Architectural Record

Interim Results of World Trade Center Investigation "Exonerates" Twin Towers' Design in Sept. 11, 2001 Collapse

October 22, 2004

Structural steel of the twin 110-story towers of the World Trade Center was stripped of its fireproofing by debris from the aircraft impact and weakened by the resulting fires, eventually causing the towers to collapse, according to an interim report by the National Institute of Standards & Technology. The report says the region of dislodged fireproofing was determined from the predicted path of the debris.

“Had the fireproofing not been dislodged, the temperature rise of the structural components would likely have been insufficient to cause the global collapse of the towers,” says NIST in the Oct. 19 release of another interim report of its $16-million study of the WTC destruction on Sept. 11, 2001, by terrorists. “Fireproofing dislodged by debris left the components more sensitive to heat than any areas where there was missing or thin fireproofing before the aircraft impacts,” says the report.

Many experts familiar with the twin towers design are not surprised by the findings. But they are worth noting, say sources, because there are others, both structural engineers and fire experts, who have questioned whether the design by Skilling Helle Christiansen Robertson in some way contributed to the collapse.

According to S. Shyam Sunder, NIST’s lead investigator for the study, an ordinary office fire would likely have resulted in burn-out, not collapse.

In addition, NIST has determined that the majority of the steel was stronger than minimum requirements. “The safety of the towers was most likely not affected by the small percentage of steel below the minimum,” says the report. “Building designs routinely allow structures to withstand greater loads than are expected by including significant factors of safety. Moreover, the structural loads on Sept. 11, 2001, were well below this design level. “

In fire tests in August, NIST also determined that the floor systems in the towers met the New York City building code of the time (ENR 9/13 p. 16).

The findings include an explanation for the time delay between the collapses of the two towers. (The south tower, Two WTC, survived for 56 minutes; the north tower, One WTC, for 103 minutes). NIST says the difference was primarily due to five items: the asymmetrical structural damage of the aircraft impact to Two WTC compared to the aircraft damage to One WTC; the time it took for heat to soften, buckle and shorten core columns that had fireproofing dislodged by debris impact; the structure’s ability to redistribute loads as the core columns shortened; the time it took for fires to traverse from their initial location to the face of the towers where perimeter columns were bowing inward (as seen only minutes before the collapse of each tower); and the time it took for heat to soften and buckle those columns.

NIST plans to release its final draft of the twin towers’ report in December or January. A four to six-week public comment period will follow. The final release is expected in May. The draft report on Seven WTC is set to be released in May. The final report is expected out in July.

COLLAPSE OF WORLD TRADE CENTER TWIN TOWERS

COLLAPSE OF WORLD TRADE CENTER TWIN TOWERS

Both towers were tube-like, and the majority of their support came from exterior columns. There was also a cluster of columns in the center of the building connected to the exterior columns by floor trusses.

THE COLLAPSES:
WHAT MAY HAVE OCCURRED

1. After the planes struck the towers, thousands of gallons of jet fuel burned.
2. Thick layers of insulation on exterior and interior steel columns were breached, and sprinkler systems were disabled.
3. Fires broke out with temperatures reaching perhaps 1,000 to 2,000 degrees Fahrenheit.
4. The steel column systems, softened in the heat, separated from each other and buckled.
5. Weighted down by debris, furniture, concrete and steel, floors progressively collapsed.
   
   

Sources: Urban Data Solutions; Skilling Ward Magnusson Barkshire; The Port Authority

STRUCTURAL DETAILS OF THE WORLD TRADE CENTRE TOWERS

The twin towers, framed in structural steel, had exterior moment frames with 14-in. steel box columns spaced 39 in. centers. The configuration created a complete tube around the building. The central steel core carried gravity loads only. The exterior tube provided all the lateral resistance. Horizontal steel trusses spanned 60 ft from the exterior wall to the core. Concrete on metal deck completed the floor diaphragm.

The twin towers were part of a seven-building complex designed by architect Minoru Yamasaki that covers eight city blocks. An 800 x 400-ft foundation box, 65-ft-deep and with 3-ft-thick retaining walls, is under more than half the complex, including the twin towers and the adjacent hotel. The complex was completed in phases beginning in 1970 (ENR 7/9/64 p. 36).

World Trade Center Collapsing (AP Photo)

Each tower contained about 100,000 tons of steel and 4 in. of concrete topping on the 40,000-sq-ft floors, according to Henry H. Deutch, assistant to the chief structural engineer for construction manager Tishman Realty & Construction Co. Inc., New York City, during the construction of the WTC and currently head of HHD Consultants Inc., Osceola County, Fla.

Deutch says that originally, the north tower contained asbestos in its cementitious fireproofing as did the first 30 stories of the south tower. He believes the asbestos, which had been encapsulated, was removed after the 1993 bombing. In a press conference, Mayor Rudolph W. Giuliani said the city's health department had tested the air in the area and found no undue amount of chemical agents.

Millions across the nation also "saw" the towers collapse, through live television news coverage. The south tower fell at 10 a.m. and the north tower at 10:29 a.m.

Reports indicate that the impact of each plane compromised the structural integrity of each tower, knocking out perimeter columns and the interior structure. The explosions then caused further damage, sweeping through several floors. "These were airliners scheduled for long flights, full of fuel, causing massive explosions," says Richard M. Kielar, a Tishman senior vice president. "No structure could have sustained this kind of assault," says Kielar.

As the fires burned, the structural steel on the breached floors and above would have softened and warped because of the intense heat, say sources. Fireproofed steel is only rated to resist 1,500 to 1,600° F. As the structure warped and weakened at the top of each tower, the frame, along with concrete slabs, furniture, file cabinets, and other materials, became an enormous, consolidated weight that eventually crushed the lower portions of the frame below.

Jon D. Magnusson, chairman-CEO of Skilling Ward Magnusson Barkshire Inc., Seattle, structural engineer for the original World Trade Center, agrees: "From what I observed on TV, it appeared that the floor diaphragm, necessary to brace the exterior columns, had lost connection to the exterior wall."

When the stability was lost, the exterior columns buckled outward, allowing the floors above to drop down onto floors below, overloading and failing each one as it went down, he says.

A big question for implosion expert Mark Loizeaux, president of Controlled Demolition Inc., the Phoenix, Md., is why the twin towers appeared to have collapsed in such different ways. Observing the collapses on television news, Loizeaux says the 1,362-ft-tall south tower, which was hit at about the 60th floor, failed much as one would like fell a tree. That is what was expected, says Loizeaux. But the 1,368-ft-tall north tower, similarly hit but at about the 90th floor, "telescoped," says Loizeaux. It failed vertically, he adds, rather than falling over. "I don't have a clue," says Loizeaux, regarding the cause of the telescoping.

Security measures were tightened at the 12-million-sq-ft WTC complex after a terrorist bomb on Feb. 26, 1993. That bomb blew out one section of a north tower basement X-brace between two of the perimeter columns. The blast ripped out sections of three structural slabs in the basement levels between the north tower and the hotel, threatening the structural integrity of the foundation box. It did little damage to the north tower's structural tube, other than the affected X-brace. Damage was extensive to the other building systems, however, because the bomb compromised major utility lines in the basement, and the brace compromised the central core wall, allowing soot and smoke to shoot up the building core (ENR 3/15/93 p. 12).

Fall Of 3rd Tower

A NATION CHALLENGED: GROUND ZERO; Burning Diesel Is Cited in Fall Of 3rd Tower

By JAMES GLANZ and ERIC LIPTON (NYT) 1715 words

Massive structural beams that functioned as a sort of bridge to hold up the 47-story skyscraper known as 7 World Trade Center were compromised in a disastrous blaze fed by diesel fuel, leading to the building's collapse on Sept. 11, investigators have concluded in a preliminary report.

The tower was set on fire by debris from the twin towers and burned for about seven hours before collapsing in the late afternoon under previously unexplained circumstances. The analysis of its collapse is one of the first detailed findings by a team of engineers organized by the Federal Emergency Management Agency and the American Society of Civil Engineers to understand the fate of all the buildings around the site.

As much as 42,000 gallons of diesel fuel was stored near ground level in the tower and ran in pipes up to smaller tanks and emergency generators for Mayor Rudolph W. Giuliani's command center, the Secret Service's office and other tenants.

Investigators have determined that the burning fuel apparently undermined what is known as a transfer truss. The trusses, a series of steel beams that allowed the skyscraper to be built atop multistory electricity transformers, were critical to the structural integrity of the building and ran near the smaller diesel tanks.

A failure of the same type of structural bridge contributed to the collapse of the Alfred P. Murrah Federal Building in Oklahoma City when it was bombed in 1995. Federal guidelines for public buildings, created in 1996, warned of the dangers of such trusses in terrorist attacks.

''It's certainly right in the vicinity where the columns go into this transfer system,'' said a person knowledgeable about the investigators' draft report on the World Trade Center. ''The rest of the building is built on top of the bridge.''

While 7 World Trade Center, which stood across Vesey Street just to the north of the twin towers, was not formally a federal building, it did house crucial government offices that included the city's nerve center for emergency response.

The investigators said that their conclusions, combined with other findings about the failure and collapse of 5 World Trade Center, could prompt serious changes in the codes used in building construction.

The findings are in a draft report that has already been circulated among government agencies, and are based on videos made on Sept. 11, witnesses' reports, interviews with firefighters, evidence from the debris pile and structural analysis. Team members, who described many of the findings, cautioned that the conclusions on the collapse of 7 World Trade Center could still be modified as reviews proceed.

But Irwin Cantor, one of the building's original structural engineers, who is now a consulting engineer and member of the City Planning Commission, said the diesel-related failure of transfer trusses was a reasonable explanation for the collapse.

He said he believed that diesel tanks were not envisioned in the original design of the building. ''It ended up with tenants who had diesels,'' Mr. Cantor said. ''I know none of that was planned at the beginning.''

According to floor plans submitted to the Port Authority of New York and New Jersey, which owns the land on which 7 World Trade sat, the building complied with city fire codes, said Frank Lombardi, the authority's chief engineer. Those codes permit no more than one fuel tank with a capacity of 275 gallons or less on above-ground floors, he said.

Jerome M. Hauer, who was the director of Mayor Giuliani's Office of Emergency Management at the time the command center was opened at 7 World Trade, said several teams of engineers reviewed plans to open the office there. But no one ever mentioned any hazard associated with placing fuel tanks above ground, near a transfer truss, he said.

''There were a host of people who looked at this,'' said Mr. Hauer, who is now a managing director of the crisis and consequence group at Kroll Worldwide, a security consulting company based in New York. ''We relied on their judgment.''

Fire officials did at one point question the storage of large amounts of fuel well above the ground level, saying that one large tank for the mayor's command center, if ever compromised, might fuel a fire that would threaten the building.

The Sept. 11 draft report also has photographs and a description of debris collected from a previously undisclosed, multistory collapse within 5 World Trade Center, a nine-story office building that also burned on Sept. 11 but largely remained standing. The team has found that one specific type of bolted connection, called a column tree connection, that joined floor-support beams, failed in the heat of the fires, causing the four-story collapse in the part of 5 World Trade at the corner of Vesey and Church Streets.

Although no one died as a result of the collapses in 5 and 7 World Trade Centers, since both stood long enough to be evacuated, the team's findings are likely to lead to recommended changes in the way public and government buildings are constructed, much the way similar studies did after the Northridge earthquake near Los Angeles in 1994 and the Oklahoma City bombing.

The team is still deliberating on how tightly it can pin down the precise train of events that led to the collapse of the twin towers themselves. But until now, the collapse of 7 World Trade has stood as one of the outstanding mysteries of the Sept. 11 attack, since before then, no modern, steel-reinforced high-rise in the United States had ever collapsed in a fire.

High-rise buildings are designed to be able to survive a fire, even if the fire has to burn itself out. The strategy is to ensure that the steel support structures are strong enough or protected well enough from fire that they do not give way in the time it takes for everything inside an office building, like furniture, to burn.

In major high-rise fires elsewhere in the country, such as the 1 Meridian Plaza fire in Philadelphia in 1991 and the First Interstate Bank fire in Los Angeles in 1988, this approach has worked. The 1 Meridian fire burned for 19 hours, leaping from floor to floor and burning out as combustible materials were used up. But the fires at 7 World Trade Center raged mainly on lower floors and never burned out, and in the chaos of Sept. 11, the Fire Department eventually decided to stop fighting the blazes.

''What the hell would burn so fiercely for seven hours that the Fire Department would be afraid to fight it?'' said one member of the investigating team.

According to the Port Authority floor plans, 275-gallon diesel tanks sat on the fifth, seventh and eighth floors and were fed through pipes from the larger tanks near ground level. The team member said that while the diesel fuel remains the most likely candidate for feeding the fires, it was still unknown whether there could have been other sources of fuel in the building, kept there by tenants like the Secret Service that have disclosed little of what their spaces contained.

The huge steel transfer trusses ran mostly through the fifth, sixth and seventh floors where the fires burned. The purpose of the trusses, which included zigzagging and horizontal members and were concentrated around the building's core, was to allow 7 World Trade to be built over two Consolidated Edison substations that already existed on that spot when the building went up in the late 1980's. Together the stations held 10 transformers, each about 35 feet high and 40 feet wide.

Using the trusses to avoid having vertical structural columns pierce the transformers, the building was constructed around them like a hen sitting on a giant egg.

''We had to do design tricks to accommodate the existing Con Ed facility,'' said Mr. Cantor, the structural engineer. ''This building had an awful lot of transfers.''

Transfer trusses are a well-tested technique and are used in countless high-rise buildings, as well as in bridges around the world. Engineers say that transfer trusses, for most buildings, present no extraordinary hazard. But if there is an explosion, earthquake or long-burning fire, they can present a problem.

In Oklahoma City, during the 1995 bombing of the Federal Building, a large transfer girder on the building's third floor gave way, helping to precipitate a progressive collapse that later analysis showed was responsible for most of the 168 deaths. After this attack, federal guidelines for buildings that would hold government agencies were changed, recommending that buildings be designed so that single-point failures did not cause a catastrophic collapse.

Videos of the 5:28 p.m. collapse of 7 World Trade lend vivid support to the truss-failure theory. Roughly 30 seconds before the building goes down, a rooftop mechanical room starts to disappear, falling into the building's core. Then a second larger rooftop room sinks. The building then quickly collapses.

Both rooms were above sections of the building held up by the trusses. Other video evidence shows fire concentrated in the floors containing the trusses and the fuel tanks.

Dr. John D. Osteraas, director of civil engineering practice, Exponent Failure Analysis Associates, in Menlo Park, Calif., reviewed videos of the collapse, discussed it with other engineers and came to a similar conclusion; the fuel, the trusses and the fire brought 7 World Trade down. ''The pieces have come together,'' he said. ''Without the fuel, I think the building would have done fine.''

Dr.James Harris, PhD, P.E.

Testimony of Dr.James Harris, PhD, P.E.
President J.R. Harris & Company
Denver, Colorado
On behalf of the Structural Engineering Institute of the American Society of Civil Engineers Before the Committee on Science
U.S. House of Representatives October 26, 2005
Washington Office 101 Constitution Ave., N.W.
Suite 375 East
Washington, DC 20001
(202) 789-7850
Fax: (202) 789-7859
Web: http://www.asce.org

Mr. Chairman and Members of the Committee:
Good morning. My name is James Harris, and I am pleased to appear on behalf of the Structural Engineering Institute of American Society of Civil Engineers (ASCE/SEI)1 as you examine “The Investigation of the World Trade Center Collapse: Finding, Recommendation and Next Steps” in light of the release of findings and recommendations of the National Institute of Standards and Technology investigation. The events at the World Trade Center in New York City on September 11, 2001, were the worst building disasters in the history of the United States. The National Institute of Standards and Technology conducted a building and fire safety investigation of the disaster under the authority of the National Construction Safety Team Act (15 USC 7301 et seq). As a result of its WTC Investigation, on June 23, 2005 NIST issued a draft report with recommendations, and invited public comments on June 23, 2005. ASCE/SEI supports a thorough review and deliberation of all of the NIST Recommendations and looks forward to further discussions clarifying the situations to which the NIST Recommendations should apply. ASCE/SEI believes that engineers must avoid over-optimistic reassurances about building safety, and agrees that increased efforts should be focused on preventing terrorist attacks. That said, the 30 recommendations presented by NIST within eight categories address a range of issues that we at ASCE/SEI think require serious discussion. Many of the recommendations were presented by NIST as “changes to codes and standards,” which some may interpret to mean that the painstaking process of developing consensus code and standard provisions should be unreasonably accelerated. We believe that the consensus process, which is already underway at ASCE/SEI for some of the concerns NIST has raised, is essential so that all aspects of an issue can be considered. All of the issues deserve further consideration in that community. In the view of ASCE/SEI, at least some of the NIST recommendations will require development of new technologies and close examination of their effects upon the practice. At the same time, the existing codes and standards processes that are already in place, both in and outside ASCE/SEI, provide appropriate mechanisms for advancing several of these discussions. Ultimately, the implementation of these recommendations will require the development of appropriate thresholds and bounds for their application. ASCE/SEI looks forward to taking an integral role in clarifying the application of these recommendations. In fact, some of the NIST recommendations follow actions previously initiated by ASCE/SEI. For example, with respect to Recommendation #2, ASCE/SEI is close to issuing a Wind Tunnel Testing standard and anticipates opening it for public comment. With respect to Recommendation #9, ASCE/SEI has been working with the Society of Fire Protection Engineers, and has already prepared a draft to update ASCE/SEI/SFPE 29-99 (Standard Calculation Methods for Structural Fire Protection), by incorporating performance-based fire resistant design. With regard to Recommendation #27, we look forward to engaging ASCE’s professional practices committee for comment and guidance, though our initial reaction is that it may not be necessary or beneficial to all parties for the Engineer of Record to retain all documents for all time; our preliminary view on document retention is that the owner should retain the drawings. ASCE/SEI favors the development of tools to assist engineers in addressing the issue of progressive collapse (Recommendation #1). The development of a consensus document providing multiple approaches to mitigating progressive collapse would benefit the profession by providing concepts and techniques upon which to build. It is worth noting that GSA requirements have already advanced technology for evaluating progressive collapse. In general, ASCE/SEI prefers a building-specific and/or ownerspecific approach to mitigating progressive collapse rather than a code-mandated requirement. However, also with respect to Recommendation #1, the ASCE/SEI reserves judgment on whether and how to develop standardized software to evaluate the susceptibility of a particular structural system to progressive collapse. Not all buildings are at risk of being exposed to the type of events commonly associated with initiating progressive collapse. This NIST recommendation needs study of its application and its effect upon the profession because of the various design thresholds involved. When considering possible causation events, other, non-structural, solutions are sometimes effective. Having said that, we look forward to discussing who would develop and maintain the potential software, who would distribute it and who would take responsibility for training the profession in its use. ASCE/SEI agrees that designing for fire performance of structures (Recommendations #4-7) needs to be discussed within the broad engineering profession, and is interested in taking an active role in supporting studies examining these recommendations. A draft has been prepared and we would welcome NIST’s input in furthering the development of this standard. The concept embedded in Recommendation #8 of treating fire as a load case for structural design will necessitate assumption concerning fire protection systems. Their historical performance will need to be included in the discussions along with the technical and economic impact. ASCE/SEI feels that some of the NIST recommendations need further clarification and discussion. ASCE/SEI would like a clearer description of the rationale and motivation for developing limit state criteria in Recommendation #3. It is possible that serviceability, perception of motion issues, and existing seismic criteria on drift may satisfy this recommendation. While much of Recommendation #25 appears to ASCE/SEI to be reasonable, the concept of certification of “as-designed or as-built” safety needs additional discussion and understanding. Without further understanding of the envisioned intent of this recommendation, its implementation may face numerous technical, economic, and authoritative hurdles. Improving safety in existing buildings, as directed in Recommendation #26, is certainly a laudable goal and one that ASCE/SEI supports. While the existence of as-built drawings would assist in the rehabilitation of existing structures as specified in Recommendation #26, a requirement for the retention of a broad range of documents would not improve the safety or performance of structures. Lastly, the roles of various professionals within a project will change and vary from project to project. The assignment of roles and responsibilities is an issue best handled by the contract documents rather than codes and standards, as proposed in Recommendation #28. ASCE also supports Recommendations #29 and #30 which call for increased continuing professional development for engineers and the curriculum be expanded strengthen the base of available technical capabilities and human resources. It is essential that practicing civil engineers remain current with issues and advancements in technology. ASCE supports the attainment of a Body of Knowledge for entry into the practice of civil engineering at the professional level. The Body of Knowledge prescribes the necessary depth and breadth of knowledge, skills, and attitudes required of an individual entering the practice of civil engineering at the professional level in the 21st Century. Establishing innovative solutions to protect public health and safety requires coordination, training and sustained research and development. We are particularly encouraged by the recommendations pertaining to education and we enthusiastically support continuing education of the profession. However, specific issues, such as cross-training of fire and structural engineering professionals, need to be clarified in further discussions. Our profession is responsible for protecting the public to the best of our abilities and to seek new technologies to help us meet that charge. In order to do that, we feel it is important to draw a distinction between advancing the technology through the development of various tools, such as consensus documents on progressive collapse and fire-structure interaction, and potentially adversely affecting the profession by imposing regulations and restricting the engineers’ freedom to develop the best solution for each individual building and the embedding of mandatory provisions in building codes. While not every NIST recommendation may be ready for enactment as is, ASCE/SEI is moving forward with discussion of the issues and their implications for structural engineering practice, and looks forward to working closely with NIST to clarify the application of these recommendations.

NIST Recommendations Referenced:
Recommendation 1. NIST recommends that: (1) progressive collapse should be prevented in buildings through the development and nationwide adoption of consensus tandards and code provisions, along with the tools and guidelines needed for their use in practice; and (2) a standard methodology should be developed—supported by analytical design tools and practical design guidance—to reliably predict the potential for complex failures in structural systems subjected to multiple hazards. Recommendation 2. NIST recommends that nationally accepted performance standards be developed for: (1) conducting wind tunnel testing of prototype structures based on sound technical methods that result in repeatable and reproducible results among testing laboratories; and (2) estimating wind loads and their effects on tall buildings for use in design, based on wind tunnel testing data and directional wind speed data.
Recommendation 3. NIST recommends that an appropriate criterion should be developed and implemented to enhance the performance of tall buildings by limiting how much they sway under lateral load design conditions (e.g., winds and earthquakes).
Recommendation 4. NIST recommends evaluating, and where needed improving, the technical basis for determining appropriate construction classification and fire rating requirements (especially for tall buildings greater than 20 stories in height)—and making related code changes now as much as possible—by explicitly considering factors including:
• timely access by emergency responders and full evacuation of occupants, or the time required for burnout without local collapse;
• the extent to which redundancy in active fire protection (sprinkler and standpipe, fire alarm, and smoke management) systems should be credited for occupant life safety;
• the need for redundancy in fire protection systems that are critical to structural integrity;
• the ability of the structure and local floor systems to withstand a maximum credible fire scenario without collapse, recognizing that sprinklers could be compromised, not
operational, or non-existent;
• compartmentation requirements (e.g., 12,000 ft2 (24)) to protect the structure, including fire rated doors and automatic enclosures, and limiting air supply (e.g., thermally resistant window assemblies) to retard fire spread in buildings with large, open floor plans;
• the impact of spaces containing unusually large fuel concentrations for the expected occupancy of the building; and
• the extent to which fire control systems, including suppression by automatic or manual means, should be credited as part of the prevention of fire spread. Recommendation 5. NIST recommends that the technical basis for the century-old standard for fire resistance testing of components, assemblies, and systems should be improved through a national effort. Necessary guidance also should be developed for extrapolating the results of tested assemblies to prototypical building systems.
Recommendation 6. NIST recommends the development of criteria, test methods, and standards: (1) for the in-service performance of spray-applied fire resistive materials (SFRM, also commonly referred to as fireproofing or insulation) used to protect structural components; and (2) to ensure that these materials, as-installed, conform to conditions in tests used to establish the fire resistance rating of components, assemblies, and systems.
Recommendation 7. NIST recommends the nationwide adoption and use of the “structural frame” approach to fire resistance ratings.
Recommendation 8. NIST recommends that the fire resistance of structures should be enhanced by requiring a performance objective that uncontrolled building fires result in burnout without local or global collapse.
Recommendation 9. NIST recommends the development of: (1) performance-based standards and code provisions, as an alternative to current prescriptive design methods, to enable the design and retrofit of structures to resist real building fire conditions, including their ability to achieve the performance objective of burnout without structural or local floor collapse: and (2) the tools, guidelines, and test methods necessary to evaluate the fire performance of the structure as a whole system.
Recommendation 25. Nongovernmental and quasi-governmental entities that own or lease buildings and are not subject to building and fire safety code requirements of any governmental jurisdiction are nevertheless concerned about the safety of the building occupants and the responding emergency personnel. NIST recommends that such entities should be encouraged to provide a level of safety that equals or exceeds the level of safety that would be provided by strict compliance with the code requirements of an appropriate governmental jurisdiction. To gain broad public confidence in the safety of such buildings, NIST further recommends that it is important that as-designed and as-built safety be certified by a qualified third party, independent of the building owner(s). The process should not use selfapproval for code enforcement in areas including interpretation of code provisions, design approval, product acceptance, certification of the final construction, and post-occupancy inspections over the life of the buildings.
Recommendation 26. NIST recommends that state and local jurisdictions should adopt and aggressively enforce available provisions in building codes to ensure that egress and sprinkler requirements are met by existing buildings44. Further, occupancy requirements should be modified where needed (such as when there are assembly use spaces within an office building) to meet the requirements in model building codes.
Recommendation 27. NIST recommends that building codes should incorporate a provision that requires building owners to retain documents, including supporting calculations and test data, related to building design, construction, maintenance and modifications over the entire life of the building45. Means should be developed for offsite storage and maintenance of the documents. In addition, NIST recommends that relevant building information should be made available in suitably designed hard copy
or electronic format for use by emergency responders. Such information should be easily accessible by responders during emergencies.
Recommendation 28. NIST recommend that the role of the “Design Professional in Responsible Charge” should be clarified to ensure that: (1) all appropriate design professionals (including, e.g., the fire protection engineer) are part of the design team providing the standard of care when designing buildings employing innovative or unusual fire safety systems47, and (2) all appropriate design professionals (including, e.g., the structural engineer and the fire protection engineer) are part of the design team providing the standard of care when designing the structure to resist fires, in buildings that employ innovative or unusual structural and fire safety systems.
Recommendation 29. NIST recommends that continuing education curricula should be developed and programs should be implemented for training fire protection engineers and architects in structural engineering principles and design, and training structural engineers, architects, and fire protection engineers in modern fire protection principles and technologies, including fire-resistance design of structures.
Recommendation 30. NIST recommends that academic, professional short-course, and webbased training materials in the use of computational fire dynamics and thermostructural analysis tools should be developed and delivered to strengthen the base of available technical capabilities and human resources.

Max Porter, professor of civil and construction engineering

Engineers study structural failure of WTC buildings

by Ryan Lamb
Iowa State Daily

After the collapse of the World Trade Center towers in New York City last week, a team of structural engineers formed by the American Society of Civil Engineers has been actively studying the structural failures of the buildings, hoping to gather information that may benefit the design of future skyscrapers.

Heat was the key factor in the building’s collapse, said Max Porter, professor of civil and construction engineering.

Porter said he believes that, in addition to the obvious gap in the structure left by the crash of the plane, the collapse may have been be due to a decrease in the strength of the steel frame of the building caused by the massive amount of heat generated by burning jet fuel.

“The WTC towers’ structures were based upon a tube-within-a-tube design,” said Porter, president of the Structural Engineering Institute, a semi-autonomous branch of the ASCE.

“At the center was a steel and concrete core, while the outside was comprised of steel columns spaced very close together running the entire vertical height of the building,” he said. “Steel, like all building materials, is only able to handle a certain amount of heat before reaching a failure point in terms of strength.”

Porter said the towers’ design, which set the standard for skyscraper architecture in the 1960s, was ahead of its time for more than just its extraordinary height.

“The building relied extensively on the outer steel frame which, when connected to the concrete core, formed a diaphragm,” he said. “This system was very effective in dealing with the stresses caused by high velocity wind at that height.”

Porter said many of his colleagues believe the connections between the trusses running underneath the floors and the steel columns may have failed as a result of the heat. This failure would have caused the structure to become unstable, and the floor resting on the trusses to fall.

“Once one floor fell and landed on the next floor, the building was bound to collapse,” Porter said. “This, along with the overheated steel, allowed for the structures to implode, or collapse inward, as many people witnessed on TV.”

Mardith Baenziger, a former structural engineering consultant with more than 30 years of industry experience, said she did not have enough information to say for sure what caused the collapse.

Based upon information she heard in news reports and read on the Internet, however, Baenziger also believed overheated steel may have been the cause for the collapse.

“Buildings, skyscrapers or not, are generally only designed against regular office fires,” said Baenziger, associate professor of civil and construction engineering. “These fires were fueled by thousands and thousands of pounds of jet fuel. Imagine the heat.”

Both structural engineers emphasized that buildings only can be fire-proofed effectively to a certain point. After that point, it becomes both structurally and economically unsound.

“Steel is often fire-proofed with concrete,” Baenziger said. “That concrete, despite its thin layering, is still very heavy, and applying too much of it would put too much weight at the top, causing the building to be unstable.”

Porter said the real problem is not whether a fire-proofing system could be designed to withstand such fires, but whether it was realistic from a money standpoint.

“The cost would just be unfeasible,” he said.

Both structural engineers said they believe, despite the eventual collapse of the towers, that the frames performed remarkably well.

“The fact that both of them held up for more than an hour after a giant jet had crashed into them, which allowed hundreds to be saved, testifies to their sound design,” Baenziger said. “They were designed to withstand natural phenomena, such as hurricane winds, but not commercial jets.”

Porter said the buildings were designed to withstand the crash of a 707 commercial airliner.

“After a plane crashed into the Empire State Building, the designers worried about planes crashing into the buildings just peaking above the city’s skyline,” Porter said. “However, commercial jets larger than the 707 were not a factor.”

Why the Twin Towers Fell

Engineers who studied the World Trade Center after the September 11 attacks tell why the Twin Towers stood as long as they did, and why they eventually collapsed. For the most current information on New York's World Trade Center, be sure to visit our WTC Index.

WTC Information • Construction Facts • Reconstruction Plans • Minoru Yamasaki, Architect • WTC Books • WTC Index • Disasters Index Engineers Explain • WTC Coverage: icivilengineer • WTC Building Performance Study • Minerals, Metals & Materials Society • NOVA • Computer Animations of the WTC Collapse New York's World Trade Center Twin Towers were designed to withstand fire and hurricane-force winds. Some engineers believed the Twin Towers could even survive impact from a Boeing 707. But no engineer or architect could have anticipated the terrorist attack that turned the Twin Towers to rubble, and experts often don't agree what steps might have been taken to make the buildings stronger. For the most recent findings and detailed technical information, be sure to visit the World Trade Center Coverage page at icivilengineer. How did the Twin Towers fall? 1. Impact from the Terrorist Planes When Boeing jets piloted by terrorists struck the Twin Towers, some 10,000 gallons (38 kiloliters) of jet fuel fed an enormous fireball. But, the impact of the planes and the burst of flames did not make the Towers collapse right away. Like most buildings, the Twin Towers had redundant design. The term redundant design means that when one system fails, another carries the load. Each of the Twin Towers had 244 columns around a central core that housed the elevators, stairwells, mechanical systems, and utilities. When some columns were damaged, others could still support the building. 2. Heat from the Fires The sprinkler system was damaged by the impact of the planes. But even if the sprinklers had been working, they could not have maintained enough pressure to stop the fire. Fed by the remaining jet fuel, the heat became intense. Most fires don't get hotter than 900 to 1,100 degrees F. The World Trade Center fire may have reached 1,300 or 1,400 degrees F. Structural steel does not easily melt, but it will lose about half its strength at 1,200 degrees F. The steel structure of the Twin Towers was weakened by the extreme heat. The steel also became distorted because the heat was not a uniform temperature. 3. Collapsing Floors Most fires start in one area and then spread. The fire from the terrorist planes covered the area of an entire floor almost instantly. As the weakened floors began to collapse, they crashed into the floors below. With the weight of the plunging floors accelerating, the exterior walls buckled. Why did the collapsed towers look so flat? Before the terrorist attack, the Twin Towers were 110 stories tall. Constructed of lightweight steel around a central core, they were about 95% air. After they collapsed, the hollow core was gone. The remaining rubble was only a few stories high. Could the World Trade Center have been made stronger? In a report produced by the Federal Emergency Management Agency (FEMA), the American Society of Civil Engineers (ASCE), and other organizations, experts concluded that no skyscraper could have withstood the impact of the terrorist airplanes. Further, the experts warned that it would not be "technically feasible" to design a building that could survive this type of terrorist attack. Instead, engineers and architects are suggesting that we focus our efforts on designing better warning and evacuation systems so that we can save more people inside the buildings.