Holistic approach in engineering design - controlling risks from accidental hazards in bridge design

Abstract

Engineering design, in concise terms, is what engineers do using what they know. It is the underlying decision making activity that determines what is to be built and how it should be built. An ever present requirement in engineering design is that the structure should be safe. While historical approaches to safety in design relied on experience and engineering judgment, modern approaches have rationalized uncertainty in an effort to treat risks in a more consistent and objective way. Concurrent to these advancements, design codes have been developed which include safety formats that are calibrated using these rationalized approaches. This thesis investigates the limitations of the design codes in controlling risks in engineering design and proposes that a complementary approach – involving case-specific risk assessments – is necessary for addressing the risks that are not properly treated by the design codes. The main advantage of such an approach is that:

• it broadens the scope of assessment to consider structural systems and possibly non-structural constituents;

• it is also applicable during the conceptual design phase for the bridge structure; and

• it is complementary to current codified approaches

While similar approaches are common in large scale construction projects they are rarely applied in the design of more conventional bridge structures. However, in this thesis it is argued that the application of such approaches is also useful in more common bridge projects to better control risks inadequately treated by design based on code compliance. A framework for a holistic risk-informed approach is provided which focuses on the conceptual design of bridge structures and on the control of risks from accidental hazards. Case studies are conducted to highlight the usefulness of the approach and to help develop crucial aspects of the approach while providing useful background information for its possible implementation in future projects. Specific attention is also paid to the modeling of risks from heavy goods vehicle (HGV) impacts to bridge substructures – a design situation which was found to be inadequate treated using current codified approaches.

Abstract (Swedish)

Popular Abstract in English

A central requirement in the design of bridge, or any structure, is that they are safe and do not fail unexpectedly. To ensure that such occurrences are acceptably rare involves predicting the various ways in which the bridge can fail and ensuring that provisions are made to prevent or control these potential failure. In other words, the risks associated with failure should be controlled such that they are within acceptable limits. The broader issue investigated in this thesis relates to the manner in which risks of failure are treated in engineering design of structures – specifically bridges – to ensure their safety.

Historically, the safety of a structure was assured by relying on past successful practices and by using factors of safety to account for any uncertainties. These factors alter the design values of loading or structural capacity to ensure a sufficient margin for error against the unknown. Historically, the selection of such factors was based on experience and engineering judgment and failures from less well understood phenomena had higher factors of safety. However, in the middle of the 20th century rational approaches to the selection of safety factors and of the treatment of structural safety were developed which eventually started replacing these previous methods.

Currently, design codes are the main instruments used in regulating procedures for verifying that a structures design is adequate in terms of safety against collapse. This includes specifying the magnitude of safety factors to be used during design. Such codes are written by code writing bodies and help provide assurances to society and other stakeholders that a design is safe and compliant with building regulations. There are, however, some important limitations of design based on code-compliance. These limitations are such that revisions of codes do not suffice in remedying the situation; a complementary approach is needed. The main outcome of this thesis is a framework for an approach which is intended for treating risks inadequately covered by the design code. This approach focuses on the design of bridge structures to withstand accidental or extreme loading situations – or hazards. These situations are rare and their magnitude can be very significant compared to more conventional loading situations such as the weight of traffic on a bridge. The approach also focuses on the how to consider the risks from these hazards during the early conceptual stages of design – in which case there are usually multiple competing technical solutions. The main advantages with the approach are:

1. The approach is holistic and broadens the scope of assessment – while codes are focused on the safety of single structural elements

2. The approach can be applied during the conceptual design stage – while the codes are only applicable during the detailed design stage

3. The approach complements codified design instead of competing with it

In developing the approach further, three crucial aspects were identified and investigated. To start, the usefulness of the approach during the conceptual design phase was tested by applying it for an actual construction project in Sweden. Second, the importance of broadening the scope was highlighted when conducting a more detailed assessment of a specific hazard – train collisions – to a second bridge case. Finally, risk modeling approaches were investigated in detail for the case of heavy vehicle collisions to bridge supporting structures. It was found that current codified approaches relating to this hazard are inadequate and an alternative design method was developed using stochastic modeling approaches – where uncertainties related load and structural behavior are directly considered. Furthermore, a simplified method for assessing the risks related to this hazard was developed.

• author: Björnsson, Ivar
• supervisor: Sven Thelandersson; Kurt Petersen
• opponent: Professor Faber, Michael, Technical University of Denmark, Kgs. Lyngby, Denmark
• organization: Division of Structural Engineering
• publishing date: 2015
• type: Thesis
• publication status: published
• subject: Civil Engineering
• keywords: design, codes, accidental, extreme, bridges, holistic, approach, robustness, risk, reliability, uncertainty
• in: TVBK
• volume: 1048
• pages: 258 pages
• defense location: Lecture hall MA1, Mathematics Annex building, Sölvegatan 20, Lund University, Faculty of Engineering, LTH
• defense date: 2015-11-06 10:15:00
• ISSN: 0349-4969
• ISBN: 978-91-87993-03-9
• language: English
• LU publication?: yes
• id: 14bbb178-1617-4518-b19c-3797da6e8a03 (old id 8052270)
• date added to LUP: 2016-04-01 14:24:13
• date last changed: 2019-05-23 17:29:14