Blast effects on buildingsSecond edition

To Emma

Blast effects onbuildingsSecond edition

Published by Thomas Telford Limitd, 40 Marsh Wall, London E14 9TP.

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First published 1995

This second edition 2009

Also available from Thomas Telford Limited

Application of codes, design and regulations. R. K. Dhir , M. D. Newlands and A. Whyte.

ISBN: 978-0-7277-3403-7

Progressive collapse of structures. U. Starossek. ISBN: 978-0-7277-3610-9

A catalogue record for this book is available from the British Library

ISBN: 978-0-7277-3521-8

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Contents

Foreword to the second edition xi

Acknowledgements xiii

About the editors xiv

List of authors xvi

1 Introduction 1Major General Christopher Elliott CB MBE, Arup SecurityConsulting

Objective 1Scope 3Modern terrorism 5Risk 5The special effect of catastrophic loss 6Partial factors in blast design 6A design philosophy: planning for protection 7

2 Basic guidelines for enhancing blast resilience 8Geoff Mays, Cranfield University at the Defence AcademyDavid Hadden, Arup Security Consulting

The requirements of the client 8Design features 9Blast loading and stand-off 12Response of a building to blast load 17Protected spaces 25References 29

3 Blast loading 30Peter Smith, Cranfield University at the Defence AcademyDavid Cormie, Arup Security Consulting

Notation 30

v

Introduction 32Explosions 32Explosion classification 33Explosives classification 33Blast waves in air from high explosives 34Blast waves in air from vapour cloud explosions 36Blast wave interactions 37Basic blast wavefront parameters 37Blast wave parameters for loading on structures 39Blast wave scaling laws 41Reflection coefficients 44Regular and Mach reflection 45External blast loading of structures 48Internal blast loading of structures 53Conclusions 56References 58

4 Prediction of blast loads 59David Cormie, Arup Security ConsultingConrad Izatt, Arup

Notation 59Introduction 59Categorisation of techniques 60Empirical methods 61Phenomenological methods 65First-principle methods for blast loads from high explosives

and deflagrative events 66References 77

5 Structural response to blast loading 80Peter Smith, Cranfield University at the Defence AcademyDavid Cormie, Arup Security Consulting

Notation 80Introduction 81Elastic SDOF structure 82Evaluation of the limits of response 85Iso-damage diagrams 86Energy solutions for specific structural components 92Lumped mass equivalent SDOF systems 94Resistance functions for specific structural forms 97Advanced SDOF methods for blast analysis 99

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Discussion 101References 102

6 Design of elements in structural steel 103Geoff Mays, Cranfield University at the Defence AcademyIan Feltham, ArupMike Banfi, Arup

Notation 103Introduction 104Objectives 104Design loads 105Design strengths 105Deformation limits 107Behaviour of structural steelwork subject to blast loading 108Flexural design of structural steel elements to resist blast

loading — quasi-static/dynamic response 111Flexural design of structural steel elements to resist blast

loading — impulsive response 114Dynamic reactions 115Design example: structural steel beam subject to

quasi-static/dynamic load 115Design of connections in structural steelwork 117Steelwork detailing for steel structures subject to blast

loading 118References 118

7 Design of elements in reinforced concrete and masonry 119Geoff Mays, Cranfield University at the Defence AcademyIan Feltham, Arup

Notation 119Introduction 121Objectives 122Design loads 122Design strengths 123Deformation limits 126Behaviour of reinforced concrete subject to blast loading 127Flexural design of reinforced concrete elements to resist

blast loading 130Flexural design of reinforced concrete — impulsive response 131Flexural design of reinforced concrete — quasi-static/

dynamic response 133

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Design of reinforced concrete elements for shear underblast loading 136

Dynamic reactions 139Design example 1: reinforced concrete cantilever subject

to impulsive load 139Design example 2: reinforced concrete wall panel subject

to quasi-static/dynamic load 142Detailing reinforcement in concrete subject to blast

loading 146Design of connections 147Spalling and breaching of reinforced concrete under blast

loading 148Design of masonry structures to resist blast loading 150References 152

8 Design of elements in steel—concrete—steel compositematerials 154Neil Coyle, Corus Bi-SteelDavid Cormie, Arup Security Consulting

Notation 154Introduction 156Objectives 156Design loads 156Design strengths 157Deformation limits and design cross-sections 157Behaviour of steel—concrete—steel composite elements

subject to blast loading 158Stiffness 164Shear design 164Transverse shear design 165Longitudinal shear design 165Design of SCS elements to resist blast loading 166Design example 1: SCS cantilever subject to impulsive load 168Comparison of SCS and reinforced concrete 174Detailing of SCS structures 175References 176

9 Design of glazing 177David Smith, Arup Security ConsultingDavid Cormie, Arup Security Consulting

Notation 177

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Introduction 177Types of glazing and behaviour under blast loading 180Levels of blast enhancement 184Framing systems 187Design of laminated glass for blast loads 190Derivation of the resistance function for a laminated

glass pane 194Design example 1: single-glazed laminated glass pane

subject to blast load 202Design example 2: single-glazed laminated glass pane

subject to increased blast load 208Iso-damage analysis 208Calculation of edge reaction forces 210Glazing hazard classification 211References 214

10 Whole-building response to blast damage 216David Cormie, Arup Security Consulting

Notation 216Introduction 216Disproportionate versus progressive collapse 217Protection and disproportionate collapse 218Development of robustness requirements in national standards 218Tolerability of risk 223Methods of design for structural robustness 226Tie-force-based design methods 228Alternate loadpath methods 230Alternate loadpath analysis procedures 237Key element design methods 245Discussion 246References 247

11 Vehicle-borne threats and the principles of hostilevehicle mitigation 250Paul Forman, Centre for the Protection of NationalInfrastructure (CPNI)Dorian Evans, Ministry of DefenceGary Heward, MFD International

Introduction 250Types of vehicle-borne threat 253Site assessment for vehicle-borne threats 255

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Principles of hostile vehicle mitigation (HVM) 257Principles of design of vehicle security barriers for

high-energy impact 269Foundation requirements for vehicle security barriers 271References 273

12 Design of building services under blast loading 274John Taylor, Castra Consulting

Introduction 274Effects of explosions on building services 274Principles of design 275Design of services to support life safety 277Design of services to aid escape and evacuation 282Design for service continuity 283Design for ease of recovery 284Fixing, mounting and containment of building services 285References 289

13 Implications for building operation 290Chris Veale, Centre for the Protection of National Infrastructure(CPNI)

Introduction 290Managing the risk 292The threat assessment 292Pre-event contingency planning 292Post-event contingency planning 295Useful publications 295References 297

AppendicesA Equivalent SDOF properties for beams and slabs 299B Maximum deflection and response time for elasto-plastic

SDOF systems 309C Design flowchart 319D Conversion factors 323

Index 325

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Foreword to second edition

In the fourteen years since the publication of the first edition of thisbook, terrorism has evolved in scope and scale. In this second editionwe have attempted to update and expand the material to reflect thedevelopment of the field of blast engineering in response to theevolving terrorist threat. For the first time, material on hostile vehiclemitigation is included, as is a chapter on the design of buildingservices against blast loads. Several parts of the field have developedsubstantially since the first edition, in particular methods for thedesign of glazing against blast loads which have benefited from anextensive research and development programme in both the UK andthe US. Computational and numerical analysis permits us to evaluateproblems in ways which were not possible fourteen years ago. Inaddition, we have expanded the scope to include material on thedesign of buildings against other forms of blast such as industrialexplosions, vapour cloud explosions and deflagrations.

David CormieGeoff MaysPeter Smith

London and Shrivenham, June 2009

xi

Acknowledgements

I am indebted tomy co-editors, Professor Geoff Mays andDr Peter Smith,editors of the first edition for being receptive to my original proposal toprepare a second edition of this book, and for their unwavering supportduring its writing. I hope this edition does justice to its predecessor andmanages to retain its accessible style and content. I am enormouslygrateful to the chapter authors for their invaluable contributions.The preparation of the second edition was supported financially by

Arup’s Design and Technology Fund. I am grateful to Arup for thissupport, and to Cranfield University at the Defence Academy and tothe Centre for the Protection of National Infrastructure. I also wish toextend my thanks to my colleagues in Arup Security Consulting andelsewhere in Arup, notably Arup Advanced Technology & Research.Eduardo Aja of Arup Security Consulting has prepared the examples inChapter 4, for which Tim Rose of the Atomic Weapons Establishmenthas permitted us to use Air3d and has undertaken a valuable review ofthe chapter. Corus has generously made available their design methodsfor Corus Bi-Steel in Chapter 8. John Colvin of John Colvin GlassConsultant kindly assisted with material relating to the forthcoming ENStandards for glass. Bassam Izzuddin of Imperial College has reviewedChapter 10 in detail and has provided many valuable suggestions for itsimprovement. Tom Ward has helped me with a number of the figuresand Simon Blakeney with some of the photographs. Vivianne Kollevrisof Arup Security Consulting has reviewed large parts of the text inmeticulous detail with an unassailable and seemingly inexhaustibleenthusiasm. I am enormously grateful for her meticulous eye for detail,without which a number of errors would have made it into print. Anythat remain are mine alone.Without the support and encouragement of my wife Emma, none of

this would have been possible. Thank you.

David CormieLondon, June 2009

xiii

About the editors

David Cormie is an Associate with Arup Security Consulting and asenior member of the counter-terrorist team. He has spent his careerspecialising in the behaviour of buildings and structures underextreme loads and specialises in blast engineering, with particularinterest in structural robustness. He leads the counter-terrorismdesign of projects worldwide including airports, mass transitinfrastructure, commercial and government buildings, internationalsporting venues and strategic assets. He has delivered short courseson blast engineering and been invited to speak at conferences acrossthe world on the design of buildings to be resilient against terrorism.He is a chartered member of the Institution of Civil Engineers andthe Institution of Structural Engineers.

Professor Geoff Mays is Professor of Civil Engineering and Head ofSchool for Cranfield University at the Defence Academy of theUnited Kingdom. He conducts research and consultancy and lectureson postgraduate courses in the fields of structural resilience to blastloading and the strengthening of concrete structures. An author ofover ninety papers for refereed journals and conference proceedings,he has also written books on the durability of concrete structures andadhesives in civil engineering. He was the Convenor of the CENCommittee which led the development of European Standards forthe protection and repair of concrete structures and remains thechair of its working group concerned with structural bonding.

Dr Peter Smith is Reader in Protective Structures with CranfieldUniversity at the Defence Academy of the United Kingdom atShrivenham. He lectures on Masters programmes and shorter coursesin the UK and worldwide on blast loading, structural response anddesign. He has supervised research contracts and undertakenconsultancy in this general area for both commercial and government

xv

organisations. He is the author of over 100 journal and conferencepapers and technical reports as well as books in the blast area. He iscurrently a member of a European working group concerned withexplosion effects on urban buildings.

xvi

List of authors

Mike Banfi, ArupDavid Cormie, Arup Security ConsultingNeil Coyle, Corus Bi-SteelMajor General Christopher Elliott CB MBE, Arup Security ConsultingDorian Evans, Ministry of DefenceIan Feltham, ArupPaul Forman, Centre for the Protection of National InfrastructureDavid Hadden, Arup Security ConsultingGary Heward, MFD InternationalConrad Izatt, ArupGeoff Mays, Cranfield University at the Defence AcademyDavid Smith, Arup Security ConsultingPeter Smith, Cranfield University at the Defence AcademyJohn Taylor, Castra ConsultingChris Veale, Centre for the Protection of National Infrastructure

xvii

1

Introduction

ObjectiveThe purpose of this book is to give engineers and architects a betterunderstanding of the opportunities (and of their own and their clients’responsibilities) to provide buildings which minimise damage to peopleand property in the event of an explosion. Of course, the focus is on thedamaging effects of terrorist attacks, but the principles herein can beapplied to all explosive events. It is not a design manual, requiringcompliance, but a design handbook, giving guidance and practical advice.

Since the first edition was published in 1995, the threat of terrorismhas evolved in scope and scale, particularly with the emergence ofsuicide bombers prepared to die in the act of delivery. Regrettably,terrorism shows every sign of enduring for a long season yet. Sonormal has it become that counter-terrorist measures arenow considered usual in most commercial and recreational activities;protection is now common-place in infrastructure projects and thereis heightened curiosity about what can be achieved in this area. Thishas helped the blast engineer very considerably in attracting attentionto the benefits of sensible protective design.

However, it was the deliberate flight of two fully-fuelled aircraft intothe twin towers of the World Trade Center in New York in 2001 thatchanged awareness most abruptly. Initial astonishment at the act wasreplaced by a recognition that terrorists would explore any opportunitypresented, bounded only by what was possible within the laws of physics.

The simultaneous attacks on the World Trade Center and thePentagon have left a deep imprint on perceptions: the ambition ofthe terrorists involved; the elegance and the novelty of their methodof attack; the complexity of their preparations; the callous and casualmurder of thousands of innocents; the initial disruption to a highlyinterconnected commercial world (and also the surprising resilienceachieved through that interconnectivity); the insult delivered tonational self-esteem with consequences far beyond the attack itself.

1

Yet the buildings performed extremely well under what was an‘unscripted’ attack for them, for they remained standing long enoughfor almost all those below the location of the impact to escape. Muchof this was due to an inherently robust structure and to a sensiblereview of evacuation procedures by the owners following the 1993detonation of a vehicle-borne device in the basement car park. Throughthe experience learned, the evacuation rules for events which couldchallenge the structural integrity of the building were changed from‘stay put’ to ‘evacuate with best speed’.

These go to the heart of the philosophy laid out in this book: thatprotection should be intelligent, thoughtful and holistic in approach,not blind and expensive hardening against the greatest explosivecharge weight that a terrorist might be theoretically able to deliver.

It is a fact that too much is not done that could be done, because ofconflict of interest with planning and financial areas or lack ofimagination. An underlying purpose of this book is to give engineersthe arguments that will sway the debate in favour of sensible, practical,economic measures being adopted, not discarded.

All this underlines the thesis of the first edition: that protectivemeasures were not solely about hardening and blast protection, but amuch more inclusive consideration about the balance betweenprotection and continuing normal life. However, since its first printing,the field of blast engineering has expanded greatly and a great deal ofeffort has been invested around the world in research and developmentof counter-terrorist design, from the development of better methods ofanalysis to manufacturers bringing new products to the market forimproving the counter-terrorist resilience of buildings. Much work hasbeen undertaken to improve the way in which we manage and respondto incidents, in terms of evacuation and sheltering strategies, emergencyresponse and coordination, and recovery after an incident.

In parallel, significant advances have been made in numerous otherfields. The development in the field of computational fluid dynamicshas important applications in the prediction of blast loads that werenot possible fourteen years ago. As the oil and gas industry has matured,technology developed for protection against gas deflagrations hastransferred onshore and into the high explosives arena. Ourunderstanding of structural behaviour has improved greatly withregard to phenomena such as progressive collapse. The regulationspertaining to progressive collapse in the UK Building Regulationswere revised in 2004. Research into structural collapse has also beena very active field, and particularly so since the collapse of the World

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Blast effects on buildings

Trade Center on 9/11. The importance of ductility and rotation capa-city of connections is now more generally recognised, as are theconcepts of robustness and resilience.

Facade design has moved on very significantly since the publicationof the first edition. Our understanding of the behaviour of glass, glazingsystems and facade framing systems has benefited from wide-rangingprogrammes of research and development supported by full-scale testingundertaken by the UK Government and the US Army Corps ofEngineers. Design methods have been developed for the design ofglazing systems based on fundamental theory of structural analysis,rather than the semi-empirical methods contained in the first edition,which are now outdated and obsolete.

Finally, the field of structural engineering continues to makeadvances. With ever more demanding architectural and structuralforms being proposed, challenges present themselves apace to theindustry’s structural engineers for more efficient, elegant, slender orarchitecturally demanding solutions.

For all these reasons it has been appropriate to revise and expand theoriginal text, if it is to continue to provide authoritative and usefulguidance.

ScopeThe chapters of this book have been thoroughly revised to update thematerial to take account of the advances made in the field. The basicguidelines for enhancing building resilience in Chapter 2 have beenretained and updated. Blast loading has now been split into twochapters, with the theory of blast loading being covered in Chapter 3,followed by a new chapter (Chapter 4) on the prediction of blastloads. Chapter 4 describes the empirical methods available for theprediction of blast loads and when they might be used or might indicatethat more advanced methods are necessary. A section is presented onthe use of semi-empirical methods and more advanced methods suchas computational fluid dynamics for the prediction of blast loads.

Chapter 5 describes the structural response to blast loading, and hasbeen revised and enhanced to include recent developments in advancedsingle degree of freedom techniques for modelling structural behaviour.The previous single chapter on the design of elements in steel andreinforced concrete has now been split into separate chapters on eachmaterial (Chapters 6 and 7), with a further chapter (Chapter 8) onthe design of elements in steel—concrete—steel composite materials.

3

Introduction

The material has been aligned with the Eurocodes in recognition oftheir forthcoming adoption in the UK.

The methods for design of facade elements is perhaps the one area thathas altered most radically since this book was first published, and there isnow a separate chapter (Chapter 9) on the design of glazed facades.Modern methods of analysis and design bear almost no resemblance tothe state-of-the-art methods that were available in 1995, thanks mainlyto a very extensive programme of full-scale tests carried out on glazingby the UK government throughout the second half of the 1990s.

A new chapter (Chapter 10) has been included on the whole-building response to blast damage. This chapter includes discussionon progressive and disproportionate collapse. Material is presented onthe issues of robustness as they apply to blast effects on buildings.The chapter aims to give the practising engineer an insight into thetheory and modern approach to the fundamental analysis of collapsefollowing sudden structural damage. The aim is to provide the engineerwith an understanding of the phenomenon, its characteristics, and howto go about a logical and coherent assessment of the vulnerability of abuilding to collapse under blast loading. Where specific vulnerabilitiesare identified in a structure, the engineer may choose to provideprotection to or enhance the robustness of a local structural element,or to enhance the ability of the structure to redistribute load afterdamage. Practical mitigation measures are presented for both theseapproaches.

One area not covered in the first edition was the design of protectionagainst vehicle-borne attack. Penetrative vehicle attacks on buildingshave become a common occurrence around the world, and the engineeris now frequently asked to consider the design of vehicle protection forhigh-energy impact. In Chapter 11 the principles of design of vehiclerestraint measures to protect military, government and commercialbuildings against high-energy impact are presented, enabling theengineer to approach the problem competently.

Chapter 12 is a new chapter presenting the design of building servicesunder blast loading. The design of building services to exhibit resilienceunder blast loading has become increasingly important as the emergencyresponse to terrorist threat has changed. Frequently, high-rise and large-occupancy office buildings now adopt principles of inward evacuation,seeking refuge in hardened or adequately protected areas of thebuilding. Consequently, the need arises for continuity of life safetyservices under blast loading and due consideration of the design ofother building services during a blast event. Higher performance criteria

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Blast effects on buildings

for business-critical activities place greater resilience requirements onbuilding services, and this chapter discusses the design of buildingservices to withstand and provide continued safe operation throughand after a terrorist attack.

The final chapter of the book (Chapter 13) discusses implications forbuilding operation. The management of and response to incidents hassince the publication of the first edition benefited significantly fromsubstantial research and development in this area. The original contentof this chapter has been completely revised and extended to include thefindings of much of this work and to present an analysis of how weoperate our buildings to best manage and minimise the risks associatedwith terrorist attack.

Modern terrorismTerrorism has been described as the deliberate use of violence to createa sense of shock, fear and outrage in the minds of a target population.Several factors in the way we now live make that easy to achieve.

First, terrorists are able to make use of the media as never before to carrya sense of terror to their target population, and television, in particular,gives terrorists a political leverage out of all proportion to their otherpowers. Second, developed societies have become very dependent oncomplex, ‘brittle’ systems (e.g. railways, airlines, gas pipelines, largeshopping areas and business centres) which are both vulnerable andcritical to society’s function, and allows the terrorist many suitable targets.Third, terrorists hide behind the camouflage of normal daily life. Thismeans that almost all effective measures to combat terrorism also carryconsiderable constraints on individual freedoms, which governmentsare rightly reluctant to impose, and often will not.

This leads to several conclusions: terrorism today is much easierto contain than to eliminate; there are few completely acceptableantidotes to it; the prudent design will allow for its effects wherever itis possible and affordable.

RiskProtection is not an absolute concept and there is a level of protectionwhere the cost of protection provided with respect to the cost of thepotential loss is in balance. Protection can never offer a guarantee ofsafety; conversely, too much protection is a waste of resources withregard to what is being saved.

5

Introduction