Building Operating Management

The Future of High-Rise Building Design
How will the collapse of the World Trade Center towers change tall buildings?

By BOM Editorial Staff
Email the BOM editors



Coverage:
In the weeks following the terrorist attack on the World Trade Center, media reports were filled with as many questions as they were facts. How did terrorists bypass airport security? How did terrorists gain access to airliner cockpits? How did foreigners learn to fly commercial airliners while living in the United States? The answers to most of the questions resulted in changes that were evident in longer airport security lines and the creation of a Department of Homeland Security.



But even as measures to thwart future attacks were put in place, one issue remained shrouded: Why did the towers collapse?

Now, after nearly four and a half years of intensive study, there is consensus about how the towers collapsed. But even today, there is no agreement on the lessons that can be drawn from that tragedy. The most ambitious attempt to define those lessons came in the final report on the collapse of WTC 1 and WTC 2 by the National Institute of Standards and Technology (NIST). That report offered 30 recommendations on ways that tall buildings can be made safer.

Some recommendations are concerned with structural stability. Others discuss how emergency responders should react to situations involving high-rises. Others lay out improved procedures for evacuating buildings.

The most far reaching of the recommendations have provoked a debate among facility executives, code officials and others involved in building, designing and operating high-rise structures.

This series of articles explores some of the issues that will determine how high-rises are constructed in the years to come. The first article looks at the NIST effort as a starting point for the discussion on how high-rises can be made safer. The second article looks at the issues surrounding a single recommendation — full building evacuation — that would change how building occupants and emergency workers respond to emergencies in high-rises. A third article looks at an element of building operations that NIST was not charged with investigating: building security.

As industry bodies grapple with possible changes to codes and standards, they will have to decide what sorts of disasters high-rises should be designed to survive. Although NIST officials say the recommendations were not developed solely on the basis of the terrorist attacks, others question whether they go too far in protecting against other kinds of threats. Those critics say that the World Trade Center attacks are unique, and therefore should not be used to justify taking very costly steps to implement some of the NIST recommendations.

Some industry observers might also use the contents of a forthcoming NIST report investigating the collapse of a third World Trade Center tower — WTC 7. Because that tower was spared the impact of an airliner, many are viewing its collapse as more typical of the type of emergency high-rises around the nation might face. That tower was spared the impact of an airliner. Although the NIST report due out in spring will detail the cause of the tower’s collapse, the agency has already reported fires resulting from the initial attack on the other two towers played a role.

Decisions of whether to require building codes that call for thicker fire-resistive coatings on high-rise structural members, use of redundant sprinkler systems, for example, will be influenced by whether such improvements are worth the expense. Many of the NIST recommendations will cost building owners money. In the case of some recommendations, perhaps a lot of money.

The codes and standards groups reviewing the recommendations face an arduous task. On one side, they’ll hear from a group of people who say nothing in the NIST report is worth implementing. On the other side, they’ll hear that everything in the report is not only valid, but that some specific recommendations don’t go far enough toward protecting buildings.

Code groups will be charged with plotting the middle ground without knowing exactly what future terrorist acts or natural disasters will test the limits of high-rise design. Those groups are charged with finding the elusive answer to the question, “How safe is safe enough?”


forming codes and standards
Speaking In Code
National organizations consider fundamental changes to how
high-rises are designed, built and operated

By Mike Lobash, Executive Editor


The collapse of the World Trade Center towers triggered an investigation unprecedented in cost and scope. Nearly 200 investigators spent more than $40 million and three years to determine why the towers collapsed and to issue recommendations on how to make high-rises safer.

Now, with 43 reports constituting more than 10,000 pages filed, the question has changed. No longer is the question, “Can high-rises be safer?” Rather, it is, “How safe should high-rises be?”

“The bottom line is no one can tell us how safe is safe,” says Gerald Jones, co-chairman of a National Institute of Building Sciences (NIBS) committee reviewing recommendations in the federal government’s final report on the collapse of two of the three World Trade Center towers that were destroyed in the attacks. “Everything is a compromise between safety and economics.”

Code organizations will likely spend the next several years discussing 30 recommendations contained in a National Institute of Standards and Technology (NIST) report aimed at improving the safety of buildings taller than 120 feet. At issue in the discussion will be whether the recommendations — taken individually — should be made part of the model building codes and national standards that govern how buildings are designed and constructed.

The recommendations are broken into eight subject areas. They are:

  • Increased structural integrity.
  • Enhanced fire resistance of structures.
  • New methods for fire resistant design of structures.
  • Improved active fire protection.
  • Improved building evacuation.
  • Improved emergency response.
  • Improved procedures and practices.
  • Continuing education and training.

As the recommendations are reviewed, some are likely to become model building codes with little debate. And a few recommendations, particularly those regarding education and training of emergency responders and of architects and engineers, require little or no action by code and standards organizations. One recommendation suggesting that academic, professional short courses and Web-based training materials be offered in the use of computational fire dynamics and thermodynamics, for example, affects professional associations and doesn’t require code changes.

Fundamental Shift
Other recommendations will prove to be more controversial, with the amount of debate surrounding the adoption of each being proportionate to the cost involved with implementing it. Perhaps none of the recommendations are more controversial than the ones that aim to change the way high-rises are evacuated. One recommendation calls for full building evacuations during building-specific and large-scale emergencies, such as widespread power outages, fires, explosions, terrorist attacks and unforeseen natural disasters. Another would require high-rises to contain more remote exits and two sets of structurally hardened elevators. During emergencies, one set of elevators would be used by occupants and the second set by emergency personnel. The exact number of exits and elevators required would depend upon building size and the number of occupants.

Those recommendations constitute a sea change in the approach to the way high-rises are designed to accommodate evacuations. By and large, high-rises are designed to handle the evacuation of a single floor. If necessary, multiple floors are evacuated one at a time in what’s known as a staged evacuation.

That was the design approach at the World Trade Center towers, says Shyam Sunder, deputy director of NIST’s Fire Research Laboratory and lead investigator on the collapse of the towers. Stairwells were designed to be wide enough and positioned so that 390 occupants — the maximum on any one floor — could be evacuated at one time.

On Sept. 11, 99 percent of the people located below the floors of impact were able to escape the World Trade Center towers before they collapsed, even though the building wasn’t designed to accommodate full building evacuations, according to the final NIST report on the collapse of the towers. The story would have been different if the buildings were fully occupied.

Beyond Terrorism
Even so, Sunder says the recommendations in the NIST report were not based on the ability of a high-rise to withstand the impact of an airliner or to evacuate occupants after such a terrorist attack. Rather, they were developed to aid the evacuation of buildings for any emergency situation, including earthquakes, widespread power outages, tornadoes and other events.

“The need to evacuate a high-rise might need to happen even without a terrorist attack,” Sunder says. “If you look at the nominal life of a building as 100 years, there are likely emergencies that require full evacuation of buildings.”

If the recommendations will make high-rises safer in case of events other than terrorist attacks, that would help justify their being adopted in model codes. Some perceive the NIST report as being narrowly focused on preventing the catastrophic results of Sept. 11.

Samir Mokashi of the engineering firm IDC says immediately following the terrorist attacks, everybody thought that hardening buildings to withstand the impact of airliners was a potential design goal. But now that the event hasn’t been repeated in more than four years, support for that idea has waned.

“Is a plane going into a building something we need to protect against?” he asks.

Phil DiNenno, one of eight members of the National Construction Safety Team Advisory Committee, which advised NIST in its investigation, says of the 30 recommendations issued by NIST, one requiring additional remote exits might have changed the outcome of the attacks on Sept. 11. The addition of those exits — stairwells on the corners and on the perimeter of the building — might have allowed more occupants located above the floor of impact to evacuate.

“But that is singularly an expensive change,” says DiNenno, who is also president of Hughes Associates, a fire protection engineering firm.

The expense comes not only in the design and construction of additional exits, but in the loss of rentable real estate, most notably, the amount of premier daylit space landlords could lease to tenants along a building’s perimeter.

As the recommendations work their way through the code adoption process, expect debate among code-making bodies, building owners — who will ultimately fund the cost of the new codes through increased high-rise design and construction expenses — and building component and material manufacturers whose businesses stand to benefit or suffer from the adoption of certain recommendations as codes.

Seeking Help
NIST has already contracted with NIBS to work on turning the recommendations in the final WTC report into code language that can be adopted by model code groups, including the International Code Council and the National Fire Protection Association (NFPA). Jones’ committee, aptly called the Committee for Translating the NIST WTC Investigation into Model Building Codes, is moving quickly to get at least some changes incorporated into the 2007 supplement to the International Building Code

. The deadline for submitting code change proposals for consideration in the next code cycle is March 24, with final action scheduled for May 2007.

“We’re going to do our best to get at least some of the recommendations adopted in the current code process,” Jones says.

Some of the measures recommended by NIST began working their way into codes and standards even before the agency completed its investigation. For example, NFPA included a provision in its 2003 edition to require wider stairwells for buildings with more than 2,000 occupants. In the 2006 code cycle, there were no dramatic code changes based on the NIST report, says Gary Keith, vice president of building and life safety for NFPA.

For the 2009 code cycle, which will be available for public comment in 2007, Keith says the association’s High-Rise Safety Advisory Committee will review the recommendations and monitor proposals for code changes.

“The questions will be whether anyone looking at the report sees recommendations that are worthy of code changes,” he says.

Shortly after NIST’s final report was issued, BOMA International, representing building owners and managers, issued a statement calling for further study, including a risk-benefit analysis, into some of the recommendations. One comment took issue with a NIST recommendation calling for the development of national standards and codes to prevent the progressive collapse of buildings.

“The big issue here would be to what extent it would be required to prevent progressive collapse?” wrote BOMA’s David Johnston, a member of the organization’s advocacy staff, in submitting written comments to NIST. “How many column failures (specific number, a percentage of a floor, etc…) would need to be able to be withstood? ‘Progressive collapse should be prevented’ is a pretty vague goal without a better explanation of what factors must be considered.”

Another recommendation that has drawn early criticism from BOMA is one requiring the fire resistance performance of structures. NIST recommends that uncontrolled building fires result in burnout without a structure suffering either partial or total collapse.

“The effort is a bit unrealistic and unwarranted,” BOMA wrote. “NIST assumes the complete loss of the sprinkler system, and no intervention by fire departments.”

As the process of reviewing the recommendations moves forward, code-making bodies will likely hear those types of comments as well. While the arguments might not be geared at questioning whether a certain code will make buildings safer, they will be pointing out that the cost of implementing a code might not be worth the expense.

“The real question as the process moves forward is how much will a change cost,” DiNenno says. “That’s the real world where code is written.”


elements of future high-rises
click on image for a PDF version



new evacuation procedures proposed
Evacuation 911
Full building evacuations and use of elevators during emergencies are among sweeping proposals for egress procedures

By Brandon Lorenz, Senior Editor



After more than four years, it’s possible to forget not only how tragic the events of Sept. 11, 2001, actually were, but also how much more tragic they could have been. Had the buildings been fully occupied, the National Institute of Standards and Technology (NIST) has estimated that it would have taken more than three hours to fully evacuate the buildings. In the process, 14,000 people — 28 percent of the occupants — would have died because of insufficient stairwell capacity.

Those stark numbers prompted NIST to call for changes in high-rise building design to improve evacuation time in an emergency. If the recommendations are adopted, facility executives could one day oversee high-rise buildings that have timely full building evacuation plans for non-fire emergencies. The buildings would have hardened elevators and stairwells. Stairwells would be wider and spaced farther apart, and upgraded elevators would be used for evacuation and for emergency responders.

But the NIST recommendations are both controversial and broadly defined, which means industry code bodies will have a significant role to play shaping the recommendations into code — if they are adopted at all.

New Approach to Evacuation
High-rises have been built assuming that the entire building wouldn’t need to be evacuated at once. Instead, during a fire on one floor, occupants evacuate to adjacent floors until it is safe to return.

What does timely full building evacuation entail? NIST’s recommendation says that a building’s size, population, function and iconic status should be taken into account in designing an egress system. “The key question is how much time should be allowed or specified for total safe evacuation,” says Milosh Puchovsky, principal fire protection engineer for the National Fire Protection Association (NFPA).

There is little agreement about how much time should be allowed for a full building evacuation because the topic is relatively new, says Ron Klemencic, chairman of the Council on Tall Buildings and Urban Habitat. “Clearly, if you put in 10 stairwells, you could get people out of the building much faster,” he says. “But how fast is fast enough?”

A full building evacuation during a prolonged power outage, for example, would not necessarily need to be rapid. An explosion, however, might require faster evacuation.

Even if there is disagreement regarding how fast a full building evacuation should be, elevators, stairwells and evacuation plans will all need major revisions to make full evacuation practical.

Structural Changes Required
The final NIST report on the collapse of the World Trade Center towers indicated that the overall evacuation rate in WTC 2 was 108 survivors per minute, about 50 percent faster than WTC 1 at 73 survivors per minute. That’s because occupants in WTC 2 used the elevators in the 16 minutes before the second tower was struck.

For more than 30 years, building occupants have been told not to use an elevator during a fire. With new recommendations from NIST for full building evacuation, the American Society of Mechanical Engineers (ASME) is examining whether elevators could safely be used to evacuate high-rises during an emergency.

Designing elevators that could safely be used during a fire or other emergency is a serious challenge, says Edward Donoghue, administrator for the National Elevator Industry Inc. (NEII) and a safety and codes consultant. For example, one hazard that needs to be overcome is keeping water used during firefighting out of the hoistway, Donoghue says. ASME is conducting separate hazard analyses to see whether elevators can safely be used by occupants for evacuation and whether they can also be used for emergency responders during a fire or other emergency. The NIST report also recommends emergency responders have access to a dedicated elevator.

“It’s a very extensive analysis,” Donoghue says. “It’s probably going to take another couple of years to complete.”

Upsides and Downsides
While elevators factor into the timely evacuation of high-rise buildings, NIST has not ignored stairwells. NIST recommends designing stairwells wide enough to accommodate both descending occupants and ascending emergency responders. That means wider stairwells. “For certain size buildings, provisions have already been implemented in NFPA 101 and NFPA 5000 that require additional stair width to accommodate the counterflow of emergency responders,” says Puchovsky. NFPA 101 is a life safety code. NFPA 5000 is a model building code.

The 28-story Cira Centre in Philadelphia, for example, has stairwells that are 50 inches wide, as opposed to the 44 inches required by code. Construction on the building began in 2003, and the stairwells were voluntarily widened, says Stephen Rush, leasing agent for Brandywine Realty Trust.

NIST has also recommended that stairwells be marked with consistent signage, be located farther apart without increasing average travel distance and maintain their integrity under foreseeable building-specific or large-scale emergencies. Each World Trade Center tower had three emergency stairwells, but only one stairwell in WTC 2 remained passable after the towers were struck, according to NIST. Increasing stairwell distance means more stairwells could move from the building core to the perimeter.

However, maximizing the distance between stairwells could slow evacuation as it takes longer for occupants to reach a stairwell, Klemencic says.

New York City’s World Trade Center Building Code Task Force issued a variety of recommendations in 2003 that are now required by the city’s Local Law 26. Three requirements regarding stairwells apply to buildings 75 feet and higher and are retroactive. Photoluminescent markings are required in all exit doors and exit stairs. Additional signage is required when the exit path is not clear and such signs must have battery or generator power.

Disaster Planning
Disaster planning in a post-9/11 world means evacuation plans should move beyond fires. Power outages, earthquakes, tornadoes, fires, explosions and terrorist attacks should all be taken into account when evacuation plans are formulated, according to NIST.

“When we are talking about full building evacuation, we are talking about evacuation for potentially any type of situation,” says Jim Carrigan, supervising engineer for fire and life safety for the Syska Hennessy Group. Prior to 9/11, for example, facility executives really didn’t talk about hazardous material issues when formulating evacuation plans.

Unlike NIST’s recommendation for full building evacuation, the recommendation for better evacuation planning appears to be gaining traction.

In one case, a tenant occupying multiple floors in a high-rise paid to have its evacuation plan reviewed and upgraded after the developer declined, says Carrigan. The original plan only contained provisions for a fire emergency, while the revised evacuation plan was broadened to take other potential emergencies into account. It was later adopted by all 40 building tenants, Carrigan says.

In New York City, Local Law 26 requires that emergency evacuation plans include non-fire-related events. “The idea was that while the codes have had sufficient fire protection and fire safety plans, the conditions of other emergencies that are not directly related to a fire have not been considered,” says Ronny Livian, chairman of the World Trade Center Building Code Task Force.

In earthquake-prone Seattle, a full building evacuation plan doesn’t necessarily make sense, says Ben Barron, vice president of development for Clise Properties, which manages 1700 Seventh Avenue, a 23-story office building. “We feel that the tenants are safer in our building than out on the street, at least until we analyze what is going on.”

Barron says that a man threatened to blow himself up while standing in the lobby of a nearby courthouse. Rather than completely evacuate the 1700 Building, tenants were moved away from the side of the building nearest the courthouse. The reason? Tenants who left the building likely would have gathered in a courtyard near the courthouse, placing them in danger.

Beyond the Exits
Facility executives can’t forget about occupants once they leave a building. Proper planning means making sure occupants remain safe once they are evacuated, says Livian.

“In a larger building, you may have an occupant load of 10,000 or 20,000,” he says. “Where do you put them and still be safe? You can’t just leave them out in the street.”

To be successful, full building evacuation plans must be tailored for specific buildings and the types of occupants in the buildings, says Carrigan. In addition, the plans require training so that facility executives know when it is appropriate to call for a full evacuation and when to call for a phased evacuation. What’s more, facility executives should not call for a full-building evacuation by paging the entire building and asking occupants to leave, as was the case on 9/11, Carrigan says.

“The purpose of a full building evacuation is to do a staged evacuation in a controlled manner,” he says. “With the current buildings, if the stairwells haven’t been widened, you have to do a staged evacuation or there will be a bottleneck.”

Counting Occupants
Comprehensive disaster planning also means accounting for mobility-impaired occupants. According to NIST, 6 percent of the occupants in the World Trade Center were mobility-impaired. In a traditional phased evacuation, mobility-impaired occupants can be moved away from a fire or other emergency with relatively little difficulty. A full building evacuation is more troublesome. Knowing which occupants are impaired and where they are located is key. Ideally, an evacuation plan also takes into account visitors and occupants who are temporarily impaired because of illness. In the case of non-fire emergencies, Carrigan recommends using elevators to evacuate mobility-impaired occupants.

In Seattle, Clise Properties has identified mobility-impaired tenants. In case of an emergency, a “buddy system” has been arranged to make sure the mobility-impaired tenants can get the assistance needed to evacuate, if necessary. The company has ordered an evacuation chair that will allow occupants to be evacuated down a stairwell, Barron says.

Even if the NIST recommendations governing full building evacuation and elevators are approved by code bodies, implementation will likely take years. While not all cities require high-rises to develop separate emergency action plans for fire and non-fire events, a proper plan that considers fire and non-fire-related emergencies can ensure that a building disaster doesn’t become a greater tragedy.

“In a post-9/11 world, everybody needs to have an emergency evacuation plan for fire and non-fire events,” says Carrigan. “We have different threats today.”


security guidelines to keep high-rises safe
De Facto Standards
Although codes don’t govern building security systems,
these high-rise systems have been upgraded across the board

By Greg Zimmerman, Managing Editor



Designing and maintaining high-rise security following 9/11 represents a multifaceted challenge to building owners. Absent a report like the one the National Institute of Standards and Technology produced with its list of recommendations for fire safety, evacuation procedures and technologies, high-rise building owners are largely on their own to determine how best to protect people and property at their buildings.

The need for particular high-rise security measures depends on a building’s location, proximity to other types of buildings, tenants, purpose and visibility, among other things. However, access control is one area of security that building owners have ramped up in high-rises nearly across the board since 9/11.

In the past, standard operating procedure had been to control access in many high-rises only after business hours, says Carlos Villarreal, vice president, national security and life safety for Trizek. Many high-rises had a single card reader in the elevator vestibule that wasn’t turned on until the close of the business day. While some access control was usually in place at the building’s main entrance, once a visitor or occupant made it through the first point, the person was free to wander.

“Traditional high-rise buildings were much like schools before Columbine,” says Sean Ahrens, senior security consultant at Schirmer Engineering and a member of the ASIS commercial real estate council. “They were open and people could walk around at will.”

Controlling Entry
Since 9/11, that has changed. Access is controlled 24/7 at elevators, including freight and service elevators. Visitors and occupants may need specific permissions tied to their credentials to gain access to certain floors. Also, gone are the days of flashing a badge to a security guard at a building’s main entrance.

“Optical turnstiles are being used more in large buildings,” says Geoff Craighead, vice president of high-rise and real estate services at Securitas and chair of ASIS commercial real estate council. “It’s all very well to have card readers at elevators, but if you can’t control piggy-backing at the main access points, then you’re not controlling access at all.”

Visitor management policies have also become more sophisticated. Gone is the simple sign-in sheet. In its place are automated systems that allow occupants to pre-register visitors, who are then given a badge, which must be returned when they leave. In many federal high-rises, or other buildings with many visitors, a dedicated visitor management station exists so as not to slow the entry of regular building occupants.

And it’s not just visitor management that’s getting its own lobby space. In general, experts say high-rise building lobbies are becoming more segmented and compartmentalized.

“It’s important to separate people that haven’t been screened yet,” says Lauris Freidenfelds, vice president at Sako & Associates. “Lobbies are getting bigger, too. The lobby is the area where there is most likely to be an incident. If there is an incident, it’s essential that the lobby is designed so the incident has the least impact on the rest of the building.”

Security by Design
To make controlling access easier, many high-rise buildings, including newly designed ones, are cutting back on the number of places people are able to get in and out. In addition, many high-rises have designated a single entry for mail, packages and large deliveries. New high-rises are being designed so that the mailroom is situated at the exterior of the building and can be cordoned off at a moment’s notice in case of a threat, says Ahrens.

Policies that govern vehicle access are receiving new scrutiny as well. It’s become common to design new structures with lower deck parking — structures with smaller floor to ceiling distances — near the building so that only small vehicles can fit. Higher deck parking for large vehicles that could potentially carry large amounts of explosives is farther away from the building. Also, experts suggest locating occupant parking near the building, with visitor or public parking located farther away. If the high-rise is serviced by underground parking, Ahrens warns that building owners should make sure measures are taken so that the building above is properly protected. “There needs to be increased vehicle screening,” he says. “That means putting mirrors under the cars as they come in and opening trunks.”

Other exterior measures intended to mitigate the effects of bombs include bollards, concrete barriers and even sculptures or artwork. Tiered landscaping or ponds, when the site allows, can also make approaching the building by vehicle more difficult.

However, if the high-rise’s site limits stand-off distances or physical measures, window film and blast-resistant glass can help reduce the impact of a bomb. New high-rises are sometimes being designed with fewer windows near grade. For instance, the new Freedom Tower, which will be built on the World Trade Center site, is being designed with no windows on the first 20 floors of the building, says Craighead.

Preventing CBR
Concerns about a possible chemical, biological or radiological (CBR) attack have led many building owners to monitor access to a once-ignored part of the building: grade-level exterior and interior air intakes. The danger is that a terrorist could dump into the air intakes an agent that is circulating through the building’s HVAC system within minutes. To combat that threat, some high-rise building owners are installing CCTV cameras with intelligent video algorithms on the air intakes. If a video frame is broken by motion, a security staff member is alerted automatically and can determine whether there is danger and, if so, begin evacuation procedures. This strategy probably won’t prevent an attack, but it can help speed the response and evacuation.

However, if the vulnerability is deemed severe enough, Craighead says some high-rise building owners have undertaken the significant cost of relocating the air intakes out of pedestrian reach.

Also, many new high-rise buildings are being designed so that exterior air intakes are at least 75 feet above ground.

In the case of an outside or nearby CBR attack, many building owners have instituted defend-in-place procedures, which were nearly nonexistent before 9/11. “Traditionally, tenants had been trained to evacuate,” says Villarreal. “But now we’re seeing seminars on how to shelter in place, and how to completely seal off a building so no one can get in or out.”

Freidenfelds says that practicing a procedure is just as important as instituting a procedure in the first place. “Make sure that response plans are exercised,” he says. “We don’t want to be dusting off these plans when there is an emergency.”

Guidelines for Securing Facilities

Several documents produced since 9/11 aim to provide direction to building owners on improving security. While these documents are all applicable to high-rise buildings, and security personnel responsible for those structures, none was developed specifically for tall buildings. They apply to a range of occupancies.

The newest documents that can help building owners cope with these challenges were developed by the National Fire Protection Association (NFPA). The project to develop the documents began in 1994 on a request from the insurance industry. After several starts and stops, constant goading by the insurance industry, and the terrorist attacks of 9/11, NFPA redoubled its efforts and released the documents in July 2005.

“NFPA was already going down the path to security guidelines,” says Rich Bielen, chief systems and applications engineer at NFPA. “But 9/11 pushed everything.”

NFPA 730, Guide for Premises Security, covers vulnerability assessments, designing a security plan and suggested security measures for different occupancies. NFPA 731, Installation of Premises Security Equipment, covers the application and installation of physical security technologies. Basically, NFPA 730 is the “what”; NFPA 731 is the “how.”

Bielen says 730 is considered a guide while 731 is considered a standard, which is more stringent. Originally, 730 was going to be a standard as well.

“Many of the users on the committee were not comfortable with the mandatory requirements format, so the committee debated whether to make it a standard, a recommended practice or a guide,” says Bielen. “The guide format was the most agreeable format as a mix of recommendations and information.” Bielen says the ultimate goal is for both 730 and 731 to become regulation.

Instead of Code
Currently, no regulations or codes exist that govern how building security should be planned and executed. That’s because creating security codes would be like creating separate fire codes for each building, says William Sako, executive vice president of RJA Group and chairman of the board for Sako & Associates. A central challenge for a high-rise building owner is determining what risks or threats exist for a specific property. Identifying where dollars should be spent to address the most urgent vulnerabilities and selecting systems that best mitigate threats are the other critical challenges.

ASIS has also developed several guidelines, which are oft-consulted tools for building owners to help assure that their security plans are on par with those of similar facilities in similar security situations.

“We create guidelines that are broad enough so that they’re applicable to Fortune 500 companies, as well as companies with 50 employees,” says Regis Becker, chairman of ASIS’s guidelines commission and global director of security and compliance at PPG Industries. “Security is more an art than a science.”

Topics such as business continuity, risk assessment, responsibilities and accountabilities of a chief security officer, private security officer selection and training, and workplace violence prevention and response are covered.

A guideline on selecting physical security elements, which Becker says will be most applicable to high-rise buildings, is under development.

As is the case with the NFPA and its documents, ASIS is an American National Standards Institute (ANSI)-accredited national standards developer, and all its documents were developed according to ANSI protocol.

—Greg Zimmerman, managing editor

Fire Testing Is Questioned in Findings on Towers

Fire Testing Is Questioned in Findings on Towers

By ERIC LIPTON

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.

PROGRESS REPORT

PROGRESS REPORT
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.
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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)
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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-
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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.
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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.
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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).
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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.
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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.
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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.
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