Jitendra Agarwal

Vulnerability of structural systems

Abstract A structure is vulnerable if relatively small damage leads to disproportionately large consequences. A structure which is unacceptably vulnerable in any one way is not acceptably robust. A theory of structural vulnerability has been previously reported by the authors. The purpose of the theory is to identify particular failure scenarios by analysing the connectivity of the structural form. In this paper vulnerability theory is applied to three-dimensional frames and, in a preliminary way, to structural dynamics which is crucially important in assessing impact damage in for example progressive collapse. The concepts are illustrated through examples. The vulnerable failure scenarios may then be examined by conventional response analysis and/or by systems reliability theory and risk assessment. The theory is also applicable to structural damage assessment or for assessing structures under unforeseen terrorist attack.

Vulnerability of 3-dimensional trusses

Abstract A system is robust if it can withstand arbitrary damage. There are many practical ways to design in robustness but there is yet no accepted theory of robustness. One insight into the lack of robustness is gained if it were possible to identify how a system is vulnerable. This insight is in the form of ‘a theory of structural vulnerability’ developed at the University of Bristol for 2-dimensional structures [Lu Z, Yu Y, Woodman NJ, Blockley DI. A theory of structural vulnerability. The structural engineer 1999:77(18):17–24]. It is a theory of form and connectivity the purpose of which is to identify weak links within a structure. In this paper further development of the theory and its application to 3-dimensional structures is presented. Algorithms for implementing the theory are described and illustrated through three examples.

VULNERABILITY OF SYSTEMS

Abstract A system is vulnerable if any damage from any source produces consequences that are disproportionately large in comparison with that damage. Conversely a system is not robust if it cannot withstand arbitrary damage. Reliability theory is not sufficient for robust safety. In this paper, we propose a contribution to a general theory of vulnerability that is a theory of form and connectivity. The purpose is to identify weak links. This preliminary theory can be applied to a wide range of systems including structures, water pipe works, traffic flows and organisations and is potentially of use for safety management and to reduce the risk of overlooking vulnerable failure scenarios.

Vulnerability Analysis of Structures

A system is robust if it can withstand arbitrary damage. There are many practical ways to design in robustness but there is no satisfactory measure of robustness. Reliability theory is a useful tool for examining the probability of failure for a pre-defined type of loading, however low probability-high consequence events may be missed. Whilst it is not easy to put forward a unified theory of robustness, one insight into the lack of robustness is gained by identifying how a system is vulnerable. If a system is vulnerable in any one way then it is not robust. Like robustness, vulnerability has been defined differently in different contexts.

Robustness of Structures: Lessons from failures

Abstract Robustness is considered as an attribute of a structural system that relates to its ability to fulfil its function in the face of adverse events. It is difficult to quantify robustness. The focus of COST action TU0601 has been on developing a framework to quantify robustness and on identifying methods and strategies to improve the robustness of structures. The objectives of this paper are to present an analysis of different failures from the point of view of robustness and to identify measures that directly or indirectly contribute to robustness. It is concluded that structural form plays a major role but it is essential to ensure good management processes for design and construction.

Structural dynamic analysis on a connection machine

Abstract The Interacting Objects Process Model is a naturally parallel way of representing processes based on a ‘connectionist’ approach. It provides powerful new opportunities for the modelling of complex systems including flexible local behaviour models, interrogation methods similar to those used in artificial intelligence and self monitoring methods for more effective reliability management. The development of the IOPM for structural dynamic analysis using the massively parallel Connection Machine CM-200 is reported here. Examples include idealised systems of springs and masses and continuum structural systems using finite element relations. The performance compared to that of the IOPM on a sequential machine is analysed.

Improving resilience through vulnerability assessment and management

The increasing complexity of infrastructure systems and the possibility of severe consequences due to interdependency and uncertain demands have led to an increased emphasis on resilience. Resilience, in simple terms, is the ability of a system to withstand adverse conditions and to recover quickly from these. Its interpretations and linkages to the related concepts of vulnerability and risk are examined. It is argued that vulnerability is an inherent characteristic of any system, hard or soft, and its identification and management is essential for improving the systems resilience. A systems approach to identify the vulnerable failure scenarios uses the concepts of form, connectivity and hierarchical modelling. Modelling of interactions with social systems and assessing their consequences requires dealing with uncertainty and it remains a challenge.

A systems approach to vulnerability assessment

A system is vulnerable if any small damage produces consequences which are disproportionately large. The damage may come from unknown sources. Consequently any inherent weaknesses in the form of the system need to be explored. In this paper, we present a systems approach to analyse the vulnerabilities of a system and hence to manage risks. The form of the system is organized into a hierarchical model that can be systematically examined for weak points. The approach can be applied to many networked systems including lifelines. Here it is briefly illustrated through a simple structural system and a road network. In different disciplines different approaches to define and assess vulnerability are used. For example, seismic vulnerability of structures is usually associated with a seismic event of a given intensity. Vulnerability for social systems is related to their adaptability and stability to damage and change. Vulnerability of transport networks is often related to reductions in their serviceability levels. Topological features are also used to arrive at a measure of network vulnerability. In this paper, a systems approach to identify vulnerabilities in the form and connectivity of a system is presented. The proposed method uses a graph model of the system and leads to a hierarchical representation of the system which can then be systematically examined for vulnerable failure scenarios. The approach can be applied to many different networked systems e.g. structures, road networks, water supply systems, energy distribution systems etc. The purpose of the paper is to review the concepts of vulnerability, robustness and risk in civil engineering systems; to present a vulnerability and risk analysis procedure within a generic systems approach and finally to illustrate the methodology through two examples - a structure and a road traffic network.

Systems 2030 – Emergent themes

Systems 2030 was the theme of the Felicitation Symposium for Professor David Blockley held on 7 and 8 April 2008 at the University of Bristol to mark his retirement and celebrate his career. This special issue contains eight papers arising from the symposium, including this editorial paper. The papers by David Blockley and Paul Jowitt probably form the core of this special issue and help to circumscribe the sense and context in which ‘systems thinking’was used at the symposium. Jowitt presents a historical perspective of the development of systems thinking, starting from early roots in ‘hard’ systems and operational research techniques and ending with ‘soft’ systems and reflective practice. The paper by Pidgeon is another essential historical strand, bringing out the importance of thinking in socio-technical categories through examples taken from industrial safety. Together they contribute to the current state of play and pointers to the future presented by Blockley, who argues that ‘everything is a process’ and advocates learning through feedback for tackling uncertainty. The need for a definition of what is meant by systems thinking was identified during the symposium and an INCOSE UK Z Guide1 – ‘What is systems thinking?’ – has recently been published. In it, systems thinking is defined as a way of thinking used to address complex and uncertain real-world problems. It recognises that the world is a set of highly interconnected technical and social entities which are hierarchically organised, producing emergent behaviour. The papers by Godfrey and Oxenham illustrate applications of systems thinking in two specific areas, namely sustainable construction and defence procurement, respectively. Both papers bring out the importance of the temporal dimension in systems thinking – the sustainability paper in its description of life cycle analysis and the defence paper in identifying the need to ‘future proof’our designs. The final two papers are on climate change (Hall and Pidgeon) and ethics (Blockley and Dias). These papers have been developed from the brainstorming sessions described below. They focus, interestingly, on two of the broad spheres that need to be addressed in systems thinking – the climate change one on the natural environment impinged upon by technology and the ethics one on the social environment influenced by human belief systems.

Structural integrity: hazard, vulnerability and risk

We all have to get better at managing risk. Risk combines vulnerability and chance of hazard. The core idea of hazard is potential for danger. Vulnerability captures the concept of susceptibility to damage but it is more than that. It involves characteristics of the form of the system as well as characteristics of structural response to hazards including material defects, loading actions and accidental damage. A new method of finding failure scenarios as series of damage events is based on a systems approach to clustering the structure into simple sub-structures and searching for failure scenarios with high vulnerability index.