October 2003 • Modern Steel Construction

Anatol Longinow, Ph.D. and Farid Alfawakhiri, Ph.D.

GENERAL EXPLOSION SCIENCE (SEE REF. 1,2,3)What is an explosion? What are some common types of explosions?

An explosion is a rapid release of stored energy characterized by a bright flash and anaudible blast. Part of the energy is released as thermal radiation (flash); and part is cou-pled into the air as airblast and into the soil (ground) as ground shock, both as radiallyexpanding shock waves.

To be an explosive, the material: 1. Must contain a substance or mixture of substances that remains unchanged under or-

dinary conditions, but undergoes a fast chemical change upon stimulation.2. This reaction must yield gases whose volume—under normal pressure, but at the

high temperature resulting from an explosion—is much greater than that of the orig-inal substance.

3. The change must be exothermic in order to heat the products of the reaction and thusto increase their pressure.Common types of explosions include construction blasting to break up rock or to de-

molish buildings and their foundations, and accidental explosions resulting from naturalgas leaks or other chemical/explosive materials.

What is a shock wave? The rapid expansion of hot gases resulting from the detonation of an explosive

charge gives rise to a compression wave called a shock wave, which propagates

BBLLAASSTT RREESSIISSTTAANNTT DDEESSIIGGNN wwiitthh

SSTTRRUUCCTTUURRAALL SSTTEEEELLCOMMON QUESTIONS ANSWERED

Got questions

about blast-

resistant de

sign?

Check out this

handy tear-o

ut

reference!

Strategies for blast protection have become an important considera-tion for structural designers as global terrorist attacks continue at analarming rate. Conventional structures, particularly those abovegrade, normally are not designed to resist blast loads; and becausethe magnitudes of design loads are significantly lower than those

produced by most explosions, conventional structures are susceptible to dam-age from explosions. With this in mind, developers, architects and engineers in-creasingly are seeking solutions for potential blast situations, to protectbuilding occupants and the structures themselves.

The questions and answers that follow offer some explanation of explosionsand the potential dangers they present to steel-framed buildings. The authorstake a look at the historical response of steel-framed structures to blast situa-tions and which types of structural frames, connections and steel shapes bestresist blast loads. They also examine strategies designers can use to implementheightened building security and greater structural resistance to blast threats.Design specifications, code requirements, progressive collapse, seismic re-quirements and composite construction also are considered. Lastly, a list of ref-erences on the topic of blast protection is provided, along with informationabout computer software programs that can aid designers.

through the air. The front of the shockwave can be considered infinitely steep,for all practical purposes. That is, the timerequired for compression of the undis-turbed air just ahead of the wave to fullpressure just behind the wave is essen-tially zero.

If the explosive source is spherical, theresulting shock wave will be spherical. Sinceits surface is continually increasing, the en-ergy per unit area continually decreases.Consequently, as the shock wave travelsoutward from the charge, the pressure inthe front of the wave, called the peak pres-sure, steadily decreases. At great distancesfrom the charge, the peak pressure is infini-tesimal, and the wave can be treated as asound wave.

Behind the shock wave front, the pres-sure in the wave decreases from its initialpeak value. At some distance from thecharge, the pressure behind the shock frontfalls to a value below that of the atmosphereand then rises again to a steady value equalto that of the atmosphere. The part of theshock wave in which the pressure is greaterthan that of the atmosphere is called thepositive phase, and, immediately followingit, the part in which the pressure is less thanthat of the atmosphere is called the nega-tive or suction phase.

What is a deflagration? How does itdiffer from a detonation?

A deflagration is an exothermic reaction(a moving flame front), which propagatesfrom the burning gases to the unreactedmaterial by conduction, convection and ra-diation. In this process the combustion zoneprogresses through the material (flammablemixture) at a rate that is less than the speedof sound in the unreacted material. In con-trast, a detonation is an exothermic reactioncharacterized by the presence of a shockwave in the material that establishes andmaintains the reaction. A distinctive charac-teristic of detonation is that the reactionzone propagates at a speed greater than thespeed of sound.

Under proper conditions, flammable andcombustible gases, mists or dusts sus-pended in air or another oxidant can burnwhen ignited. This could cause a deflagra-tion-induced explosion to occur when thefollowing conditions are met: 1. The presence of fuel mixed in proper

proportions with the atmosphere (oxi-dant). Most gaseous fuels have lower-and upper-flammability limits for theirconcentrations in the air; and the con-

centration must be within these limits fora deflagration to occur.

2. The presence of air (oxygen) or other ox-idant. Higher oxygen concentrations ac-celerate the rate of combustion, and lowconcentrations of oxygen reduce it.

3. The presence of an ignition source withenergy output sufficient to initiate defla-gration. Ignition can result from a hotsurface, flame or spark. Location of theignition source at the geometric center ofa confined fuel-oxidant mixture results indevelopment of the highest pressure andrate of pressure rise.

4. The combustion of a gas must generatea pressure greater than the structural ca-pability (strength) of the confining struc-ture. An explosion occurs when theenclosing structure ruptures.

What are the damaging effects ofexplosions to structures? (see Ref. 5)

Conventional structures, in particularthose above grade, are susceptible to dam-age from explosions, because the magni-tudes of design loads are significantly lowerthan those produced by most explosions.For example, design snow loads in the Mid-west range from about 5 psf to about 50 psf.The peak pressure in the blast pulse pro-duced by 10 lb of TNT at a range of about50’ is approximately 2.4 psi (which is 348psf!) with a duration of the positive phase of7.7 ms. Conventional structures are not nor-mally designed to resist blast loads.

Recent terrorist attacks demonstrate thetypes of damage that can be produced. The1993 terrorist attack on the World TradeCenter in New York City removed severalthousand square feet of concrete floor slabsin the general area of the explosion and se-verely damaged several buildings’ commu-nication, transportation and utility systems.Due to the inherent redundancy of the steelframes, the structures did not collapse.

The 1995 attack on the Alfred P. MurrahFederal Building in Oklahoma City revealedthe vulnerability of conventional structuraldesigns when subjected to blast loads. Whena weapon is located at street level, the blastshock wave acts up against the underside ofthe floor slabs at upper stories. Floor slabsare not designed for this magnitude and di-rection of load—for this direction of load, thereinforcement is in the wrong place.

PHYSICAL SECURITYWhat are the general objectives of defen-sive design involving a terrorist attack?

The main objective of defensive (pro-tective) design of a civilian facility is to

minimize casualties and damage. Lifesafety should be the primary design pa-rameter. In certain situations it is also nec-essary to provide for the functionalcontinuity of the facility. For example, ahospital must function after an attack inorder to provide services for critical pa-tients. Similar requirements apply to fireand police stations. While it is impossibleto design all buildings against all threats, itis possible to design some buildings to beresistant to some threats. Defensive designoften conflicts with aesthetics, accessibil-ity, fire safety regulations and budgetaryconstraints.

What defensive strategies can beemployed to reduce risks of terroristattacks involving explosions?

The first step in the defensive designprocess is to establish the probable riskand the parameters of the threat to a facil-ity. Risk of “collateral damage” to nearbybuildings should also be considered. It isthen possible to consider countermeasures(defensive strategies) to the threat. Com-mon external blast threats are car, van ortruck bombs. Internal blast scenarios in-volve a smaller explosive charge packed ina letter or a brief case, or a car bomb in aparking garage.

One way to protect a building from apossible attack is to make weapon deliverydifficult. A set back distance and a securefence around the building can serve thispurpose. However, this approach often isnot viable in a city where buildings adjoinother buildings along busy streets. In thesecases, measures such as surveillance, lim-its on traffic movement and guards can en-hance protection.

In the design of upgrades and retrofitsof existing facilities, countermeasures thatinvolve establishing a defensive perimeter(fences, bollards, etc.) and positioning thebuilding at some distance from this secureperimeter often are not possible. Instead,threat countermeasures include the reloca-tion of important functions to safer areasof the building. Other measure includehardening the mail area, moving peoplefrom external walls to inner offices, replac-ing or strengthening windows and windowframes, hardened safety rooms, hardeningportions of the building, or moving the en-tire operation to a more secure facility. Inall circumstances, defensive strategiesmust incorporate some measures of facil-ity-access control, contingency planningand emergency training for all occupants.

Modern Steel Construction • October 2003

What is the difference between physicaland operational security measures?

Physical security measures, also calledpassive security measures, include actionssuch as perimeter protection with walls,fences, bollards, planters and intrusion-de-tection alarms. It also includes actions likehardening the structure or portions thereofto mitigate blast effects if perimeter protec-tion is not sufficient.

Operational security measures, alsocalled active security measures, involve ac-tions such as intelligence, surveillance andguards.

What specific blast effects areconsidered in defensive structuraldesign?

As mentioned previously, in an explosionproduced by a vehicle bomb, part of the en-ergy is released in the form of thermal radi-

ation, and part is coupled into the air as airblast and into the ground (soil) as groundshock.

For above-grade structures subject tosurface attack and airbursts, air blast is theprimary mechanism producing the potentialfor damage and casualties, and this is theloading that is used in design.

For buried or below-grade structures,depending on weapon yield, ground shockcan be an additional design effect.

What is a “stand-off” distance? What isa “height of burst” (HOB)?

Stand-off distance refers to the direct,unobstructed distance between a weaponand its target. Height of burst refers to aer-ial attacks. It is the direct distance betweenthe exploding weapon in the air and the tar-get. For a bomb capable of being detonatedabove a target, an optimum height produces

the maximum coverage by a given level ofpressure, resulting in maximum damage.This is referred to as the optimum HOB.

How large are design blast loads? Howare blast loads evaluated?

Selection of the blast charge size W isbased on the perceived risk to the designbuilding and any buildings nearby. Variousfactors play a role here, such as the socialand economic significance of the building,security measures that deter terrorists, anddata from previous attacks on similar facili-ties. The minimum standoff distance R isdetermined from the layout of a building’ssurroundings and reflects the expectation ofhow close to the building the design chargecould explode.

W and R are two necessary inputs forthe scaled distance parameter Z = R/W0.33

that is used to determine “equivalent” de-

October 2003 • Modern Steel Construction

Textbooks and ManualsNewmark, N. M. An Engineering Approach to Blast Resistant

Design. University of Illinois Engineering Experiment Station,Reprint Series No. 56, 1953

Norris, C. H., Hansen, R. J., Holley, M. J., Jr., Biggs, J. M.,Namyet, S., Minami, J. K. Structural Design for DynamicLoads. McGraw-Hill Book Company, 1959

Biggs, J. M. Introduction to Structural Dynamics. McGraw-HillBook Company, 1964

“Fire and Blast – Criteria and Loading,” Technical Committee 8,Planning and Design of Tall Buildings, International Confer-ence on Planning and Design of Tall Buildings, Lehigh Univer-sity, Bethlehem, Pennsylvania, August 21-26, 1972

Glasstone, S., Dolan P.J., (Editors). The Effects of Nuclear Weapons.Prepared and Published by the U.S. Department of Defense andthe U.S. Department of Energy, Third Edition, 1977, Reprintedby the Federal Emergency Management Agency

“Design of Structures to Resist Nuclear Weapons Effects,” ASCE –Manuals and Reports on Engineering Practice – No. 42, 1985

TM 5-855-1, “Fundamentals of Protective Design for ConventionalWeapons,” U.S. Department of the Army, November 1986 (www.military-info.com/MPHOTO/p021c.htm)

TM 5-1300 Structures to Resist the Effects of Accidental Explo-sions,” U.S. Departments of the Army, Navy and Air Force,November 1990(www.military-info.com/MPHOTO/p021c.htm)

“Design of Blast Resistant Buildings in Petrochemical Facilities,”ASCE Task Committee on Blast Resistant Design, 1997

UFC 4-010-01, DoD Minimum Antiterrorism Standards for Build-ings, Department of Defense, 31 July 2002 (www.tisp.org/files/pdf/dodstandards.pdf)

Documents Pertaining to Blast Effects, Structural Design andPeople Survivability in a Blast EnvironmentAssheton, R. (Compiler). History of Explosions – On Which the

American Table of Distances Was Based, Including Other Ex-plosions of Large Quantities of Explosives. Bureau for theSafe Transportation of Explosives and Other Dangerous Arti-cles, 1930

Longinow, A., Hahn, E.E., Bertram, L. A.,”Personnel Survivabilityin a Blast Wind Environment,” Journal of the Engineering Me-chanics Division, ASCE Proc. 103 (EM2), April 1977

Longinow, A., Chu, K. H., Thomopoulos, N. T., “Probability ofSurvival in a Blast Environment,” Journal of the EngineeringMechanics Division, ASCE Proc. Paper 16991, April 1982

American National Standard for Estimating Airblast Characteris-tics for Single Point Explosions in Air, With a Guide to Evalua-tion of Atmospheric Propagation and Effects. S2.20-1983

Protection of Federal Office Buildings Against Terrorism. Commit-tee on the Protection of Federal Facilities Against Terrorism,Building Research Board, Commission on Engineering andTechnical Systems, National Research Council, National Acad-emy Press Washington, D.C. 1988

“Structures for Enhanced Safety and Physical Security,” Proceed-ings of the Specialty Conference sponsored by the StructuralDivision of the American Society of Civil Engineers, Arlington,Virginia, March 8-10, 1989 (Edited by T. Krauthammer)

Protecting Buildings From Bomb Damage—Transfer of Blast-Ef-fects Mitigation Technologies From Military to Civilian Appli-cations. National Academy Press, Washington, D.C., 1995

Open-Source References on Blast Design

The following list includes documents specific to the question asked as well as documents that are useful as background informa-tion on explosions, blast-resistant structural design and survivability of building occupants in blast environments.

sign pressure impulses using publishedcurves [see Ref.10]. For greater accuracy,computer programs such as AT Blast areavailable for free download atwww.oca.gsa.gov.

Blast loads are applied to externalbuilding cladding if it is assumed to trans-fer the loads to structural elements.Where windows, doors and external wallsare not expected to remain intact, blastloads also should be applied to internalstructural elements. Floor slabs especiallyshould be checked for uplift-pressure im-pulse. Blast loads usually are not factoredand used in combination with unfactoredgravity loads.

What are the most popular and cost-effective methods for upgrading existingbuildings for physical protection?

Some level of blast resistance is re-quired for new Federal Buildings. ExistingFederal Buildings undergoing expansionalso must include blast resistance. In eachcase the General Services Administration(GSA) establishes design requirements.Specific actions can involve: protectingwindows; installing a secure perimeterfence and/or hardening a portion of thebuilding; and determining the likelihood ofprogressive collapse and designingagainst it. There is no comparable, univer-sal guidance in the civilian sector. How-ever, some of the guidance developed bythe Federal Government is available to thegeneral public.

BLAST RESISTANTSTRUCTURAL DESIGNWhat is the historical experience withsteel-framed structures subjected toBlast?

A study of 17 British buildings hit byGerman bombs during World War II exam-ined eight steel-framed buildings, five re-inforced concrete buildings and fourwall-bearing buildings (see Ref. 6). Thesteel-framed buildings included office,apartment and industrial buildings, and atwo-story railway station.

The weight of bombs ranged from 110lb to 3,100 lb. In each case the chargeweight was approximately 50 percent ofthe bomb weight. With one exception allwere internal explosions and the type ofdamage was fairly typical.

One example is the explosion damageto a seven-story apartment building. Thisbuilding consisted of a concrete-encasedsteel frame (for fire protection). The floorswere 6” hollow tile with 3.5” concrete top-

ping, supported on steel beams and gird-ers. Exterior walls consisted of 9” brickand tile facing. Interior walls were 3” brickwith plaster surfaces.

A 1,100-lb bomb perforated the roofand three floors and detonated just abovethe fourth floor. Damage to the seventhfloor consisted of a failed girder due toimpact from the bomb and about 100 sq.ft of floor area removed. Damage to thesixth floor included a buckled girder withtorn out connections, several deflectedbeams and approximately 190 sq. ft offloor area removed. On the fifth floor, onegirder deflected about 7”. Several otherfloor beams were bowed. Approximately650 sq. ft of floor area was demolished.On the fourth floor, one girder was blowndown together with four beams. One col-umn deflected 7” and twisted, and about700 sq. ft of floor area was demolished.The fifth floor was blown up; the fourthwas blown down. One bay on each of thefirst, second and third floors is believed tohave collapsed due to weight of debrisfrom above. There was no fire. Due to thesufficient redundancy of the steel frame,the building did not collapse.

Another example of a steel-framedbuilding subjected to an internal explosionwas the World Trade Center on Feb. 26,1993. A van containing approximately1,800 lb. of fertilizer-based explosives wasparked on an exit ramp just south of col-umn 324, one of the main steel columnssupporting the 110-story tower structure.The column measured about 4’ by 4’across. It and six adjacent columns losttheir fireproofing and lateral restraint (thebracing provided by the concrete floorsthat were blown out around them), butotherwise were not damaged by the explo-sion. The fact that the column did notbuckle from the significant increase in itseffective length speaks well for the redun-dancy in a building that probably was notdesigned for blast loading.

How different are seismic and blasteffects on structures?

The first difference is in the way agiven structure is loaded. In the case of anearthquake the structure is subject toground motions that shake the structurefrom the base up. In the case of an explo-sion produced by an air or a surface burst,the structure is loaded by means of acompression wave (shock wave) oversome area. Since a portion of the blast en-ergy is coupled into the ground, the struc-ture is also subject to ground motions

similar to an earthquake, though muchless intense.

A second difference is the duration ofloading. For earthquakes, the duration ofinduced motions (shaking) can rangefrom seconds to minutes. Additional load-ings are produced by “aftershocks,”which are generally less intense than theinitial shaking. For conventional explo-sives, the duration of a pressure wave ison the order of milliseconds.

For example, in the Oklahoma Cityevent, the yield of the weapon was ap-proximately 4,000 lb TNT equivalent. Thetruck containing the explosive was posi-tioned about 10’ from the building. Thepeak pressure at the face of the buildingswas about 1,900 psi, and the duration ofthe positive phase of the pulse was ap-proximately 3 ms. Judging by the size ofthe crater, a fair portion of the energycoupled into the ground, producingground shock. However, judging by thedamage, clearly air blast was the primarydamage mechanism. Further, earthquakesshake an entire building, but producemostly horizontal loads at floor-slab lev-els, concentrating in the specially de-signed, laterally stiffer structural systems.Blast usually does not attack the entirestructure uniformly, but produces themost severe loads to the nearest struc-tural elements, both vertical and horizon-tal, with little regard to their stiffness.Uplift pressure load on floors is also aspecific blast effect.

What is the role of structural ductilityin blast resistance?

The term ductility refers to the abilityof the material to absorb energy inelasti-cally without failure—the greater the duc-tility, the greater the resistance to failure.Blast-resistant designs often conserva-tively assume elastic response in order tosimplify design, minimize permanent(plastic) deformations, and reduce post-blast repairs, especially where functionalcontinuity of the facility is considered.Due to their highly ductile features, struc-tural steel frames provide additional ulti-mate resistance for a blast eventexceeding in severity the design blast.

Ductile inelastic structural responsecan be expected during both severe blastand severe earthquake events. However, itis generally recognized that plastic hingezones and ductility demands in the twoevents do not necessarily match becauseof the differences in the loading patternsand effects.

Modern Steel Construction • October 2003

Does the mass of the structural frameplay a role in blast-resistant design?

Yes. The inertia, as measured by themass of the structure or structural member,is an important factor in the response to adynamic-impulse lateral load such as ashock wave. Because steel is the mostdense construction material, heavy and ro-bust steel members are especially effectivein resisting blast loads. This is evident in theperformance of heavy tanks and battleships,the ultimate blast-resistant structures.

Do building codes require structures tobe blast resistant?

For ordinary buildings, like apartments,offices, and stores, building codes do notrequire blast resistance. For buildings thathouse hazardous processes, building codesrequire special safety considerations. Forexample, the Uniform Building Code statesthat “walls, floors and roofs separating ause from an explosion exposure shall be de-signed to resist a minimum internal pres-sure of 100 pounds per square foot inaddition to other conventional loads.”

Which is better at resisting blast loadeffects—a moment frame or a bracedframe?

The lateral stability of a moment frame isdependent on the bending stiffness of rigidlyconnected beams and columns. Adequatediagonal bracing or shear walls at selectedlocations provide the lateral stability of abraced frame. Elements of lateral stabilityoften are distributed more uniformly in mo-ment frames, in which case each part of thebuilding is more likely to be stable on itsown. Therefore, moment frames are the bet-ter choice for blast-resistant design. Inbraced frames, the diagonal braces or shearwalls can be knocked out by an engulfingblast wave, reducing the effectiveness of thebraced frame, unless special features are in-cluded to mitigate this potential behavior.

PROGRESSIVE COLLAPSEWhat is progressive collapse?

Progressive collapse is the propagation,by a chain-reaction, of a local structural fail-ure into the failure of a substantial portion ofthe building, disproportionate in magnitudeto the original failure.

What events caused progressivecollapse incidents in the past?

The 1968 failure of one corner of a 23-story residential precast concrete buildingin London (Ronan Point) was caused bypoor connection detailing and was triggered

by an explosion from a gas deflagration. Inthe aftermath, the UK introduced buildingregulations addressing progressive col-lapse. In North America, some examples ofprogressive collapse include the 12-storysteel-framed Union Carbide office building,in Toronto, 1958; a 16-story cast-in-placereinforced concrete apartment building,Boston, 1970; and a 16-story post-ten-sioned concrete lift-slab building in Bridge-port, CT, 1987.

The Alfred P. Murrah Federal Building inOklahoma City was a dramatic example ofprogressive collapse of a weakly redundantreinforced-concrete building, with collapsetriggered by the vehicle bomb near the frontof the building. As mentioned earlier, thebuilding had minimal resistance to upwardloads generated by the blast at street level.

What is robustness? How can one addrobustness to a building?

When referring to a building, the termrobustness implies the strength and sturdi-ness to resist excessive loads. A highly re-dundant steel-framed building can beconsidered robust.

It is more difficult and expensive to addstrength to an existing building than to con-sider this aspect in a new design, especiallyfor high-rise buildings. One notable buildingthat has undergone strengthening (an in-crease in its robustness) is the CiticorpBuilding in New York City. After the buildingwas built, it was discovered that it would notlikely survive a particular wind condition.The building was strengthened effectively,but at a significant expense.

What official design specifications existfor reducing the risk of progressivecollapse?

The sector of our economy that re-searches the protection of governmentbuildings from terrorist attack and mitigatesprogressive collapse of these buildings isthe General Services Administration (GSA),Department of Defense (DoD) and their con-tractors. GSA and DoD have developedguidelines for the protection of buildingsagainst blast effects. Civilian-sector engi-neering firms that work for GSA on FederalBuildings receive these guidelines as dic-tated by a particular project. Some of theseare available to the general public.

The GSA’s “Progressive Collapse Analy-sis and Design Guidelines for New FederalOffice Buildings and Major ModernizationProjects,” is available for free download atwww.oca.gsa.gov. Also, “DoD MinimumAntiterrorism Standards for Buildings,” is

available for free download atwww.tisp.org/files/pdf/dodstandards.pdf.

STRUCTURAL MEMBERS ANDCONNECTIONSHow does high-rate loading, as producedby blast loads, affect steel properties?(See ref. 8, 9)

The yield stress of low-carbon struc-tural steel subjected to dynamic loadstends to increase. The ultimate strengthis less affected. Elastic modulus remainsthe same. Steels with higher-static yieldstresses achieve a lower percentage inyield-stress increase under dynamic load-ing, as do weaker steels.

For example, an experiment on struc-tural steel members consisting of mildsteel (static yield stress of Fy = 37 ksi) as-sociated with time to yield, showed dy-namic-yield stresses in the range of 45ksi and 50 ksi (an increase in the range of22 percent and 35 percent). In this series,the time to yield ranged from approxi-mately 1 s to 1 ms, and the fundamentalperiod of the respective structural mem-bers was approximately 100 ms. Forstructural members with fundamental pe-riods of less than 100 ms, test results in-dicated a dynamic yield stress of morethan 50 ksi.

What are the common ranges forsteel-deck gages and concrete-slabthickness in floors designed for blastresistance? Can lightweight concretebe used in blast-resistant design?

The traditional reinforced-concreteslab on top of steel deck, composite andnon-composite, is an efficient blast-re-sistant floor system. Concrete-slab thick-ness depends on the magnitude ofdesign-blast pressure, and the span be-tween supporting beams. Two layers ofreinforcement usually are required tosustain upward and downward loads.Steel deck can effectively prevent con-crete fragmentation. Steel-deck type andgage are selected to support constructionloads during concrete placement. Light-weight concrete is less effective in resist-ing blast loads than normal-weightconcrete.

In blast-design applications, what canbe done to ensure that concrete floorslabs do not separate from structuralsteel beams when subjected to upliftblast pressures?

One approach is to weld slab rein-forcement to connector studs (in com-

October 2003 • Modern Steel Construction

posite floors) or directly to steel supportbeams. Another option is to design andcast the beams integrally with the slabs.

What structural shape is the optimalchoice for beams in blast-resistantfloors?

The choice of structural members sup-porting a slab depends on the load magnitudeand where it is expected to act. If the blastload is expected only on the top of the slab,such as a slab over a basement, then either aW-shape or hollow structural section (HSS)is likely to be effective. If the maximum blastload is as likely to act on top of the floor slabas on its lower surface, then both shapes arelikely to be effective. When the underside isloaded, the support beams will be loadedboth on the bottom and on their sides. Thenet direct load on the webs of W-shapes islikely to be minimal. Where significant torsioneffects are likely, HSS are preferred for theirsuperior torsion resistance.

What types of column sections arepreferred in blast-resistant design?

Military manuals for blast-resistant de-sign base procedures on material propertiesincreased by approximately 10% to accountfor strain-rate effects. Columns designed toresist high blast loads usually have suffi-ciently small slenderness ratios, and buck-ling occurs plastically rather than elastically.Also, because dynamic-impulse load tendsto suppress the occurrence of buckling, it isconservative to adapt static formulas to thedynamic case. The choice of structuralshape will depend on a number of factors,like whether the column is subjected to anaxial load, or to flexural and axial load. Sincein the latter case the load can come fromany direction, it is useful to use a shape thathas equal flexural strength in all directions,such as a round or square HSS.

What types of steel-frame connectionsare effective in mitigating blast andprogressive collapse effects?

Both bolted and welded connections per-form well in a blast environment. If a weldedconnection can develop the strength of theconnected elements (or at least the weakestof the connected elements), the connectionwill remain intact. The same is true for abolted connection. However, welded con-nections need to be carefully detailed andconstructed.

With large members (especially in mo-ment frames), it can be difficult to developmember strength using bolts. However, cer-tain bolted connections, such as those using

top and bottom flange angles, can sustainsignificant inelastic deformations and some-times are preferred in blast-resistant design.

ANALYSIS METHODS ANDLITERATURE SOURCESWhat analysis methods are used in blast-resistant design?

Most structures are complex in behavioreven under static loads, and their responseto dynamic loads might include additionalcomplications from combinations of elasticand inelastic vibration modes.

A common approach to determine thedynamic response of a structure to somespecific loading is to model the structure asa system of finite structural elements andmasses connected together at a discretenumber of nodal points. If the force-dis-placement relationships are known for theindividual elements, structural analysis canbe used to study the behavior of the assem-bled structure.

It is prudent for practical design pur-poses to adopt approximate methods thatpermit rapid analysis of complex structureswith reasonable accuracy. These methodsusually require that both the structure andthe loading be idealized to some degree.

During the 1950s and 1960s, much workwas done to develop simple methods for thedesign of structures subjected to blast loadsproduced by blast from nuclear weapons.[The book by J. M. Biggs (Ref. 12) which isa revision of an earlier book (Ref. 11) writtenby several authors including J. M. Biggscontains an excellent introductory presenta-tion of such methods.]

What design/analysis software isavailable?

Blast design/analysis software for thegeneral public is not available at this time.Software and design manuals exist in theU.S. Government and military sector, butthese items generally are made availableonly to contractors doing work for U.S. Gov-ernment agencies, such as the U.S. ArmyCorps of Engineers and the General ServicesAdministration (GA).

As mentioned earlier, the software prod-uct AT Blast, deals with blast pressures andis available from the GSA website. There areprograms available for the dynamic-re-sponse analysis of single-degree-of-free-dom systems, such as Nonlin, which can bedownloaded for free at:

www.app1.fema.gov/EMI/nonlin.htm

Anatol Longinow, Ph.D., is a an adjunctprofessor of civil and architectural engineer-

ing at Illinois Institute of Technology inChicago, and an adjunct professor of civilengineering at Valparaiso University, Val-paraiso, IN. Farid Alfawakhiri, Ph.D. is Se-nior Engineer–Fire Design with AISC inChicago.

REFERENCES1. Fire Protection Handbook. National Fire

Protection Association, Thirteenth Edi-tion, 1969

2. Glasstone, S. and Dolan P. J. (Editors),The Effects of Nuclear Weapons. U.S.Department of Defense and the U.S. De-partment of Energy, Third Edition, 1977Reprinted by the Federal EmergencyManagement Agency

3. Guide for Explosion Venting. NFPA68–1978

4. Clayton S. White, “The Nature of theProblems Involved in Estimating the Im-mediate Casualties from Nuclear Ex-plosions,” CEX-71.1, Civil Effects TestOperations U.S. Atomic Energy Commis-sion, July 1971

5. Longinow, A., “The Threat of Terrorism –Can Buildings be Protected?” BuildingOperating Management, July 1995

6. “Effects of Impact and Explosion,” Sum-mary Technical Report of Division 2,NDRC (National Defense Research Com-mittee) Volume 1, Washington, D.C.1946

7. GSA Security Criteria. Building Technolo-gies Division, Office of Property Devel-opment, Public Buildings Service,General Services Administration, 1997

8. Air Force Design Manual, Principles andPractices for Design of Hardened Struc-tures. Research Directorate, Air ForceSpecial Weapons Center, Air Force Sys-tems Command, Kirtland Air Force Base,New Mexico, December 1962

9. Crawford, R. E., Higgins, C. J., Bult-mann, E. H. The Air Force Manual for De-sign and Analysis of HardenedStructures. Air Force Weapons Labora-tory, Kirtland Air Force Base, New Mex-ico, October 1974

10.“Structures to Resist the Effects of Acci-dental Explosions,” Dept. of the ArmyTech. Manual, TM5-1300, Dept. of theNavy Pub. NAVFAC P-397, Dept. of theAir Force Manual, AFM 88-22, June 1969

11.Norris, C. H., Hansen, R. J., Holley, M. J.,Biggs, J. M., Namyet, S., Minami, J. K.Structural Design for Dynamic Loads.McGraw-Hill Book Company, Inc. 1959

12.Biggs, J. M. Introduction to StructuralDynamics. McGraw-Hill Book Company,1964 �

Modern Steel Construction • October 2003