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1、ENGINEERING P A R T 3 CHAPTER 22 PROTECTIVE DESIGN OF STRUCTURES Richard L. Tomasetti, PE Co-Chairman, The Thornton-Tomasetti Group New York, New York John Abruzzo, PE Vice President, LZA Technology The Thornton-Tomasetti Group New York, New York Human history becomes more and more a race between ed
2、uca- tion and catastrophe. H.G. WELLS(1866-1946) English author and historian, Outline of History History is rife with violent incidents directed toward people, buildings, and property. The events of September 11, 2001, and others during the late twentieth century directed toward Americans at home a
3、nd abroad underscored the need for the design and construction community to reach even higher in the quest for advanced engineering techniques to save lives during a terrorist attack or disaster. From benchmark events such as the 1983 attack on the American embassy in Beirut, the 1993 bombing of the
4、 World Trade Center, and the 1995 bombing of the Alfred P. Murrah Federal Building in Oklahoma City, to subsequent attacks on public and private American facilities, embassies and installations overseas, building performance and the ability of a structure to with- stand blast have become two of the
5、more important issues that engineers, architects, and building owners must address. Engineering research after each terrorist event resulted in technological advances and evolving best practices designed to save lives, allow swift and complete building evacuation, and increase life safety. The event
6、s in Oklahoma City prompted an industry-wide examination of progressive col- lapse, reduction of the impact of flying glass, and minimization of damage to neighboring buildings during and after an event or disaster. Much of the testing and evaluation of building systems and construction materials ge
7、nerated after the Oklahoma City bombing was produced for the federal government, initially under the auspices of the U.S. Department of Justice (USDOJ), with the participation of several federal agencies. In June 1995, USDOJ published Vulnerability Assessment of Federal Buildings, assessing the exis
8、ting building inventory at the time. In October 1995, the Interagency Security Committee (ISC) was formed to create long-term design and construction standards for federal building security. The doc- ument, ISC Security Design Criteria for New Federal Office Buildings and Major Modernization Project
9、s, issued in May 2001, was based on the USDOJs earlier findings. 22.3 Although the security design criteria apply to federal facilities, the standards and mitigation strategies are applicable to state, local, and private sector buildings housing government agencies, government contractors, tenants r
10、equiring improved security levels, and owners seeking a greater degree of safety for their tenants and properties. With these standards, building owners, working with design and construction professionals, can develop a comprehensive security plan for a building or site. The security plan: Addresses
11、 risks, threats, design, operations, and technology requirements Identifies low probability, high risk threats to the building, known as abnormal loads Establishes the required structural performance Transparent Security Integrating design, operations, and technology into the security plan is the mo
12、st effective way to achieve transparent security, invisible to the public eye. Sound structural systems and protective design principles are essential elements of maintaining transparent security and enhancing the built environment. Design encompasses planning, programming, design, and construction
13、of physical protective barriers, such as walls, screens, floors, roofs, and standoff or the distance between the target and the blast threat. Operational security includes policies and procedures within a facility or organization. Operations address every aspect of emergency and disaster planning, a
14、nd the role of personnel to ensure the safety of people and property. Technology is an integral part of security planning, and it works best when integrated early in pro- ject development with design and operational policies to achieve operational and capital savings. Selection of appropriate detect
15、ors, sensors, surveillance cameras, and other technology is ulti- mately a decision made by building owners, often based on the advice of security consultants or in-house experts. PROTECTIVE DESIGN OF STRUCTURES Protective design of buildings is accomplished by integrating into the architectural and
16、 engineering building design program various means of mitigating threats, such as biological, chemical, and radi- ological attack, and force protection from blast, fire, ballistic attack, and illegal entry. Protective design of structures deals with the mitigation of force protection threats or abno
17、rmal loads acting on building structural framing and exterior walls. Protection is generally achieved through a combination of standoff, redundancy, and hardening. Standoff, in suburban settings, can be measured in hundreds of feet from public streets. Standoff of 350 feet or more is recommended for
18、 suburban sites, but is impractical in urban settings. However, even a few feet can make a substantial difference in terms of damage to the structure. Therefore, standoff in urban settings is no less critical to protective design of the structure. Redundant systems provide a means of surviving the u
19、nanticipated. Redundant structural systems are necessary for preventing progressive collapse. Hardening and energy absorptive shields can be used to enhance critical structural elements, walls, stairwells, loading docks, and windows where standoff alone is insufficient to reduce the threat to tolera
20、ble levels. While each of these three strategies can be effective as protective measures, project constraints usually demand a combination of these three measures to provide a solution. For instance, in most 22.4ENGINEERING urban environments, it is not practical to even consider a standoff of 350 f
21、eet. In urban centers, stand- off of even 20 feet can have appreciable cost. Therefore, hardening and redundancy need to be incor- porated into the design process to accommodate the effects of abnormal loads. ABNORMAL LOADS Abnormal loads on buildings may be caused by vehicular impact, blast loads f
22、rom accidental or pur- poseful explosions, or local failure due to fire. Once the threat is defined, the structure must perform to a level consistent with established performance criteria, such as preventing structural collapse of part or all of the building. Protection against abnormal loads incurs
23、 cost. To maximize the benefit of protection expense, the first step is determining where protection is needed. This is based on the assets housed in a building and the structural capabilities required for asset protection. For example, if assets are housed within a building in a hardened, undergrou
24、nd space, known as a bunker, they may not be vulnerable to failure of a structural member supporting the b ing entrance canopy. Hardening the entrance canopy to pro the security design goal of asset protection. However,the asset may not be protected if a street level blast destroys a column supporti
25、ng the b ing above, precipitating building collapse and destroying the bunker. In this case, increasing building resistance against collapse may be the most efficient means of increasing the protection level for the bunker and assets. The bunker is indirectly vulnerable to the failure of a street le
26、vel column, but not the failure of the canopy support. Identifying vulnerabilities requires establishing performance limits of critical and noncritical ele- ments. Performance limits discern a failed member from a functional member, or a failed system from a functional system. In the example, the fa
27、ilure of a street level column can affect the perfor- mance of the bunker. Risk assessment addresses the likelihood of potential threats and the structural vulnerability of a building. For example, the owner of a mid-rise building leasing space to a tenant threatened by a domestic organization with
28、a history of using pipe bombs as a weapon of choice could decide to minimize the risk of severe structural damage or collapse. If a determination is made that severe structural damage will not occur from a pipe bomb, the risk of threat may be high, but the struc- tural vulnerability to the potential
29、 threat is low. Operational security measures, such as increased security personnel, installation of surveillance cameras, and enhanced police presence may be suf- ficient deterrents to potential threats. Adding security film over windows or installing laminated glass may provide increased protectio
30、n by reducing the risk of hazard from breakage of street- level glazing. Determining likely threats against a building or site depends on the assets to be protected, the assets of adjacent facilities, and the profile of those likely to carry out potential threats (Table 22.1). Of the 199 internation
31、al attacks reported by the U.S. Department of State in the 2002 report Patterns of Global Terrorism 2002, almost 70 percent were bombings, and 50 percent of the targets were businesses. If a building is across the street from a target of organizations known to detonate vehicular bombs, the security
32、solution for a pipe bomb threat may be inadequate. Vehicular bombs pose a greater threat to buildings because structural systems are more vulnerable to collapse from a car bomb than a pipe bomb, and the extent of glass damage increases significantly beyond the street level. PROGRESSIVE COLLAPSE Eval
33、uating building performance against abnormal loads should consider the local impact of the threat and the consequences to adjacent structures and sites in the vicinity. For example, a bomb detonated on the street may affect the near column with a tremendous blast load. This single column member PROT
34、ECTIVE DESIGN OF STRUCTURES22.5 must then be assessed or analyzed for the expected level of damage. If the column is expected to fail, the consequences should be considered for the surrounding structure and adjacent structures. At minimum, progressive collapse or more precisely, disproportionate col
35、lapse, of the structure should be prevented. According to Minimum Design Loads for Buildings and Other Structures, SEI/ASCE 7-02: Progressive collapse is defined as the spread of an initial local failure from element to element even- tually resulting in the collapse of an entire structure or a dispr
36、oportionate large part of it. Some authors have defined resistance to progressive collapse to be the ability of a structure to accommodate, with only local failure, the notional removal of any single structural member. Aside from the possibility of further damage that uncontrolled debris from the fa
37、iled member may cause, it appears prudent to consider whether the abnormal event will fail only a single member. An example of limited local damage is containment of damage to adjacent bays and within two or three stories of a multistory structure. This damage may be the result of an explosion that
38、dam- ages the floors above and below the point of detonation and causes column failure. Restricting pro- gressive collapse requires containing the damage to areas directly loaded. Floors above should remain stable, yet may sustain damage due to loss of a column. Floor fram- ing in bays not immediate
39、ly adjacent to the affected framing should also remain stable. Lastly, lat- eral load systems provided to carry wind and seismic loads should not be depleted, but must have sufficient alternate load paths to remain viable systems. Vulnerabilities identified can be mitigated. Mitigation may include h
40、ardening to increase strength, ductility enhancements to increase energy absorption, the addition of alternative or parallel systems to create redundancy, or any combination of these methods. Building vulnerabilities should be iden- tified and performance levels assessed as part of the risk assessme
41、nt and security plan. STRUCTURAL SYSTEMS Buildings must resist several forces: Vertical loads due to gravity Lateral loads due to wind Load effects due to seismic motion Loads due to blast and impact 22.6ENGINEERING TABLE 22.1Description of Terrorist Organizations OrganizationDescription CONUS group
42、s Ethnic and white supremacy groups (continental U.S.) Generally threats are less severe than OCONUS terrorists Objectives include death, destruction, and publicity OCONUS groups Better organized and equipped than CONUS groups (outside continental U.S.) Attacks are more severe and more frequent than
43、 CONUS groups Paramilitary OCONUS groups Predominately ethnically or religiously based Military capabilities, frequently state sponsored Military and improvised weapons Most serious attacks including suicide attacks Predominately ethnically or religiously based Source: Adapted from Structural Design
44、 for Physical Security, ASCE, 1999.) Resistance is accomplished by integrating two load-carrying systems: Gravity load system Lateral load system Gravity Load System The gravity load system transfers or carries loads imposed by the forces of gravity. For steel or concrete-framed buildings, the load
45、path starts on the floor slab, which carries the load to floor beams or joists. The beams or joists carry the load to the girders, which carry the load to the columns, and finally, from columns to footings, and to the ground. Some concrete buildings consist of floor plates and columns without horizo
46、ntal beams. In these cases, the columns support the floor plate, as four legs support a tabletop. In either case, each element of the gravity load system has a gravity load influence area. Influence Area The influence area establishes the load path hierarchy. The influence area of a member is deter-
47、 mined by defining the total floor area that produces a load effect in the member upon the load- ing of that floor area. Influence areas are not restricted to a single floor, as with the case of columns or transfer girders. The influence area is important when assessing potential for structural dama
48、ge by failure of an element. Greater influence areas indicate more critical roles for the structural element. The influence area of a column can be an order of magnitude greater than for a floor beam or girder. The potential to cause substantial structural damage is much higher for failure of a colu
49、mn or bearing wall than for a beam or girder, although one is not always independent of the other. For instance, beams often provide lateral restraint necessary to ensure column stability. If the beams are no longer capable of providing this restraint, a failure of the column may ensue. The hierarchy of the potential to cause structural damage follows the influence area of the ele- ment, with the highest level typically being the columns, then girders, and lastly, floor beams (Table 22.2). PROTECTIVE DESIGN OF STRUCTURES22.7 TABLE 22.2Lessons Learned fr