Opening Remarks1

 of 

Dr. S. Shyam Sunder 

Deputy Director 

Building and Fire Research Laboratory 

National Institute of Standards and Technology 

at the 

Meeting to Discuss IBC Code Change Proposal for 

Progressive/Disproportionate Collapse 

May 1, 2006 

NIST released the final report in October 2005 from its building and fire safety investigation of 

the collapses of the World Trade Center (WTC) towers on 9-11. The report included 30 

recommendations for improving building and occupant safety derived from the findings. On 

March 24, 2006, the first 19 proposed changes to model building codes based upon and 

consistent with the NIST WTC recommendations were submitted to the International Code 

Council. 

The 19 proposed changes—submitted by building code experts associated with two ICC 

committees, NIBS and GSA—address areas such as: increased resistance to building collapse 

from fire and other incidents, use of spray-applied fire resistive materials (commonly known as 

"fireproofing"), performance and redundancy of fire protection systems (i.e. automatic 

sprinklers), elevators for use by first responders and evacuating occupants, the number and 

location of stairwells, exit path markings, and fuel oil storage/piping. 

Taken together, they are a robust, reasonable and appropriate set of advancements, and if 

adopted, would represent a significant improvement in public safety over current practice.2

One of these 19 code change proposals deals with progressive collapse or disproportionate 

collapse. This proposal should be considered within the broader framework of structural 

robustness and integrity. In recent decades, there has been an emphasis on maximizing the 

efficiency of the structural system to mitigate effects of the weight premium on cost, especially 

for super tall buildings3

. 

Structural engineers, however, do not have an objective metric today for measuring the safety 

performance of the structure as a complete system. Thus, we cannot quantify the degree of 

safety of a structural system or compare the safety of one structural system relative to another 

system for a given performance objective. As we build increasingly taller and more efficient 

buildings—that pose inherently greater consequence-driven risks to known hazards and have less 

redundancy—it becomes vitally important to assure a minimum level of safety of the structural 

system in satisfying the performance objective, in addition to assuring the efficiency of the 

structural system (e.g., pounds per square feet of structural materials used). 

 

1

 Edited and updated after the meeting. This document incorporates responses to discussions at the meeting. 2

 Several organizations, including NIST, are reviewing these proposals and may offer amendments or suggestions 

for improvement during the code development process. 

3

 In theory, the most efficient structural system that can be designed is a determinate system, which lacks 

redundancy. 

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Our codes and standards focus on quantifying performance and assuring the safety of 

components and connections, with the exception of some instances such as in earthquakeresistant design where system performance can be quantified (e.g., using R factors). Building 

codes and standards typically use safety factors in deterministic design and load and resistance 

factors in probabilistic design to quantify the safety performance of components and connections. 

Similar safety metrics should also be considered for the performance of the structural system4. 

For example, the ratio of the ultimate reserve capacity of a structural system to the design load 

carrying capacity of the system may be used to quantify safety performance. This metric—or 

global safety factor for the system—may be obtained either under purely gravity loads or under 

different combinations of lateral and gravity loads. The metrics may also be used to quantify the 

safety performance of major structural sub-systems such as the core, perimeter, and floor 

framing systems. 

Under gravity loads, the ultimate reserve capacity of the structural system may be determined 

while the structure is subject to design gravity loads with appropriately applied load reduction 

factors. Under lateral loads, the ultimate reserve capacity of the structural system may be 

obtained while the structure is subject to both lateral and gravity loads consistent with typical 

load reduction factors. 

While advanced analysis tools exist to quantify the safety performance of structural systems 

using these metrics, more work is needed to determine appropriate global system safety factors—

or load and system resistance factors—to implement meaningful provisions in codes and 

standards. This will require developing comparative data regarding the safety performance of 

different structural systems using these metrics.5

Until the above information on global system safety factors becomes available, current methods 

for mitigation of progressive or disproportionate collapse may be used in our codes and standards 

to ensure a minimum level of robustness and integrity (or safety performance) of the structural 

system. These methods include: tying buildings components together to provide continuity and 

strength; providing structural redundancy via alternate load paths; and enhancing specific 

resistance of structural components to known hazards. 

 

4

 System performance metrics are routinely used in other industries and applications. For example, fuel efficiency 

of automobiles is similar to materials efficiency for buildings. Also, the system safety performance of an 

automobile, measured in terms of crashworthiness ratings, is similar to the ultimate reserve capacity of the structural 

system. Fuel efficiency, in turn, depends on the design of major subsystems such as the engine, aerodynamic shape 

of the body, and the grade of fuel. Similarly, the system safety performance depends on the design of major 

subsystems such as the engine, body, doors, and supplemental restraint systems. Each of the subsystems and 

components may be governed by standards as well. 

5

 Some experts cite the excellent safety record of tall buildings in recent decades and ask whether there is a need to 

quantify the safety of the structural system. Statistics over several decades, however, are not adequate to quantify 

risks. Instead data is needed over some multiple of the expected life of a building, typically 100 years. In designing 

structures for hurricanes and earthquakes, it is common practice to consider rare events with return periods of 500 to 

2,500 years. Similarly, long term risks should be anticipated in designing structural systems for general robustness 

and integrity. 

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Codes and standards provisions should focus on the use of threat or hazard independent methods 

to mitigate progressive or disproportionate collapse and to assure system safety performance6

. 

When specific threats or hazards are known or can be anticipated, they should be considered 

explicitly in design; codes and standards should have provisions treating them as such. 

Robust tools already exist for specific use in design to mitigate progressive or disproportionate 

collapse. The April 2006 issue of Structure magazine—a joint publication of NCSEA, CASE, 

and SEI—contains an excellent summary of these tools. The articles in the magazine also 

illustrate the considerable technical and professional capabilities already available within the 

United States. 

The U.K. has had a progressive collapse standard “Standards to Avoid Progressive Collapse – 

Large Panel Construction” since 1968. The standard lists two methods for mitigating 

progressive collapse: (1) by providing alternative load paths, assuming the removal of a critical 

section of the load bearing system, and (2) by providing stiffness and continuity to the structural 

system to ensure the stability of the building against forces liable to damage the load supporting 

members. The standard also specifies an accidental static pressure of 5 pounds-per-square-inch 

and minimum tie forces for continuity. These provisions are based on engineering experience 

and judgment. Similar provisions have been adopted in the Eurocode. Currently, there is no 

field evidence to indicate that these provisions are not working or that the resulting building 

designs are less safe. 

 

In the United States, the American Society of Civil Engineers (ASCE) has in its standard 

(ASCE/SEI 7), structural integrity requirements for progressive collapse mitigation. ASCE plans 

to develop guidance for the prevention of progressive collapse. A technical committee has been 

recommended, but has not yet been formed. It will be some years before a guidance document is 

developed and made available for code adoption. 

 

NIST has an ongoing multi-year research project on the development of criteria for prevention of 

progressive collapse and is currently assessing best practices in current use. The NIST best 

practices document is intended to provide owners and practicing engineers with the current best 

practices to mitigate progressive collapse, including methods similar to those adopted in the U.K., 

and those used by federal agencies such as GSA, DoD, and the State Department. The draft of 

the document will be made available for broad review in conjunction with training seminars to 

be conducted by ASCE in 2006. The final document will be available by the end of 2007. 

 

In the course of its Investigation into the collapse of the World Trade Center Towers, NIST did 

not find any evidence that well-tied buildings performed unfavorably (or collapse earlier) than 

buildings that are not well-tied. In fact NIST found that, had the major structural subsystems of 

the WTC towers not been tied together, the core of the towers would have collapsed earlier. The 

hat-truss tied the core to the perimeter walls of the towers, and thus allowed the building to 

 

6

 The practice of removing one or more columns as part of the analysis to provide alternate load paths within a 

structural system has led to considerable confusion among many experts who want to know what specific hazard 

would cause the column or columns to fail. Instead, the focus should be on enhancing the general robustness and 

integrity of the structural system by providing alternate load paths using threat or hazard independent methods.

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withstand the effects of the aircraft impact and subsequent fires for a much longer time—

enabling large numbers of building occupants to evacuate safely. 

NIST believes that it is imperative for U.S. building codes and standards to address requirements 

for robustness and integrity of the structure as a system, especially tall buildings. This is 

essential to quantify and assure a minimum level of safety for the structural system, much as U.S. 

codes and standards do now to quantify and assure a minimum level of safety for structural 

components and connections. With few exceptions, the lack of minimum requirements for 

global safety factors for structural systems represents a major gap in U.S. codes and standards. 

This gap must be closed with a sense of urgency and commitment by the professional and 

building official communities. We should find an efficient and effective way forward today by 

discussing the specific code change proposal on disproportionate collapse submitted to the IBC. 

In closing, NIST welcomes and fully respects the ongoing debate among the professional and 

building official communities as they consider the 19 code change proposals based on the WTC 

recommendations for adoption. 

All ICC members will have the opportunity to vote on the proposals at hearings scheduled for 

this fall. All changes passed and those which did not pass but for which public comments are 

received will then be up for approval—and inclusion in the ICC codes—when ICC Government 

Member representatives meet in the spring of 2007. 

For more information: -- including a Web-based system for tracking the progress toward 

implementing all of the NIST WTC recommendations -- go to http://wtc.nist.gov . A link to 

"Status of NIST's Recommendations" from the WTC website lists each of the recommendations, 

the specific organization or organizations (e.g., standards and code developers, professional 

groups, state and local authorities) responsible for its implementation, the status of its 

implementation by organization, and the plans or work in progress to implement the 

recommendations. The status of the implementation of the recommendations is current as of 

April 10, 2006 and includes links to the nineteen code change proposals submitted to the 

International Code Council for the March 24, 2006 deadline and supplementary information 

produced by the NIBS building code experts. 

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