Behavior and Design of Commercial Multistory Buildings Subjected to Blast

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Behavior and Design of Commercial Multistory BuildingsSubjected to Blast

Mike P. Byfield1

Abstract: The behavior of nonmilitary buildings subjected to blast is considered. Case studies from World War II are described, as wellas more recent events from the detonation of large vehicle borne devices in the Middle East, North America, and Europe. Conventionalmethods for nonseismic design are shown to lead to frames with overstrong beams connected together by relatively weak connections.This may explain much of the evidence from bomb damaged buildings in which building connections have been observed to fracture ina brittle manner when subjected to blast. The risk of progressive collapse may be minimized by strengthening beam to column connectionslocated at close proximity to potential vehicle borne devices and a capacity design method for such strengthening is advocated.

DOI: 10.1061/�ASCE�0887-3828�2006�20:4�324�

CE Database subject headings: Blast loads; Brittle failure; Connections; Ductility; Progressive failures; Collapse; Stiffness; Buildingcodes; Steel structures.

Introduction

The vehicle borne improvised explosive device is the weapon ofchoice for most terrorist organizations. These have a proven abil-ity to cause progressive collapses and it is therefore highly desir-able that all buildings at risk from attack should provide theiroccupants with an adequate level of protection against theseweapons. For most commercial buildings, such protection mustbe provided without significant additional expense and withoutcompromising the functionality of a building’s interior. This isdifficult to achieve, however a review of the history of bombdamaged structures reveals factors that will govern the likelihoodof a progressive collapse occurring following localized damage toa building’s frame.

A significant proportion of progressive collapses which havebeen triggered by bomb blast have been attributed to the weak-ness of the connections between beams and columns. FrancisWalley and the late Lord Baker both played important roles insurveying bomb damaged structures in London during the WorldWar II bombing campaign. Both investigators concluded that themajority of collapses caused by high explosive bombs could betraced back to the failure of these connections.

In 1968 the progressive collapse caused by an accidental natu-ral gas explosion at Ronan Point in London highlighted the needto adequately tie load bearing members together, see Fig. 1. Theapartment block was constructed using precast concrete panelsbolted together. The collapse was triggered when Ivy Hodge lit amatch on the 18th floor. The blast dislodged a precast concrete

1School of Civil Engineering, Univ. of Southampton, Southampton,Hampshire SO17 1BJ, U.K. E-mail: [email protected]

Note. Discussion open until April 1, 2007. Separate discussions mustbe submitted for individual papers. To extend the closing date by onemonth, a written request must be filed with the ASCE Managing Editor.The manuscript for this paper was submitted for review and possiblepublication on January 27, 2006; approved on March 31, 2006. Thispaper is part of the Journal of Performance of Constructed Facilities,Vol. 20, No. 4, November 1, 2006. ©ASCE, ISSN 0887-3828/2006/4-
324–329/$25.00.
324 / JOURNAL OF PERFORMANCE OF CONSTRUCTED FACILITIES © AS

J. Perform. Constr. Facil.

walling unit and triggered a progressive collapse that killed fourpeople, although Ivy Hodge miraculously survived. This land-mark event led to important changes to the building regulations inthe United Kingdom. From that point onward all structural mem-bers had to be adequately tied together and those members criticalto stability had to be designed to resist the blast over pressurescaused by a natural gas explosion. This was assumed to beequivalent to a statically applied load of 34 kN/m2. These simplerequirements have been generally regarded as a low cost andsensible means in which to reduce the risk of collapses due toblast.

High Explosive Effects on Structures

Explosives can be categorized as either deflagrating �low� or deto-nating �high�. The chemical reaction in low explosives is via aburning type process. It therefore propagates through the explo-sive compound or gas at a relatively slow rate. In contrast thereaction passes through a high explosive compound in the form ofa shock wave.

The duration of the pressure wave tends to be longer with lowexplosives, which include the new family of blast enhanced fuel-air mixture thermobaric warhead munitions. These are proliferat-ing across the globe, as witnessed by the inhabitants of Grozny inChechnya at the hands of the Russian Army. Fuel-air mixtureblasts �including accidental natural gas explosions� generate veryhigh impulsive loads and can be particularly damaging tostructures.

Commonly used high explosives include TNT, RDX, andSemtex, all of which have approximately equal yield. Militaryhigh explosives produce an instantaneous rise in air pressure,making them particularly effective at fragmenting metal shell cas-ings to produce shrapnel. Terrorists rarely use large quantities ofmilitary explosives due to the difficulties of acquisition �with theexception of insurgents in Iraq, who have access to large quanti-ties of munitions�. Vehicle borne devices often use homemadeexplosive compounds, such as ammonium nitrate fertilizer based
explosives. These homemade compounds detonate and are there-
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fore classified as high explosives. The TNT equivalence is ap-proximately half that of military explosives and the rate at whichthe shock wave propagates through compounds is slower. Thismakes them less efficient for breaking shell casings. However, theviolent expansion of hot gases that produce the blast wave is alsoslower. As pressure time histories from high explosives are oftensubstantially shorter than the natural periods of building compo-nents, this slower reaction time can be more effective for impart-ing energy into a building’s superstructure.

The positive pressure phase of a blast wave is followed by anegative pressure phase, created as air returns to fill the void leftby the explosion. This suction is of much lower intensity than thepositive phase. Despite this the suction on the front face of abuilding from the pressure phase has been known to cause steel-work connections to fail that would otherwise have survived.

An important feature of blast waves is that they reflect offbuilding surfaces. This means that they can travel for some dis-tance down roads surrounded by tall buildings. Blasts initiated inurban areas result in multiple reflected waves which interact toproduce interference damage patterns. Such interference can beseen in Fig. 2 which was caused by a blast wave from a fuel oildepot at Hemel Hempstead in England in 2005. It is possible tomodel the reflections due to street architecture in order to predictthe expected blast pressures. This is useful for determining theextent of strengthened glass needed for tall buildings. Multiplereflections enhance the destructive capability from an explosion.Therefore blasts in confined spaces can cause extensive structuraldamage, as witnessed in the World Trade Center in 1993. Ventingby means of sacrificial panels is an effective means of limitingdamage from such an event. Blasts initiated in open spaces canalso produce multiple reflections in re-entrant corners of buildingfacades, such as the overhanging floors employed in the Murrah

Fig. 1. Progressive collapse in London �Ronan Point, 1968� causedby a natural gas explosion

Building in Oklahoma City.

JOURNAL OF PERFORMANCE


Observed Behavior of Framed Structures Subjectedto Blast

London during World War II

A vast amount of data and observations were compiled duringWorld War II �WWII� on the performance of buildings subjectedto the effects of high explosive bombs. The data gathered in-cluded 60,000 basic reports on bomb damage, in addition to 5,000detailed reports on individual damaged structures. The results ofthese investigations are available in the Walley collection, storedin the archive of the Institution of Civil Engineers Library inLondon. One of the participants in the study was the late Lord J.F. Baker �1948� who went on to become Head of Engineering atthe University of Cambridge in England. At that time he wasemployed by the Ministry of Home Security �now the Home Of-fice�. Baker concluded that of the 50 or so steel framed buildingsthat he surveyed in detail, almost all collapses were the result ofinadequate connections between perimeter columns and beams.Of particular fragility were buildings whose external walls ran inparallel with the direction of slab span. In such cases the concretecasing to wall beams was often weakly tied into the floor slabs,leaving the connections between the primary beams and the pe-rimeter columns as the only effective restraint against outwardmovement of the walling system. The fragile nature of these con-nections often led to tensile failures of perimeter beam to columnconnections due to near miss attacks. High explosives cause animmediate rise in pressure, which is followed by a negative pres-sure phase of lower intensity. It was observed that even the rela-tively low suction pressures from near miss events were sufficientto cause widespread failures of these connections, leading to se-rious floor collapses. One such typical failure due to a near miss isshown in Fig. 3.

Based on his observations Baker recommended that the tyingbe improved between flooring and wall framing systems. He alsorecommended strengthening of beam to column connections,which generally failed due to a combination of the prying actionresulting from insufficient ability to accommodate large beam endrotations and tensile loading, see Fig. 4. Walley �1994a� con-cluded that “there is no doubt that the most important means ofincreasing the resistance of frames was to increase the stiffness ofconnections.” Walley played an important role in the WWII fo-rensic investigations, and took part in the examination of dam-aged structures in Hiroshima shortly after the detonation of theatomic bomb. That survey, together with subsequent research,demonstrated that the membrane action of flooring systems im-parts enormous strength to structures subjected to nuclear blasts.

Multistory framed buildings in London of that period were

Fig. 2. Damage to a building’s cladding showing evidence ofinterference between reflected blast waves

found to have an impressive ability to redistribute loads following

OF CONSTRUCTED FACILITIES © ASCE / NOVEMBER 2006 / 325

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substantial damage from direct hits. This was attributed primarilyto the bracing effects of masonry panel walling, which was able toeffectively redistribute loading from severely damaged parts offrames. Baker �1948� noted that “the effects of the explosion onthe building as a whole depends to a large extent on the internalplanning,” by which he was referring to the dividing of buildings

Fig. 3. Perimeter column connection failures following suction loadsfrom a near miss event �Baker et al. 1948, reprinted with permissionfrom Thomas Telford Ltd.�

into cells by internal partitions, which at the time were generally



constructed out of 11.5 cm �4.5 in.� thick masonry. These parti-tions provided a vital means for redistributing load followingdamage to columns. A typical example is shown in Fig. 5 whichshows a building in which the entire wing of a building is leftunsupported, due to unseating of the support girder. Progressivecollapse was prevented by the great strength of the panel wallingwhich cantilevered off from the undamaged sections of the build-ing. Further robustness was also provided by the fire protected tosteel frames, which was by means of reinforced concrete infillduring that time. This imparted additional mass to the elements

Fig. 4. A typical beam-to-column connection failure �Baker et al.1948, reprinted with permission from Thomas Telford Ltd.�

Fig. 5. Front wall of building unsupported due to unseating ofsupport girder �Baker et al. 1948, reprinted with permission fromThomas Telford Ltd.�

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which is important when resisting impulsive loads. The concretewill also provide a greater resistance to local buckling.

Experience of Attacks by the Provisional IrishRepublican Army

The experience of terrorism in the British Isles gained during theterrorist campaign by the Provisional Irish Republican Army�PIRA� is particularly relevant to today’s threat from terrorists,because PIRA’s preferred weapon was the vehicle borne attackusing large quantities of homemade explosives. PIRA detonatedlarge numbers of extremely powerful bombs and the typicalcharge size was 1,000 kg of TNT equivalent, which is similar tothe size used by Timothy McVeigh during his attack on the Mur-rah Building. Much of the early records are documented byRhodes �1974� who worked for the Government of Northern Ire-land from 1953 to 1974. He noted that concrete framed structuresfrequently sustained severe damage to their frames, including thesevering of beams and columns without causing progressive col-lapses. Again, panel walling and diaphragm walls played a vitalrole in bracing severely damaged structures. Concrete framedstructures, such as the St. Mary’s Axe Building in the City ofLondon, which was attacked in 1992, can often sustain significantdamage to the perimeter frame without progressive failure. This ismainly due to the monolithic nature of the frame providing sig-nificant redundancy via a combination of three dimensional vier-endeel actions and bracing from panel walling.

Reinforced concrete frames were generally found to fracture atthe joints between beams and columns where the reinforcement islapped. This also presents a significant zone of weakness whensubjected to the reverse uplift loads from blast. Load reversal canalso cause structural elements to become dislodged if not ad-equately tied together. An example of such a collapse was theattack on the Dropping Well Bar in Ballykelly, Northern Irelandin 1982, in which 17 people were killed. The detonation of arelatively small amount of explosive contained in a hand bagcaused precast concrete slab units to become dislodged from theirsupports, which thereafter crushed occupants in the crowded bar.While tragic, this incident highlights the importance of tying allstructural components together regardless of overall structural im-portance. Blast loading imposes extreme loads over very shortdurations. Unlike conventional loads, the mass of a member im-parts a resistance to load in addition to the conventional structuralstrength. This inertial response can result in unusual effects. Re-inforced concrete members may be undamaged along theirlengths, although the connections between members can be se-verely damaged or ruptured �Rhodes 1974�.

The 1990s witnessed a series of approximately 1,000 kg TNTequivalent bombs detonated in the financial districts of London.These included the 1992 St. Mary’s Axe bomb, the 1993Bishopsgate bomb, and the 1996 Docklands bomb. Fortunately noframed buildings collapsed as a result of these attacks, althoughthe financial costs were enormous because the sector of Londoncontaining the banking industry was repeatedly attacked. It wasestimated that the total cost to business and damage to propertywas over £1 billion for the Bishopsgate bomb alone.

FBI Murrah Building in Oklahoma City in 1995

The attack by Timothy McVeigh showed that seemingly well de-signed and robust modern buildings can be susceptible to progres-sive collapse following attack by improvised explosive devices.
In common with most modern buildings, the Murrah Building


incorporated curtain wall cladding, designed to accommodatefloor deflections through movement joints. These light-weightglazed systems cannot provide viable emergency bracing of thetype illustrated in Fig. 5. Further, the absence of structural inter-nal partition walls �also shown in Fig. 5� very substantially limitsthe ability to redistribute loads by cantilevering from the undam-aged interior of a building. In the absence of these alternative loadpaths modern multistory buildings are susceptible to column dam-age. The blast in the Murrah Building destroyed three columnslocated on the front face of the building. These columns sup-ported transfer beams that supported intermediate columns. Thusthe framing system adopted is partly responsible for widening thezone of the building that collapsed.

Attacks by Islamic Terrorist Organizations

The attacks by PIRA did not aim to create large-scale losses ofcivilian lives, because PIRA recognized the need to maintain adegree of political as well as public support. Bombs were there-fore detonated with warnings and therefore the threat from pro-jectiles and flying glass was less important. In contrast Al-Qaedaand their followers aim to maximize civilian casualties and there-fore they provide no warning of attacks. The use of suicide bomb-ers allows them to locate weapons extremely close to buildingenvelopes, or even within them, as witnessed at the World TradeCenter in 1993. This significantly increases the threat from thesehighly effective weapons.

In 1983, 241 U.S. Marines, soldiers and sailors died in Leba-non when a Mercedes truck loaded with explosives penetratedtheir building’s interior before being detonated, resulting in a pro-gressive collapse. The attack used a massive quantity of explosive�3,600–5,400 kg TNT equivalent� and produced a crater some14 m�13 m�2.7 m deep. This was the largest conventional ex-plosion ever seen at that time by the FBI forensic experts �MarineCorps 1984�. The building was of modern reinforced concreteconstruction and designed to a high standard for the U.S. Em-bassy in West Beirut. The event illustrates that the only way todefend a building against such an aggressive attack is by prevent-ing unauthorized vehicles from approaching. This is known asmaintaining a stand off. Politically this was a landmark eventbecause it demonstrated that terrorism could change the foreignpolicy of a superpower, with U.S. forces withdrawing from Leba-non immediately afterwards.

The lesson of the need to provide a stand off was not lost.Jersey barriers were installed around the housing complex forU.S. military forces in the Khobar Towers in Dahran, in the East-ern Province of Saudi Arabia. The building survived massivedamage to the façade without progressive collapse; there were300+ casualties and 19 deaths. It was estimated by the DefenseSpecial Weapons Agency that the yield was equivalent to9,000 kg of TNT �House Armed Services Committee 1996�. Theresulting blast produced a crater some 24.4 m wide and 9.1 mdeep. Fortunately the device was detonated 32 m from the frontface of the building. Had the bombers succeeded in detonating thedevice closer to the building the casualties would have beenhigher. The resulting blast propelled the Jersey barriers into thefirst four floors of the building, which combined with the blastloading succeeded in destroying the lower precast panels of thefaçade. As the precast units in the remaining three floors abovewere left unsupported the entire façade of the building collapsed.This residential building was entirely constructed using a closelyspaced configuration of precast concrete panels, which were well
tied together. The multiple lines of closely spaced vertical sup-

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ports created numerous alternative load paths and formed a struc-ture not unlike the WWII structure shown in Fig. 5. Thus, thisevent demonstrates the importance of load bearing internal parti-tions in redistributing loads. Importantly the building was de-signed in accordance with the British reinforced concrete designcode BS8110. As stated previously, British codes require struc-tural elements to be adequately tied together to prevent collapsesfollowing blast. This requirement undoubtedly helped reducedamage in this case.

The HSBC headquarters in Istanbul also survived a massivetruck bomb without progressive collapse. This may in part be as aresult of the high strength designed into the building due to thethreat from earthquakes. Similarly, in 1993 a vehicle bomb deto-nated 2 m outside the perimeter columns in the basement ofWorld Trade Center 1 did not result in a progressive collapse. Thesteelwork was exceptionally strong and reflected the blast, whichcaused a collapse of the reinforced concrete substructure for adistance of some 100 m from the detonation without affecting theglobal stability of the tower �Robertson 2005�.

Brittle Buildings

The safety factors contained in modern codes of practice are cali-brated using a combination of what has been shown to be satis-factory in the past and using probabilistic theory. The basic theoryunderpinning the probabilistic approach assumes that both load-ing and resistance can be modeled using the log-normal probabil-ity distribution functions illustrated in Fig. 6. If this theory holdstrue, then the probability of a structural component failing is pre-dictable. As the probability of failure is deemed to be so low theconsequences of failure need not �and are not� considered. Thisseemingly safe approach can create brittle structures because noeffort is made to ensure that ductile failure modes govern buildingperformance.

The majority of components in a structure will have strengthwell in excess of that assumed during design. In fact it has beenshown that the steel-concrete composite beams that are ubiquitousin modern high-rise steel frame buildings can typically resistdouble their design loads, when subjected to large sagging deflec-tions �Byfield 2004�. This overstrength can create brittle buildingsbecause the weakest link in a load path can become the beam tocolumn connections. The important factor in resisting blast load-ing is the ability to fail in a ductile manner. For example, if abeam is relatively weak and its connections strong, then verylarge plastic deformations in the beam may result from overload-ing due to blast. Conversely, if a beam is supplied over strong,failure may occur in the connections, with the beams remaininglargely undamaged. Clearly, very little energy is absorbed in thelatter case.

In the United Kingdom it is usual practice to use nominallypinned connections between beams and columns, with lateral

Fig. 6. Probabilistic theory underpinning limit state design

�wind� forces resisted by bracing or shear walls. Connection de-



signers do not generally consider the high beam end rotations thatwould occur in severely overloaded beams, Fig. 7. End rotationscreate a prying action that has been shown to lead to bolt fracture�Byfield 2004�. Thus, routine designs often create structures withoverstrength beams connected together by brittle connections.This may explain the consistent observations made during WWIIby Baker and Walley that it was invariably weakness of connec-tions that triggered collapse due to blast damage. The importantfactor in surviving extreme short duration loads is the ability toabsorb energy without brittle connection failures. Steelworkbeams and columns are particularly good at absorbing energythrough plastic deformation. In many ways limit state design rep-resents a design methodology similar to that used in the automo-tive industry prior to the advent of crumple zone design. At thattime vehicles were generally constructed using overstrong com-ponents connected together by relatively weak, low ductility con-nections. It is recognized that design codes for seismic regionsalready result in ductile energy absorbing frames.

By concentrating on ensuring ductile failures, automotive en-gineers have been able to significantly reduce the number ofdeaths from road accidents. Crumple zone design has also re-duced the weight of vehicles, as designers concentrated on ensur-ing relatively weak components fail, rather than their connections.Likewise, savings can be made in the volume of steel and con-crete used in buildings by moving to a similar system. Beamscould be designed to resist working loads in the conventionalmanner. Thereafter, true strength of the beams should be deter-mined, with the connections designed to resist the maximum loadtransferable from the beam. In situations where terrorist attack isconsidered a threat, strength calculations should be inclusive ofimpulsive and strain rate effects. It is recognized that changes indesign practice are very difficult to implement although a capacitydesign approach along these lines has been proposed �Byfield

Fig. 7. Prying action in nominally pinned connections subjected tohigh beam end rotations

2004�.

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Design of Connections

The designers of steel and reinforced concrete framed buildingsconsidered at risk of attack should consider the importance ofensuring the strength of connections substantially exceeds that ofthe beams for the reasons stated above. Careful considerationshould also be given to the detailing of connections in order toensure ductility. Connections for steel and concrete framed struc-tures designed to resist seismic loads are likely to have a goodability to resist blast loading. A range of sensible details for steelframes subjected to blast are contained in Part 5 of the U.S. De-partments of the Army, Navy and Air Force �1991� code TM5-1300. Importantly these details avoid the use of bolts in tensionand they concentrate on providing continuity in load paths. Struc-tural grade steels can harden by 50% under the high rates of strainproduced during blast. Moreover, it was widely believed that highstrains also increased the strength of bolts in tension. The recentanalysis of the response of standard structural grade bolts sub-jected to rapid rates of loading shows this not to be the case�Munoz-Garcia et al. 2005�. The tests revealed that high strainrates cause a significant reduction in both tensile strength andductility, with failure exclusively via thread stripping. Suchbrittleness under high strain rates is also observed in butt welds.This strain rate weakening combined with strain rate hardeningfor plate material can be expected to reduce the ductility of jointsand lead to brittle failure mechanisms for many popular structuraldetails used in nonseismic regions. Brittleness of bolts can bepartially overcome by the use of stainless steel bolts, sinceMunoz-Garcia et al. �2005� have recently shown that stainlesssteel bolts harden under high rates of strain.

Conclusions

The surveys of bomb damaged buildings in London during WWIIshow that most multistory buildings of that time possessed animpressive ability to survive damage from the effects of highexplosive bombs. Buildings generally had closely spaced columnsby modern standards. They used masonry panel walling for inter-nal partitions and cladding. These provided emergency shear re-sistance in the event of often substantial damage to framingsystems.

In comparison many modern commercial buildings may bemore susceptible to progressive collapse following column dam-age, as witnessed during the attack on the Murrah Building in1995. Such buildings often incorporate wider column centerlines.More importantly, commercial office buildings no longer incorpo-rate stiff masonry partition walls and cladding. Designers of highprofile buildings considered at risk of attack should consider theprovision of emergency load paths for the redistribution of dam-aged column loads. Such load paths could take a form similar tothat of the outrigger trusses installed in the top floors of the WorldTrade Center Twin Towers, trusses that played a vital role inredistributing loads from the damaged perimeter columns.

History shows that progressive collapses are often triggered bythe failure of beam to column connections, which often fail in abrittle manner when subjected to blast. Research has shown thatconventional design standards may create overstrong beams con-nected together by relatively weak and brittle connections. More-over, components such as bolts have been shown to weaken under



high rates of strain. In order to minimize the likelihood of pro-gressive failures designers may choose to strengthen the beam tocolumn connections located at close proximity to potential ve-hicle borne devices. Such connections should avoid reliance onbolts and welds that are placed in direct tension.

Conventional beams have been shown to possess a signifi-cantly higher flexural strength than that assumed during design.Connections should be designed to support these higher loadsrather than the much lower loads normally assumed duringdesign.

Finally, vehicle borne improvised explosive devices arecapable of destroying all framing systems located at closeproximity. Therefore the only truly effective means of protectinga structure at serious danger of attack is to maintain a safe dis-tance between vehicles and buildings.

Acknowledgments

This paper extends work previously published in the InternationalConference on Forensic Engineering, London in 2005.

References

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Byfield, M. P. �2004�. “The economic design of multi-storey steel framedbuildings to resist terrorist attack.” Struct. Eng., 31–38.

Byfield, M. P. �2005�. “Prevention of progressive collapses due to terror-ist attacks.” Proc., 3rd Int. Conf. on Forensic Engineering: Diagnos-ing Failures and Solving Problems, Taylor and Francis, London, 233–240.

Federal Emergency Management Agency �FEMA�. �2003�. “Buildingdamage.” Risk Management Series, Primer for design of commercialbuildings to mitigate terrorist attacks, Chap. 4, FEMA-427, Washing-ton, D.C.

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Robertson, L. E. �2005�. “Rising to the challenge.” Struct. Eng., 20–27.U.S. Departments of the Army, Navy and Air Force. �1991�. “Structural

Steel Design.” Technical manual TM5-1300: Structures to resist theeffects of accidental explosions, Chap. 5, U.S. Dept. of Commerce,National Technical Information Service, Washington, D.C.

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Documents

Behavior and Design of Commercial Multistory Buildings Subjected to Blast