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.


pdf icon 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.


pdf icon 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.


UK report anticipated NIST calls

New Civil Engineer International 01/08/2005

VIRTUALLY ALL conclusions in NIST's official report into the collapse of the World Trade Center towers were reached in the UK three years ago, engineers have claimed.

A major report Safety in tall buildings was published in July 2002, just 10 months after the towers collapsed (NCEI August 2002). It was published by an Institution of Structural Engineers-led panel, which included ICE senior vice president and Jacobs Babtie director Gordon Masterton.

As recommendations are similar to those published by NIST. These included calls for buildings to be designed to survive complete burn out, include "robust, resilient and durable passive fire protection" and allow large-scale evacuation.

It also called for regular independent audits of fire protection systems.

"The interesting thing is that the more you read the more it looks similar to what we did in the UK after 9/11, " said Masterton. "Of the 30 recommendations 24 are identical messages to those contained in our report.

"It made me quite proud that what we produced in eight months at no cost contains commendations so similar to a $16M (£8.8M) investigation that has taken four years to come to its conclusions."


Huge investigation but questions remain

New Civil Engineer 22/09/2005

Painstaking reconstruction of the 9/11 attacks reveals much detail about the process of destruction.

America's National Institute for Science & Technology (NIST) revealed more detail than ever of its computer based analysis of the Twin Towers disaster of 11 September 2001 last week.

The ndings were presented and discussed at a conference on its campus at Gaithersburg near Washington DC. Not everyone agreed with everything that was said, but the findings of one of the most comprehensive structural failure investigations in history were fascinating to witness at first hand.

Million node structural and fire performance models of both towers and of the Boeing twin engined, wide bodied passenger jets which smashed into them enabled NIST to reproduce almost all the observed phenomena of the collapses.

And what became evident was the complex role of the fuel in the tanks of the Boeings at the moment of impact and in the microseconds that followed.

Without the fuel the thin skinned, largely aluminium planes would have been shredded by the protective 'fence' of high strength steel perimeter columns on the Leslie Robertson-designed towers. Undoubtedly, the engines could have caused serious damage ? NIST's simulations showed that a single engine had enough kinetic energy to smash through the perimeter 'fence' and still take out up to two columns in the core. And the tough steel landing gear was capable of considerable penetration as well.

But on impact each plane's wings and fuselage would have been reduced to aluminium confetti.

This, according to NIST, is precisely what happened to the outer wings and the front section of the fuselage. But inside the inner wing sections and the fuselage fuel tanks of both aircraft were some 38,000 litres of fuel.

The concentrated inertia of this fluid mass converted the fragile tanks into 800km/h sledgehammers. The perimeter columns yielded and failed, creating the gaping wounds whose iconic images still grip the attention of the world.

The same impact ruptured the fuel tanks. NIST's recreation of these moments assumes around 30% of the fuel released was consumed in the enormous fireballs which roared through both towers. Its model shows that most of the fuel cloud was confined within the impact floors.

NIST made no comment on why both Boeings were banking sharply to the left at the moment of impact. The result of the angled impacts was to spread fire and ruin over a number of floors, greatly increasing the initial loss of life.

The impacts and fireballs produced shockwaves and high velocity debris. NIST believes that fire-proofing in the impact zones was largely scoured away by the debris and the shockwaves, and that this loss of protection was the key factor in the ultimate collapses of both towers.

Other experts disagree (see News). But what is largely unchallenged so far is NIST's contention that on the impact floors the violent releases of kinetic and chemical energy blasted much of the floors' contents across to the far side of the buildings, where they were churned together with the debris from the planes and significant quantities of unburnt jet fuel.

This debris pile effect was more pronounced on WTC2 than on WTC1, NIST believes, due to the different geometries of impact. Add to that the greater number of windows blasted open by the fireball on WTC2 and the probable extra damage to the core columns, and NIST's explanation of why WTC2 collapsed so much earlier than WTC1 ? even though it was struck 18 minutes later ? is complete.

The debris pile in WTC2 was bigger and had plenty of oxygen available when it burned, so it burned hotter. Floor trusses and columns in this area were affected by the fire sooner. As a result, the hat truss at the top of WTC2 had a much harder job to redistribute loads from severed and softened columns, and failed sooner.


All this work is just the start

New Civil Engineer 22/09/2005

Can engineers really design for a 9/11 style attack on steel framed buildings? Dave Parker reports.

Do engineers really know how tall steel framed buildings will behave in an intense fire?

To judge from the views expressed at last week's conference, held in Washington to discuss the official inquest into the collapse of the World Trade Center towers on 11 September 2001, the answer is no.

In October 2002 the United States National Institute for Science & Technology (NIST) was asked to determine exactly what happened to the World Trade Center towers on 9 September 2001.

It was also asked to make recommendations about how to minimise the risks of similar calamities in the future.

NIST engineers must have known that whatever they discovered and concluded would not satisfy everyone.

Last week, on its campus just outside Washington DC, NIST set its conclusions and recommendations before an informed, and potentially sceptical, international audience of academics and fire engineering consultants.

The draft final report took nearly three years of work by 200 researchers. It cost $16M (£8.8M) and contains 10,000 pages. The verdict on the shocking collapse of WTC1 and WTC2 (the twin towers) is simple.

They absorbed the enormous kinetic energy of 100t Boeing aircraft striking them at 800km/h and could not cope with the fires that followed. There was no evidence of deliberate demolition or missile strikes, or internal explosions.

NIST investigators identified and located the 14 grades of steel used in the structures, and fed their unique properties into the computers. They took ultrasound measurements of the thickness of the steel tubes that make up the Boeing's landing gear.

They viewed 300 hours of videotapes of the tragedy, and correlated their massive computer models against them.

They performed simulations that took weeks to run.

They can explain almost every moment on every video ? except why aircraft landing gear components went right through the towers and out the far side.

It was a mammoth undertaking, without precedent, and NIST deserves congratulations for the depth and intensity of the investigation. But at the end of it all, engineers around the world are entitled to ask what it all means.

Yes, we have a reasonably convincing model of what happened on 9/11 ? but alternative models exist (see News). And yes, we have some obviously sensible recommendations on means of escape which will ensure that future tall buildings will have much more robust stairwells, and probably hardened elevator access for fire-fighters as well.

But what remained unchallenged last week was the acceptance that the traditional prescriptive approach to specifying fire resistance and more advanced performance based computer fire engineering techniques both have their weaknesses.

Many engineers now believe that current standard fire testing and the data it produces is no reliable guide to how structures and structural members behave in real fires. It would seem that this will still make it difficult for engineers to produce convincing designs for intense fires.

NIST's re test programme for the WTC investigation was just one piece of evidence to highlight the inadequacy of current test methods which generally rely on scale models.

Connections between structural elements are almost never tested at full scale, yet these are often the most vulnerable parts of a structure. The uncertainties that still remain after the NIST investigation ought at least to persuade the US government and perhaps its British counterpart that more money should be spent on full scale testing in future. Only then will the work done by NIST have created a truly worthwhile legacy.

Row erupts over why twin towers collapsed

New Civil Engineer 22/09/2005


A ROW over the causes of the World Trade Center twin tower collapses on 11 September 2001 broke out between British and American fire engineers last week.

British engineers strongly disputed official American claims that the towers became more vulnerable to collapse after the hijacked aircraft scraped vital fire protection from their steel frames.

The twin towers collapsed when each caught fire after terrorists flew hijacked Boeing passenger jets into them.

The disagreement provoked a strong exchange of views at a major conference held at Gaithersburg near Washington DC to discuss the official findings of America's National Institute for Standards & Technology (NIST) investigation into the 9/11 collapses.

"We don't believe that NIST has satisfactorily demonstrated that the loss of fire proofing was the deciding factor in the collapse, " said Arup associate director Dr Barbara Lane.

We have carried out computer simulations which show that the towers would have collapsed after a major fire on three floors at once, even with fireproofing in place and without any damage from plane impact." Lane said the difference of opinion was significant because clients had begun to demand that designs had NIST-compliant fire protection (NCE 30 June).

NIST is now recommending that all structural elements of tall buildings have the same degree of fire protection.

Firms like Arup have developed international reputations for producing designs which avoid the need for such extensive fi re protection.

Lane commended NIST's modelling exercise and agreed with most of its recommendations.

"But they have not taken proper account of the thermal expansion of the structural elements, especially the floors.

"As a result, there is too much emphasis on passive fire protection in NIST's recommendations and not enough on the benefits of good design." Dave Parker, Gaithersburg (See analysis p23) . ST's report into the baffling collapse of the 47 storey WTC7 tower on 9/11 has been delayed again, NIST confirmed last week. A draft for public comment is now not expected until May next year. In June NIST promised that the report would be out by the end of 2005 at the latest. It blames the latest delay on the need to concentrate resources on finalising the report into the twin towers collapse.

An Initial Microstructural Analysis of A36 Steel from WTC Building 7

An Initial Microstructural Analysis of A36 Steel from WTC Building 7

J.R. Barnett, R.R. Biederman, and R.D. Sisson, Jr.

OTHER ARTICLES IN THE WTC SERIES

"Why Did the World Trade Center Collapse? Science, Engineering, and Speculation" by Thomas Eagar and Christopher Musso

Better Materials Can Reduce the Threat from Terrorism by Toni G. Maréchaux

An Initial Microstructural Analysis of A36 Steel from WTC Building 7 by J.R. Barnett, R.R. Biederman, and R.D. Sisson, Jr.

News & Update


A section of an A36 wide flange beam retrieved from the collapsed World Trade Center Building 7 was examined to determine changes in the steel microstructure as a result of the terrorist attack on September 11, 2001. This building was not one of the original buildings attacked but it indirectly suffered severe damage and eventually collapsed. While the exact location of this beam could not be determined, the unexpected erosion of the steel found in this beam warranted a study of microstructural changes that occurred in this steel. Examination of other sections in this beam is underway.

ANALYSIS

Rapid deterioration of the steel was a result of heating with oxidation in combination with intergranular melting due to the presence of sulfur. The formation of the eutectic mixture of iron oxide and iron sulfide lowers the temperature at which liquid can form in this steel. This strongly suggests that the temperatures in this region of the steel beam approached ~1,000ºC, forming the eutectic liquid by a process similar to making a “blacksmith’s weld” in a hand forge.



Severely eroded I beam cross sections, nominal composition (%) of A36 steel plate is: (0.29C max, 0.80–1.2Mn, 0.04P, 0.05S, 0.15–0.3Si bal Fe).


Oxidation and intergranular melting; unetched.



Eutectic formation (iron oxide-iron sulfide), etched 4% natal.


An EDX Analysis of eutectic region.



The microstructure of unaffected A36 steel: (a-left) white-ferrite, dark-pearlite and (b-right) pearlite region. Pearlite forms in bands due to manganese segregation and prior hot working.



The authors acknowledge the assistance of Jeremy Bernier and Marco Fontecchio in preparing this information.

J.R. Barnett is a professor of fire protection engineering, and R.R. Biederman and R.D. Sisson, Jr. are professors of materials science and engineering, at Worcester Polytechnic Institute, Worcester, Massachusetts, 01609.

Prof. Dr.-Ing., P.E. Uwe Starossek (TUHH), Germany

PROGRESSIVE COLLAPSE OF STRUCTURES
Uwe Starossek
Prof. Dr.-Ing., P.E.
Structural Analysis and Steel Structures Institute
Hamburg University of Technology (TUHH), Germany
E-mail: starossek@tuhh.de

In terms of tragedy and losses the above mentioned cases of failure were far exceeded by the collapse on September 11th, 2001 of the twin towers of the World Trade Center. The impact of the airplane and the subsequent fire initiated local failures in the area of impact. The ensuing loss in vertical bearing capacity was limited to a few stories but extended over the complete cross section of the respective tower [9, 10]. The upper part of the structure started to move downwards and accumulated kinetic energy. The subsequent collision with the lower part of the structure, which was still intact, caused enormous impact forces which were far beyond the reserve capacities
of the structure. This, in turn, led to the complete loss of vertical bearing capacity in the area of the impact. Failure progressed in this manner and led to a total collapse.

Beyond Disaster (Architecture week)

Beyond Disaster

by ArchitectureWeek

In our second week since the terrorist disaster in New York, Washington, and Pennsylvania, emergency crews continue to work on rescue and recovery. Families, friends, a nation, and the world mourn heavy losses. The people of the United States, this magazine among them, struggle toward a distant normalcy.

New York Mayor Rudolph Giuliani updated the official casualty figures for the World Trade Center area on Thursday afternoon, September 20. The latest estimates suggest some 25,000 people successfully escaped the World Trade Center buildings. But as of this afternoon, 6333 people are missing or dead, with only 241 bodies recovered, and 6291 are counted injured.

Altogether secondary to the tragic, overwhelming human losses, the loss of buildings is also shocking. All seven buildings of the World Trade Center (WTC) complex are completely destroyed. Of nearly 300 acres (120 hectares) of commercial space, only a small portion of the north Plaza building still encloses recognizable space.

The list of collapsed buildings includes WTC 6, the U.S. Customs House to the north; WTC 3, the 22-story Marriott World Trade Center hotel just west of Tower Two; and WTC 4 and 5, the south and north Plaza Buildings to the east. Insurance companies currently estimate their costs for the World Trade Center disaster at about 30 billion dollars.

A neighboring church has also apparently collapsed, and repairable damage to surrounding buildings is extensive, including damage to One Liberty Plaza across Church Street to the east.


Open Questions

Professionals and other observers have already begun the necessary discussion of whether the structural performance of the World Trade Center towers should be applauded, because their initial resistance to tremendous impacts and accelerated fires allowed tens of thousands to escape, or criticized, because their sudden total collapse killed thousands more.

Experts are widely quoted as suggesting the collapse of the towers was inevitable — even though clearly not expected by the New York Fire Department. Wider speculation has come from structural experts and less authoritative observers.

From the time each was hit by an almost fully fueled hijacked 767 commercial jet, the south tower stood for 56 minutes, and the north tower stood for one hour and 40 minutes. Explanations of the resistance of the buildings point to standard fireproofing treatments and structural safety factors eventually overwhelmed by unexpectedly high temperatures.

Discussion of the building failures point to the loss of many who had survived the initial impact, and the historical statement of the original designers that they had designed in anticipation of the impact of a 707 jet airliner. Speculative critiques point to weaknesses in the perimeter columns, the floor systems, the internal core, possibly vulnerable shear connections between floors and core, a lack of internal columns between floors, and an overall lightweight, intentionally minimalist structural system.

The horrible power of "progressive collapse," in which the weight of some floors falling causes the floors below to fail as well, has been discussed by demolition experts and people of the street. Further speculation has concerned the adequacy of the fire suppression system, the fireproofing of structural steel, and the exiting stairwells.

It is extremely important to understand both the success and failure of these towers under extreme stress, partly to assuage a collective need to understand, partly as matter of forensic crime scene reconstruction, but most importantly to help the design professions create safer building codes and highrise buildings in the future.

Many of the theories now being offered contradict each other. No one has the necessary data yet to reach responsible professional conclusions regarding specific failure modes in the World Trade Center structures. Real answers can only come after a great deal of painstaking research, followed by calm, impartial, and in-depth structural analysis. Until investigation and analysis is complete, it will remain premature to make judgments about the causes of the collapse.

The issues are currently under investigation by the American Institute of Steel Construction (AISC), the Structural Engineering Institute of the American Society of Civil Engineers (ASCE), and others. In future issues, ArchitectureWeek will report on substantial findings.

An Architect in New York

For one architect who lives in Greenwich Village, a mile and a half from the New York disaster site, the recovery involves mourning both lost people and lost buildings. James Brogan, AIA, director of information technology of Kohn Pedersen Fox Associates, reports:

"The World Trade Towers, visible from any vantage point in our neighborhood, had always loomed over us as endearing landmarks and symbols of New York City, and as representations of late 20th century New York architecture. The events of the past week have touched the psyche of our neighborhood in profound ways.

"Greenwich Village was closed and secured on Tuesday afternoon September 11th, with vehicle access denied, stores and schools closed, people on the streets, fighter jets flying overhead. Our residential street was continually busy with racing ambulances for the first two days of the disaster — 10 to 15 at a time — literally all day and night. The steady sound of sirens and racing rescue vehicles were constant reminders that we were in the midst of something extraordinary, and horrifying.

"A disturbing quiet overtook the neighborhood on the third day, anticipating the grim news of a decline in locating injured victims. The racing rescue vehicles and wailing sirens were replaced by a pervasive and telling odor of burning debris permeating the neighborhood and our homes. Air Force jets and police helicopters continued to fly overhead at regular intervals, yet at street level all was unusually quiet.

"Looking south, a disturbing void reminds us of the horror of last week. Throughout the city, there are fliers with photographs of missing victims posted on bus shelters, light poles, and buildings that are constant reminders of how deeply this disaster has pervaded everyday life. This is a disquieting and distressing time — one that has touched us deeply and will affect each day of our lives in New York for many years."

Beyond Disaster

In New York and elsewhere, thousands of problems must be solved before we can return to normal life. One of these is the ongoing problem of hate crimes against immigrants and Americans of Middle Eastern origin. Unfortunately, there are too many people who may allow feelings of revenge to override fairness.

Paul Taylor, president of the National Organization of Minority Architects (NOMA), is using his position of leadership to remind our profession how they can help. In inviting architects to the NOMA Annual Conference in November, he writes:

In haste and in grief journalists and victims have inappropriately equated terrorists with Arabs and Muslims. This is a dangerous, unjust, and intolerable form of racism. It is now time for NOMA to play a role in the healing of America.

We want Arab-American and Muslim architects to know that we embrace you as friends. We extend to you a special invitation to our conference, where NOMA will demonstrate to the rest of the country how not only to tolerate diversity but to celebrate it. September 11, 2001 was a wake up call for Americans; now is the time for the profession of architecture to wake up and loudly exclaim that everyone is welcome.

In the 1960s, Minoru Yamasaki, architect of the World Trade Center, and himself a minority architect, also expressed the significance of humanism:

The World Trade Center should, because of its importance, become a living representation of man's belief in humanity, his need for individual dignity, his belief in the cooperation of men, and through this cooperation his ability to find greatness.

ArchitectureWeek joins NOMA and Yamasaki in calling for a spirit of humanity and diversity. In that spirit, as the United States prepares for war against a shadowy enemy, let Americans and our friends worldwide rally around these words of the pledge that helps define our nation: "with liberty and justice for all."

Related links:


About the World Trade Center
About the Pentagon
AISC to Investigate World Trade Center Collapse

Discuss this article at DesignCommunity.com


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The twin towers of the World Trade Center at night, before the contruction of Battery Park City.
Photo: Minoru Yamasaki Associates, Inc.

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The World Trade Center during construction, 30 years ago, bears an eerie resemblance to the destroyed ruin.
Photo: Minoru Yamasaki Associates, Inc.

ArchWeek Image

Site plan of the World Trade Center.
Image: descriptive drawing

ArchWeek Image

Typical floor plan of a World Trade Center tower, with open space free of interior columns.
Image: descriptive drawing

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Looking across the World Trade Center plaza and fountain.
Photo: GreatBuildings.com photo © Lawrence A. Martin

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This remarkable image sequence, photographed by architect Curtis J. Gibbs, shows the horrible progressive collapse of the north tower of the World Trade Center, one of the world's tallest buildings. Many had already evacuated, but thousands were still inside.
Photo: Curtis J. Gibbs.

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World Trade Center architect Minoru Yamasaki with a model of lower Manhattan.
Photo: Minoru Yamasaki Associates, Inc.

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Serious damage to the Pentagon building from impact and fire.
Photo: spaceimaging.com

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ASCE Comments on NIST WTC Report Recommendations

ASCE Comments on NIST WTC Report Recommendations


June 27, 2005

The American Society of Civil Engineers (ASCE) issued comments regarding a report by the National Institute of Standards and Technology (NIST) investigating the fires and collapses of New York City's World Trade Center (WTC) towers following the terrorist attacks of Sept. 11, 2001.

NIST urged the technical community to examine changes to design, materials and techniques - including possible changes to codes and standards - that could improve building performance and increase the safety of occupants and first responders. ASCE referenced a conclusion in the report that the WTC towers would likely not have collapsed if the spray-on fireproofing had not been dislodged by the impact of the aircraft.

ASCE cautioned that the NIST findings should not be interpreted as a "how to" guide for preventing a building's collapse when hit by an airplane.

"The lessons learned from this tragedy can help improve the ability of buildings to resist more routine fires," said Jeremy Isenberg, Ph.D., P.E., president of ASCE's Structural Engineering Institute. "Instead of revising building codes to address extreme events such as the impact of a large jet airliner, resources should be focused on improving fire-resistance methods for the conditions more likely to affect the types of buildings where most of us live and work. For extreme situations, it is best to direct resources toward preventing the attack."

The NIST recommendations, said ASCE, are largely consistent with the findings of the assessment report released in May 2002 by ASCE and the U.S. Federal Emergency Management Agency (FEMA). The ASCE/FEMA report recommended six items - including improvements in fireproofing, sprinkler systems and egress design - in the design and construction of buildings deemed likely targets of terrorist attacks.

Gene Corley, PhD., P.E., team leader for the ASCE/FEMA study team, concurred with the NIST recommendation for further research, and welcomed NIST's plan to host a conference for key standards-setting organizations to review and discuss the findings and recommendations. "We've certainly learned a lot from studying how the towers and the surrounding buildings performed when subjected to extreme forces. But we also have decades of experience with the performance of other structures subjected to intense fire," Corley said. "Do we know enough to change the building code and should those changes apply to all buildings, or just to tall buildings or buildings considered 'high profile' targets? We've got to reach a consensus on those questions, and we most certainly need further research on some of the key issues raised in this report."

Representatives of the Structural Engineering Institute of ASCE supported further study into the specific kinds of events and environments that compromise fireproofing materials, and called for the development of testing facilities capable of studying the effects of fire on long-span steel beams.

Experience at the WTC and the Pentagon, also the subject of an ASCE performance assessment following the terrorist attacks of Sept. 11, suggested that common fireproofing methods used in both steel-frame and reinforced concrete construction are vulnerable to being "scoured" by the force of the debris field following a significant impact.

Less clear, said ASCE, is the magnitude of the force needed to cause widespread damage to the fireproofing, and whether the type of spray-on fireproofing material used at the WTC remains intact in conditions of normal wear and tear, or even when subjected to the force of a major earthquake.

Building performance studies conducted following the Northridge earthquake of 1994, for example, found that seismic motion damaged many of the connections in the steel beams and columns of buildings, yet the fireproofing remained in place.

Source: American Society of Civil Engineers (ASCE).

ASTM Int'l Studies NIST Report on WTC Collapse

ASTM Int'l Studies NIST Report on WTC Collapse


October 17, 2005


In response to a National Institute of Standards and Technology (NIST) report on the fires and collapses of New York City's World Trade Center towers following Sept. 11, 2001, ASTM International Committee E05 on Fire Standards formed an advisory group that will: 1) review the draft report, 2) develop recommendations and 3) propose work items for consideration by Committee E05.

The report details recommendations made to organizations that develop building and fire safety codes, standards and practices for specific changes that would improve the safety of tall buildings, their occupants and first responders.

Specifically, the review will identify items in the NIST report that are involved with the following:

  • The development or revision of ASTM standards intended for analysis and assessment of structural fire protection and fire resistance of building assemblies.
  • ASTM standards that deal with buildings, structures, building materials, building assemblies, interior finish, furnishings and contents.
  • ASTM standards intended to measure and describe the response of building materials, products and assemblies to sources of heat and/or flame under controlled conditions, including fire growth rate and products of combustion, for example, smoke and toxic gas.
  • The evaluation of E05 standards related to computer fire modeling and determination as to whether new standards or revisions to existing standards are needed.
  • The development or revision of fire test methods for measuring fire responses and properties of materials, products and assemblies when exposed to laboratory sources of heat, or flame, or both, especially input data from computer fire modeling.
  • Fire-related research relevant to E05 activities.
  • Analysis of additional ASTM standards mentioned in the report, including those not under the jurisdiction of Committee E05.

John Roberts

How have engineers learned from the attack on New York’s World Trade Center? More than two years since the tragedy of 9/11, John Roberts writes on the structures of tall buildings and how they can be changed to reduce the likelihood of this happening again.

September 11 2003 was the second anniversary of the infamous attacks on the World Trade Center, prompting renewed interest in the questions about what effect this tragedy has had on structural engineering and on the design of tall buildings. In the immediate aftermath of the deliberate attack, the unprecedented collapse of such massive and iconic structures and the appalling loss of life, engineers have given intense consideration to a number of key questions. These include:
● Was this event so extreme and ‘unrepeatable’ so as to not be relevant for any other building?
● Did the WTC towers perform in a proportionate way to the extreme conditions imposed by the attack?
● Should engineers design some types of key buildings to protect against analogous extreme/deliberate attacks in the future?
● Are there practical and cost-effective changes to current design practice that could significantly enhance the safety of people in and around such buildings in the future, if an extreme event occurred again? In fact, although there was a great deal of (entirely understandable) initial comment on many of these issues, as is usual with such disasters a detailed investigation was required into exactly what happened – the chain of events leading to the total collapse of the two 110-storey towers and the loss of over 3000 lives. This examination was carried out with exemplary speed, thoroughness and openness by a US government agency, FEMA (Federal Emergency Management Agency).1
A summary of the sequence of events is given below, and it is this understanding that informs the views of engineers as to the questions posed above.
● The plane impact caused extensive physical damage to (in each case) about three storeys and at least twothirds of all the columns on the face of the building where the plane impacted.
● Two other events occurred immediately, i.e. the commencement of fires burning across several floors, and the impact damage to some of the insulating materials which provide fire protection to the structural steelwork in these buildings.
● As far as the fire is concerned, it has been widely, and incorrectly, reported in the media that the WTC fires were particularly intense due to the burning of hydrocarbons (i.e. the unused aviation fuel). In fact the fuel burned out very rapidly and the real fire damage came from the very widespread ‘ordinary combustion’ of the contents of the building, i.e. the desks, furniture, paper, etc. Simple calculations show that the fire load on each floor of the towers from these sources greatly exceeded the total fire load of the aviation fuel delivered into the building from the aircraft impacts.
● Structural steel is not a combustible material, but it is seriously weakened at temperatures above about 600ºC. As a consequence it requires fire protection when used in multi-storey buildings. At the WTC the form of the steelwork, the form of the fire protection and the severe disturbance caused by the plane impact combined to leave the steel frame – already locally damaged by the impact, but still at that stage very successfully carrying the massive loading from the 250,000-tonne weight of the building – vulnerable to weakening in a fire.
● The gradual heating of the steel frame due to the fire, now acting on damaged and probably ‘unprotected’ steel, caused sufficient local collapses of either the floors or the supporting columns to act as a trigger event to initiate a progressive collapse of the whole building. Lower sections, not damaged by fire or impact, were unable to survive the dynamic loading from falling upper sections of the building, each floor of which weighed over 2000 tonnes. The main conclusions that can be drawn from the FEMA investigations as regards the performance of tall or large buildings subjected to extreme events fall naturally into three distinct but interrelated groups.2
First is the prevention of what is termed ‘progressive collapse’ of structures (this is where an initial failure of a relatively small part of a structure can trigger a collapse of a significant further part or even, as at the WTC towers, the whole structure). Measures to reduce the risk of progressive collapse include:
● the use of robust, ductile and energy-absorbing structural elements and connections
● providing multiple ‘load paths’ (i.e. adopting the principle of redundancy and avoiding the use of critical elements such as transfer structures whose failure would compromise the overall safety of the building). The second issue relates to improvements to the fire resistance of structures. In most modern forms of construction, adequate fire resistance is provided to allow people using the building time to escape when threatened with what might be termed a ‘conventional’ fire, i.e. a relatively localised fire that occurs due to an accidental cause or even one that is started maliciously. However, the experience of the WTC events highlights the dual problem of widespread fires started very rapidly, accompanied by impact or other physical damage at the same time. Structures and key parts of the fabric of a building have routinely been designed and constructed so as to comply with fire resistance standards, but (not unnaturally) these standards have previously concentrated on the single issue of performance in fire. In fact, for resistance to extreme events, consideration needs to be given to the general robustness and impact resistance of fire-protection systems. These include not only applied materials (such as spray finishes, or boards, designed to enhance the fire resistance of structural steel), but also so-called active, or secondary, fire-protection systems such as sprinklers which can be damaged (e.g. by impact or explosion) and thereby rendered dysfunctional at exactly the time when they are needed to perform their crucial fire-protection role. Finally, consideration must be given to the escape routes used by people to safely exit from buildings during an extreme event. This issue is conditioned in part by the sobering fact that in the two WTC towers, only four people at or above the floor level impacted by the two planes survived, whereas over 99% of the occupants below these levels managed to escape. The fire escape routes, i.e. the protected staircases, were all completely severed by the impact and the ensuing fire, trapping the occupants who were at or above those levels. Had one or more of the staircases survived intact, or had working lifts in lift-shafts also survived, then many more people might have escaped. Again the same issues are highlighted – that safety-critical elements of buildings should be robust, be able to withstand impacts and absorb energy, and be separated physically to provide a multiple choice of routes. In very large occupancy buildings (some of which may have populations of tens of thousands of people) the management of evacuation and escape needs critically reviewing. Current practice is to adopt what is called phased evacuation (i.e. the evacuation of only immediately threatened zones of occupation) and to use staircases only rather than lifts. But if simultaneous evacuation of a building in the fastest possible time is required, then the provision of ‘safe’ lifts would dramatically reduce the time required to evacuate all occupants if a lifethreatening event were occurring. In raising these issues, engineers involved in building design are effectively acknowledging that the basis on which current standards of building performances are based – i.e. that the design should perform and provide for all anticipated ‘natural’ or ‘foreseeable/accidental’ events – may sadly no longer be a sufficient safeguard for some particularly vulnerable buildings. To have to consider a new and sinister threat of deliberate attacks on major building structures is a difficult and uncomfortable situation to accept, but one which engineers are equipped to deal with, if the need arises.
■ References 1 ‘World Trade Center Building Performance Study’, FEMA 403/May 2002. Washington DC. 2 ‘Safety in Tall Buildings’, Institution of Structural Engineers, July 2002, London. Acknowledgements This article is based on a presentation given by the author at the British Association for the Advancement of Science meeting on 11 September 2002, a summary of which was published in the BA magazine. The author is chairman of the Institution of Structural Engineers working group on ‘Safety in Tall Buildings’ which published the report at reference 2. John Roberts is a Director at Babtie Group, technical and management consultants, and a Fellow of the Royal Academy of Engineering. He was President of the Institution of Structural Engineers, 1999–2000 and is also a Visiting Professor in the Principles of Engineering Design at the Manchester Centre for Construction and Construction Engineering (UMIST/University of Manchester).

Another story by John Roberts in the Structural engineer.

Beyond the Towers: Performance of Masonry

Beyond the Towers: Performance of Masonry
By David T. Biggs, P.E., Ryan-Biggs Associates
Masonry Home > Products and Properties > Beyond the Towers

Comparing the present with the past in the world around us can be an important learning experience. Such was the case for the Federal Emergency Management Agency (FEMA) and the American Society of Civil Engineers (ASCE), in the difficult task of conducting an evaluation of the World Trade Center (WTC) and surrounding buildings.


The Event

On September 11, 2001, airplanes struck two 110-story office towers in New York and the Pentagon in Washington, D.C. The towers (WTC 1 and WTC 2) collapsed in less than two hours, and another building in the complex (WTC 7) collapsed later in the afternoon. Thesebuildings had few or no masonry components. All of the surrounding buildings suffered damage from falling debris, wreckage, and fire fromthe towers. While the impact of portions of the
collapsing buildings did the majority of harm, there was also damage from flying debris and air to the masonry used in their construction.


The Investigation

Six days after the attacks, teams of engineers, notably from the Structural Engineering Association of New York and the New York City Department of Buildings, began evaluating the structural integrity of over 400 buildings surrounding the WTC site. Using a rapid visual evaluation system, they performed immediate assessments and made decisions regarding continued use and occupancy of buildings in lower Manhattan. Damage was described as either structural or collateral. The buildings with structural damage were inspected in detail.

Buildings immediately surrounding the plaza were most heavily affected by the disaster. They weregenerally constructed from 1906 through the 1980s; most had some masonry components.


Masonry Performance: Verizon Building, 140 West Street

One of the closest neighbors to the WTC site, the 30-story Verizon building, is a steel-framed brick building constructed circa 1924. The typical floors, composed of concrete-encased steel beams and girders, are redundant and robust. The exterior face of the perimeter framing is encased in brick and serves as both exterior wall and infill. Header bricks connect the exterior brick to the backup and support the weight of the exterior wythe. Columns are also brick encased.

The façade, floors, and framing of the south and east sides of a building that sustained heavy damageThe façade, floors, and framing of the south and east sides of the building sustained heavy damage, as did several exterior columns. Although the framing deflected as much as 2 ft into the building, the masonry infill restrained the columns from collapse. The steel structure was not affected. None of the damage threatened the structural
integrity of the building. And although there was a substantial fire in WTC 7, there were no fires reported in this building. It was never out of service. The structure was shored and repaired.

Masonry improved the performance of the Verizon building because:

  • Perimeter brick masonry walls absorbed much of the impact from both WTC 1 and WTC 7, resulting in less damage to the steel structure.

  • Masonry infill of the exterior wall provided a redundant load path and helped prevent collapse.

  • Framing damage was localized, partially due to masonry infill.

  • Exterior brick headers held the upper brick from collapsing above a damaged section.

  • Brick encasement of the columns and concrete encasement of the steel framing provided fire and impact resistance.

  • Built-up sections appeared to be more ductile and better able to absorb energy along with the masonry infill.

  • The building did not experience fire damage—masonry and (safety glass) windows limited penetrations through exterior walls.

Summary

In the towers themselves, the stairwell and elevator walls were constructed
of gypsum products. Evidence indicates most of the floors of impact were
damaged and rendered unusable. While it is presumptuous to assume that
masonry enclosures would have survived the attacks, it is obvious that more
durable wall systems would have improved chances for survival for occupants
above the level of impact. Future research should be devoted to evaluating
and developing durable, fire-rated egress enclosures for high-rise buildings.
Reinforced masonry and concrete are two effective solutions that can be used
without further development.

Example after example demonstrates how masonry helped prevent greater
destruction during the World Trade Center disaster. Some of the lessons learned:
  • Older framed buildings with masonry components performed generally better than newer buildings with lightweight curtain wall construction.

  • Masonry (walls, beams, partitions, infill) served as fireproofing and provided significant structural redundancy.

  • Masonry infill absorbed impact energy to minimize damage locally.

  • Masonry veneers and panelized systems are readily repaired.

Masonry proved in this event that it does more than simply enclose space; it
provides fire protection, structural capacity, and even structural redundancy.
It can provide safer enclosures for stairways or other exit routes, affording egress in high-rise buildings during emergencies.

Kurt McMullin

Fire, not impact, may have doomed Trade Center, says prof

Silicon Valley / San Jose Business Journal - September 11, 2001

The two towers of the World Trade Center may have collapsed because the planes which crashed into them this morning scraped away protective coatings on steel beams allowing incredibly intense fires to soften supporting beams, according to a San Jose State University professor who has worked on buildings of similar size.

While not drawing a conclusion as to the cause of the collapse, a Stanford University professor has estimated the intensity of the fires, likening them to the explosion of an atomic bomb.

"The planes hitting the sides of the buildings probably did not do that much structural damage," says Kurt McMullin, professor of civil engineering at San Jose State.

"The impact of the jets probably knocked a lot of fireproofing off [the steel girders]," he says. "Losing the glass windows allowed the fire to travel to several floors. It just led to a complete collapse of the steel frame which then dropped all the floors above.

"We design buildings to withstand high intensity fires for a limited time," he says. "[But] once steel becomes hot it becomes soft and it loses its strength."

The buildings, engineered to withstand the force of a hurricane, should have been able to withstand the impact of the planes, he says.

But the impact was hardly inconsequential, according to a Stanford University professor.

According to applied physics professor Steve Block's calculations, the amount of energy released by each plane that hit the World Trade Center was equivalent to about 1,000 tons of TNT-- or 1/20th of the energy released by the Hiroshima atomic bomb in 1945. Both planes combined released one-tenth of the energy produced at Hiroshima, according to Mr. Block's calculations.

A typical commercial Boeing 767 jet weighs about 412,000 pounds on takeoff -- including people, luggage and 24,000 gallons of fuel, he says. If you combine the energy released by the burning fuel [equivalent to about 1,000 tons of TNT] with the kinetic (motion) energy caused by the collision [equivalent to about one ton of TNT], you get the equivalent of roughly 1,001 tons (1.001 kilotons) of TNT -- or one-twentieth of the Hiroshima bomb, Mr. Block says.

Mr. McMullin noted that the Empire State Building withstood the impact of a World War II bomber which plowed into it in a dense fog in 1945. The difference this time, he believes, may have been the size of the fires generated by the impacts.

 
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