Building+Failures+due+to+Blast+Effects

//﻿Todd J. Rasey, E.I.T., BS, MS Candidate, Department of Civil and Environmental Engineering, The Pennsylvania State University, 2010//
 * ﻿Building Failures due to Blast Effects **

Explosion, Blast, Bomb, Terrorism, Collapse, Oklahoma City, Murrah, Ronan, Glazing, Trade Center
 * Key Words**

=**Introduction**= toc Malicious explosive attacks on civil infrastructure have proven to be an ongoing problem in the world. The bombings of the Alfred P. Murrah Federal Building in Oklahoma City (1995) and the U.S. embassies in Kenya (1998) are two well-known examples of instances in which severe damage followed the detonation of explosive devices near the structures. The ensuing damage was highlighted by (but not limited to) partial or total collapse of the targeted or surrounding structures. Additionally, the effect of the explosions was seen in the amount of human casualties and injuries that occurred as a result of the blasts, not all of which were due to building collapse. Direct blast effects, fragmentation of window glazing, ensuing fires, and other factors contributed largely to the number of resulting injuries and fatalities. Even accidental exposions, such as the one which led to the collapse of the Ronan Point tower (1968), represent a threat to buildings and their occupants. While post-event images of collapsed buildings are the most recognizable symbols of devastating bomb effects, they represent just one of several significant types of building failure associated with blast effects. This article will review the types of damage blasts may inflict on buildings, look at specific failure case studies, and discuss design methods for protecting against such failures.

=**Collapse due to Blast Loadings**= Full or partial collapse of buildings are the most recognizable type of failure that may result from a blast. Typically, such as in the cases of the Murrah Building and the Ronan Point tower, the blasts themselves do not destroy the entire building, but rather eliminate critical members, leading to progressive collapse of the structure (Stewart 2006).

On April 19, 1995, a truck bomb was detonated next to the Alfred P. Murrah Federal Building in Oklahoma City. The nine story building, which housed numerous government offices as well as a day care center, suffered progressive collapse of nearly half of the structure (see Figure 1); 167 people were killed and 782 were injured in the event. Some reports state that the collapse was initiated by failure of only four columns; however, the column failure may not have been due to direct blast effects. Several forensic studies show that the failure was a result of the loss of a critical transfer girder which had been cast monolithically with the floor slab (Perez 2009). According to the reports, the increase in pressure associated with the blast caused the second-story floor slabs to undergo negative flexure—a loading condition for which they were not reinforced. Only one column had been destroyed by direct blast effects, but the response and failure of the slab led to loss of the critical transfer girder, which in turn led to failure of three additional columns and progressive collapse of the building (Osteraas 2006).

The findings that the Murrah collapse was not simply due to obliteration of the columns due direct blast effects, but rather column loss coupled with an unanticipated slab loading condition—show how difficult it can be to design for blast loadings. A similar problem led to the collapse of the Ronan Point tower in 1968. On May 16, 1968, a small explosion occurred in an apartment on the 18th floor of the 22-story high rise. The tenant had struck a match in order to light the kitchen stove; the stove, however, due to a poor connection, had been leaking gas into the apartment. The match ignited the gas, causing a relatively small explosion in the apartment. The pressure induced by the explosion was likely less than 10psi; this was indicated by the fact that the tenant’s hearing had not been damaged (Pearson 2005). Despite being a fairly small explosion, the pressure was great enough to displace the walls of the apartment. The walls, as per design, were load-bearing; each floor was supported by the walls of the story beneath it. When the walls of the 18th floor apartment were displaced by the explosion, collapse initiated and progressed down the corner of the building. Four people were killed and seventeen were injured in the event. Forensic studies concluded that poor detailing and construction practices were largely to blame for the collapse of the tower. In both the Murrah and the Ronan Point cases, explosions led to partial collapse of the structures and considerable loss of life. In each case, failure was ultimately triggered by an explosion inducing unexpected loading conditions.

=**Non-Collapse Damage and Fragmentation of Window Glazing**= Not all explosions lead to collapse; in many cases, the building can be significantly damaged yet remain standing, such as in the case of the World Trade Center Bombing in 1993. On February 26, 1993, a truck bomb was detonated in the parking garage beneath the World Trade Center. Six people were killed in the initial blast and 1,024 were injured, most of whom were injured during the evacuation procedure. The blast destroyed several levels of concrete slab, but most of the damage was contained to the garage area; the tower was able to withstand the damage and remain standing. However, the effects of the blast led to other significant problems. The initial explosion destroyed the tower's main electrical power line which resulted in a loss of both primary and emergency lighting, as well as loss of power to the elevators. The explosion also started a fire; the smoke from this fire travelled up the nonpressurized stairwells to the 93rd floors of both WTC towers. Because of the loss of power and the spread of smoke throughout the buildings, most of the injuries (many of which were related to smoke inhalation) occured during the evacuation procedure. While both towers remained standing after the event, the buildings failed to adequately control the evacuation.

While the 1993 Word Trade Center bombing gives an example of how buildings can fail to perform while not collapsing, the most significant threat to building inhabitants in the event of an explosion is fragmentation of window glazing. In general, explosive charges detonated near buildings usually do not cause significant structural damage to modern steel and concrete structures (Stewart 2006). While collapses do sometimes occur (e.g., the Murrah and Ronan Point buildings), glass fragmentation often leads to many of the injuries seen after an explosion. In the case of the Murrah bombing, approximately two-thirds of the non-fatal casualties were attributed to flying glass (Claber 1998). Depending on the type of building, an explosion can cause large amounts of glass to fracture, posing serious threats to both the building inhabitants as well as pedestrians. In 1992, a London bombing resulted in damage to approximately 450 metric tons of glass; 350 tons were deposited outside the building while a significant portion of the remaining amount was projected inward. K J Claber, the Head of Explosion Protection for the London-based Special Services Group (SSG) described how a truck bomb would affect a relatively robust building if detonated at varying distances; this is displayed in the table below:

Source: Claber (1998)
 * Local collapse of building ||= 10-20 m ||
 * Structural damage ||= 50 m ||
 * Damage to blast resistant glazing ||= 100 m ||
 * Damage to glass with film ||= 200 m ||
 * Damage to glass without film ||= 400 m ||

=Protective Design Measures= Explosions may either be intentional (as in the Murrah bombing) or unintentional (as in the Ronan Point collapse). Learning from the latter event, the effects of unintentional blasts—and unintended loading conditions—may be mitigated by improved progressive collapse design criteria as well as higher-quality workmanship. Conversely, malicious explosive attacks on particular buildings are somewhat more predictable. According to a 2008 study, certain buildings are at greater risk due to terrorism than others (Stewart 2008). High-risk buildings may include ones with high damage consequences (loss of life, damage to infrastructure, or economic loss) or those which are subject to a specific threat (such as government offices). Embassies are sometimes targeted, such as in the case of the U.S. Embassy bombings in Kenya; Figure 2 shows the aftermath of the embassy attack in Nairobi.The aforementioned 2008 study also concluded that only in these high-risk scenarios is it economical to incorporate protective measures into the design of the building.

Stewart (2008) gives the following examples of protective measures: ==

Enhanced perimeter security:
 * Perimeter wall.
 * Vehicle barriers and inspection.
 * Security personnel.
 * Increased stand off.

Facility design:
 * Blast and impact resistant glazing.
 * Strengthened perimeter columns and walls.
 * Enhanced structural stability measures.
 * Enhanced dictility and connectivity.
 * Provide alternate load paths.

Allow rapid evacuation and access to first responders' facility relocation:
 * Threat minimization.
 * Increased stand off or separation from other threats.

Osteraas (2006) offers more general measures; he provides the following list of design recommendations based on forensic analysis of the Murrah building collapse:
 * 1) A complete three-dimensional space frame (without antiredundant features such as the transfer girder) that interconnects all load path elements provides stability and provides redundant load paths.
 * 2) That frame must be protected with provision of "mechanical fuses" that allow slabs and walls to fail without destroying the frame.
 * 3) The frame must be robust and ductile to absorb overloads with large deformations while maintaining continuity.
 * 4) Lower portions of perimet columns should be designed, to the greatest extent possible, to resist the direct effects of blast.

=**References**= Article discussing window glazing design considerations in regard to blast effects. The article describes explosion effects, as well as provides statistics related to past explosive attacks and the number of injuries resulting from damaged glass.
 * Claber, K.J. (1998). "Designing Window Glazing for Explosive Loading," //Security Technology, 1998. Proceedings., 32nd Annual 1998 International Carnahan Conference on Security Technology,// 65-72, 12-14 Oct 1998**

A discussion of the collapse of the Alfred P. Murrah Federal Building in 1995 as a result of a truck bomb. The analysis focuses on the causes of the collapse, specifically the damage done to the columns. Although 4 columns were lost, evidence suggests only one was destroyed by the bomb itself, while the remaining 3 buckled due to load redistribution and other effects.
 * Osteraas, John D. (2006). "Murrah Building Bombing Revisited: A Qualitative Assessment of Blast Damage and Collapse Patterns." //Journal of Performance of Constructed Facilities, ASCE, 330-335, Nov 2008//**

This article provides background on the events leading up to the collapse of the Ronan Point apartment tower in 1968. The article discusses how the accidental explosion triggered a progressive collapse.
 * Pearson, Cynthia and Delatte, N. (2005). "Ronan Point Apartment Tower Collapse and its Effect on Building Codes." //Journal of Performance of Constructed Facilities, ASCE, 172-177, May 2005.//**

This page provides a well-rounded description of the bombing and the ensuing collapse, including: descriptions of the building itself, blast details, failure mechanisms, and a discussion of the lessons that may be learned from the event.
 * Perez, Andres R. (2009). "Murrah Federal Office Building (April 19, 1995)." Failures Wiki. http://failures.wikispaces.com/Murrah+Federal+Building. Accessed 28 Sep 2010.**

This paper looks at malicious explosive attacks from a risk analysis perspective. It develops a probalistic risk assessment procedure that can be used to predict the risks of damage. Topics such as blast effects and modeling techniques are discussed.
 * Stewart, M.G., Netherton, M.D., Rosowsky, D.V. (2006). "Terrorism Risks and Blast Damage to Built Infrastructure." //Natural Hazards Review, ASCE, 114-122, August 2006//**

In investigating the cost effectiveness of risk mitigation strategies, this paper provides descriptions of the types of risk mitigation strategies, such as vehicle barriers, perimeter walls, blast-resistant glazing, and strengthened columns.
 * Stewart, Mark G. (2008). "Cost Effectiveness of Risk Mitigation Strategies for Protection of Buildings against Terrorist Attack." //Journal of Performance of Constructed Facilities, ASCE, 115-120, March/April 2008.//**

=﻿Additional Resources=

This book is a reference for the protective design of structures with respect to many kinds of malicious attack modes. Includes descriptions of the kinds of damage that blast effects may inflict.
 * Krauthammer, Theodor. __Modern Protective Structures__. CRC Press. Taylor and Francis Group. Boca Raton, FL. 2008.**

Analysis of the structural failure of a reinforced concrete building subjected to blast loads.
 * Luccioni, B.M., Ambrosini, R.D., Danesi, R.F. (2003). "Analysis of building collapse under blast loads." //Engineering Structures 26// (2004) 63-71.**

This paper discusses methods for predicting the effects of bomb blasts on buildings. It includes an in-depth review of the mechanisms by which bombs may damage buildings.
 * Remennikov, A.M. (2003). A Review of Methods for Predicting Bomb Blast Effects on Buildings. //Journal of Battlefield Technology, 2003//, 6(3) 5-10. Argos Press Pty Ltd.**

This paper outlines a procedure for a probabilistic risk assessment focused solely on window glazing subjected to explosive blast. The analyses consider such uncertainties as blast modeling, glazing response, and glazing failure criteria.
 * Stewart, M.G., Netherton, M.D. (2008). "Security risks and probabilistic risk assessment of glazing subject to explosive blast loading." //Reliability Engineering and System Safety 93 (2008) 627-638.//**