Blast-Resistant Highway Bridges: Design and Detailing Guidelines (2010)

Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

41.1 Overview The events of September 11, 2001, dramatically illustrated the catastrophic damage that terrorists can inflict on our civil structures. The attacks against the World Trade Center and Pentagon, unfortunately, were not isolated incidences of ter- rorist actions taken against U.S. assets. Over the last several decades there have been an increasing number of terrorist attacks that have led to tremendous losses. As a result of these events, the engineering community has become more aware of the need to design structures that can better withstand the effects of bomb blasts. For example, lessons learned from the Oklahoma City bombing in 1995 and the embassy attacks in Tanzania and Nairobi in 1998 have begun to shape current design guidelines for the prevention of progressive collapse. In the area of transportation security, approximately 60% of terrorist attacks against highway infrastructure have consisted primarily of explosive attacks (Jenkins and Gerston, 2001), which highlights the need for blast-resistant structures. Although the chances of terrorist attacks directed against a bridge are typically assumed to be very small, the economic and socio-economic consequences can be extremely high. The majority of the current state of knowledge for the design of structures subjected to blast loads is based on and directed toward the performance of military structures and civilian buildings. There has been very little notable research on the blast-resistant design of highway bridges. Thus, to implement the design of bridges for security, experimental and analytical research are needed to evaluate the effectiveness of current blast-resistant design guidelines for buildings applied to bridges or to develop new design and detailing guidelines. Since September 11th, several major efforts focused specif- ically on transportation security have been initiated. Nation- ally, several high-level groups have been assembled to develop recommendations and formulate both short- and long-term strategies for dealing with terrorist threats to bridges and other transportation assets. Among these efforts, the Blue Ribbon Panel on Bridge and Tunnel Security, organized through a joint effort of the FHWA and AASHTO, has been among the most significant. Following the efforts of the Blue Ribbon Panel, leading researchers at the FHWA developed both near- term and long-range research priorities to address important issues in the area of transportation security. A summary of the recommendations that have come from these and other groups can be found in the literature review presented in Chapter 2 of this report. One of the first research projects initiated on bridge secu- rity following the attacks on September 11th was directed by the Texas Department of Transportation, along with fund- ing from seven state DOTs and the FHWA. The focus of the research was on the development of mitigation strategies to improve the performance of a variety of different bridge types to potential terrorist courses of action. This study investi- gated cost-effective, unobtrusive design and retrofit options for a variety of bridge components and bridge structural sys- tems based on parametric studies carried out using simplified analytical models (Winget et al., 2004). More recently, through other pooled-funded projects, the Army Corps of Engineers has looked at the performance of steel suspension bridge towers subjected to very severe close-in blasts, and they have also worked on a similar project for concrete towers for sus- pension and cable-stay bridges. Other research has focused on common bridge components, such as the multi-hazard pier study conducted by researchers at SUNY Buffalo (Fujikura et al., 2008), as well as the tests on prestressed girders coordi- nated by the Corps of Engineers (Transportation Pooled Fund Program, 2008). While these recent projects have provided tremendous insight into the performance of bridges subjected to explosions, this field is still quite new. Hence there is much that still needs to be learned regarding the design of bridges for security. An in-depth presentation on the current state of practice and relevant research on this topic can be found in Chapter 2 of this report. C H A P T E R 1 Introduction

51.2 Background and History In order to develop an appreciation of the extent to which terrorists have targeted U.S. assets, it is helpful to review some previous incidents. The events highlighted below are not intended to be an exhaustive list, but they do illustrate the need for enhanced security. On February 26, 1993, a bomb was detonated in the park- ing garage of one of the World Trade Center towers. As a result of this attack, 6 people were killed and 1,042 were injured. Damage was observed over seven floors, and prop- erty damage was over one-half billion dollars. According to the ASCE accident investigation (Wikipedia, 2008), the com- partmentalized layout of the building structure was credited with minimizing the propagation of damage and preventing progressive collapse. Two years after the bombing of the World Trade Center, on April 19, 1995, the Alfred P. Murrah building in Oklahoma City was attacked. The death toll from this event was much larger than that from the 1993 event. As a result of a large truck bomb, 169 people were killed, over 500 were injured, and damages exceeded $100 million. From an engineering per- spective, there was great concern over the structural config- uration of the Murrah building. This nine-story structure incorporated a transfer girder at the third floor that allowed the column spacing from the floors above to be doubled from 20 feet to 40 feet on the bottom three stories. Because the bomb blast likely caused the failure of three of the columns that supported the transfer girder, the high loads from the floors above could not be redistributed to the remaining columns. As a result, the Murrah building failed due to pro- gressive collapse. Because of this event, research into progres- sive collapse has once again become a great concern to the structural engineering community, and engineering guide- lines to resist progressive collapse have been developed by the Department of Defense (2005) and the General Services Administration (2003). These events that took place on U.S. soil are very familiar to much of the population, yet several other events in recent years have shown that U.S. assets all over the world are sus- ceptible to terrorist attacks. Though it is not necessary to describe all of these events in great detail, it is helpful to dis- cuss the incidents that have implications related to structural engineering. One such event includes the bombing of the Khobar Towers in Saudi Arabia on June 25, 1996. This facil- ity was used to house U.S. and allied forces. There were 19 fa- talities and approximately 500 U.S. personnel wounded in the attack. Other events that raised awareness of the need to protect against terrorist activities took place on August 7, 1998. On this date, two U.S. embassy buildings were bombed in Africa. As a result of these attacks, 11 Americans were killed and over 30 were injured. The response of the two embassy buildings differed greatly. The embassy building in Tanzania fared quite well, and damage was limited. The Nairobi em- bassy building, however, suffered severe damage and under- went a partial collapse in a similar progressive fashion as the Murrah building. Transportation targets have not been exempt from these types of attacks. In a 1997 report, Brian Jenkins describes over 550 terrorist attacks worldwide against transportation targets between 1975 and 1997. He states: We have seen an increase in attacks on public transportation as terrorism has increased over the past quarter century and more recently as terrorists have demonstrated greater willing- ness to kill indiscriminately. The Irish Republican Army’s long- running terrorist campaign in the United Kingdom has included numerous attacks on rail lines, trains, subways, and stations. Palestinian terrorists have carried out numerous attacks on Israeli buses and bus stations. Algerian extremists directed their 1995 terrorist campaign in France against the subway and rail system. The first large-scale terrorist use of chemical weapons was carried out in Tokyo’s subways, an ideal environment for chemical attack. Islamic extremists in New York planned to attack the city’s bridges and tunnels in 1993 and its subways in 1997. (Jenkins, 1997) Recent events around the world support Jenkins’ position. On March 11, 2004, ten bombs exploded on four trains in three stations during the busy morning rush hour in Madrid, Spain. The bombings killed 191 people and injured hundreds of others. Results of the investigation following the attacks led to the arrest of several militants believed to be affiliated with al-Qaeda. More recently, public transportation in London became the target of terrorist attacks. On July 7, 2005, terror- ists carried out a coordinated series of suicide bombings dur- ing the morning rush hour. In total, four different bombs exploded on three subway trains and one commuter bus, resulting in 52 deaths and approximately 700 people injured. These bombings were the largest and deadliest attacks on the transportation infrastructure ever to occur in London. Just two weeks later, on July 21, 2005, four attempted bomb attacks again disrupted London’s transportation network. This time, however, authorities were able to prevent the attacks from taking place, and all main bombing suspects were arrested within a week of the incident. In the aftermath of the events following September 11th, the potential threat against highway bridges became more pronounced to federal and state officials. While federal agen- cies such as FEMA and the Center for Defense Information had warned about highway structures being potential targets of ter- rorist acts, few state agencies had the resources or the exper- tise to implement any safety measures. Officials in California learned first-hand that the threat of terrorist action can lead to major disruptions in the transportation network. On

6November 1, 2001, Governor Gray Davis held a press con- ference to announce that California had received intelli- gence that several of the state’s bridges were threatened to be destroyed between November 2 and November 7. In re- sponse, security around the threatened bridges was sub- stantially increased by assigning National Guard troops to protect the bridges. While the threats against these bridges were later determined to be non-credible, the disruption to traffic and the expense involved in providing security per- sonnel to protect the bridges was significant. In June 2003, a man in Ohio suspected of being a member of al-Qaeda was arrested for investigating the use of special- ized equipment to sever the cables of one of the bridges in New York. The person arrested (Mohammed Rauf) was a truck driver who admitted he was an al-Qaeda agent who met with Osama bin Laden and high-ranking al-Qaeda leaders at an al-Qaeda terrorist training camp in Afghanistan. Rauf was later convicted of plotting to sabotage the bridge and launch a terrorist attack against the nation’s capital, including derail- ing trains. During our current conflict with Iraq, a self-proclaimed “anti-American” group threatened to carry out terrorist attacks against diplomatic compounds, airlines, and public transportation systems in eight U.S. allied countries (Fox News, 2004). Furthermore, insurgents in Iraq have rou- tinely targeted bridges, and several significant collapses have occurred (which are detailed in Chapter 2 of this report). Thus, despite the heightened public awareness of potential terrorist acts, and despite the fact that more stringent security measures are in place now than before September 11th, ter- rorists appear to be as motivated as ever to attack U.S. targets. In fact, even after September 11th, terrorist attacks against American interests as a percentage of the total terrorist attacks worldwide have steadily increased. This conclusion is also sup- ported by data discussed in the AASHTO/FHWA Blue Ribbon Panel Report (Blue Ribbon Panel on Bridge and Tunnel Secu- rity, 2003) in which data from the FBI show that terrorism against U.S. assets has been steadily increasing. These events help illustrate that there is a growing need for engineers to be able to design structures to help minimize the consequences of a terrorist attack. Despite this awareness, the engineering profession does not currently have any consistent guidelines to help limit the effects of an attack. In the Octo- ber 12, 1998, edition of Engineering News Record, Gene Corley, one of the principal investigators of the Oklahoma City bombing, commented that “while the military and federal government have developed methods to improve the blast resistance of their own buildings, they have not transferred these methods to civilian engineers.” In fact, even today, more than 13 years after the bombing of the Murrah building, such guidelines are not readily available to the structural engineer- ing community. In the November 7, 2001, issue of Engineer- ing News Record, 90% of an estimated 3,000 architects that were involved in a web-based seminar on security issues “expressed frustration at the lack of risk assessment guide- lines available to them for advising their clients.” In response to these needs, at least for transportation assets, several orga- nizations have begun to develop tools that engineers and plan- ners can use to carry out vulnerability and threat assessments. For example, Science Applications International Corporation’s (SAIC) 2002 publication, A Guide to Highway Vulnerability Assessment for Critical Asset Identification and Protection, was prepared for the AASHTO Security Task Force and provides a risk assessment procedure that is tailored specifically to the transportation infrastructure. Several other publications have appeared in the research literature offering procedures for carrying out risk assessments of transportation infrastruc- ture, and Chapter 2 of this report includes a review of the rel- evant studies. Although improvements have been made in recent years, there is still a great need for the available information on ter- rorist preparedness to be made accessible to structural engi- neers. In particular, because bridge engineers have not typically needed to consider structural integrity in response to a terror- ist attack, current design guidelines do not consider the issue of bridge security. As a result, there is a need to compile avail- able research literature and provide this information to trans- portation officials and highway bridge engineers. In fact, in response to the terrorist attacks on September 11th, AASHTO assembled a task force to consider transportation security, and now a subcommittee is focused entirely on this issue. Other professional organizations such as the Transportation Re- search Board (TRB) and the American Society of Civil Engi- neers (ASCE) have also established national committees to address the topic of transportation security. The primary charge of the AASHTO task force is to “establish guidance and share practices that help state DOTs prepare vulnerability assessments of their highway infrastructure assets, develop deterrence/surveillance/protection plans, develop emergency response plans and capabilities for handling traffic for major incidents on and off the transportation system, and assess and respond to military mobilization needs in each state.” In response to the September 11th tragedy and the sub- sequent events that suggested the vulnerability of our trans- portation infrastructure, research efforts have been initiated to address a large number of pressing needs. Due to the open- ness and ease of access to transportation assets, the transporta- tion community faces a challenge much greater than that of individual building owners. Current priorities include port security, inspection of hazardous cargo, tunnel security, and bridge security. It is this last topic that is of relevance to the current report and the focus of this document.

1.3 Research Approach The information included in this report summarizes the work completed for the National Cooperative Highway Research Program’s (NCHRP) Project 12-72. The primary objective of this research was to develop design guidance for improving the structural performance and resistance to explosive effects for bridges. In order to meet this objective, it was essential to compile information on the response of struc- tures subjected to blast loads. Limited information specifically focused on bridges was available in the open literature during the time that this research was conducted, but information on the behavior of blast-loaded building components was found to be useful in understanding the response of bridges sub- jected to blast loads. In addition, a thorough assessment of design principles and available technologies that are useful in protecting bridges from other extreme loading conditions such as earthquakes and vehicle crashes was made to determine their potential for application in improving bridge response to blast effects. This information has been compiled in the background and literature review appearing in Chapter 2 of this report. Because of the great variety of bridges in the national inven- tory, it was necessary to develop a research program that addressed high priority needs in the design of bridges to resist explosions. While there are many types of bridges and struc- tural systems in use, it was important to focus the research on areas that would yield critical information of national importance. In the second half of Chapter 2, the focus of the research project is explained, and details of the experimen- tal and analytical testing programs are given in Chapters 3 and 4, respectively. In Chapter 5, research findings are presented. This chapter provides information on the results of a two-phase experi- mental testing program and the corresponding analytical research program. Using the results of the experimental and analytical research, blast-resistant design guidelines are given in Chapter 6. The information is assembled using the AASHTO Load and Resistance Factor Design (LRFD) format so that the procedures can be presented in a format that is familiar to bridge engineers. Following these design guidelines, Chapter 7 includes recommendations for analyzing bridges subjected to blast loads. In this chapter, both simplified and detailed modeling approaches are covered. To illustrate the application of the proposed analysis and design guidelines, detailed examples are provided in Chapter 8. In Chapter 9, the main findings are summarized, and recom- mendations for future research are given. 7