Anonymous

Machine Design, vol. 80, no. 5, pp. 38, 40, 6 Mar. 2008.

Federal investigators recently released a few preliminary findings about the infamous Minnesota I-35W bridge failure. By chance, the announcement came as FEA and CAD vendors were converging on SolidWorks World, a major trade show. FEA experts at the show had plenty to say about the federal findings. Their conclusion: Had FEA been around when the bridge was built, it would have caught the errors that seem to have lead to the bridge's collapse.

Andre Marshall and James Quintiere.

ASEE Annual Conference & Exposition, 2007

Fire Protection Engineering undergraduate students enrolled in a fire assessment laboratory course conducted their own investigation of the September 11, 2001 World Trade Center disaster by simulating the fire that followed the aircraft impact. The project focused on characterizing the fire on the 96th floor of WTC1 (North Tower) and evaluating the contribution of the fire to the structural collapse. Students contacted vendors and suppliers for the World Trade Center to get information regarding construction details and fire properties of building materials and furnishings. Students also obtained information reported from the National Institute of Standards and Technology Building and Fire Research Laboratory investigation of the World Trade Center collapse. A 1/20th scale model of the original structure (including damage effects from the aircraft and liquid fuel dispersed from the aircraft impact) was designed, constructed, and instrumented over ten weeks corresponding to the last half of the semester. Students held briefing for invited guests from the university, government agencies and industry prior to the actual scale model test. Results from the test were recorded continuously with video and with an automated data acquisition system for detailed analysis. Analysis of the results in the scaled spatial and temporal coordinates provided insight into peak temperatures, smoke production rates, and fire growth behavior that may have occurred in the actual WTC1 fire. This classroom study provided an excellent opportunity for students to apply classroom principles to a problem of significant social and engineering relevance.

S. Chen, E. Lim, W. Lee, et al.

EOS Transactions, American Geophysical Union, Vol. 88, No. 52, 2007, Suppl. Volume 1.

The impact of assimilating observations from conventional instruments, Radar, and satellites on Hurricane Katrina (2005) was assessed using high-resolution model simulations. Four nested domains were configured with grid spacings of 54 km, 18 km, 4.5 km, and 1.5 km for domains 1 to 4, respectively. Three sets of experiments, EXP1-3, with various initial times and data cycling periods were conducted. The data cycling periods were 1800 UTC August 25 to 0000 UTC August 26 (6 h), 0000 UTC to 0600 UTC August 26 (6 h), and 1800 UTC August 25 to 0600 UTC August 26 (12 h) for EXP1-3, respectively. The first two sets (i.e. EXP1 and EXP2) were to evaluate the impact of assimilating different observations on Katrina simulations, while the last one (EXP3) was to examine the influence of different assimilation periods for radar data. Thirty-six hour model integrations were then performed after the data cycling on all four domains. The results from EXP1 shows that the assimilation of radar and conventional data had a positive impact on simulated storm's intensities during the first 24 hours, while the influence of satellite data was also positive, though less significant, and the influence was able to extend the whole 36-h simulation since the coverage of satellite data was much larger than that of radar. However, none of EXP1 experiments with the assimilation of observations had reduced track errors, which increased with time and reached about 200-250 km after a 36-h integration. The forecasts from EXP2 show great improvement in simulated storm's intensity and track after the use of observations, in particular for the first 24 h of the forecasts. The track error was close to 50 km during the whole simulation period. The results from EXP3, which started the data cycling at 1800 UTC August 25 as in EXP1, confirmed that the observations from 0000 UTC to 0600 UTC August 26 used in this study played a key role in improving simulated tracks. Moreover, the results further concluded that the impact on the simulated track was contributed from GTS data and/or satellite data. Doppler radar data assimilation mainly contributed to the improvement of the hurricane intensity forecast. Finally, all three sets of experiments indicated that the assimilation of radar data in an interval of every three hours for a 6-h time period is an optimal setting.

Aileen Cho, Tom Ichniowski and William Angelo.

Enr, Vol. 259, No. 6, 13 Aug. 2007, pp. 12-16.

Just as West Virginia's Silver Bridge collapse in 1967 marked a new era for bridge inspections and awareness of U.S. infrastructure issues, so will Minnesota's Interstate 35W bridge collapse be another ante-upping chapter. The chapter is still being written. U.S. Dept. of Transportation Secretary Mary Peters has vowed a 'topto-bottom review' of federal bridge inspection guidelines. The specific structural issues that may be reshaped depend largely on what the National Transportation Safety Board will determine from its investigation. Fatigue cracks, lack of redundancy, bearings corrosion, welding codes - a variety of possible factors have been thrust on the national stage. But engineers caution against premature theories regarding why the 40-year-old steel truss bridge collapsed Aug. 1. What does seem clear is that this will lead to updates in inspection guidelines, increased use of monitoring technologies and renewed attention to the complex issue of funding.

Thomas L. Holzer.

Abstracts with Programs - Geological Society of America, Vol. 39, No. 6, Oct 2007, pp. 198.

Seismic networks increasingly are being used to provide insight into the timing and nature of chemical explosions associated with accidents, crimes, and acts of terrorism. Examples of such use of seismographic data, known as forensic seismology, include analyses of the April 19, 1995, Oklahoma City terrorist bombing; the August 10, 2000, explosion that sunk the Russian submarine Kursk; the August 19, 2000, New Mexico gas pipeline explosion; the September 11, 2001, plane crashes into the World Trade Center and Pentagon; and the February 1, 2003, Space Shuttle Columbia accident. Following the bombing of the Alfred J. Murrah Federal Building in Oklahoma City, seismic recordings were used to confirm that only a single bomb had been detonated and also to estimate the size of the bomb. Conspiracy advocates had proposed that the two accused bombers, who were ultimately convicted, had been set up. It was speculated that their bomb was used as cover for a larger and more damaging bomb, detonated by the U.S. government. The seismic recordings clarified the bombing scenario and publication of the results discouraged a conspiracy defense by the accused bombers. Seismic recordings of explosions that doomed the Russian submarine Kursk revealed details that otherwise might have remained Russian state secrets and helped rebut the Russian contention that a foreign vessel had collided with the submarine. The recordings indicated that a small 50 to 100 kg (equivalent TNT) explosion preceded the main 3- to 7-ton explosion by about 2 minutes, suggesting that a torpedo may have detonated and set off the sequence of events that sank the submarine. Analysis of the seismic signal of the main explosion also indicated that the resulting gas bubble was generated at near-bottom water pressures. Seismic recordings of the New Mexico gas pipeline explosion indicated multiple ignitions. The time between ignitions, recorded at nearby seismometers, helped determine the degree of pain and suffering (and cost to the pipeline company) of the 11 victims of the explosions in a lawsuit brought by relatives. Distant seismic recordings of the September 11, 2001, collapses of the two towers of the World Trade Center were used to constrain and show that ground shaking generated by the collapses was not a major contributor to damage or collapse of surrounding buildings.

I. N. Robertson, H. R. Riggs, S. C. S. Yim and Y. L. Young.

Journal of Waterway, Port, Coastal and Ocean Engineering, Vol. 133, No. 6, Nov-Dec 2007, pp. 463-483.

The storm surge associated with Hurricane Katrina caused tremendous damage along the Gulf Coast in Louisiana, Mississippi, and Alabama. Similar damage was observed subsequent to the Indian Ocean tsunami of December 26, 2004. In order to gain a better understanding of the performance of engineered structures subjected to coastal inundation due to tsunami or hurricane storm surge, the writers surveyed damage to bridges, buildings, and other coastal infrastructure subsequent to Hurricane Katrina. Numerous lessons were learned from analysis of the observed damage, and these are reported herein. A number of structures experienced significant structural damage due to storm surge and wave action. Structural members submerged during the inundation were subjected to significant hydrostatic uplift forces due to buoyancy, enhanced by trapped air pockets, and to hydrodynamic uplift forces due to wave action. Any floating or mobile object in the nearshore/onshore areas can become floating debris, affecting structures in two ways: impact and water damming. Foundation soils and foundation systems are at risk from shear- and liquefaction-induced scour, unless designed appropriately.

G. L. Sills, N. D. Vroman, R. E. Wahl and N. T. Schwanz.

Geo-Denver 2007: New Peaks in Geotechnics; Denver, CO; USA; 18-21 Feb. 2007

In August 2005, Hurricane Katrina made landfall just east of New Orleans and inflicted widespread damage on the Hurricane Protection System (HPS) for southeast Louisiana. The storm surge produced by Hurricane Katrina in some cases overwhelmed the HPS beyond its design, but in other cases levee failures occurred at water levels well below their design. The response to this disaster by the US Army Corps of Engineers (USACE) included forming an Interagency Performance Evaluation Taskforce (IPET) to study the response of the system and, among many lines of inquiry, to identify the causes of failure of levees and floodwalls. Beginning in September 2005, the IPET gathered forensic geotechnical data from failed portions of levees and floodwalls. These data were considered perishable and had to be gathered quickly due to levee rebuilding operations. The performance of the levee and floodwall system provided valuable lessons. These lessons consist of: the need for resilience of the HPS; the need for risk-based planning and design approach; and the deficiency in knowledge, technology, and expertise in the hurricane protection system arena. The rebuilding efforts and future assessments and designs of hurricane protection systems will incorporate the lessons learned.

H. PETROSKI.

American Scientist, Vol. 94, No. 1, pp. 7-11, 2006, pp. January/February.

New Orleans was built on its present site because of its strategic location near the mouth of the Mississippi River. However, this waterway is prone to flooding, and although the original settlement occupied relatively high ground, the growing city expanded into lower-lying areas, some on land reclaimed from Lake Pontchartrain, north of the city. New Orleans was protected from potential flooding by an elaborate system of levees along the banks of the lake, and because it lies below sea level, huge pumping stations were built to pump out rain and other unwanted water. Mathematical and computer models predict how a levee will respond to varying pressures of water and wind, but whether a levee protects a city depends on the appropriateness of the design, the care taken with construction and maintenance, and how it is tested. The larger and stronger the levee, the greater the monetary and environmental costs. A levee designed to protect against a 200-year storm will cost more than one designed to protect against a lesser storm. In New Orleans, the failed levee system was supposed to protect against potential flooding from a Category 3 hurricane. When Hurricane Katrina struck, the levees system breached in several places, and water poured into the city. The New Orleans disaster was compounded by a lack of foresight in integrating the whole infrastructure system, plus a lack of backup systems. In the years before Katrina, increased federal funding to strengthen the levees had been requested but was not forthcoming. Decisions must now be made about whether to patch the levee system and restore the city to its pre-Katrina state; whether to raise and harden the levees to withstand a Category 4 or 5 storm; and whether to build backup levees. It is possible to build a redundant parallel levee system, but this would double maintenance costs and would not guarantee protection. An alternative to restoring the levee system would be to raise lower sections of the city above sea level, as was done in Galveston, Texas, after the island city was inundated by a huge storm surge in 1900.

Anthony Hall.

American Reconstruction, Vol. 1, No. 8, Dec. 2006, pp. 14, 16.

Since the attack on September 11, computer scientists and engineers at Purdue University's School of Civic Engineering and at other academic institutions have been working hard to determine what factors led to the collapse of the towers. Working with National Science Foundation grants, engineering teams led by Professor Mete Sozen, and a computing team led by Christoph M. Hoffman, a professor of computer science, have recently completed a simulation of what might have happened when a commercial airliner carrying 10,000 gallons of jet fuel crashed into the North Tower, the first of the two towers to collapse. This exercise in structural forensics is the third phase of a project that began with a simulation of the attack on the Pentagon that occurred on the same day. Purdue teams finished that simulation in 2002 and have worked since then to recreate the North Tower disaster.

Anonymous

PB2008107010, Apr 2006, pp. 80.

Hurricane Katrina was one of the strongest storms to impact the coast of the United States during the past 100 years. Katrina reached Category 5 levels over the Gulf of Mexico, then weakened and made landfall in Louisiana and Mississippi with strong Category 3 storm winds. The storm surge, however, did not diminish before landfall, and the record surge caused widespread devastation in the coastal areas of Alabama, Louisiana, and Mississippi. The storm surge caused failures of the levee system that protects the City of New Orleans from Lake Pontchartrain, and 80 percent of the city subsequently flooded. Prior to Hurricane Katrina, Alabama, Louisiana, and Mississippi did not have statewide building codes for non-state-owned buildings. Many of the communities in areas that were heavily impacted by Hurricane Katrina had either not adopted up-to-date model building codes that incorporate flood and wind protection or had no building codes at all. The lack of adequate building codes greatly compounded the effect of Hurricane Katrina on building performance.

Mohammed R. Karim and Michelle S. Hoo Fatt.

Journal of Engineering Mechanics, Vol. 131, No. 10, Oct. 2005, pp. 1066-1072.

A numerical simulation of the aircraft impact into the exterior columns of the World Trade Center (WTC) was done using LS-DYNA. For simplification, the fuselage was modeled as a thin-walled cylinder, the wings were modeled as box beams with a fuel pocket, and the engines were represented as rigid cylinders. The exterior columns of the WTC were represented as box beams. Actual masses, material properties and dimensions of the Boeing 767 aircraft and the exterior columns of the WTC were used in this analysis. It was found that about 46% of the initial kinetic energy of the aircraft was used to damage columns. The minimum impact velocity of the aircraft to just penetrate the exterior columns would be 130 m/s. It was also found that a Boeing 767 traveling at top speed would not penetrate exterior columns of the WTC if the columns were thicker than 20 mm.

Anonymous

CE News, Vol. 17, No. 11, Dec. 2005, pp. 14.

WASHINGTON, D.C. - Many of the New Orleans levee and floodwall failures in the wake of Hurricane Katrina occurred at weak-link junctions where different levee or wall sections joined together, according to a preliminary report released Nov. 2 by independent investigators from the University of California, Berkeley (UC Berkley) and the American Society of Civil Engineers (ASCE). Raymond Seed, UC Berkeley professor of civil and environmental engineering, and Peter Nicholson, an associate professor of geotechnical engineering at the University of Hawaii, presented several findings at a hearing before the Senate Committee on Homeland Security and Governmental Affairs.

K. L. Bibb and R. K. Prabhu.

NTIS, Vol. N20040084368, 2004, pp. .

In support of the Columbia Accident Investigation, inviscid computations of the aerodynamic characteristics for various Shuttle Orbiter damage scenarios were performed using the FELISA unstructured CFD solver. Computed delta aerodynamics were compared with the reconstructed delta aerodynamics in order to postulate a progression of damage through the flight trajectory. By performing computations at hypervelocity flight and CF4 tunnel conditions, a bridge was provided between wind tunnel testing in Langley's 20-Inch CF4 facility and the flight environment experienced by Columbia during re-entry. The rapid modeling capability of the unstructured methodology allowed the computational effort to keep pace with the wind tunnel and, at times, guide the wind tunnel efforts. These computations provided a detailed view of the flowfield characteristics and the contribution of orbiter components (such as the vertical tail and wing) to aerodynamic forces and moments that were unavailable from wind tunnel testing. The damage scenarios are grouped into three categories. Initially, single and multiple missing full RCC panels were analyzed to determine the effect of damage location and magnitude on the aerodynamics. Next is a series of cases with progressive damage, increasing in severity, in the region of RCC panel 9. The final group is a set of wing leading edge and windward surface deformations that model possible structural deformation of the wing skin due to internal heating of the wing structure. By matching the aerodynamics from selected damage scenarios to the reconstructed flight aerodynamics, a progression of damage that is consistent with the flight data, debris forensics, and wind tunnel data is postulated.

Qing Zhou and T. X. Yu.

Journal of Engineering Mechanics, Vol. 130, No. 10, Oct. 2004, pp. 1177-1187.

The World Trade Center collapse has brought attention to progressive collapse of tall buildings and the study of possible countermeasures. From the viewpoint of energy transfer, this analysis explains why the collapse could not stop by itself once began. By introducing a design parameter called collapse stability index that controls design against progressive collapse, it is found that conventional design of a tall building usually leads to an inherently unstable structure in the event of a progressive collapse. In a subsequent feasibility study in this paper, a heavy-duty metal-based honeycomb energy absorbing structure is proposed. Using a finite element analysis, it is demonstrated that the structure is capable of absorbing potential energy released in a tall building collapse. The added energy absorbing devices will only occupy a small percentage of the total floor space. By properly designing and installing such devices, a progressive collapse, should it happen in a tall building, may be arrested within a few floors, and hence, the building is inherently stable to the progressive collapse. The theory is also elaborated with the example of the World Trade Center collapse.

A. De Luca.

STESSA 2003: Proceedings of the Conference on Behaviour of Steel Structures in Seismic Areas

The World Trade Center collapse started a wide scientific debate in the engineering community about tall building design and safety measurements. Some aspects of major interest are (1) the investigation aimed at understanding the causes of the structural failure and (2) the identification of possible improvements in the design approach for increasing robustness of such structures under exceptional loadings. The research towards these targets necessarily must start from the examination of the original design of the World Trade Center towers in order to understand the approach adopted in the structural design and to assess the ensuing performance implications. In this paper, elastic analyses (including P-delta effects) using a preliminary finite element model of World Trade Center Tower 1 are carried out with the aim of assessing the behaviour of the structure under gravity and wind loads.

A. S. Usmani, Y. C. Chung and J. L. Torero.

Fire Safety Journal, Vol. 38, No. 6, Oct. 2003, pp. 501-533.

This paper uses a finite-element model to investigate the stability of the Twin-Towers of the World Trade Center, New York for a number of different fire scenarios. This investigation does not take into account the structural damage caused by the terrorist attack. However, the fire scenarios included are based upon the likely fires that could have occurred as a result of the attack. A number of different explanations of how and why the Towers collapsed have appeared since the event. None of these however have adequately focused on the most important issue, namely 'what structural mechanisms led to the state which triggered the collapse'. Also, quite predictably, there are significant and fundamental differences in the explanations of the WTC collapses on offer so far. A complete consensus on any detailed explanation of the definitive causes and mechanisms of the collapse of these structures is well nigh impossible given the enormous uncertainties in key data (nature of the fires, damage to fire protection, heat transfer to structural members and nature and extent of structural damage for instance). There is, however, a consensus of sorts that the fires that burned in the structures after the attack had a big part to play in this collapse. The question is how big? Taking this to the extreme, this paper poses the hypothetical question, 'had there been no structural damage would the structure have survived fires of a similar magnitude'? A robust but simple computational and theoretical analysis has been carried out to answer this question. Robust because no gross assumptions have been made and varying important parameters over a wide range shows consistent behaviour supporting the overall conclusions. Simple because all results presented can be checked by any structural engineer either theoretically or using widely available structural analysis software tools. The results are illuminating and show that the structural system adopted for the Twin-Towers may have been unusually vulnerable to a major fire. The analysis results show a simple but unmistakable collapse mechanism that owes as much (or more) to the geometric thermal expansion effects as it does to the material effects of loss of strength and stiffness. The collapse mechanism discovered is a simple stability failure directly related to the effect of heating (fire). Additionally, the mechanism is not dependent upon failure of structural connections.

W. RYBCZYNSKI.

Discover, Vol. 23, No. 10, pp. 68-75, 2002, October.

More is learned from buildings that fall down than from those that do not. The performance of a building, complicated by aging, behavior of users, natural elements, and unnatural events, is difficult to simulate. The first investigative report of the World Trade Center (WTC) collapse reveals no design deficiencies and no specific structural features considered substandard. A major study will examine testing standards, building codes, and ways to protect existing buildings from terrorist attacks. Until the WTC attack, no high-rise blaze led to an actual structural collapse of an entire building. At the WTC, many survivors of the initial crashes were trapped in or above the impact zones. Each tower had three sets of fire stairs (two 44-inches wide and one 56-inches wide), all clustered together in the service core at the building's center, which also contained elevators, air-handling shafts, and bathrooms. High-rise buildings are designed with centrally-located cores to provide structural support and bracing and to hide mechanical functions in the least desirable part of the building. The vertical shafts--stairs, ducts, and elevators--act as chimneys during a fire and must be specially protected. Although the cores in the twin towers were built of closely spaced, massive steel columns and beams, the fire stairs were encased only by gypsum wallboard attached to metal studs: two 5/8-inch-thick layers of wallboard on the exterior and one on the interior. This can withstand fire for 2 hours but offers little resistance to even a hammer blow. All three sets of stairs in the north tower and two of three in the south tower were completely destroyed. Until the 1960s, structural steel was encased in poured concrete or brick. Its heavy mass absorbed the heat by dissipating it through dehydration. Because the weight of such fire protection increased costs significantly, lightweight substitutes were developed, usually spray-on coatings of mineral fibers. At the WTC, the quarter-inch coating of inorganic fibers was in the process of being thickened to 1-1/2 inches. The Pentagon, dating from the 1940s, is a bearing-wall structure of reinforced concrete, whose mass was more successful in absorbing the impact from the aircraft than the lightweight steel structure of the WTC towers.

W. E. LEARY.

New York Times, Vol. , No. , 2001, pp. F2, September 25.

Understanding the natural forces contributing to the collapse of the World Trade Center buildings can explain the destruction, why the buildings crashed down, and the enormous clouds of dust that erupted. The World Trade Center buildings contained 200,000 tons of steel, 425,000 cubic yards of concrete, and 600,000 square feet of glass. Each floor was a reinforced-concrete pad resting on a metal deck. Each floor was about 1 acre and weighed about 4.8 million pounds. When the airplanes struck the towers, the intense heat generated by burning jet fuel weakened the buildings' steel framework, causing the upper floors to collapse, which began the chain reaction borne along by gravity. Considering the size and weight of each floor pad shows why so much of the mass of each building was smashed into small pieces. As each floor fell, the combined weight of each additional floor pulverized the pieces until mostly dust remained. The estimated total energy released was equivalent to detonation of 600 tons of 2,4,6-trinitrotoluene (TNT). As the buildings fell, large plumes of dust moved away like high-speed jets. Engineers claim the compression of the buildings' air propelled the dust like an accordion pushes out air. The dust cloud spread at over 50 miles per hour and dumped dust as far as 2 miles away. Not only did the buildings' air compress, but as air rushed into the space that the buildings evacuated, a downdraft was formed that hit the debris below, pushing out even more dust. The buildings had a metal-tube-structure design, consisting of hundreds of steel columns around the outer face of each tower. The steel columns on each tower's sides kept the floors aligned as they fell. The buildings' frames just peeled back, staying just long enough for each floor to pass.

H. M. O'Connell, R. J. Dexter, P. E. Bergson and P. M. Bergson.

Vol. MNRC200110; PB2007112078, Mar 2001, pp. 89.

This research project resulted in a new, accurate way to assess fatigue cracking on Bridge 9340 on I-35, which crosses the Mississippi River near downtown Minneapolis. The research involved installation on both the main trusses and the floor truss to measure the live-load stress ranges. Researchers monitored the strain gages while trucks with known axle weights crossed the bridge under normal traffic. Researchers then developed two-and three-dimensional finite-element models of the bridge, and used the models to calculate the stress ranges throughout the deck truss. The bridge's deck truss has not experienced fatigue cracking, but it has many poor fatigue details on the main truss and floor truss system. The research helped determine that the fatigue cracking of the deck truss is not likely, which means that the bridge should not have any problems with fatigue cracking in the foreseeable future. As a result, Mn/DOT does not need to prematurely replace this bridge because of fatigue cracking, avoiding the high costs associated with such a large project. The research also has implications for other bridges. The project verified that the use of strain gages at key locations combined with detailed analysis help predict the bridge's behavior. In addition, the instrumentation plan can be used in other similar bridges.

W. Gene Corley and Ronald Sturm.

17, January/February 2001, pp. Forensic Examiner, vol. 10, no. 1/2.

This article discusses the data collected by the Federal Building Performance Assessment Team (BPAT) regarding the performance of the nine-story portion of the bombed Murrah Building (Oklahoma City), which sustained irreparable damage and significant collapse in the April 19, 1995 bomb blast. Structural drawings show that the Murrah building consisted of cast-in-place ordinary reinforced concrete framing with conventionally reinforced columns, girders, beams, slab bands, and a one-way slab system. Exterior spandrels that supported the exterior curtainwall were exposed concrete with a vertical-board-formed finish. Although nothing in the original documents, applicable code, or interviews indicated that blast or earthquake loading was to be considered in the design, the required wind-load resistance did provide substantial resistance to lateral load. Plans called for and confirmed that the design's live load requirements followed those of the Oklahoma Building Code. According to general notes on the structural drawings, the Murrah Building was all reinforced concrete that was proportioned, fabricated, and delivered in accordance with the American Concrete Institute Building Code Requirements for Reinforced Concrete. The level of structural detailing and the use of schedules with full dimensions for all slab, T-beam, spandrel beam, transfer girder, and column reinforcing bars were significantly better than normally expected for buildings of this type. Part II presents the findings of the laboratory studies and the resulting conclusions of the team. 3 references

W. Gene Corley and Ronald Sturm.

31, March/April 2001, pp. Forensic Examiner, vol. 10, no. 3/4.

This second part of a two-part article focuses on significant lessons learned from the Building Performance Assessment Team (BPAT) that investigated the damage caused by the bombing of the Alfred P. Murrah Federal Building in Oklahoma City, lessons that can help prevent similar disasters in the future. The Federal Emergency Management Agency (FEMA) assembled the BPAT, which was composed of American Society of Civil Engineers (ASCE) and Federal Government engineers. Forensic engineering techniques allowed the BPAT to determine the condition of the building before the blast, the blast size, and the mechanism of failure. These findings provided a means for assessing potential mitigation techniques. The team concluded that the building was designed as an ordinary reinforced-concrete-frame structure in accordance with Federal specifications. Records indicate that the building was extremely well detailed. When the building was designed, Oklahoma City codes did not require consideration for earthquake, blast, or other extreme loadings. The large explosion (equivalent to 4,000 pounds of TNT) was centered approximately 15.6 feet from column G20, which was immediately removed with a shattering effect. Loss of this column removed support for the transfer girder on the third floor between columns G16 and G24. The team found that the loss of columns G24, G20, and G16, along with significant portions of the floors above, was only partially attributable to the effect of the blast. The progressive collapse that followed was the direct cause of the majority of the damage and up to 90 percent of the fatalities. Columns G16 and G24 probably would have survived if detailed with more tie reinforcement. Increased concrete strength could also have increased the lateral load capacity of columns G16 and G24. If all techniques, including higher concrete strength, more continuity of reinforcement, and greater standoff distances were used, progressive collapse could have been reduced by 95 percent or more. 3 references