products, approaches and codes of practice, a European code on existing structures should account for these differences and, hence, should give Countries a wider possibility to add specific rules for different peculiarities.
Second Generation of Eurocode 8 Part 3 on Seismic Assessment and Retrofit of Existing Structures¶
by Paolo Franchin, Sapienza University of Rome, member of Project Team for EN1998
In 1997 an international workshop on Performance based earthquake engineering was held in Bled, Slovenia. Leading experts agreed that a deformation-controlled design was the best way to implement performance-based engineering principles for earthquake resistant design. They envisioned a staged process, the first stage being the introduction of displacement-based checks alongside force-based design, the second one in which the two approaches were both allowed and coexisted and the third and final one, when force-based design would be abandoned. Eurocode 8 Part 3 was released in 2005 as an experimental document, which introduced displacement-based assessment based on nonlinear analysis and a set of capacity models for deformation and strength of existing non-conforming members. The second generation document, which is being finalized at the time of writing as part of the more general revision of the entire Eurocodes set, continues in that direction, and also revises most capacity models moving them from informative annexes to normative text. It covers a wider range of materials, including now RC, steel, timber and masonry structures. The new document also replaces the judgement-based, non calibrated so-called confidence factors and associated knowledge level, with a more rational set of partial factors which take into account uncertainty in geometry, construction details and material properties as assessed through tests and inspections, as well as the usually large epistemic uncertainty in the capacity models themselves.
Standardisation for Assessment of Existing Structures in the USA¶
by Jim Harris, J.R Harris & Company, USA and Bruce Ellingwood, Colorado State University, USA
Standardization for assessment of existing structures in the USA is accomplished through a set of several standards produced by different organizations. The “International Existing Building Code” is a model code and is produced by an organization of building officials. “Guidelines for Structural Condition Assessment” and “Seismic Evaluation and Retrofit of Existing Buildings” are both produced by the American Society of Civil Engineers; the latter include intricate details, given the nature of the limit state for earthquake resistance. “Code Requirements for Assessment, Repair, and Rehabilitation of Existing Concrete Structures” is produced by the American Concrete Institute. The “Specification for Structural Steel Buildings,” which is primarily focused on design of new buildings, includes an appendix “Evaluation of Existing Structures.” These various standards have some commonalities, but also differ in important aspects. Next to buildings the standards for assessment of bridges will be discussed.