Fire Testing Is Questioned in Findings on Towers

Fire Testing Is Questioned in Findings on Towers


Published: August 26, 2004

NORTHBROOK, Ill., Aug. 25 - For more than two years, federal investigators have been struggling to resolve a critical and contentious question concerning the collapse of the World Trade Center towers: was the spray-on fireproofing initially placed on the twin towers' innovative lightweight floors sufficient to protect them in a major fire?

Now, a series of federally sponsored tests that ended here today has produced a provocative but complex finding: the fireproofing, as it was installed during the construction of the trade center in the 1960's, met the standards of the day.

But, in a conclusion that may have ramifications for understanding other tall buildings and future structures, investigators from the National Institute of Standards and Technology found that the test used to determine fireproofing sufficiency, then and now, may itself be flawed - unable to predict accurately what will be required in a real-life fire. As a result, the towers indeed may been more vulnerable to a fire than anyone could have known.

The questions about the fireproofing in the towers have become part of an emotional debate over whether the Port Authority of New York and New Jersey, which oversaw the building of the trade center, and the structural engineer involved in its design deserve part of the blame for the towers' collapse. Doubts have been raised not only about whether the original fireproofing was sufficient, but also about whether the Port Authority did enough to make sure that the lightweight, spray-on material did not fall off as the years passed, as inspections conducted at one point suggested might have been happening.

The debate over the sufficiency of the fireproofing on the World Trade Center's lightweight floors - essentially metal and concrete decks supported underneath by a series of inch-thick zigzagging rods - intensified in May 2003 when federal investigators concluded that the Port Authority, back in the late 1960's, apparently never performed the formal laboratory fire test on the design.

That meant there was no way for the Port Authority to say for sure that the towers' floors would hold up against an extremely intense two-hour fire, as was required then under the New York City building code. The Port Authority, when the towers were built, said it was committed to meeting or exceeding the city code, even though as an agency created by the two states, it was not required to do so.

For federal investigators, then, determining just how well protected the building was with fireproofing - the questions of whether it met basic standards and whether the standards were based on good science - took on critical importance.

The investigators set up a testing program at the nation's most famous fire-testing site, the Underwriters Laboratories, which owns a giant oven into which sections of floor can be placed and burned. The investigators had to decide first what thickness of fireproofing to test, since the original Port Authority plans called for half-inch amounts while the buildings, as actually constructed, ended up with three-quarters of an inch on the floor trusses. They decided to conduct it both ways.

They also decided to take the unusual step of testing longer sections of floor than normally required, to reflect more accurately their lengths as they existed in the trade center buildings. In each of the tests, the pieces of floor would be subjected to flames reaching 2,000 degrees and monitored to see if they maintained their ability to support a large amount of weight and prevent the spread of intense heat for at least two hours. The two-hour measure is the standard required in the city's building code at the time.

The piece of flooring with the half-inch coating of fireproofing failed. That result meant that the trade center's design, despite contentions by the Port Authority, would not have met the city's code. But the shorter piece of flooring covered with the three-quarter inch layer of fireproofing - equivalent to what was in the towers when they were completed - did last the two hours. As a result, this would allow the Port Authority to claim that the towers, as built, would have met the city's standard, even if the thicker fireproofing might have been a quirk that resulted from the way the contractors sprayed it on nearly 40 years ago.

It was the different results that surfaced when the longer pieces of floor - the ones that more accurately reflected floor sections used in the trade center - were tested that have provoked concerns about the legitimacy of the widely accepted furnace tests. One of the larger pieces of floor - the one that was set up to simulate the restraint applied by a real-life building - failed in a fire test.These results left investigators with a disturbing reality: in the test in which they used the equivalent of a scale-model toy car, the results suggested that the fireproofing was sufficient. But when they used what would have been the equivalent of a real car, the fireproofing failed.

"We want a link between the performance in the lab test and the performance in a real-scale, live situation," said William Grosshandler, chief of the fire research division of the standards institute. "We need to understand two tests came out differently."

Steve Coleman, a Port Authority spokesman, said the agency would have no comment on the test results until it had a chance to review them. Monica Gabrielle, who had traveled to the Underwriters Laboratories to witness the final test on behalf the Skyscraper Safety Campaign, said she ended the day simply with confusion.

"This is supposed to be science," she said, referring to the differing test outcomes. "I am not quite sure what the tests revealed."

From the time questions were first raised about the fireproofing in the buildings, the Port Authority has argued that the damage done by the two giant airplanes flying into the towers at extraordinary speeds caused such damage that the fireproofing was almost irrelevant.

The federal investigators have always acknowledged the uniqueness of the damage done, and the challenges posed for the buildings. S. Shyam Sunder, the lead investigator at the standards institute, said as much before the fireproofing tests began. The attack on Sept. 11 created fires in both buildings that were far larger and more sudden than ever could be expected to start all at once in a traditional office fire. The impacts of the planes also knocked off at least some of the fireproofing. And it destroyed a swath of exterior and core columns in the buildings, structural elements that were actually responsible for holding up the towers.

Further complicating the picture is the fact that on the day of the attack, the fireproofing on the floors in the two towers differed in thickness. The south tower, which fell in 56 minutes, had only three-quarters of an inch of fireproofing on its upper floors, the same as when it was built. But the north tower, meanwhile, which stood for 102 minutes, had 2.5 inches of fireproofing on the same floors because the Port Authority had in 1995 decided to gradually upgrade the fireproofing after apparently questioning whether the original thickness was sufficient.

The fireproofing, then, is one of only many variables that may have played a role in the speed of the collapse; others such as the angles, speed and height at which the planes hit are perhaps even more significant. But the investigators still believe that the floors may have played a role in initiating the collapse, perhaps simply because the various tests show that the floors sagged a great deal during intense fires, which may have been enough to undermine already weakened outer columns.

New York City officials have already, at least temporarily, banned the use of the floor design that was featured in the twin towers, concerned that the combination of lightweight materials and spray-on fireproofing is unwise at best.

But city officials have not anticipated that in their final report, which is expected in December, the federal authorities might recommend revising the basic system used nationwide to determine if floors sections, columns or other structural elements that are slated to be used in new construction projects can sustain the stress and heat of an intense fire, an outcome that the tests here suggest is possible.

Science in Africa

Engineering aspects of the WTC Twin Towers

Graham Shepherd, Rhodes University

Graham Sheperd gives some insight into the engineering aspects of the World Trade Center Twin Towers ahead of a lecture at the National Festival of Science, Engineering and Technology, Africa's largest science festival to be held in March in South Africa.

On September 11th 2001 the world watched in horror as the unthinkable happened. Two jetliners hijacked and slammed at high speed into two of the most recognizable buildings in the world, the World Trade Centre Twin Towers of downtown Manhattan's financial hub, followed by the total collapse of both mighty towers a short time later.

Created in the 1960s (and completed in the early seventies) as a landmark piece of renewal of a somewhat rundown part of lower Manhattan, funded by the Port Authority of New York and New Jersey, the towers were the career masterpiece of Japanese-American architect Minoru Yamasaki and brilliant young engineer Leslie E. Robinson.

The towers were of colossal proportions, by any standards, even those of New York's Manhattan Island, home of some of the world's tallest and best known skyscrapers. The towers, called North Tower and South Tower, stood 417 and 415 metres tall, respectively. In addition, North Tower supported a 108 metre high radio and TV broadcast tower on its roof. Each tower used enough steel to build a Nimitz class aircraft carrier, in its construction.

The towers were built up from prefabricated welded steel sections which all had to arrive on site in exactly the correct order, as storage of such a vast amount of steel would not be possible in the space available. Computers were used to keep track of the process: a first in the building industry. Each tower had a cross sectional area of over 4000 square metres and was constructed in a very unique way with all the load bearing vertical members concentrated in the exterior walls and in a very strong central "core" which carried the lift system of 99 lifts per tower. Otis elevators designed a system of express and local lifts unique to the WTC towers at the time. One caught an express to a sky lobby situated either on the 44th floor or 78th floor. This greatly reduced the space devoted to lifts, thereby increasing the percentage utilization for office space.

With the structural backbone situated in the core and in the exterior walls, it was possible to rent a whole floor of the WTC and be able to see 64 metres in two directions if one stood in the corner of one's space. There were no further columns in the intervening space! Of course you could order your floor partitioned into smaller offices in any way you wanted, but the partitions were only partitions and bore no load.

In this Science festival talk we will see exactly why the towers were designed in this way. We will use simple concepts of applied physics to see why Leslie Robinson's worst nightmare in the design was not earthquake or even the huge weight of the building. It was quite simply the wind! We will see the unique solution he devised to create an enormously strong but light tower, with enough built in redundancy to withstand a 240 km per hour wind! (Try to imagine the effect of a wind of this speed blowing against huge steel aluminium and glass "sail" 417 meters high and 64 meters wide!).

We will consider the effect of the plane crashes on the towers. Each plane was a wide cabin jetliner loaded with enough fuel to fly to the America West Coast, 5000 km away. Each was flying at very high speed at the moment of impact. At the time of the design of the towers the largest commercial plane flying was the Boeing 707. Leslie Robinson designed his towers to withstand being hit by a 707. But the scenario of his design was very different from what happened on September 11th : He envisaged a 707 lost in fog looking for the airport, low on fuel at the end of its flight, with a pilot not daring to go faster than the stalling speed of 280 km per hour under such dangerous conditions. The planes which hit the towers were estimated to be doing between 750 and 950 km per hour, respectively! Their destructive power can be shown to rise with the square of speed, so you can see that this event was about an order of magnitude worse than Robinson had imagined.

The initial collisions did tremendous damage to the various different elements of the buildings, which we will analyze in detail, but still the building stood. One of the reasons why they stood was without doubt the huge resistance to the wind that had been built in.

Further devastation followed as fires of enormous proportion, triggered by the burning jet fuel spilled at the crash sites began to rage without any hope of being checked. More damage to various building elements occurred. We analyze the probable form of this damage with the help of engineers with specialist knowledge of fire damage.

After standing for 56 minutes and 102 minutes, respectively, tower two (South tower) and tower one (North tower), in that order, both collapsed. We analyze what particular system went critical "first", for each tower and we see that the nature of collapse was significantly different for the two towers.

One collapse was initiated at the crash site floors; we will see that the total collapse was completely unavoidable. We consider the amazing "ear-witness" report of a person who survived the collapse of South Tower, from floor 22 and tie in what he heard and felt with what the engineers know must have happened. (His survival-he regained consciousness to find himself lying on top of the ten storeys high rubble pile which was once South Tower-was without doubt one of the most miraculous escapes of the day.

Dr. Barbara Lane

Arup Study Sees WTC Collapse Likely Even Without Loss of Fireproofing

-- Consulting-Specifying Engineer, 10/5/2005

Dr. Barbara Lane, an expert in structural fire design solutions with London-based global consulting and engineering firm Arup, has presented the firm's findings that the collapse of the WorldTradeCenter towers due to fire could have occurred even without the loss of structural fireproofing caused by aircraft impact.

Dr. Lane presented the results of Arup's detailed structural fire collapse study at a two-day National Institute of Standards and Technology (NIST) technical conference on the FederalBuilding and Fire Safety Investigation of the World Trade Center (WTC) Disaster. She spoke at the public comment period of the session on Structural Fire Response and Collapse Analysis on Sept.15.

Following a three-year investigation and analysis of the WTC collapse, NIST is in the final stages of preparing the results of its study and recommendations for improvements to tall building design and management procedures.

Arup commended the work of NIST to model the WTC collapse—a vast undertaking. However, Arup's review of NIST's findings and its own analysis led it to conclude that NIST has not satisfactorily demonstrated its main conclusion but that the impact-induced loss of fireproofing was the deciding factor in the collapse.

Quantifying the performance of the structure in real fire scenarios is key in designing structures to withstand progressive collapse. For several years, Arup has been working with the University of Edinburgh to model the performance of structural frames in realistic building fires using finite-element analysis. This approach has been used by Arup to model a building with very similar structural design and fire characteristics to WTC Tower 1.

Arup’s analysis concluded that the effect of thermal expansion on the perimeter columns of the towers—even without the airplane impact—could have led to collapse due to the severity of fire occurring on multiple floors and the resulting thermal expansion of structural elements, particularly the floor systems. The Arup analysis conclusively illustrates that even with code-approved fire protection, a severe fire—without aircraft impact—could still lead to collapse.

Thermal expansion, an integral parameter of Arup’s modeling of the event, was not included in the NIST model—a likely reason for the differing conclusions. Arup supports the widespread application of such in-depth structural detailing for future tall building design and construction, as opposed to more prescriptive code- or materials-based solutions. This form of analysis can bring additional robustness to a structural design. Quantifying the response of a structural design subjected to fire allows a designer to determine the strengths and weaknesses of the design and make alternative detailing or other alterations to the structure to improve its performance.

ICC News Release

World Trade Center 9/11 investigation could result in new
generation of building safety and fire prevention codes

The nation's leading developer of building safety and fire prevention codes will use findings from an investigation into the World Trade Center attack to better understand what led to the towers' collapse and develop construction guidelines to better protect lives and property.
The International Code Council will use its code development process to address building safety and fire prevention code issues raised in the National Institute of Standards and Technology (NIST) findings from its World Trade Center investigation.

"NIST has done an important public service by conducting this comprehensive study," said International Code Council CEO James Lee Witt. "The International Code Council intends to fully review its findings as it strives to continue to improve building safety and protect lives and property."

International Code Council members last year approved a change to the International Building Code (IBC) related to the World Trade Center collapse. The IBC now requires that buildings 420 feet and higher have a minimum three-hour structural fire-resistance rating. The previous requirement was two hours. The change provides increased fire resistance for the structural system leading to enhanced tenability of the structure and gives firefighters additional protection while fighting a fire. The IBC establishes minimum standards for the design and construction of building systems. It addresses issues such as use and occupancy, entry and exit during emergencies, engineering practices and construction technology.

The International Code Council updates its codes every three years through a governmental consensus process. Proposed code changes and comments on the proposals are accepted from anyone and everyone in public hearings. However, the final decision on code changes rests in the hands of the International Code Council's governmental members, building and fire officials, who have no vested interest other than public safety.

As a result of the World Trade Center attacks and proposed code changes to address terrorism-related issues in the built environment, the International Code Council formed an Ad Hoc Committee on Terrorism Resistant Buildings. The committee—made up of code officials, engineers, architects and other building professionals—will look at the NIST report and its forthcoming recommendations, and other research.

The International Code Council also participates in an American Society of Mechanical Engineers task force to investigate the use of elevators in fires and other emergencies. This group began meeting following the World Trade Center attacks to examine the use of elevators for occupant exit and firefighter entry into burning buildings.

In the late-19th century, the United States enacted the first set of building regulations because of widespread property losses caused by fire. By the early 1900s, code enforcement officials were writing codes for their individual communities. These codes, which were often inconsistent from town to town, led to the need for model building codes that could be used all across America and around the world.

"Historically, major advances in building safety and fire prevention codes have been the result of lessons learned from past events," said Witt. "While no code can eliminate all risks, what we learn from the past does save lives and better protect property in the future."

The International Code Council, a membership association dedicated to building safety and fire prevention, develops the codes used to construct residential and commercial buildings, including homes and schools. Most U.S. cities, counties and states that adopt codes choose the International Codes developed by the International Code Council.


TO: Penny Beebe
FROM: Casey Stevenson
SUBJECT: Study of the collapse of the World Trade Center towers
DATE: November 5, 2004
I. Introduction
The focus of my research project is the collapse of the World Trade Center (WTC) towers fol-lowing the terrorist attacks of September 11, 2001. Specifically, I hope to establish the sequence of events that took place inside the towers that eventually led to their total collapse. This focus has not changed since my last progress report, dated October 15. In this progress report, I will provide all of the information I have obtained since then, although some references to the previ-ous report will be necessary. I have spent an average of 4 hours per week in preparation of this report.
II. Sources
I have acquired a progress report issued by the National Institute of Standards and Technology (NIST) in June 2004 that contains all of their findings to that date. NIST is the government agency performing a complete investigation of the WTC collapse. The Federal Emergency Management Agency (FEMA), whose findings were included in my first progress report, per-formed only the initial investigation; NIST is in the process of a complete investigation.
In addition to a progress report, investigation leaders from NIST made updated presentations of their findings on October 19 to the National Construction Safety Team. The most recent findings are included in the presentations, which have the most current information available. I have also
acquired Forensic Engineering: Proceedings of the Third Congress, a collection of technical pa-pers, two of which deal directly with the structural response of the WTC towers to the terrorist attacks.
III. Information Acquired
A. Tower Design
As mentioned in my proposal, the WTC towers were designed to withstand the accidental impact of a large aircraft. Specifically, the designers planned for a Boeing 707 traveling at 600 mph crashing into the 80th floor. The analysis performed by the designers indicated that such an im-pact would cause local damage to the impact floors but would not cause an entire tower to col-lapse. However, “[t]he effect of the fires due to jet fuel dispersion and ignition of building con-tents was not considered.” (NIST, 2004, p. 4) Skeptics have said that designers planning for an aircraft impact could not have ignored the ensuing fires, but documentation of the design process indicates just that.
B. Computer Modeling
The initial investigation of the collapses revealed almost everything that could be deduced from simple observation of the towers on September 11. One of the most effective ways to learn more about the structural events that took place inside the towers is to use computer programs to model the towers, the aircraft, and the impact of the two bodies. A discussion of the computer modeling of collapse follows.
Abboud et al. used two different computer models in their analysis. First, they modeled the air-craft and towers in a computer program known as FLEX. They went to great lengths to ensure that the towers and aircraft were modeled accurately and that the analysis procedure was consis-tent with the events of an impact. For example, many analysis programs allow structural ele-ments to deform only slightly and do not allow them to crush or break apart. The program used in this analysis was manipulated to allow just such behavior to occur; the aircraft and building elements could be crushed or torn into fragments. Overall, the analysis was very realistic and a good representation of what actually might have happened during the impact. While FLEX was the appropriate program to analyze the impact, a program known as SAP2000 was used to ana-lyze the structure after impact. The results of the FLEX analysis were used as the input for the SAP2000 model. SAP2000 allows for temperature variations (in this case fires) to be included in an analysis and is thus better suited to examine the behavior of the damaged structures under the load of the fires. (2003, pp. 362-364)
NIST used a program known as LS-DYNA to analyze the impact of the aircraft into the towers. They then used SAP2000 and a program called ANSYS to analyze the structure in its damaged state by using the results of the LS-DYNA analysis as input. A typical ANSYS model of a WTC tower core and hat truss is shown in Figure 1. The green areas represent steel members and the white letters are member labels. A fire dynamics program known as FDS was used to simulate the ignition and spread of fires after impact. (Sunder, 2004, p. 18)
C. Aircraft Impact
In my first progress report, I stated that the damage to the cores of the WTC towers could not be quantified and would likely never be known for sure. I have found sources that attempt to de-scribe the damage hidden in the cores by using computer modeling and simulation of the impacts of the aircraft. A more complete analysis of the structure of the WTC towers can be performed by making use of the information in these sources.
Figure 1. ANSYS Computer Model
(Adapted from Gross, 2004, p. 7)
In addition to repeating the findings of the FEMA team investigating damage to the perimeter columns, NIST reports damage to the core columns. In WTC 1, the aircraft severed 3 core columns and severely damaged 10 more in the center of the north face of the core (Sunder, 2004, p. 22). According to analysis by Abboud et al., half of the core columns of WTC 1 were severed or damaged so severely that they lost all their load-carrying capacity (2003, p. 364). In WTC 2, NIST reports that 5 core columns were severed and 5 damaged at the east end of the south face and at the southeast corner of the core (Sunder, 2004, p. 22).
The aircraft impacts damaged other parts of the towers besides the core and perimeter columns. As stated in my first progress report, local floor damage and collapse was visible along the im-
pact faces of the towers. Abboud et al. report that this visible collapse was the extent of floor damage and that floors across the majority of the towers remained intact after the impacts (2003, p. 364). Significant damage to the ceilings occurred, allowing “unabated” heat transfer from the fires to trusses and other structural members normally concealed above the ceiling (NIST, 2004, p. 6). The FLEX model of the impact also defined a debris field, or an area of the tower where flying debris could have removed fireproofing from structural members (Abboud, 2003, p. 364). Later analyses of the fires and the performance of the tower structures reveal that a significant amount of fireproofing in the debris field must have been removed by the aircraft impacts.
D. Fire Development
In my first progress report, I described various methods that investigators used to estimate the size of the fires in the towers. These analyses resulted in estimates of heat output as well as a maximum temperature of the fires. I have acquired new sources that looked more closely at the available evidence and more accurately reconstruct the fires in the towers.
Examination of video evidence and fire modeling in FDS shows that the fires started near the impact regions and progressed across the towers. In both towers, little flaming was observed in the impact areas, but flames were seen moving along the faces of the towers away from the im-pact zone. In WTC 1 significant flames were observed on the south and west faces. In WTC 2, which was impacted on the south face, the most consistent flames were observed along the east side and in the northeast corner of the tower. (Beyler, 2003, p. 373) The fires would ignite in a given location and burn for about 20 minutes, then move down the face of the tower, igniting a new area (NIST, 2004, p.11). This fire progression pattern means that areas of the towers were subject to intense fires for only a short time period, not the entire time between impact and col-lapse.
NIST’s fire simulations indicate that temperatures approached 1000° C in fire areas, but such high temperatures were not sustained over the entire tower area at a given time (Sunder, 2004, pp. 26-27). Beyler et al. agree, but they are very careful to point out that such temperatures were only present where fires were actively burning. In an attempt to dispel media claims that mas-sive, raging fires brought the towers down, the authors show that, on the whole, the fires were 5
less intense than a standard fire. Large amounts of incombustible debris from the towers’ me-chanical systems and from the aircraft would slow the development of the fires. Incombustible gypsum and concrete dust created by the impacts would further slow fire development. A maximum compartment temperature (temperature of the open space on a WTC floor) of 400 – 700° C is estimated, which is structurally significant but not detrimental. Widespread tempera-tures above 1000° C did not develop and fires and extreme temperatures were localized. (2003, pp. 372-380)
E. Effect on Structure
In my first progress report, I discussed how the perimeter columns redistributed loads to adjacent perimeter columns and core columns after the aircraft impact severed several perimeter columns. Damage to core columns was not considered in my first progress report and will thus be de-scribed here. It is the damage to the core columns, combined with that to the perimeter columns, that ultimately caused the towers to collapse.
As mentioned above, the aircraft impacts severed or damaged a number of columns in the core of both towers. The structure responded by redistributing loads previously carried by the damaged core columns to other parts of the structure. First, load was transferred to adjacent core columns via the core framing. Second, load was transferred from the core to the perimeter by the floor system, which directly connected the two. Last, loads were redistributed to intact core and pe-rimeter columns via the hat truss. (NIST, 2004, p. 5) Severed or damaged core columns hung from the hat truss, acting as tension members that suspended the floors above the impact region. The towers remained stable following impact and would collapse only after the fires had had a significant effect on the remaining intact structure.
1. WTC 1
As the fires spread across the floors of WTC 1 they heated the core columns, causing them to lose stiffness and buckle. Loads were again redistributed, as they were after impact, with the core framing, floor system, and hat truss redirecting loads to remaining core columns and pe-rimeter columns. As more core columns lost stiffness and strength, three things occurred.
First, the ability of core framing to transfer loads to adjacent core columns diminished. The core framing was being heated just like the columns, and elements were losing their stiffness and strength. Additionally, if all adjacent columns were themselves buckling and failing, the core framing could not redistribute load among them. (NIST, 2004, p. 5)
Second, connections between core columns and the hat truss failed. They were not designed to resist the enormous tension load required to support the floors hanging below. As each hat truss connection failed, the structure was forced to redistribute loads in a different manner. The only element still capable of transferring loads away from the core was the floor system. (NIST, 2004, p. 5)
Third, the hat truss itself began to fail. The diagonal members of the hat truss (a normal truss, defined in my first progress report) buckled in compression. As the diagonals yielded to the ever-increasing loads, the ability of the hat truss to transfer loads among core columns and be-tween the core and the perimeter diminished. The floor system again was the only remaining structural element that could redistribute loads, and it too was being weakened by the fires. (NIST, 2004, p. 5)
One may ask how the floor system (see Figure 2) could transfer loads among columns. Besides supporting the floors of the WTC towers, the floor system acted as a structural element known as a diaphragm, which is a wide, flat element (think of a piece of plywood as a diaphragm). It is very strong in the plane of the diaphragm, but relatively weak out of the plane. If one pushes down in the center of a piece of plywood, it will deflect downwards. However if one pushes or pulls on the edge of a piece of plywood, the wood will not bend or deflect. The purpose of the floor system in the undamaged structure was to connect the perimeter and core and transfer hori-zontal loads between them (horizontal loads would push and pull on the edge of the floor system, in its strong direction). The floor system was supported at its edges by the perimeter columns and at the center by the core.
As the core columns lost their stiffness and as the hat truss became progressively weaker, the core columns began to displace downward, pulling one end of the floor trusses down with them. The floor system diaphragm now looked like a shallow bowl; the perimeter columns supported its edges but the core columns were pulling its center down. The floor system was now pulling upward on the core from the perimeter columns, acting much like a cable in tension.
Figure 2. Perimeter, Floor System and Core
(Sunder, 2004, p. 33)
As the floor diaphragm began to act as a cable, the localized fires and elevated compartment temperatures were heating it. It began to lose stiffness and strength and started to sag. My first progress report describes floor system sag in response to heating. As the core moved downward and as the floor system lost its stiffness, it acted more and more like a cable in tension; part of the core load was hanging from the floor system that was supported by the perimeter. This cable action, along with the action of the hat truss and core framing, redistributed loads from the core to the perimeter as the core col-umns buckled.
The floor system pulled up on the core columns, also pulling inward on the perimeter columns, which were designed to carry forces along their length, not perpendicular to their length. They bowed in, as seen in Figure 3, near the impact floors. (Sunder, 2004, p. 11) At the same time, the perimeter columns were being heated by the fires, which did not have the same effect on the pe-rimeter columns as they did on the core columns. Three sides of the perimeter columns were ex-posed to the atmosphere and ambient temperatures, while only the interior side was heated. As described in my first progress report, when a member is heated it tends to expand. As the inside face of the perimeter columns was heated, it expanded. The outside face remained at its initial length. Inward bowing of the perimeter columns resulted from this thermal load, augmenting the inward bowing caused by the pull of the floor system (NIST, 2004, p.7).
From the discussion of buckling in my first progress report, we see how columns that are bowed are more likely to buckle. The perimeter columns were still in relatively good condition com-pared to the core columns, but were forced to carry more and more of the load as the core col-umns lost strength. As they became more heavily loaded, they were also pulled inward. Col-lapse initiated when the perimeter columns buckled.
Figure 3. Inward Bowing of Perimeter Columns on WTC 2 East Face.
(Sunder, 2004, p.15)
The collapse initiation was not as instant as it appears to be in videos with which the public is familiar. It began with the buckling and failure of one or two perimeter columns on the south face of WTC 1. The perimeter frame then redistributed the loads carried by the buckled columns to adjacent columns by Vierendeel action (described in my first report). This time, however, adjacent columns had no reserve capacity, as they did when the aircraft first impacted the tower; they were already approaching their ultimate capacities. As the first one or two columns failed and the load they carried was transferred to adjacent columns, these adjacent columns buckled and failed right away. Buckling progressed down the south face of WTC 1 until all columns 9
were buckled; the adjacent column buckling process then turned the corner of the tower, buck-ling columns down the east and west faces. (Sunder, 2004, p. 11)
As one entire floor level of columns failed, the floors above the failure floor began to drop. The buckling of the perimeter and core columns somewhat slowed their downward motion because it requires large amounts of energy to buckle so many columns. However, the potential energy of the upper floors was simply too great and the columns below the failure floor could not stop the mass of the upper floors once they were set in motion.
2. WTC 2
WTC 2 failed in a similar manner to WTC 1, but the failure mechanisms have important differ-ences. Floor trusses in WTC 2 along the east face were subject to sagging, a mechanism de-scribed in my first progress report. The sagging of the floor trusses increased the cable action of the floor system and pulled the perimeter columns in with an even greater force than that in WTC 1. WTC 2 failed first along the east face, with the column failures progressing quickly around the corner to the south and north faces. (Sunder, 2004, p. 12)
As one may recall, damage to the core of WTC 2 was primarily in the southeast corner. When a structure redistributes loads, corner columns are essential – they are like the legs on a table. The WTC 2 core was missing an essential leg. The ability of the WTC 2 core to carry loads was thus quickly reduced, placing the loads on the perimeter columns via the floor system. WTC 2 was also hit about 14 floors lower than WTC 1, meaning that the failure floors of WTC 2 had to carry the weight of 14 more floor levels than those of WTC 1.
IV. Work to be Completed
In the next several days, I will wrap up my research on this topic. I would like to find some of the sources that my sources used so I can get a more first-hand knowledge of the concepts that are being discussed. Among others, I would also like to briefly review an MIT analysis of the collapses that was cited by the recent NIST presentations.
The draft of my final report will require a considerable amount of time to put together from the two progress reports. While all the concepts are currently explained, they need to be tied to-gether and their interdependencies need to be illustrated. The sequence in which I will describe the structural principles will also need to be carefully thought out so that the reader can easily understand what was happening inside the towers.
V. Works Cited
Abboud, N., M. Levy, D. Tennant, J. Mould, H. Levine, S. King, C. Ekwueme, A. Jain, G. Hart. (2003) Anatomy of a Disaster: A Structural Investigation of the World Trade Center Collapses. In: Proceedings of the Third Congress on Forensic Engineering. San Diego: American Society of Civil Engineers.
Beyler, C., D. White, M. Peatross, J. Trellis, S. Li, A. Luers, D. Hopkins. (2003) Analysis of the Thermal Exposure in the Impact Areas of the World Trade Center Terrorist Attacks. In: Proceedings of the Third Congress on Forensic Engineering. San Diego: American Society of Civil Engineers.
Gross, J. (2004) Project 6 – Standard Fire Tests of WTC Tower Typical Floor Construction. National Institute of Standards and Technology presentation to the National Construction Safety Team. October 19, 2004.
National Institute of Standards and Technology (NIST). (2004) June 2004 Progress Report on the Federal Building and Fire Safety Investigation of the World Trade Center Disaster. Washington, D.C.
Sunder, S. (2004) World Trade Center Investigation Status. National Institute of Standards and Technology presentation to the National Construction Safety Team. October 19, 2004.

Meeting to Discuss IBC Code Change Proposal

Opening Remarks1 of
Dr. S. Shyam Sunder
Deputy Director
Building and Fire Research Laboratory
National Institute of Standards and Technology
at the
Meeting to Discuss IBC Code Change Proposal for
Progressive/Disproportionate Collapse

May 1, 2006
NIST released the final report in October 2005 from its building and fire safety investigation of the collapses of the World Trade Center (WTC) towers on 9-11. The report included 30 recommendations for improving building and occupant safety derived from the findings. On March 24, 2006, the first 19 proposed changes to model building codes based upon and consistent with the NIST WTC recommendations were submitted to the International Code Council. The 19 proposed changes—submitted by building code experts associated with two ICC committees, NIBS and GSA—address areas such as: increased resistance to building collapse from fire and other incidents, use of spray-applied fire resistive materials (commonly known as "fireproofing"), performance and redundancy of fire protection systems (i.e. automatic sprinklers), elevators for use by first responders and evacuating occupants, the number and location of stairwells, exit path markings, and fuel oil storage/piping.
Taken together, they are a robust, reasonable and appropriate set of advancements, and if adopted, would represent a significant improvement in public safety over current practice.2
One of these 19 code change proposals deals with progressive collapse or disproportionate collapse. This proposal should be considered within the broader framework of structural robustness and integrity. In recent decades, there has been an emphasis on maximizing the efficiency of the structural system to mitigate effects of the weight premium on cost, especially for super tall buildings3.
Structural engineers, however, do not have an objective metric today for measuring the safety performance of the structure as a complete system. Thus, we cannot quantify the degree of safety of a structural system or compare the safety of one structural system relative to another system for a given performance objective. As we build increasingly taller and more efficient buildings—that pose inherently greater consequence-driven risks to known hazards and have less redundancy—it becomes vitally important to assure a minimum level of safety of the structural system in satisfying the performance objective, in addition to assuring the efficiency of the structural system (e.g., pounds per square feet of structural materials used).
1 Edited and updated after the meeting. This document incorporates responses to discussions at the meeting.
2 Several organizations, including NIST, are reviewing these proposals and may offer amendments or suggestions for improvement during the code development process.
3 In theory, the most efficient structural system that can be designed is a determinate system, which lacks redundancy. Our codes and standards focus on quantifying performance and assuring the safety of components and connections, with the exception of some instances such as in earthquake-resistant design where system performance can be quantified (e.g., using R factors). Building codes and standards typically use safety factors in deterministic design and load and resistance factors in probabilistic design to quantify the safety performance of components and connections. Similar safety metrics should also be considered for the performance of the structural system
4. For example, the ratio of the ultimate reserve capacity of a structural system to the design load carrying capacity of the system may be used to quantify safety performance. This metric—or global safety factor for the system—may be obtained either under purely gravity loads or under different combinations of lateral and gravity loads. The metrics may also be used to quantify the safety performance of major structural sub-systems such as the core, perimeter, and floor framing systems.
Under gravity loads, the ultimate reserve capacity of the structural system may be determined while the structure is subject to design gravity loads with appropriately applied load reduction factors. Under lateral loads, the ultimate reserve capacity of the structural system may be obtained while the structure is subject to both lateral and gravity loads consistent with typical load reduction factors. While advanced analysis tools exist to quantify the safety performance of structural systems using these metrics, more work is needed to determine appropriate global system safety factors—or load and system resistance factors—to implement meaningful provisions in codes and standards. This will require developing comparative data regarding the safety performance of different structural systems using these metrics.
Until the above information on global system safety factors becomes available, current methods for mitigation of progressive or disproportionate collapse may be used in our codes and standards to ensure a minimum level of robustness and integrity (or safety performance) of the structural system. These methods include: tying buildings components together to provide continuity and strength; providing structural redundancy via alternate load paths; and enhancing specific resistance of structural components to known hazards.
4 System performance metrics are routinely used in other industries and applications. For example, fuel efficiency of automobiles is similar to materials efficiency for buildings. Also, the system safety performance of an automobile, measured in terms of crashworthiness ratings, is similar to the ultimate reserve capacity of the structural system. Fuel efficiency, in turn, depends on the design of major subsystems such as the engine, aerodynamic shape of the body, and the grade of fuel. Similarly, the system safety performance depends on the design of major subsystems such as the engine, body, doors, and supplemental restraint systems. Each of the subsystems and components may be governed by standards as well.
5 Some experts cite the excellent safety record of tall buildings in recent decades and ask whether there is a need to quantify the safety of the structural system. Statistics over several decades, however, are not adequate to quantify risks. Instead data is needed over some multiple of the expected life of a building, typically 100 years. In designing structures for hurricanes and earthquakes, it is common practice to consider rare events with return periods of 500 to 2,500 years. Similarly, long term risks should be anticipated in designing structural systems for general robustness and integrity. Codes and standards provisions should focus on the use of threat or hazard independent methods to mitigate progressive or disproportionate collapse and to assure system safety performance6. When specific threats or hazards are known or can be anticipated, they should be considered explicitly in design; codes and standards should have provisions treating them as such.
Robust tools already exist for specific use in design to mitigate progressive or disproportionate collapse. The April 2006 issue of Structure magazine—a joint publication of NCSEA, CASE, and SEI—contains an excellent summary of these tools. The articles in the magazine also illustrate the considerable technical and professional capabilities already available within the United States.
The U.K. has had a progressive collapse standard “Standards to Avoid Progressive Collapse – Large Panel Construction” since 1968. The standard lists two methods for mitigating progressive collapse: (1) by providing alternative load paths, assuming the removal of a critical section of the load bearing system, and (2) by providing stiffness and continuity to the structural system to ensure the stability of the building against forces liable to damage the load supporting members. The standard also specifies an accidental static pressure of 5 pounds-per-square-inch and minimum tie forces for continuity. These provisions are based on engineering experience and judgment. Similar provisions have been adopted in the Eurocode. Currently, there is no field evidence to indicate that these provisions are not working or that the resulting building designs are less safe. In the United States, the American Society of Civil Engineers (ASCE) has in its standard (ASCE/SEI 7), structural integrity requirements for progressive collapse mitigation. ASCE plans to develop guidance for the prevention of progressive collapse. A technical committee has been recommended, but has not yet been formed. It will be some years before a guidance document is developed and made available for code adoption. NIST has an ongoing multi-year research project on the development of criteria for prevention of progressive collapse and is currently assessing best practices in current use. The NIST best practices document is intended to provide owners and practicing engineers with the current best practices to mitigate progressive collapse, including methods similar to those adopted in the U.K., and those used by federal agencies such as GSA, DoD, and the State Department. The draft of the document will be made available for broad review in conjunction with training seminars to be conducted by ASCE in 2006. The final document will be available by the end of 2007. In the course of its Investigation into the collapse of the World Trade Center Towers, NIST did not find any evidence that well-tied buildings performed unfavorably (or collapse earlier) than buildings that are not well-tied. In fact NIST found that, had the major structural subsystems of the WTC towers not been tied together, the core of the towers would have collapsed earlier. The hat-truss tied the core to the perimeter walls of the towers, and thus allowed the building to withstand the effects of the aircraft impact and subsequent fires for a much longer time—enabling large numbers of building occupants to evacuate safely.
NIST believes that it is imperative for U.S. building codes and standards to address requirements for robustness and integrity of the structure as a system, especially tall buildings. This is essential to quantify and assure a minimum level of safety for the structural system, much as U.S. codes and standards do now to quantify and assure a minimum level of safety for structural components and connections. With few exceptions, the lack of minimum requirements for global safety factors for structural systems represents a major gap in U.S. codes and standards. This gap must be closed with a sense of urgency and commitment by the professional and building official communities. We should find an efficient and effective way forward today by discussing the specific code change proposal on disproportionate collapse submitted to the IBC.
In closing, NIST welcomes and fully respects the ongoing debate among the professional and building official communities as they consider the 19 code change proposals based on the WTC recommendations for adoption. All ICC members will have the opportunity to vote on the proposals at hearings scheduled for this fall. All changes passed and those which did not pass but for which public comments are received will then be up for approval—and inclusion in the ICC codes—when ICC Government Member representatives meet in the spring of 2007.
For more information: -- including a Web-based system for tracking the progress toward implementing all of the NIST WTC recommendations -- go to . A link to "Status of NIST's Recommendations" from the WTC website lists each of the recommendations, the specific organization or organizations (e.g., standards and code developers, professional groups, state and local authorities) responsible for its implementation, the status of its implementation by organization, and the plans or work in progress to implement the recommendations. The status of the implementation of the recommendations is current as of April 10, 2006 and includes links to the nineteen code change proposals submitted to the International Code Council for the March 24, 2006 deadline and supplementary information produced by the NIBS building code experts.

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