Analysis of Aircraft Impacts into the World Trade Center

Analysis of Aircraft Impacts into the World Trade Center Towers Chapters 1, 2, 3, 4, 5, 6, 7 and 8. Federal Building and Fire Safety Investigation of the World Trade Center Disaster.

---

Analysis of Aircraft Impacts into the World Trade Center Towers Chapters 1, 2, 3, 4, 5, 6, 7 and 8. Federal Building and Fire Safety Investigation of the World Trade Center Disaster. (36044 K)
Kirkpatrick, S. W.; Bocchieri, R. T.; Sadek, F.; MacNeill, R. A.; Holmes, S.; Peterson, B. D.; Cilke, R. W.; Navarro, C.

NIST NCSTAR 1-2B; 290 p. September 2005.

Keywords:

World Trade Center; high rise buildings; building collapse; disasters; fire safety; fire investigations; terrorists; terrorism; aircraft impact; impact; failure; aircraft fuels; dispersons; structural dynamics; uncertainty; damage; structural damage

Abstract:

The objective of this report was to analyze the aircraft impacts into each of the World Trade Center (WTC) towers to provide the following: (1) estimates of probable damage to structural systems due to aircraft impact, including exterior walls, floor systems, and interior core columns; (2) estimates of the aircraft fuel dispersal during the impact; (3) estimates of debris damage to the interior tower contents, including partitions and workstations. Thus, this analysis established the initial conditions for the fire dynamics modeling and the thermal-structural response and collapse initiation analysis. The impact analyses were conducted at various levels of complexity including: (1) the component level, (2) the subassembly level, and (3) the global level to estimate the probable damage to the towers due to aircraft impact. Simplified analyses were also used to support the development of the global finite element models. Analysis of uncertainties using the component and subassembly analyses were conducted to assess the effects of variability associated with various input parameters and identify the most influential parameters that affect the damage estimates using orthogonal factorial design. Based on the results of the sensitivity analyses, the most influential parameters identified were varied in the global models to provide a range of damage estimates for WTC 1 and WTC 2. As part of the tower and aircraft models, constitutive relationships describing the actual behavior of the structures under the dynamic impact conditions of the aircraft were developed based on test results of the tower steels and from the open literature for other materials. Various grades of steels used in the exterior walls and core columns of the towers, weldment metal, bolts, reinforced concrete, aircraft materials, and nonstructural contents were considered. The constitutive relationships included high strain-rate effects and failure criteria for the various materials. The tower models used in the global impact analyses were developed based on the original WTC drawings and the structural databases of the towers developed within the framework of the baseline structural performance analysis. The tower models included the primary structural components of the towers in the impact zone, including exterior walls, floor systems, core columns, and connections. A refined finite element mesh was used for the areas in the path of the aircraft and a coarser mesh was used elsewhere. The models also included the nonstructural building contents, such as partitions and workstations, in the path of the aircraft debris. The Boeing 767 aircraft model was developed based on information gathered from documentary aircraft structural information, and data from measurements on a Boeing 767 aircraft. The model included the aircraft engines, wings, fuselage, empennage, and landing gear, as well as nonstructural components of the aircraft. A detailed analysis was carried out to estimate the fuel distribution in the aircraft wings at the time of impact.

Simple Model of the World Trade Center Fireball Dynamics.

Simple Model of the World Trade Center Fireball Dynamics.

---

Simple Model of the World Trade Center Fireball Dynamics. (502 K)
Baum, H. R.; Rehm, R. G.

Combustion Institute, Symposium (International) on Combustion, 30th. Proceedings. Volume 30. Part 2. July 25-30, 2004, Chicago, IL, Combustion Institute, Pittsburgh, PA, Chen, J. H.; Colket, M. D.; Barlow, R. S.; Yetter, R. A., Editor(s)(s), 2247-2254 pp, 2005.

Keywords:

combustion; World Trade Center; fireballs; flame spread; fluid mechanics; safety; equations; conservation; heat release rate; velocity field; mathematical models; computational fluid dynamics

Abstract:

An analytical model of the initial expansion of a fireball is presented. The model is based on an exact solution of the low Mach number combustion equations in the form initially proposed by the authors. The equations consist of the conservation of mass, momentum, and energy with an isobaric equation of state. The heat release rate is a prescribed spherically symmetric function characterized by a flame expansion velocity, a flame brush thickness that increases with time, and a heat release rate per unit surface area. The introduction of a prescribed heat release rate obviates the need for an explicit turbulence model. Thus, the inviscid forms of the conservation equations can be used in the analysis. The velocity field is decomposed into a spherically symmetric expansion field and a solenoidal component determined by the buoyancy induced vorticity field. The expansion field together with the induced pressure rise and temperature fields are spherically symmetric. However, the buoyancy forces induce vorticity where the temperature changes rapidly and break the spherical symmetry of the velocity field. The solution is used to study the initial expansion of the fireballs generated in the attack on the World Trade Center south tower. Video images are used to estimate the expansion rate of the fireball. This information, when combined with the analysis, leads to an estimate of the fuel consumed in the fireball that is independent of any assumptions about either the initial fuel distribution or the state of the building following the crash.

Civils fear 9/11 report will stifle fire engineering

Civils fear 9/11 report will stifle fire engineering
New Civil Engineer 16/06/2005
Dave Parker
BRITISH ENGINEERS fear that all structural steel will need impact and fire resistant coatings because of recommendations in next week's fi nal US report into the collapse of the World Trade Center towers. This could cripple modern fire engineering techniques, they warned. The move would be a response to the scale of the disaster and to the concerns of building owners and insurers. The US National Institute for Standards & Technology (NIST) will launch its final report and recommendations after its nearly four-year investigation into the 9/11 terrorist attacks in New York next Thursday. Much of the 10,000 page report into the collapses has already been published (NCE 21 October 2004). NIST has concluded that the rapid progressive collapse of the twin World Trade Center towers was triggered by fire -induced buckling of steel perimeter columns around the impact zones of the two Boeings that flew into the structures. The columns failed because their spray-applied fire protection was blasted off by the impact forces. So far NIST has made no recommendations for amendments to building codes and standards, but it will next week. Engineers are concerned that advanced fire engineering techniques developed in Britain will be sidelined in favour of more conservative alternatives. "Fire engineering makes it possible to leave some steel members unprotected, saving time and money, " said Waterman Group managing director Bob Campbell. "It will be interesting to see if NIST will come down on the side of fire engineering, especially for high rise, or if the conservative, prescriptive route is adopted." Any recommendations NIST makes are bound to have an enormous impact on US and international building codes.

Fire protection demands threaten to stifle design innovation

Fire protection demands threaten to stifle design innovation
New Civil Engineer 30/06/2005
Mark Hansford
ENGINEERS FEAR their efforts to produce innovative tall building designs will be stifled following publication of last week's comprehensive report into the World Trade Center tower collapses in 2001. But designers in the UK and the US warned this week that this could halt efforts to push the boundaries of tall building design. The US National Institute of Standards & Technology report on the 9/11 attacks makes 30 recommendations, many relating to fire protection improvements. One of the recommendations calls for US-wide adoption of the "structural frame" approach to firesistance ratings, whereby all structural members must carry the same fire rating. This is expected to make American designs more conservative. "America lags behind the UK in terms of what we are doing to reduce fire protection and any attempt to push that boundary will be met with some resistance, " said Yolles director of structures Richard Thiemann. Yolles works on both sides of the Atlantic. "It does mean you're going to get some very over-engineered buildings which is a price people are willing to pay for perceived improvements in safety." In Europe, engineers believe the effect of the recommendations will be less severe but will still put pressure on engineers to raise standards. Already, UK engineers are facing greater pressure to justify cutting edge "fi e engineered" tall building designs. "We are good at designing down fire protection. You can be taking out 50% of fire protection but still maintain building integrity. The savings can be quite signifi cant, " said Thiemann. "But you can't be seen to be doing anything less than the top level of care. Clients and tenants will be scrutinising decisions more carefully. So the standard of fire engineering will have to improve. It will simply have to become a more technical, skilled art, " he said. Buro Happold partner Mick Green agreed. "With he right applications and research, we can design buildings that can withstand fire, " he said. Jacobs Babtie director Gordon Masterton agreed that the report will increase the importance of fire safety engineering. "Getting architects and engineers to understand fire safety engineering is the most important thing, " he said.

Lasting lessons of WTC

New Civil Engineer 30/06/2005

Unthinkable. The single word on NCE's 13 September 2001 issue, pasted over an image of the fi rst World Trade Center tower in freefall, still rings true. Just hours earlier, it had been genuinely inconceivable that structures of such magnitude could succumb to this fate. Progressive collapse was by no means a new concept in 2001, yet on that bright autumn morning in Manhattan the world witnessed two of the most comprehensive examples of the phenomenon ever seen. It is obvious to state that the impact and result of deliberately crashing a fully fuelled airliner into each tower was off the scale of the predictable. But there can have been few structural engineers who were not a little surprised to see two of the world's tallest buildings reduced to rubble less than an hour later. This is not how structures are supposed to react. Four years on we know pretty much all we can about how and why the WTC towers collapsed. Yet the report and recommendations this week by the US National Institute of Standards & Technology following its exhaustive investigation into the events of 9/11 must be compulsory reading for all design professionals. The changes it urges will without question have a massive impact on building designs across the world. Many in the UK will doubtless argue that NIST has adopted an overly prescriptive approach which will rule out the "new" science of fi e engineering in favour of expensive over-engineered solutions. Perhaps. But it is absolutely right that at the heart of NIST's recommendation is the need for designers of future tall building really to get to grips with disproportionate collapse and ensure that no matter how damaged, buildings will never fall down. For British engineers, the Ronan Point collapse changed building design philosophy 40 years ago. And the Oklahoma City bombing 10 years ago prompted the US to rethink its federal building codes. Progressive and disproportionate collapse mechanisms are so well understood today it is crazy not to insist that all buildings meet minimum standards of structural redundancy. And it is absolutely right for NIST to insist that designers also get to grips with the design of routes, strategies and facilities to get occupants out of a burning or damaged buildings quickly. We have the technology, the modelling techniques and the materials to provide proper escape paths and routes in for firefi hters. And there is no reason why we cannot make fi -protected and structurally hardened escape routes in tall buildings mainstream. Major shifts in design thinking are usually prompted by major disasters. When infrastructure fails, engineers learn. So while terrorists brought down the World Trade Center towers, we cannot ignore the lessons. As always, the crucial step is being bold enough to turn the learning into action to ensure that the risk of repeat is controlled. The world is certainly a different place since 9/11 redefined the concept of terrorist threat. NIST's recommendations this week show how far the structural engineering profession must now go in response. The recommendations are tough and will no doubt be considered by many in the profession, at times, as a step too far. But we need to heed them to ensure the world's concept of what is "unthinkable" is never again experienced. Antony Oliver is editor of NCE

Collapse mechanisms

New Civil Engineer 07/07/2005

Fire engineers are still debating the exact sequence of events that caused the catastrophic collapse of the iconic Twin Towers (NCE 30 June). Conspiracy theorists still pour over every detail of every report into the disaster, looking for apparent anomalies and contradictions. But most engineers have long since accepted the basic scenario as established both by the NIST investigation and the earlier analysis by the American Society of Civil Engineers and the Federal Emergency Management Agency. Put simply, WTC1 and WTC2 collapsed primarily because terrorists flew fuel laden wide bodied jet airliners into them at high speed. Each tower was struck at a different angle and sustained slightly different damage. Had it been 300t Boeing 747s hitting the towers square on instead of 100t 757s, collapse would have been virtually instantaneous, as too many structural members would have been severed in the impacts. Instead, the very strong "perimeter tube" structure of the towers took massive damage ? but there were enough alternative load paths around the gaping holes left by the Boeings for the structures to remain stable. However, as the shredding, disintegrating remains of the aircraft plunged further into the towers, they dealt three more blows that were to prove fatal. In both towers the wreckage penetrated the core, damaging and severing core columns and blocking escape stairs. And in both cases the impact also severed the single water supply line feeding the sprinkler system. Worst of all was the effect on the spray applied fire protection that coated all the structural steelwork. How much of this was dislodged by the initial impacts will never be known. In its simulations NIST chose to assume that the steel only lost protection by debris scour. Jet fuel remaining after the initial fireballs splashed through the upper floors of the towers and poured down the cores. It burned only for a few minutes, but this was long enough to ignite the office contents on many floors. Flames from these fires began to heat up the columns and floor trusses that were now naked and unprotected. In the immediate aftermath of the 9/11 tragedy, many eminent engineers voiced their suspicions about the vulnerability of the floor truss/column joints to fire. NIST believes these joints in fact remained intact except where they had taken direct impact. NIST also believes that a combination of core column shortening and floor truss sagging eventually pulled heat softened perimeter columns inwards, triggering progressive buckling and structural failure. These opinions are based partly on an exhaustive study of nearly 7,000 videos and much the same number of still photographs and the examination of 236 fragments of steel from the towers, partly on large scale fire testing of the floor trusses, and partly on computer simulations of the effects of both aircraft impact and fire spread. Once the upper storeys began their downward plunge, the effects were brutal. Entire office floors including all contents were literally pulverised to dust by the massive kinetic energy of the falling towers.