A History of Research and a Review of Recent Developments

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8 The effects of explosive loading 8.1 CIVIL BUILDINGS There have been so many terrorist attacks in recent years on commercial, government and private buildings that a considerable bank of information exists on structural behaviour in the face of man made explosions. There have been spectacular gas and vapour cloud explosions in many countries of the world, all well documented and the subject of subsequent structural investigation. But a number of classic investigations of the effect of aerial bombing on civil buildings were made during the Second World War, and it is sensible to take as a starting point the papers by Thomas [8.1] and Baker, Leader Williams and Lax [8.2] that appeared in the Civil Engineers in War, published by the UK Institution of Civil Engineers in 1948. The diagrams and photographs look remarkably similar to those accompanying articles written in 1994 on the effect of terrorist bombing on the City of London. Baker et al. noted that fully framed concrete or steel buildings were very resistant to the effect of bombing during the Second World War. Small and medium-sized bombs dropped during that conflict were found to damage framed buildings by direct hits, but the blast from near misses of similar bombs caused only superficial (though extensive) damage. Direct hits often create excessive internal pressure within buildings, which reverses the loads for which the structure was originally designed. Very much larger bombs produce very serious external blast loads that cause downwards failure of roof trusses and lateral distortion of the entire building. The authors made a very important survey of about fifty cases of damage by high explosive bombs to multi-storey steel-framed buildings. The general Second World War experience was that the limit of vertical bomb penetration was the roof and two or three floors, but bombs released from low altitude could attack lower floors by passing through openings or penetrating external walls. The cutting of a single beam or stanchion did not usually result in collapse of the complete structure, but if a medium-sized bomb exploded inside a building it was found that about 1000 square feet of floor was demolished 183
184 The effects of explosive loading immediately above and below the explosion. The area of demolition fell off rapidly at lower floors, soon down to 100 square feet. Debris was usually held by the second and third floor slabs below the explosion, but occasionally it brought down a succession of floors. The survey emphasized that the virtual absence of progressive collapse in well-designed frame buildings meant that floors and walls near the detonation point were effective as screens. Of equal importance was the ductility and continuity of the framework, which made the building highly resistant to explosions. A possible weakness, however, was the lack of ductility and continuity in bolted or riveted connections. As in many other examples of structural loading, the crucial factor lay in the strength or weakness of connections. During the years before the Second World War welded connections were relatively rare, so there was not a great deal of evidence about the behaviour of welded frameworks. The authors, however, drew attention to the possibility of introducing ductility into building structures by clamped connections that rely on friction to produce the effects of continuity. The resulting structure could absorb a large amount of energy without collapse. The following types of damage were noted in single-storey steel-framed buildings such as hangars, warehouses and workshops: direct damage, primary collapse and spreading collapse. Direct damage was the cutting of members by a bomb (before exploding), by fragments or by crater debris; primary collapse was the collapse of members that depended for their stability on a destroyed member. Spreading collapse occurred when the forces set up by a primary collapse were transmitted to adjoining undamaged members, producing instability in these members. The spreading collapse of a roof could occur when the cutting of a single member suddenly involved the whole area of a large structure. The chief risk to roof steelwork arose from the failure of stanchion to roof-girder connections, which were sheared by violent displacement or lateral blast, or destroyed by blast uplift. Dwelling houses, of course, are rarely constructed with a ductile steel framework, so in assessing the behaviour of houses in explosions, it was necessary to examine the strength of masonry and brick structures to blast. Civilian masonry structures are not normally designed to resist explosive blast. The stone castles and military fortifications of the Middle Ages were more likely to be threatened by the penetrative action of missiles, and their wall thicknesses were set empirically by the need to resist damage by local impact and fragmentation. Civil masonry and brick structures were known to have relatively little resistance to local explosions, and in earlier times no attempt was made to predict how they might behave under attack, or what their residual strength might be. Masonry and stone were not employed much in the construction of ‘bomb-proof’ shelters once reinforced concrete appeared on the structural scene. As the Second World War approached, however, experimental research was initiated to check the behaviour of conventional building structures when subjected to the general blast from exploding aerial
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EXPLOSIVE LOADING OF ENGINEERING ST
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Explosive Loading of Engineering St
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Contents Preface vii Acknowledgemen
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Preface A long time ago I walked ov
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Acknowledgements The reports from w
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xii Notation m mass of projectile m
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xiv Notation Q work done during exp
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Introduction Explosions can threate
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Introduction xix Bertram Hopkinson
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Introduction xxi in the seventeenth
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Introduction xxiii own versions of
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Introduction xxv indoor shelters an
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Introduction xxvii explosions/ stru
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Introduction xxix particularly dama
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Introduction xxxi but the feeling i
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2 The nature of explosions propelli
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4 The nature of explosions or rathe
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6 The nature of explosions ogive, a
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8 The nature of explosions mass, an
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10 The nature of explosions Figure
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12 4 lb cylinders, TNT, 3.75 lb sph
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14 The nature of explosions As Figu
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16 The nature of explosions of the
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18 1.4 THE DECAY OF INSTANTANEOUS O
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20 The nature of explosions Figure
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22 The nature of explosions where I
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24 sometimes written in psi as The
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26 The nature of explosions 1.20 Ki
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28 The detonation of explosive char
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50 Figure 2.18 Radius/time curve of
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54 and 50%, rather than by a factor
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3 Propellant, dust, gas and vapour
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Dust explosions 61 as the period 19
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Gas explosions 63 reported by Palme
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Gas explosions 65 workings in very
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Vapour cloud explosions 67 the leak
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References 69 in Los Angeles Harbou
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4 Structural loading from distant e
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Uniformly distributed pressures 73
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Table 4.1 Loading/time relationship
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Loading/time relationships 77 overp
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Loading/time relationships 79 side
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Loads on underground structures 81
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Loads on underwater structures 87 c
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References 89 4.12 Allgood, J. (197
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92 Structural loading from local ex
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98 The general empirical equation f
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100 Structural loading from local e
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112 Figure 5.16 Relationship betwee
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120 Pressure measurement and blast
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Page 165 and 166: 132 Pressure measurement and blast
Page 175 and 176: 142 Penetration and fragmentation r
Page 177 and 178: 144 Penetration and fragmentation p
Page 179 and 180: 146 Penetration and fragmentation w
Page 181 and 182: 148 Penetration and fragmentation T
Page 183 and 184: 152 Penetration and fragmentation F
Page 185 and 186: 152 Penetration and fragmentation n
Page 191 and 192: 158 Figure 7.16 Penetration simulat
Page 195 and 196: 162 Penetration and fragmentation b
Page 197 and 198: 164 Table 7.1 Penetration and fragm
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Page 203 and 204: 170 Penetration and fragmentation l
Page 207 and 208: 174 Penetration and fragmentation p
Page 209 and 210: 176 Penetration and fragmentation o
Page 211 and 212: 178 Penetration and fragmentation o
Page 213 and 214: 180 Penetration and fragmentation 7
Page 215: 182 Penetration and fragmentation 7
Page 219 and 220: 186 The effects of explosive loadin
Page 221 and 222: 188 Table 8.1 It is worth noting th
Page 233 and 234: 200 Table 8.5 The effects of explos
Page 249 and 250: 216 Response, safety and evolution
Page 257 and 258: 224 In the log-normal distribution
Page 263 and 264: 230 Cole, R.H. 46, 48, 53, 55, 99 C
Page 265 and 266: 232 Petry 143 Philip, E.B. 13, 14,
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Subject index Aircraft damage 207 A
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A History of Research and a Review of Recent Developments

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