Failure Consequences and Reliability Acceptance Criteria for Exceptional Building Structures a Study Taking Basis in the Failure of the World Trade Center Twin Towers
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Research Collection Report Failure consequences and reliability acceptance criteria for exceptional building structures A study taking basis in the failure of the World Trade Center Twin Towers Author(s): Faber, Michael H.; Kübler, Oliver; Fontana, Mario; Knobloch, Markus Publication Date: 2004 Permanent Link: https://doi.org/10.3929/ethz-a-004807495 Rights / License: In Copyright - Non-Commercial Use Permitted This page was generated automatically upon download from the ETH Zurich Research Collection. For more information please consult the Terms of use. ETH Library Background, motivation and scope of the research project Failure Consequences and Reliability Acceptance Criteria for Exceptional Building Structures A Study taking Basis in the Failure of the World Trade Center Twin Towers Michael H. Faber, Oliver Kübler Mario Fontana, Markus Knobloch Institute of Structural Engineering Swiss Federal Institute of Technology Zurich July 2004 1 Background, motivation and scope of the research project Preface The present research project was performed in the period April 2002-March 2004 and was funded by the Swiss National Science Foundation under grant number 2100-066770. During the project period a number of publications and presentations at conferences on the topics of the project have resulted in many interesting discussions with several researchers. These discussions have provided a valuable input to the project. Specifically the project team warmly thanks Prof. Dr.- Ing. habil. R. Rackwitz, Technical University of Munich, Germany, for valuable discussions and input concerning aspects of optimality and affordable societal life saving costs. Prof. Dr. M. Haller, University St. Gallen, Switzerland is greatly acknowledged for discussions and viewpoints in regard to the socio-economical impact of hazards on the society. Moreover, Yngve Abrahamsen from the Swiss Institute for Business Cycle Research, Zurich, Switzerland, contributed with helpful discussions on the topic of macroeconomics. Finally Prof. Dr. A. Dazio and research assistant D. Buzzini, Swiss Federal Institute of Technology, Zurich, Switzerland, are sincerely thanked for their investigations regarding robustness of structures against progressive collapse. Zurich, July 2004 Michael Havbro Faber 3 Background, motivation and scope of the research project Table of Contents 1 Introduction 7 1.1 Background, motivation and scope of the research project 7 1.2 Executive summary and conclusions 7 2 Assessment and modeling of consequences 13 2.1 The WTC failure in the context of other insured losses 13 2.2 Categorization of consequences 16 2.3 Modeling of consequences 17 2.4 Consequences due to the failure of the World Trade Center Twin Towers 19 3 Decision theoretical framework 21 3.1 Objective of structural design 22 3.2 Expected life cycle benefit 23 3.3 Follow-up consequences and risk aversion 30 3.4 Interest rates and sustainability 31 4 Structural reliability and vulnerability assessment 35 4.1 Codified design of structures 35 4.2 Categories of exceptional structures 36 4.3 Principles of structural reliability analysis 37 4.4 Reliability and partial safety factors 40 4.5 Rare events, vulnerability and robustness 41 4.6 Modeling of system performance and structural damage 43 4.7 Vulnerability to fire loads 45 4.8 Robustness to impact and fire damage 50 5 Acceptance criteria and optimality for the design of extraordinary building structures 57 5.1 Design for ordinary load cases 57 5.2 Design for fire load cases 59 5 6 Conclusions and suggestions for further research 63 7 References 65 A The World Trade Center 75 A.1 Location of the World Trade Center 75 A.2 Historical review 75 A.3 Construction of the World Trade Center 76 A.4 September 11, 2001 83 A.5 Structural response of the buildings 85 B Detailed consequence assessment of the WTC failure 91 B.1 Rescue and clean-up costs 91 B.2 Property losses 93 B.3 Influence on the economy 99 B.4 Fatalities 119 B.5 Environmental consequences and loss of cultural assets 130 B.6 Risk perception 131 B.7 Total effect to society and liability 131 C Basics in macroeconomics 133 C.1 GDP 133 C.2 Multiplier effect 134 C.3 Price index and GDP deflator 136 C.4 Summary 139 D General project information 141 6 Background, motivation and scope of the research project 1 Introduction 1.1 Background, motivation and scope of the research project 1.1.1 Background and motivation The modern codes for the design of structures have been calibrated for normal structures, i.e. such that structures for normal purposes built in usual dimensions and designs, using well-known materials and constructed and maintained according to established procedures achieve an adequate and homogeneous level of reliability – in a cost efficient manner. Structures, which are not normal in the above-mentioned sense fall beyond the application area of the design codes and may be seen as exceptional structures. One particular type of exceptional structure is building structures with a high concentration of business enterprises and not least people and know-how. Failure of such structures may have enormous consequences not only for the owners of the buildings, the people and enterprises occupying the buildings but also for society in general. To establish a rational design basis for this type of buildings is a task involving the consideration of several aspects similar to the case when design codes for the design of ordinary structures is considered. However, significant differences arise in connection with the assessment of the optimal level of safety for such structures. Due to the potential enormous consequences in case of failure, the optimal level of safety to be applied as basis for the design may differ significantly from the optimal level of safety for so-called ordinary structures. This in turn may imply that when designing such structures, other load cases, failure modes and hazard scenarios need to be considered and new principles for the design may be relevant. 1.1.2 Scope of the research project Despite the atypical and senseless cause of the tragic failure of the World Trade Centre Twin Towers in New York on September 11, 2001 it might be possible to learn something from the event whereby the design basis of similar future structures could be enhanced. Of special importance is the assessment of the actual consequences in case of failure. The realistic assessment of these is a prerequisite for establishing an optimal level of safety of such structures, i.e. a corner stone in the design basis for such structures. The scope of the present project is to analyze the event of the failure of the World Trade Centre towers in regard to the consequences of the structural failure - not including the consequences attributable to the cause of the event (malevolence). The evaluations include an assessment of cost consequences on a societal level taking into account adverse effects due to the loss of lives, loss of businesses and material losses. Finally based on the findings in regard to consequences, initial principal studies are performed for the assessment of optimal safety strategies for such structures. 1.2 Executive summary and conclusions 1.2.1 Mapping, assessment and modeling of consequences The present research report is initially concerned about the mapping of the consequences following the incident of the failure of the World Trade Center Twin Towers on September 11, 2001. A thorough reporting and assessment is given on the reported number of fatalities, economical losses as well as potential impacts to the environment and cultural assets. The main focus is, however, 7 Chapter 1 – Introduction devoted to the assessment and differentiation of economical losses. The types of consequences considered and their differentiation is illustrated in Figure 1.1. Rescue and clean-up Property losses WTC Twin Towers Surrounding buildings & facilities Buildings’ & facilities‘ inventory losses Influence on the economy Business interruption Material and economic and Material Lost rents Lost cultural assets losses Cultural Environmental consequences to environment Consequences Consequences due to fatalities Immaterial loss Dependents‘ lost income to persons Consequences Consequences due to High-Rise Building Failure Societal life saving costs Figure 1.1 Different types of consequences mapped and assessed within the present report. Table 1.1 Summary of assessed consequences following the failure of the WTC Twin Towers. Scenario Consequence type Low High Rescue & clean-up 1.7 1.7 Property 19.2 19.2 WTC Twin Towers 4.7 Other destroyed buildings 2.0 Damaged buildings 4.3 Inventory 5.2 Infrastructure 3.0 Fatalities 5.5 5.5 Environment & cultural assets 0.1 0.1 Impact to economy 9.1 66.2 Businesses 7.2 64.3 Infrastructure 0.7 0.7 Rents 1.2 1.2 Total 35.6 92.7 (in billion USD) In Table 1.1 the assessed losses corresponding to the differentiation in Figure 1.1 are summarized. The monetary value assigned to fatalities has been derived using the concept of the Life Quality Index which provides the amount of money which is necessary and affordable for the society to invest into life saving activities (1·million USD per statistical life). Moreover, the compensation of fatalities may also depend on the societal and legal system of the considered country. In case of the failure of the WTC Twin Towers the Victim Compensation Fund compensated the dependents of the fatalities. Based on the compensation scheme an average compensation of 2.0 million USD per fatality can be calculated summarizing to a total cost of compensation 5.5 billion USD. A major observation from Table 1.1 is that the consequences which would normally be understood as the direct consequences (traditionally forming the basis for optimal design considerations), i.e. the property losses corresponding to the Twin Towers alone plus perhaps rescue and clean-up only amounts to about 12% to 33% of the total loss depending on the assessment of the economic losses.