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«R. SHANKAR NAIR R. Shankar Nair R. Shankar Nair, Ph.D., P.E., S.E. is a principal and senior vice president of Teng & Associates, Inc. in Chicago. In ...»

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Hardening a secure perimeter or entry Providing a secure perimeter takes the form of a combination of building steps, blast or sacrificial walls, and bollards. Glass in lobbies presents a challenge in trying to protect lobby occupants. Retrofitting with a puncture resistant interlayer and stronger glass lites usually is too costly. Reinforcement of just the glass can be short-sighted;

where the grip of the mullions to the glass is too small, a single large projectile can be created in place of many smaller projectiles.

SUMMARY

A comprehensive security risk analysis that is clearly communicated to the client is an essential prerequisite to the blast hardening of an existing facility. Often, non-structural security measures combined with partial blast hardening are the appropriate response to the perceived threat.

The existing architectural and building services systems and the operational requirements of existing facilities place practical limitations on the hardening of existing facilities.

The relative scale of the extreme event compared with the wind and seismic criteria for which an existing facility was designed may limit the potential hardening objectives that can be achieved. Similarly, the degree of ductility inherent in the existing structural framework represents another possible limitation.

Common partial hardening objectives include the protection of critical rooms or areas, the hardening of a secure perimeter or entry and the hardening of specific structural elements or systems that are required to maintain overall stability of the structure.

The best designs for blast hardening of existing facilities are based on a clear vision of the overall security goals of the project and an equally clear understanding of the detailed limitations of that which exists.

–  –  –

Departments of the Army, Navy and the Air Force ARMY TM 5-1300, NAVY NAVFAC P-397, AIR FORCE AFR 88-22; Structures to Resist the Effects of Accidental Explosions, November 1990.

Biggs, John M. Introduction to Structural Dynamics, Macgraw-Hill, 1964.

SOFTWARE:

USAEWES/SS-R, ConWep, 20 Aug 1992) ConWep is a collection of conventional weapons effects calculations from the equations and curves of TM 5-855-1, "Fundamentals of Protective Design for Conventional Weapons".

Naval Civil Engineering Laboratory, SHOCK, Port Hueneme, California, USA, January 1,1988) Naval Civil Engineering Laboratory, FRANG, Port Hueneme, California, USA, May 1989)

–  –  –

BIOGRAPHIES

Ron Hamburger: Ronald Hamburger is a structural engineer and principal with Simpson Gumpertz & Heger in San Francisco. Mr. Hamburger has nearly 30 years of experience in structural design, evaluation, upgrade, research, code and standards development, and education. Mr. Hamburger serves as chair of the BSSC Provisions Update Committee, AISC’s Connection Prequalification Review Panel, and is vice-chair of the AWS D1.1 Seismic Task Committee. Further, he is a member of the ASCE-7 committee, is President-Elect of the National Council of Structural Engineering Associations and is the Project Director for the ATC-58 project on performance-based earthquake engineering. Mr. Hamburger was a member of the joint FEMA/ASCE Building Performance Assessment Team that studied the collapse of New York’s World Trade Center on September 11, 2001.

Andrew Whittaker: Andrew Whittaker is an Associate Professor of Civil Engineering at the University at Buffalo, with research and design-professional interests in earthquake and blast engineering. He is a licensed Structural Engineering in the State of California. Dr. Whittaker serves as a member of the ASCE committees that address loadings on structures, blast engineering, and earthquake protective systems, ACI Committee 349 on reinforced concrete nuclear structures, BSSC Technical Subcommittee 12 on seismic isolation and passive energy dissipation systems for new buildings, as Vice President of the Consortium of Universities for Research in Earthquake Engineering, and as the Structural Team Lead for the ATC-58 project on performance-based earthquake engineering.

ABSTRACT

Structural steel framing is an excellent system for providing building structures the ability to arrest collapse in the event of extreme damage to one or more vertical load carrying elements. The most commonly employed strategy to provide progressive collapse resistance is to employ moment-resisting framing at each floor level so as to redistribute loads away from failed elements to alternative load paths. Design criteria commonly employed for this purpose typically rely on the flexural action of the framing to redistribute loads and account for limited member ductility and overstrength using elastic analyses to approximate true inelastic behavior. More efficient design solutions can be obtained by relying on the development of catenary behavior in the framing elements. However, in order to reliably provide this behavior, steel framing connections must be capable of resisting large tensile demands simultaneously applied with large inelastic flexural deformations. Moment connections prequalified for use in seismic service are presumed capable of providing acceptable performance, however, research is needed to identify confirm that these connection technologies are capable of reliable service under these conditions. In addition, some refinement of current simplified analysis methods is needed.





INTRODUCTION

Many government agencies and some private building owners today require that new buildings be designed and existing buildings evaluated and upgraded to provide ability to resist the effects of potential blasts and other incidents that could cause extreme local damage. While it may be possible to design buildings to resist such attacks without severe damage, the loading effects associated with these hazards are so intense that design measures necessary to provide such performance would result in both unacceptably high costs as well as impose unacceptable limitations on the architectural design of such buildings. Fortunately, the probability that any single building will actually be subjected to such hazards is quite low. As a result, a performance-based approach to design has evolved.

The most common performance goals are to permit severe and even extreme damage should blasts or other similar incidents affect a structure, but avoid massive loss of life. These goals are similar, though not identical to the performance goals inherent in design to resist the effects of severe earthquakes, and indeed, some federal guidelines for designing blast resistant structures draw heavily on material contained in performance-based earthquake-resistant design guidelines. While there are many similarities between earthquake-resistant design and blast-resistant design, there are also important differences.

Blast-resistant design typically focuses on several strategies including, provision of adequate standoff to prevent a large weapon from effectively being brought to bear on a structure, provision of access control, to limit the likelihood that weapons will be brought inside a structure; design of exterior cladding and glazing systems to avoid the generation of glazing projectiles in occupied spaces as a result of specified blast impulsive pressures, and configuration and design of structural systems such that loss of one or more vertical load carrying elements will result at most, in only limited, localized collapse of the structure. Although blast pressures can be several orders of magnitude larger than typical wind loading pressures for which buildings are designed, the duration of these impulsive loads is so short that they are typically not capable of generating sufficient lateral response in structures to trigger lateral instability and global collapse. Steel structures with complete lateral force-resisting systems capable of resisting typical wind and seismic loads specified by the building codes for design will generally be able to resist credible blast loads without creation of lateral instability and collapse. However, explosive charges detonated in close proximity to structural elements can cause extreme local damage including complete loss of load carrying capacity in individual columns, girders and slabs. Consequently, structural design of steel structures for blast resistance is typically focused on design of vulnerable elements, such as columns, with sufficient toughness to avoid loss of load carrying capacity when exposed to a small charge and provision of structural systems that are capable of limiting or arresting collapse induced by extreme local damage to such elements and avoiding initiation of progressive collapse.

Steel building systems are ideally suited to this application. The toughness of structural steel as a material, and the relative ease of designing steel structures such that they have adequate redundancy, strength and ductility to redistribute loads and arrest collapse facilitate the design of collapse-resistant steel structures. However, effective design strategies that will provide collapse resistance at low cost and with minimal architectural impact are urgently needed as is research necessary to demonstrate the effectiveness of technologies employed to provide the desired collapse resistance. This paper explores these issues.

DESIGN STRATEGIES

Typical design strategies for collapse resistant buildings involve removal of one or more vertical load carrying elements and demonstrating that not more than specified portions of the building will be subject to collapse upon such occurrence. The element removal could occur as a result of any of several loading events including blast, vehicle impact, fire, or similar incidents. Regardless, the design strategy can be traced to lessons learned from observation of the blast-induced collapse of the Alfred P. Murrah Building in Oklahoma City. As illustrated in Figure 1 (Partin 1995) extreme damage to columns at the first story of the building, led to progressive collapse of much of the structure (Figure 2).

Figure 1 - Diagram showing elements damaged by initial blast adjacent to Murrah Federal Building Figure 2 - Remains of the Murrah Building after blast-induced progressive collapse In their report on the performance of the building, the ASCE investigating team (Sozen 1995) concluded that had the building been designed with the continuity of structural systems typically present in buildings designed for seismic resistance, the extent of building collapse following blast-induced failure of several 1st story columns would have been substantially reduced.

Moment-resisting steel frames are ideally suited to provision of this continuity and in avoiding progressive collapse.

Three examples of the effectiveness of moment-resisting steel frames in arresting collapse and preventing progressive collapse as a result of extreme localized damage can be observed in the performance of buildings at New York’s World Trade Center following the terrorist attacks of September 11, 2001. Figure 3 is a view of the north face of the North Tower of the World Trade Center, clearly indicating that the closely spaced columns and deep girders of the moment-resisting steel frame that formed the exterior wall of the structure was capable of bridging around the massive local damage caused by impact of the aircraft and arrest global collapse of the structure for nearly 2 hours. Figure 4 illustrates that the more conventional moment-resisting steel frame of the Deutsche Bank Building allowed that structure to arrest partial collapse induced by falling debris from the south tower of the World Trade Center, despite the fact that an entire column was removed from the structure over a height of 10+ stories. Figure 5 is a plan view of the WTC-6 building at New York’s World Trade Center following collapse of the North Wall of the North Tower across the top of the building. A series of one-bay moment-resisting steel frames placed around the perimeter of the building arrested collapse and limited collapse to areas not protected by momentresisting framing.

Figure - 3. North Tower of World Trade Center, Illustrating the ability of the perimeter frame to bridge around the massive aircraft impact damage and arrest progressive collapse.

Figure - 4. Deutsche Bank Building remains standing despite column loss over multiple stories (see arrow) Figure 5 - Collapse of World Trade Center 6, induced by falling debris from the North Tower. Note that the dark lines indicate approximate locations of one-bay steel moment frames around building perimeter.

The use of moment-resisting steel framing to provide collapse resistance is an obvious choice. Figure 6 illustrates how a building with a continuous moment-resisting steel frame on each line of columns can resist collapse through redistribution of load to adjacent columns. Simplified guidelines for the design of such systems have been developed for the U.S. General Services Administration (ARA, 2003) and are available to designers engaged in the design or review of federal facilities. These guidelines specify that elements of the frame be proportioned with sufficient strength to resist twice the dead load and live load anticipated to be present, without exceeding inelastic demand ratios obtained from the federal guidelines for seismic rehabilitation of buildings (ASCE, 2002). The design model utilized in these simple procedures is conceptually incorrect, but probably provides adequate design solutions.

Figure 6 - Redistribution of gravity loads from removed column in building with a continuous moment-resisting steel frame along column lines.

Under this design model, the beams and columns are assumed to be required to distribute twice the vertical forces initially resisted by the removed element, through flexural behavior. The elements are required to be proportioned to resist twice the load initially resisted by the “removed” element based on theory related to the instantaneous application of load on an elastic element. Figure 7 and Figure 8 are respectively, displacement vs. time and force vs.



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