1、26Steel Design GuideDesign of BlastResistant StructuresExterior GaugesPressureImpulseTime (msec)Pressure (psi)Impulse (psi-msec)26Steel Design GuideAMERICAN INSTITUTE OF STEEL CONSTRUCTIONDesign of Blast Resistant StructuresRAMON GILSANZ, Lead AuthorGilsanz Murray Steficek LLPNew York, New York comp
2、ression mem-bers were observed to be distorted by up to 2 in., indicating incipient buckling. Improper design of lat-tice compression braces caused total failure of the partially constructed bridge. Ronan Point, 1968, UK. Small kitchen explosion caused partial collapse of 20 stories of a corner of a
3、n apartment building. Hartford Coliseum, 1978, Hartford, CT. Long-span space frame collapsed under a moderate snow load (less than 20 psf). Compression members had been improperly designed and the failure propagated through the entire arena. LAmbiance Plaza, 1987, Bridgeport, CT. Collapse of two adj
4、oining buildings that were under construc-tion using the lift slab method. Triggered by loss of support of a slab at a column. 28 workers killed. Col-lapse propagated because final connections had not yet been made. Hyatt Regency Walkway, 1981, Kansas City, MO. Revised connection of hanger rods to f
5、raming had not been designed by a structural engineer. One con-nection failed and the lack of redundancy caused the complete collapse of both levels of walkways. Killed 114 people. World Trade Center 6, September 11, 2001, New York, NY. Several floors collapsed due to fire. The collapse was arrested
6、 by floors that were not on fire. World Trade Center 7, September 11, 2001, New York, NY. A fire caused the failure of a key structural member that resulted in the collapse of the entire building.Progressive collapse failures may be due, in part, to con-crete punching shear. Concrete codes now have
7、structural integrity reinforcement that addresses this type of failure. Examples of concrete structures that have collapsed are: 200 Commonwealth Avenue, 1971, Boston, MA. A 17-story concrete high-rise under construction. Four workers were killed and 20 injured. Skyline Plaza apartment building, 197
8、3, Fairfax County, VA. Collapsed during construction killing 14 workers; 34 others were injured. Cocoa Beach Condominium, 1981, FL. Collapsed during construction, killing 11 workers, and injuring 23 others.AISC DESIGN GUIDE 26/ DESIGN OF BLAST RESISTANT STRUCTURES / 31.2 CHARACTERISTICS OF BLAST EFF
9、ECTS An air blast creates a supersonic shock wave, increases the ambient air pressure in the environment, and may generate high velocity fragments due to the destruction of the con-tainer that holds the charge. The explosion can happen in an enclosed or open space. In the open there is no confine-me
10、nt of the explosives; therefore, there is no increase of air pressure due to confinement and venting is not relevant. In an enclosed space, venting the explosion byproducts is important.Blast loads are different from the typical loads familiar to structural engineers due to their large magnitude and
11、 short duration. The speed with which a blast load is applied exceeds the loading rate of an earthquake by several orders of magnitude. Blast pressure may exceed hundreds and even thousands of pounds per square inch, but last only a hun-dredth or even a thousandth of a second. The structure is desig
12、ned to absorb the energy from the blast. Designers use plastic design with ultimate dynamic strengths without load factors, capacity reduction factors, or safety factors. Due to the nonlinear nature of the response, member failure is characterized by large deformations and/or rotation. Further, the
13、engineer must ensure that failure of members closest to the blast will not cause a failure that propagates to elements outside the area directly affected by the air blast loading. If members outside the area fail, a progressive collapse of the structure may be generated. To prevent progressive colla
14、pse, the structure should be sufficiently redundant to allow for load redistribution or members must have sufficient strength to preclude failure.The patterns of blast damage on a particular structure will vary greatly due to several factors: Type/variety of construction, including materials, mass a
15、nd stiffness Type of explosive Standoff distance between the charge and the structure Orientation of the charge to the structure Orientation of other structures surrounding the tar-geted structureStructural damage from a blast varies significantly with distance from the charge, robustness of the str
16、ucture, and characteristics of the material. Blast pressure drops signifi-cantly with increased distance and the resulting response is correspondingly decreased. Structural damage also lessens with increased robustness and increased material ductility. An example of these effects is the bombing of t
17、he Mur-rah Federal Building in Oklahoma City, OK, where many nontargeted buildings in the vicinity of the targeted build-ing sustained significant damage from the blast. During the event, buildings up to 800 ft away from the charge expe-rienced varying levels of structural collapse, largely due to t
18、he lack of robustness. Damage varied significantly based on the building construction and the distance from the blast. In addition, windows were broken in many buildings through-out the downtown area within a 1-mile radius from the charge. The occurrence of breakage decreased, in general, with incre
19、ased distance from the blast.There are many different types of explosives, but 1 lb of trinitrotoluene (TNT) is universally used as a standard mea-sure of effectiveness of explosive materials. Homemade explosives such as ammonium nitrate with fuel oil (ANFO) are less powerful than TNT, and thus equi
20、valent weights of other explosive materials would have less effect than TNT. Some military grade explosives, such as C-4 and pentolite, produce more powerful effects using the same weight of material. TNT equivalence is a commonly used metric due to the lack of detailed information available for oth
21、er materi-als. TNT weighs about 100 lb/ft3. This means that the vol-ume of TNT corresponding to 10,000 lb is 100 ft3, which can be visualized as a 6-ft by 2-ft closet in the average home (6 ft)(2 ft)(8 ft) = 96 ft3. When an explosive device is located very close to a struc-ture, both localized and g
22、lobal damage to the structure may occur. Localized damage may consist of flexural deforma-tion, breaching (e.g., the pulverization of the material), and collapse of primary structural elements and wall systems in the immediate vicinity of the blast. As the distance from the blast increases, localize
23、d damage transitions to more wide-spread damage consisting primarily of broken windows and failure of weaker building components comprising the build-ing envelope.Varying levels of damage to a structure may also be seen as the orientation of the charge to the structure changes. In a uniformly constr
24、ucted building, the side of the building directly facing the blast will experience a higher load and more damage than the sides which are not facing the blast. The sides not facing the blast will experience an incidental loading from the blast, which will be lower than the direct reflected loading a
25、pplied to the side facing the blast.Structures in the vicinity of the targeted structure may also affect blast patterns but to a lesser extent than the items listed above. A structure located between the explosive charge and the targeted structure will reduce the peak reflected pressure on the targe
26、t structure. However, it should be noted that only under ideal circumstances will the reduction be significant. In many cases, the shock wave will re-form (almost to its original strength) over the distance between the structures. In certain instances, surrounding structures may even reflect and amp
27、lify the loads seen by the targeted structure. In gen-eral, however, the first shock loading (not subsequent reflec-tions) will control the level of damage.4 / DESIGN OF BLAST RESISTANT STRUCTURES / AISC DESIGN GUIDE 26deformation without loss of load carrying ability. Since structural steel can wit
28、hstand large inelastic deformations, it is frequently used in the design of primary structural systems for buildings designed either to resist blast or seismic loads.Although there are many similarities between design for seismic resistance and design for blast resistance, there are also a number of
29、 differences. Earthquake loads are transmit-ted to the structure via ground shaking and blast loads are transmitted through a pressure wave that hits the envelope of a building first and subsequently is transmitted through load resisting members of the building to the foundation. Seismic response in
30、volves a global response of the structural system originating in the foundation and blast begins as a local response of a few structural elements. The response of seismic loads is measured by stresses and displacement, while the response of blast loads is measured by ductility and rotation. The dura
31、tion of blast loading is much shorter than the duration of seismic loading. Typical pressure waves produced by blasts will have durations on the order of tens of milliseconds, while typical seismic loading of a structure 1.3 BLAST EFFECTS VERSUS SEISMIC EFFECTSThere are many similarities between the
32、 effects of blasts on structures and the effects of earthquakes. Both phenomena are dynamic in nature and as a result, the amount of force and deformation experienced by a structure depends sig-nificantly on the dynamic characteristics of the structure. Designs for both blast resistance and seismic
33、resistance usually anticipate that the structure will undergo substan-tial nonlinear response under design loading and that some structural elements will be damaged, perhaps to the point of failure. Due to the infrequency and magnitude of both types of loading, extensive damage is usually considered
34、 acceptable as long as the building response does not result in extensive endangerment of life safety. Because substan-tial nonlinear response is anticipated for both phenomena, good design practice often entails the use of materials and detailing practices that are capable of developing the yield s
35、trength of the structure and experiencing extensive inelastic Fig. 1-1. Pressure gauge trace from high-energy explosive detonation.AISC DESIGN GUIDE 26/ DESIGN OF BLAST RESISTANT STRUCTURES / 5elements and, depending on the design criteria, may permit collapse of limited areas of a building. Followi
36、ng the blast-induced collapse of the Murrah Fed-eral Building in Oklahoma City in 1995, investigators sug-gested that if the more ductile detailing practices commonly used in regions of high seismicity had been incorporated in the design the building may have been substantially more resistant to col
37、lapse and there may have been fewer fatalities. While this may be true in the case of that particular build-ing, design for seismic resistance alone will not, in general, provide sufficient resistance to arrest progressive collapse or ensure acceptable response under blast loads.will have a duration
38、 extending from seconds to several min-utes. Blast impulses typically produce one phase of signifi-cant positive loading and one phase of negative loading that may or may not be significant. Seismic loading will typically include many cycles of loading, making low-cycle fatigue a more significant fa
39、ctor. For comparison, Figure 1-1 and Figure 1-2 illustrate a load history for blast and earthquake effects, respectively. Note the difference in the time scales. Design for seismic loading typically attempts to preclude failure of primary vertical load carrying elements and avoids any type of collap
40、se. Design for blast resistance often antici-pates failure of one or more primary vertical load carrying Fig. 1-2. El Centro earthquake ground accelerations.6 / DESIGN OF BLAST RESISTANT STRUCTURES / AISC DESIGN GUIDE 26AISC DESIGN GUIDE 26/ DESIGN OF BLAST RESISTANT STRUCTURES / 7Chapter 2 Blast Lo
41、adsThis chapter provides an overview of key characteristics of blast loads, including types of explosion hazards and meth-ods for predicting magnitude and duration. Methodology for load prediction is reviewed along with the types of tools and data typically used. Guidelines are included for appli-ca
42、tion of loads to structures, key parameters required, and limitations. This chapter will give the structural engineer the background necessary to specify blast prediction require-ments and apply the results but it is not intended to be a comprehensive methodology for complex problems. Exte-rior and
43、interior blast loads are addressed as well as leakage pressures into a structure due to openings in or failure of the building envelope. A detailed example problem is included to guide the user through the load prediction process for a simple building.This chapter provides an overview of the types o
44、f explo-sions that may be encountered by the designer and the meth-odologies used to define blast loads. Development of design basis loads for blast resistant construction is a key element of the design development phase of a project. The most important factors in blast design are explosive type and
45、 size, location of the explosion relative to the building, and the building geometry. Blast loads vary spatially and decrease rapidly with distance, even over the surface of a wall. Loads are influenced by geometric configuration, which provides shielding and reflection. References are provided for
46、more detailed explanations of methods and design aids. Some projects have project-specific predefined blast loads (pres-sure and impulse).2.1 EXPLOSION PARAMETERSBlast loads from high energy explosives may occur due to accidental or intentional detonations. Accidents involving high energy explosives
47、 can include explosives processing and handling events. Intentional detonations can include controlled demolition, explosives testing, military weap-ons and terrorist threats. In the case of intentional detona-tions, structures may be required to withstand multiple events, such as with a test struct
48、ure. These events produce supersonic reaction fronts. For convenience in predicting blast pressures, the energy release of a high energy explo-sive is equated to trinitrotoluene (TNT). TNT equivalence values for peak pressure and impulse are reported for many explosive compounds. TNT equivalencies f
49、or many com-pounds are published in Unified Facilities Criteria 3-340-02, Structures to Resist the Effects of Accidental Explosions (DOD, 2008), but in some cases they must be determined experimentally for the configuration used. The selection of effective charge weights, safety factors on blast loads, and allowable response criteria is discussed in DOD (2008) and Baker (1983).Fireworks, other pyrotechnics, propellants and blasting agents are broadly classed as low energy explosives due to their relatively low reaction rate and blast pressure output. These materials typical
