Iano's backfill

Standards: New (costly?), New (better?), and Needed(?)

ENR's Third Exit Stair Could Make Highrises Too Costly to Build (June 4, 2007) reports on the adoption of more stringent life safety requirements for tall buildings in the 2007 Supplement to the International Building Code. Both the Building Owners and Managers Association International and the American Institute of Architects oppose as too costly a new requirement for buildings taller than 420 feet to have at least 3 independent exit stairs. Other changes to tall building requirements in the 2007 Supplement, all coming in the aftermath of the 9/11 attacks on the World Trade Center Buildings in New York City, include:

• protected fire service elevators and lobbies
• photoluminescent pathway markings in exit stairwells
• inspection and greater strength requirements for spray-applied fireproofing.

Proposals for progressive collapse resistance, increased  exit stairway enclosure integrity, and requirements for a a building's ability to burn without collapse were rejected. See the ICC's Code Development page for more information about that organization's code development process and the adoption of these new requirements.

In the same issue, Energy-Efficiency Guidelines Would Be National Model reports on the American Society of Heating, Refrigerating and Air Conditioning Engineer's (ASHRAE) ongoing development of Standard 89, an enhanced building energy efficiency guideline. Standard 89, now open for comments, is intended to reduce building energy consumption by 30% in comparison to ASHRAE's long-recognized Standard 90, and is being touted as a national standard for achieving LEED certified optimized energy performance. See ASHRAE's Advanced Energy Design Guides page for more about this organization's enhanced energy performance guidelines.

Lastly, Innovative Mix with Fly Ash is Still Not Widely Used describes a concrete mix composed of lime, portland cement, and fly ash developed by Nai C. Yang of the New Port Authority 38 years ago for use in airport runway construction. The mix is claimed to provide superior concrete and a reduction in environmental impact (due to reduction in cement content and, consequently, reductions in C02 emissions). However its broader use in airport construction has been hampered by unfamiliarity and a lack of standards for its production.

NIST Continues Progress on WTC Report

Report on Trade Center Collapses Emphasizes Damage to Fireproofing (April 5, 2005, New York Times) reports on the National Institute of Science and Technology's (NIST) April 5, 2005 public progress briefing. Some of the Institutes's findings include:

• Structural damage caused by the impacts of the airplanes in and of themselves was not sufficient to have caused the tower collapses.
• The towers collapsed when the structural frames were further weakened by the fires that followed the plane impacts.
• The structures were rendered more vulnerable to fire when fireproofing was dislodged from the structural steel members by the plane impacts.
• Though jet fuel contributed to the initial fires, the fires were mainly fueled by building contents, and to a lesser extent by aircraft contents.

The article also states that had the towers been full at the time of the attacks (they were each only about 1/3 occupied) roughly 14,000 persons could have died due to the greater time required to evacuate the larger numbers of occupants from the buildings.

Critics Blast Findings of Federal 9/11 Study (April 18, 2005, Engineering News Record) reports on critics who characterize the NIST study as expensive ($16 million) and misguided.

Jon D. Magnusson of Magnusson Klemencic Associates, Seattle, is quoted criticizing NIST for proposing building code changes based on the study of "one building that was unlike any other, for a single vent that had never happened before." Magnusson does not believe the NIST recommendations will lead to actual increases in public safety. Others echo similar concerns, citing a lack of analysis to determine the degree to which recommended safety enhancements can save lives, or characterizing recommendations as based on speculation.

Additional related articles available from ENR include:

• WTC Report Triggers Debate Over Safety (ENR 7/4/2005)
• Recommendations Gain Qualified Support (ENR 7/25/05)
• A New Era for Fire Protection and Life Safety (Architecture Record, 8/2005)

NIST will be publicly releasing its Draft Report and Recommendations on June 23.

More Info:

• Previous articles on Iano's Backfill related to building safety in general and the World Trade Center disaster in particular
• NIST's WTC web site

More WTC Findings

WTC wind load findings (Structural Engineer, November 2004) reports that additional analysis of the original wind load design for the World Trade Center towers has lead to a revision of the National Institute of Standards and Technology's previous findings. According to the revised analysis, the wind loads used in the design of the two towers exceeded code requirements throughout the life of the structures, starting with New York City's pre-1968 code requirements in effect when the buildings were constructed, and including subsequent updated code requirements through 2001, when the buildings were destroyed.

According to the brief article, the new analysis is based on original source documents including wind tunnel testing and wind load estimation methods, as well a revised interpretation of the application of this information. This has lead to an upward revision of the original design loads from earlier preliminary estimates.

Design Flaws did not lead to WTC collapse (Structural Engineer, December 2004) reports on additional NIST interim findings released in late October. Of particular note is a lead investigator S.Shyman Sunder's statement regarding the overall performance of the towers:

The buildings performed as they should have considering the airplane impact and extreme fires to which they were subjected. There is nothing there that stands out as abnormal.

Additional findings included in this NIST report address hypotheses for the causes of the tower collapses, explanation of the differences in survival time between the two towers, assessment of the post-impact capabilities of the towers, the role of fireproofing, the effects of fire on the tower structures, the quality of the structural steel itself, and factors related to occupant egress prior to the collapses.

More info:
NIST's WTC web site

Interim WTC Investigation Findings

The National Institute of Standards and Technology (NIST) has issued its second major progress report and interim findings related to the WTC disaster of 9/11/2001. The report discusses, in part:

• Working hypotheses for the collapses of the towers, as well as for the 47-story WTC 7 building.
• Factors contributing to the building collapses including the impacting aircraft, building structural systems, fireproofing, active fire protection systems, structural materials, and failure mechanisms.
• Emergency response, command and control, and communications during the disaster
• Applicable building codes at the time of the buildings' construction
• Design practices related to fire- and structural-safety, performance-based fire safety design, and the roles of building standards, codes, and regulations in such design
• Issues meriting further consideration

A few of the more interesting findings in the report include the following:

NIST's working hypothesis is that the towers failed due to column instability caused by the airplane impacts and subsequent fires. One question concerns why WTC 1 stood for nearly twice as long after plane impact as WTC 2. Though the report draws no inference, it notes separately that affected floors in WTC 1 had significantly upgraded amounts of fireproofing in comparison to affected floors in WTC 2.

The role of spray-on fireproofing in protecting the building frames from the heat of fire continues to receive considerable attention. Fireproofing thicknesses applied to the long-span floor trusses ranged from 1/2-inch to 1 1/2-inches. Separately, an unrelated 2001 model code report recommended 2-inch thickness for a similar floor system. These large discrepancies indicate a lack of technical basis for the fireproofing thicknesses used in the towers.

More than 99 percent of the tower occupants below the impact floors were able to evacuate safely prior to building collapse. Significant drops in evacuation rates prior to the collapses suggest that there was sufficient egress capacity to accommodate these building occupants. However, on the day of the attacks, both buildings were each occupied at approximately only 1/3 of full capacity. Estimates indicate that evacuation of either tower at full capacity would have required significantly more time, approximately 4 hours.

The towers were not constructed to meet requirements of the building code in effect at the time that they were built, the 1938 New York City Building Code. Rather, the Port Authority opted to conform to requirements of the upcoming 1968 Code, even though this new code was three years away from going into effect. As a result, significant reductions in life-safety requirements were applied, such as reduction in number of required stairwells, reduction in levels of fire-resistance ratings, and reduction in partition structural load requirements.

The Institute's Key Findings document is especially recommended for further reading.

More Info
NIST News Release
NIST Progress Report
Columns Likely Failed First in Terrorist-Triggered WTC Fires, ENR, June 28, 2004

Fire-Safe Debate Continues

Issues related to building fire-safety continue to receive attention both in the technical and mainstream press.

Fire Prevention and Protection: The Big Picture, Building Safety Journal, May-June 2004, offers another perspective on the ongoing fire-safety debates.

Author, Mike Slaughter, fire-safety specialist, and former firefighter and code official, addresses the question of the connection between modern building construction systems and reductions in in firefighter safety. Key points raised include:

• Slaughter cites as "common knowledge" that modern construction systems rely on lighter materials that are more vulnerable to fire.
• Between 1978 and 1999, the rate of firefighter deaths from traumatic causes inside structure fires increased by 60%.
• Other aspects of fire-safety, including building code requirements, firefighting equipment, and firefighting tactics, have undergone significant improvements during this same period.

Slaughter concludes that trends toward lighter-weight construction systems have likely reduced firefighter safety, while other improvements in code requirements, firefighting equipment and tactics have probably to some extent counteracted this negative trend.

Skyscapers' Supporters Infuriated by Fire Fearmongers, Engineering News Record, June 7, 2004, discusses the increased controversy surrounding fire safety in tall buildings in the aftermath of the collapse of the World Trade Center towers. Some interesting facts and perspectives presented in the article:

• Between 1997 and 2001, and excluding the 9/11 events, sixty-eight percent of civilian fire deaths occurred in one- and two-family dwellings, 14 percent in other residential building types, 13 percent in road vehicles. Only 3 percent of civilian fire deaths occurred in nonresidential structures.
• According to NFPA statistics, between 1985 and 1995, seven civilians died in office tower fires. During the same period there were over 57,000 civilian deaths from all kinds of building fires combined.
• W. Gene Corely, of Construction Technology Laboratories argues that buildings should be designed to withstand a full "burnout" without collapse. In the case of 9/11, the WTC 5 and 7 buildings did collapse due to fire (without the addition of large quantities of jet fuel as occurred in the two main towers).
• An article sidebar makes the case that structural steel and cast-in-place concrete are both approximately equally vulnerable to the effects of fire.
• Last month the ICC (authors of the International Building Code) voted to eliminate allowable tradeoffs for passive containment and sprinklers in buildings greater than 420 feet tall.

More Articles
Fire Codes Spur Debate, Architecture, 06/2004
New York's Building Code Reflects High-Rise Concerns, ENR Construction, Spring 2004
A Myriad of Issues, Newsweek, 6/10/2004

Defensive Design Guidelines

DHS Announces New Building Safety Guidelines, Building Safety Bulletin, May 2004, describes new publications developed by the Federal Emergency Management Agency (FEMA) and released by the Department of Homeland Security related to the design and construction of buildings for resistance to terrorist attack. Documents available on FEMA's web site include:

• FEMA 426 - Reference Manual to Mitigate Potential Terrorist Attacks Against Buildings
• FEMA 427 - Primer for Design of Commercial Buildings to Mitigate Terrorist Attacks
• FEMA 428 - Primer for Design Safe School Projects in Case of Terrorist Attacks
• FEMA 429 - Insurance, Finance, and Regulation Primer for Terrorism Risk Management in Buildings
• E155 - Building Design for Homeland Security

Elevators and Emergency Egress

Industry group proposes use of elevators during fires, Building Design & Construction, 05-04, reports on plans by the elevator industry and standards organizations to develop elevator systems suitable for occupant egress and firefighter access during building emergencies.

According to the article, benefits of such "protected" elevator systems include reduction in building evacuation times by as much as 50%, and delivery of firefighters to upper floors in high-rises up to six times faster than possible by use of stairs. These plans took a significant step forward with a March workshop sponsored by the National Institute of Standards and Technology, attended by various stakeholders including the American Society of Mechanical Engineers, International Code Council, National Fire Protection Agency, Americans with Disabilities Act board members, elevator manufacturers, and industry consultants and experts.

According to the article, in twelve countries outside the US, lifts dedicated solely to firefighter access are required in buildings 30 meters or taller. In contrast, the plans being developed here in the US will rely on normal service elevators, and will provide not only firefighter access but also occupant evacuation capabilities. The basic operational strategy works as follows:

• When any detector in the building signals a fire alarm, one designated elevator is recalled to the ground floor and taken out of service, where it can be put back into service under manual control by emergency personnel.
• As long as smoke or heat detectors in elevator lobbies and hoistways do not signal alarms, other elevators would continue to operate normally.
• If lobby or hoistway detectors do signal, then all affected elevators would be similarly recalled.
• Advanced sensing and communications systems would allow emergency personnel to gauge occupant loads and fire conditions on individual floors. Using this information, occupants could be directed to wait for elevators at their current location, proceed to elevator lobbies on other floors, or exit the building by stairway.
• Elevators themselves would be protected from smoke and fire by 1-hour rated, pressurized lobbies and pressurized hoistways.
• Proposals for integrating elevator control and communications with HVAC and fire protection systems are also part of the planning.

More Information:
Conference Proceedings: Workshop On Use of Elevators In Fires And Other Emergenices

Height & Area Requirements In The IBC

As noted in the textbook, the three previous US model code organizations have consolidated themselves into the International Code Council, and merged their model codes into the International Building Code. Recently, this author had reason to compare height and area limitations between one of the "legacy" model codes and the new International Building Code. It's no secret that the new code is at times less restrictive than its predecessors, but even knowing this in advance, the results were eye opening.

The following two charts are adapted from our design handbook, The Architect's Studio Companion. One portion of this book deals with the effects of the building code on building form by providing a series of charts that allow the designer to quickly determine allowable height and area for a project under consideration depending on such factors as applicable code, construction type, and related considerations. By comparing tables from an earlier edition of the Companion, that compiled results for the previous codes, with information from the latest edition, which refers to the new International Building Code, easy comparisons can be made.

Combustible Construction, UBC vs IBC
This first chart compares height and area limits for two codes, the "legacy" Uniform Building Code (UBC) and the International Building Code (IBC), for buildings with Occupancy Group B (Business) and constructed of combustible construction. The base chart (rendered in grey scale) represents data for the UBC. Information overlaid in blue represents data from the IBC.

Read the chart as follows:

• Various construction types and their level of fire protection are listed across the top rows of the chart. For example, "Wood light frame, 1-Hour" refers to stick built construction with 1-hour fire protection applied to the framing (typically in the form of fire-rated gypsum wallboard). Mill construction refers to buildings with noncombustible exterior wall construction and heavy timber framing for the interior structure. Noncombustible structures made from concrete or steel (included in the chart below) are listed simply according to the level of fire protection applied.
• Below the construction types, columns are further divided to distinguish between sprinklered ("Spr") and unsprinklered ("Unspr") buildings.
• Once a construction type, level of fire protection, and sprinkler condition have been established, allowable height and area can be read as follows. For maximum height in feet, read from the row directly below the sprinkler designation row. ("UH" on the chart below refers to unlimited height.)
• For maximum height in stories, proceed down a column until arriving at the solid horizontal line. Read across to the side of the chart to find the allowable stories permitted.
• To determine the maximum permitted area for the building, read the number directly below (or above, where space was limited) the solid line. This is the maximum allowable area for all floors of the building. ("UA" refers to unlimited area.)

Keeping in mind that data for the IBC is rendered in blue, a few facts are readily apparent:

1. For the cases illustrated here, more often than not, the IBC is moderately more generous in its allowable building height than the UBC.
2. In these cases the IBC is significantly more generous in its allowable building area than the UBC. This is particularly true for sprinklered buildings, where on average, the allowable area for all floors has increased by a factor of 7 times in comparison to allowable area for the UBC.

Noncombustible Construction, UBC vs IBC
This second example continues the comparison for noncombustible construction types, for the same Occupancy Group B.

The results are similar: The IBC is more often than notgenerous in allowable height than the UBC. But the IBC is significantly more generous in its allowable area than the UBC, particularly for cases where the building is sprinklered. (Though not illustrated here, the same trends are true for most other occupancy groups as well.)

Comments
Reportedly, at the time of the creation of the IBC, the approach taken to establishing height and area limitations was to take the least restrictive requirements from any of the three legacy codes and apply those requirements to the new code. As it was explained to this author by one representative of the International Code Council, this decision was based on the absence of any data indicating any one of the legacy code's limits was technically superior to the others.

For any design and construction professional interested in the topic of building safety, these are interesting times. A confluence of events have brought new attention to a number of questions that have always been part of this profession, but that often may be taken for granted by many of us. To name just a few: How is building safety measured? Who decides what constitutes a reasonably safe building? Who should be responsible for ensuring the safety of buildings? This site will continue to report on developments in these areas as they arise.

More Information
Category 01 Making Buildings on this site includes articles related to regulation of construction, building codes, and the general considerations of how buildings get built. Category wtc on this site includes articles related to building safety and in particular issues arising out of the collapse of the World Trade Center towers. Many of the articles in both of the categories link to other Web resources for further coverage of these topics.

For an introduction to building codes, Occupancy Groups, Construction Types, Height and Area restrictions, and related topics, see pages 4 - 10 of the textbook.

More On Highrise Safety

The debate on building safety continues...

Building Safety Journal
Fire Protection - The Fire Service Viewpoint?, Building Safety Journal, January-February 2004, (available online at the time of this writing) rebuts charges that current building standards allow construction of buildings that are unsafe for firefighters during building fires. This article, authored by fire protection engineer Richard C. Schulte, is perhaps most noteworthy for the prominence granted to it by its publication in the International Code Council's flagship journal. This author found the arguments put forth in the article less than compelling.

Metropolis Magazine
Shortcuts to Safety, Metropolis Magazine, March 2004, takes the position that "Two and a half years after the worst tragedy in skyscraper history, highrises are no safer." This is an informative article offering insights this author has not encountered elsewhere on this topic. Whether or not one agrees with its premise, this article is worth the attention of those concerned with the topic.

Some of the key points raised in this article include:

• The Skyscraper Safety Campaign, founded by the family of a firefighter lost in the disaster, has played a prominent role in raising issues of building safety and pushing for investigation into the causes of the building collapses. According to the article, this group was partially responsible for the creation of the ongoing investigation by the National Institute of Standards and Technology.
• Though building safety standards are enforced through public regulation, the standards themselves are crafted in the private sector, and to a significant extent by the same parties with an interest in minimizing building costs. As an example, the article cites New York City's own building regulation department, the Department of Building, that has a dual mission of maintaining building safety while also "streamlining the occupancy of new and existing office space".
• According to sources quoted in the article, the standards of safety applied to the World Trade Center towers were sufficiently lax that these buildings could not have been built in Germany, the United Kingdom, Japan, or even possibly in any American city--including New York City--if they had had to meet local building code requirements. (The developer of the towers, the Port Authority of New York is exempt from City building and fire department regulation.)
• Trends in construction since World War II toward lighter construction materials and assemblies have resulted in building systems that some argue are less resistant to fire and that are less safe for building occupants and fire fighters.

The article also discusses the reconstruction of Seven WTC, the 47-story highrise that collapsed due to fire some hours after the two main towers. The new building, dubbed "Seven II", is being built to standards significantly in excess of current code requirements including a reinforced concrete core, fire sprinklers with double the required supply capacity, exit stairs 20 percent wider than required, and other safety features. The article also notes that these enhancements may reflect both the developer's good intentions and/or economic realities of insuring a building in this sensitive location.

For more on this topic, see the previous articles on this site:
Fire-Safe Debate
The Current State of Fire Protection Design
Debate Over Fire Testing

Fire-Safe Debate

With the World Trade Center collapses, several recent catastrophic building fires, and the publication of two new model building codes, fire safety in buildings is getting more than its usual share of attention. Two recent opinion articles take the position that current trends toward relaxing building fire safety requirements are ill advised and should be reconsidered.

Reliance on sprinklers a mistake, Building Design & Construction, 01.04, takes the position that "building codes...are based on the mistaken assumption that sprinklers virtually never fail and that fire-resistant construction materials can, therefore, be minimized or eliminated". This guest commentary piece is authored by W. Gene Corley, PhD, Senior VP, Construction Technology Laboratories, and team leader for FEMA's WTC Building Performance Study.

Some of Sorley's assertions include:

• Sprinkler failure rates may be as high as 16%.
• The trend in the model codes is toward increasing reliance on sprinklers, while reducing requirements for fireproofing, fire-barriers, and smoke-barriers.
• Fire-barriers and smoke-barriers are considered "life savers" by fire fighting personnel, and they reduce the risk of building collapse.

Sorley continues with examples gleaned from his his experience as a lead investigator of the WTC disaster. He then offers more criticism for the International Building Code (IBC), which he claims has "drastically lowered" requirements for fire-resistant construction in comparison to older model codes. Sorely concludes with the advice that building fire safety requires both sprinkers and fire-resistant construction to best ensure the safety of building occupants and emergency responders.

Fire-Safe Buildings and Masonry Codes, Masonry Today, Winter 2003/2004, takes a similar position, stating "Today's building code officials should be encouraged to develop code provisions that provide an improved level of redundancy for life safety, property proteciton, and welfare of the general public". This article is written by Stephen Szoke, Director of Codes and Standards, Portland Cement Association.

Szoke states that in the process of merging the three previous model building codes, the least restrictive passive fire protection requirements from these codes were used as the basis for the requirements in the new IBC. Furthermore, new code change proposals seek to reduce even further the IBC's passive fire protection requirements.

Szoke goes on to make a number of arguments for reconsidering an overreliance on fire sprinklers in buidings, including considerations of:

• Aging population
• Risk of malicious disabling of fire sprinkler systems
• Trends in other aspects of the building code toward more stringent safety requirements
• Increased construction of multi-family building types
• Sprinkler system effectiveness
• Sprinkler malfunctions
• Reliable water supply for sprinklers

Szoke advocates a "balanced design" approach to fire-safe construction that relies on both sprinkler systems and the redundancy of passive fire-resistant construction. He further recommends building designers consider in some circumstances exceeding current code fire-separation requirements.

This Author's Comments
Readers should take note that both authors cited above are associated with construction trade groups that have an ecomonic interest in promoting non-combustible construction methods. Construction Technology Laboratories, of which Sorley is a Senior Vice President, is closely associated with the American Institute for Concrete Construction. Szoke's organization, PCA, is a promoter of concrete and masonry materials and construction. (Szoke is quite forthright in his article concerning his association and interests in this matter. He states "PCA and allied industry groups advocate changes to increase passive fire protection for buildings...")

Despite these authors' perhaps vested interests, it is this author's opinion that their points are well founded and worthy of serious consideration.

Ongoing NIST Research Into 9/11

Reliving 9/11, With Fire as Teacher, New York Times January 6, 2004, describes ongoing research by the National Institute of Standards and Technology into the causes of the collapse of the World Trade Center towers. Aspects of the NIST efforts discussed in this article include:

• Full-scale mockups and fire-testing of office workstation clusters to better understand the intensity and behavior of the fires that eventually weakened the building structure and lead to the collapse of the towers
• Testing of steel samples from salvaged structure to determine its strength characteristics under very rapid loading as was experienced during the planes' impacts
• Development of a detailed model of how heat from the fire seeped into structural elements and affected the structures' strength and stability
• Impact resistance testing of fireproofing insulation to determine the extent to which it may have been dislodged from structural elements during the intial impacts, thereby leaving the steel more vulnerable to the subsequent building fire

According to the article, NIST's findings could lead to new safety recommendations for ordinary high-rise construction. It is also expected to significantly advance the science of fire and how it affects structure.

Freedom Tower

Tower's Top Adapted From Bridge Design, ENR December 29, 2003, describes the recently released schematic design for the 1,776-foot-tall New York City Freedom Tower. Noteworthy points include:

• Primary building structure consists of of a perimeter steel diagonal grid structure and a central concrete core. Design engineers claim "substantial rigidity and redundancy."
• A 1:10 incline on two out of the four sides creates a tapered buiding form. A shifting parallelgram floor plan reflects the additional warping of the two inclined sides.
• The tower will have 70 occupied floors (compared with 110 floors in each of the previous World Trade Center towers), up to the 1,100 foot level. Above this level, two 32-foot diameter concrete cylinders extend an additional 500 feet to a hat truss connecting the two cylinders. An exterior network of cables stabilize the cylinder/hat truss structure.
• An antenna structure extends further above the hat truss.
• Wind turbines proposed for the upper, open portion of the structure may provide up to 20% of the finished building's power requirements.

WTC Investigation Progress

Trade Center Probe Clears Data Hurdles, ENR December 15, 2003, reports on progress in the federal investigation into the World Trade Center disaster. According to the article, the US Commerce Department's National Institute of Standards and Technology (NIST) has released an interim progress report stating in part, that investigators now have access to all the information needed for their research.

According to the article, NIST's examples of ongoing work include:

• Modeling of collapse scenarios
• Analysis of structural steel recovered from the collapse site
• Fire tests of office workstation mockups
• Fire endurance testing of typical tower floor systems
• Analysis of active fire protection systems (e.g., sprinklers) in use in the two main towers as well as the 50 story 7 WTC building that also collapsed due to fire
• Analysis of as-built fireproofing conditions and the impact of missing or under-applied fireproofing
• Analysis of airplane impacts on the structures

The article also reports on some controversy within the design community regarding the value of a detailed, comprehensive, and costly investigation of the tower collapses. Some see value in learning as much as possible about these events. On the other hand, Jon D. Magnusson, CEO of Magnusson Klemenic Associates is quoted as saying "The reality is that it doesn't matter if the floor trusses, the columns, or the connections went first [because] we can't out-build the terrorists." Magnusson also argues that details of the fireproofing performance may be irrelevant, since fireproofing of any type would have failed under similar conditions of airplane impact.

Separately, in Trade Center Fireproofing Tests Suggest a Wider Safety Problem, the New York Times (December 14, 2003) reports that preliminary calculations by federal investigators indicate that even relatively small gaps in spray applied fireproofing may leave a steel structure more vulnerable to the heat of fire than previously thought. According to the article, heat from a fire can enter into the structural steel at locations of missing fireproofing, and then become trapped and build up in adjacent insulated portions of the steel members. While noting that information being developed is still preliminary in nature, the article goes on to raise questions regarding the safety of fireproofing in other existing buildings. For example, the article quotes Glenn P. Corbett, a member of the investigation's advisory committee and an assistant professor of fire science at the John Jay College of Criminal Justice as saying, "When the investigation is over, this issue will radiate out to other buildings that have fireproofing in them. We really have to decide whether it's appropriate to continue to use this type of material."

NIST'S WTC investigation web site provides more information on this investigation. According to this site, the investigation has three main parts:

1. A 24-month building and fire safety investigation to determine probable causes of the collapses
2. A research and development program to provide the basis for improved codes and standards
3. A program for dissemating information and assisting the building industry in implementing recommended changes in practice

This site provides additional information including interim reports, background information, photos and simulations, and more.

High-Rise Safety

Topics related to high-rise safety continue to receive substantial attention in the industry and the trade press.

Progressive Collapse, Building Design & Construction 11-03, starts off making the interesting point that the building industry does not even have an agreed upon definition of what constitutes "progressive collapse". Furthermore, in the case of the World Trace Centers disaster, the fact that both buildings withstood the initial impacts of the airliners despite massive structural damage makes it difficult to categorize the response of these structures in any simple terms.

Some other topics discussed in this article include:

• Since the exact nature of events of this type cannot be foreseen, a general approach to alternate load path design is recommended. In the event of the failure of a primary structural element, the structure should have the capacity to redirect loads to alternate elements in the structure, preventing further collapse.
• Design criteria for resisting progressive collapse scenarios are less restrictive than normal service loads. For example, deflections larger than can be tolerated under normal service conditions may be acceptable as long as safe egress routes are maintained.
• Blast resistant design has been a concern for certain types of overseas projects for as long as a quarter century. Research on blast resistant design increased after the 1995 bombing of the Murrah Federal Office Building in Oklahoma City. Especially since 9/11, security concerns for domestic structures have further increased.
• Design for increasing survivability of the structure depends first on the determination of possible hazards or threats. Examples of specific design strategies include hardening only the floors closest to the ground where a blast is most likely to occur, or embedding steel cables in the perimeter of the building floor slabs to allow load transfer in the event of the loss of a perimeter column.
• Both the ductility of structural steel and the continuity of cast-in-place concrete are beneficial to collapse-resistant design. Increasing load capacity, redundancy, and continuity all may contribute to greater suvivability.

This article also discusses design for fire resistance, and the use of advanced modeling techniques for more accurate prediction of egress effectiveness.

Defensive Design, Modern Steel Construction November 2003, provides a more in-depth and technical discussion of design for blast resistance and prevention of progressive collapse. Topics include:

• Performance versus prescriptive design


• Blast threat analysis
• Strategies for typical buildings: Even without special design effort, there are techniques for maximizing the survivability of a structure.
• Strategies for prescriptive building designs: As an example, a prescriptive requirement might be the capability of a structure to withstand the removal of a column. This requirement may be established without necessarily performing any project-specific analysis of risks and effects.
• Strategies for performance-based building designs: In high-risk situations, project specific threats and event effects are analyzed, survivability criteria are established, and design proceeds accordingly.

For architects and other non-structural designers, it is worth noting that performance-based design for building survivability may extend beyond structural considerations to affect wall assemblies, fenestration, site design, egress design, and other aspects of a project.

As another example of the current level of attention be given to these issues, the American Institute of Steel Construction is sponsoring a Blast and Progressive Collapse Symposium, December 4 - 5, McGraw-Hill Auditorium, New York City.

References:
Progressive Collapse Analysis and Design Guidelines for New Federal Office Buildings and Major Modernization Projects, General Services Administration.
DoD Minimum Antiterrorism Standards for Buildings, US Department of Defense.

Author's Footnote
While preparing this article, this author found it difficult not to ponder what might be the the broader, non-technical implications of living in a time when threats and risks become the driving forces in design. For readers wishing to pursue this question further, here are a few starting points:
Architecture Record's online open discussion Forums, including topic areas specific to the WTC events, tall buildings, etc.
The Security Paradox, Editorial, Architectural Record April 2002.
Being Pro-City, But Anti-Terrorism, Urban Journal, Metropolis Magazine.

Building Safety & WTC

Building Standards, July-August 2002, devoted several feature articles to the topic of building safety after 9/11. Published by the former ICBO (now part of the ICC), these articles were still available online at the time of this writing at Building Standards Online.

Building Safety After 9/11: Where Do We Go From Here? discusses improving safety and security, the process for evaluation of code changes, and performance-based design goals.

Author's interpretation of building performance levels matrix illustrated in Building Safety After 9/11.

What Does September 11th Mean for Building Structure Design? discusses the prospect of making buildings more resistant to terrorist attack, the feasibility of designing buildings to withstand direcxt airplane hits, and minimizing the risk of progressive collapse.

Enhanced Emergency High-Rise Evacuation discusses current typical layouts for egress components and proposed improvements for enhanced life safety.

Additional related articles are entitled Protective Design: Saving Lives Through Structural Engineering, Educating the Public On Safety In Tall Buildings, and Engineering Systems and an Incremental Response to Terrorist Threat.

The Current State of Fire Protection Design

Experts Debate Using Structure To Resist Fire Loads (ENR, October 13, 2002), summarizes discussion at an October event sponsered by the National Institute of Standards and Technology (NIST). This invitation-only event was attended by approximately 60 representatives of the building professions, industry, and research institutions.

Points addressed in this brief article include:

• Are currently codes adequately protecting buildings?
• Do we know the relative risk of catastrophic fire in buildings?
• Do we still understand today the historical basis for prescriptive fire protection codes?
• What should be the role of performance based fire protection codes?
• What design professional should be responsible for fire performance design?

The list of quoted attendees makes interesting reading in itself:
--Johnathan C. Siu, principal engineer, Seattle Department of Design, Construction and Land Use
--Ronald O. Hamburger, principal, San Francisco office, Simpson, Gumpertz & Heger, Inc.
--Barbara Lane, associate, ArupFire, London
--Gregory G. Deirlein, director, John A. Blume Earthquake Engineering Center, Stanford University
--Jeremy Isenberg, president and CEO, Weidlinger Associates, Inc.

Debate Over Fire Testing

The National Concrete Masonry Association's Fire Safety page provides a provocative collection of information related to fire-resistant construction, the relative merits of masonry construction in comparison to more light-weight materials, and the responsibility of design and construction professionals for building safety. For any instructor looking to bring an added dose of currency and relevance to their classroom, this page offers a great opportunity for classroom presentation, student research, and discussion. A few of the most interesting items on this page include:

• Are Architects, Engineers and Code-Writing Officials Friends of the Firefighter?, by Vincent Dunn, Deputy Chief FDNY (ret.), argues that the building industry and design profession have been complicent in a gradual decline in building safety standards. This is a challenging, yet worthwhile article.
• NCMA Fires at Gypsum Wall Assemblies provides a thorough and clear description of the ASTM E 119 fire-resistance test, and challenges whether this test is appropriate for comparing the relative fire-performance of masonry and gypsum wallboard partitions.
• Fire Video (Windows Media Player or Quicktime) is a 6-minute video that includes dramatic footage of both gypsum wallboard and masonry partitions undergoing ASTM E 119 testing.

This image shows a gypsum wallboard partition subjected to hose stream testing as part of an ASTM E 119 fire-resistance test.

The debate surrounding ASTM E 119 seems to focus primarily on the hose stream portion of this test, how and when it is applied, and what it is meant to simulate. For an alternative point of view regarding the role of the hose stream test in E 119, see for example, The Gypsum Association's Fire Resistance Testing topical paper. This paper draws on historical background and comparable international standards (that have dropped hose stream testing) to suggest that the masonry associations may be misrepresenting the importance of the hose stream test in assessing fire resistance.

See also this site's wtc archive for additional articles related to lessons from this event.

Lessons from WTC

What we learned: Building performance study of the WTC collapse, first published in Structural Engineer magazine August 2002, has been reprinted in Building Safety Journal, September 2003.

This article provides a clear, overall description of the damage inflicted on the buildings, development of the fires, evacuation efforts, structural response to the building fires, and the progressive collapse of the structures. Summary findings are provided. Of particular note to architects is the recommendation to provide physical separation of alternate egress routes when buildings may be at risk of impact damage.

Masonry Performance and the WTC

Building Safety Journal (September 2003), Masonry Performance and the World Trade Center Disaster, discusses a new report by The Masonry Society, Masonry Aspects of the World Trade Center Disaster. The report evaluates the performance of buildings with masonry construction surrounding the twin towers during the WTC event. Conclusions appear to reflect favorably on masonry's greater mass and durability in comparison to other materials. For example:

• Framed buildings with exterior masonry walls generally performed better than newer buildings with lighter, curtainwall construction
• Masonry infill for walls and beams provided significant fire protection
• Masonry infill provided alternate load paths where columns were damaged, in some cases preventing partial structural collapse
• Interior masonry partitions provided redundant lateral force resistance
• Masonry flat arch floors built to early 20th standards provided better fire protection than newer steel framed construction.