Ar C.J. Walsh – Consultant Architect, Fire Engineer & Technical Controller – International Expert on : Sustainability Implementation + Accessibility (including Fire Safety) for All + Sustainable Fire Engineering
As we drive harder and deeper (at least some of us anyway ?) towards a future of Sustainable Human & Social Development … or are forcefully driven by the anthropogenic (man-made) pressures of Resource Shortages (e.g. water – food – energy) and Climate Change, in the case of millions of people living in poverty throughout the world … or are dragged screaming, which I fear will have to be the solution with the privileged classes in every society who are addicted to lavish and wasteful lifestyles and who show absolutely no interest in either Climate Change or Resource Shortages until they rear up and bite them in the ass (!!) … there is a desperate need for a more complex and precise language of Sustainability, which will give shape to the innovative trans-sectoral concepts and trans-disciplinary policy and decision-making support tools required for Tangible/’Real’ Sustainability & Climate Resilience Implementation.
At the time of writing, the Principal Challenge before us is …
Transforming Social Organization … the Ultimate Goal being to arrive quickly at a dynamic and harmonious balance between a Sustainable Human Environment and a flourishing, not just a surviving, Natural Environment … with the Overall Aim of achieving Social Wellbeing for All.
Climate Change did not directly cause Hurricane Sandy, a severe weather event which hit the Caribbean and the East Coast of the USA during October 2012 … but it was a significant contributing factor. Scenes like those in the photograph below will be experienced far more frequently in the future.
This is not Manhattan, in New York City … so, is the development shown below to be removed altogether … or renewed with the necessary and very costly construction of a massive system of flood protection measures ? Not an easy choice. Which choice would be more sustainable ?
However … WHEN, not IF … Average Global Temperatures rise above 1.5 degrees Celsius, many Small Island Developing States (SIDS) will suffer a similar fate … permanently …
The Type of Lightweight Development in the foreground of the photograph below … damaged beyond repair or re-construction during Hurricane Sandy, is not Resilient … which is a different concept to Robust, or Robustness.
Notice the building in the background, on the left, which appears to have survived fully intact … why ??
In complete contrast … the Type of Development, below, is more Resilient. Furthermore, however, as a normal human reaction to decades of aggressive, but ultimately unsuccessful, political bullying and economic assault by the USA, the Social Fabric of Cuba is very strong … making this a Resilient Human Environment …
So … what is a Resilient Human Environment … particularly in the context of Sustainable Climate Change Adaptation ?
What do we mean by Transforming Social Organization ??
And … as we drive forward, harder and deeper … why is it critical that we practice a balanced, synchronous approach … across ALL Aspects of Sustainability … to Tangible Sustainability & Climate Resilience Implementation ???
Let us confront some more interesting new words and thought-provoking concepts …
Click the Link Above to read and/or download a PDF File (2.17 Mb)
Abridged Executive Summary
The term resilience originated in the 1970’s in the field of ecology from the research of C.S.Holling, who defined resilience as ‘a measure of the persistence of systems and of their ability to absorb change and disturbance and still maintain the same relationships between populations or state variables’. In short, resilience is defined as ‘the ability of a system to absorb disturbances and still retain its basic function and structure’, and as ‘the capacity to change in order to maintain the same identity’.
Resilience can best be described by three crucial characteristics: (1) the amount of disturbance a system can absorb and still remain within the same state or domain of attraction; (2) the degree to which the system is capable of self-organization; and (3) the ability to build and increase the capacity for learning and adaptation.
In the need for persistence, we can find a first connection with sustainable development. Sustainable development has the objective of creating and maintaining prosperous social, economic, and ecological systems. Humanity has a need for persistence. And since humanity depends on services of ecosystems for its wealth and security, humanity and ecosystems are deeply linked. As a result, humanity has the imperative of striving for resilientsocio-ecological systems in light of sustainable development.
Resilience thinking is inevitably systems thinking at least as much as sustainable development is. In fact, ‘when considering systems of humans and nature (socio-ecological systems) it is important to consider the system as a whole. The human domain and the biophysical domain are interdependent’. In this framework where resilience is aligned with systems thinking, three concepts are crucial to grasp: (1) humans live and operate in social systems that are inextricably linked with the ecological systems in which they are embedded; (2) socio-ecological systems are complex adaptive systems that do not change in a predictable, linear, incremental fashion; and (3) resilience thinking provides a framework for viewing a socio-ecological system as one system operating over many linked scales of time and space. Its focus is on how the system changes and copes with disturbance.
To fully understand resilience theory, the report focuses therefore on the explanation of a number of crucial concepts: thresholds, the adaptive cycle, panarchy, resilience, adaptability, and transformability.
As shown, humanity and ecosystems are deeply linked. This is also the fundamental reason why to adopt the resilience-thinking framework is a necessity for governance. The resilience perspective shifts policies from those that aspire to control change in systems assumed to be stable, to managing the capacity of socio–ecological systems to cope with, adapt to, and shape change. It is argued that managing for resilience enhances the likelihood of sustaining desirable pathways for development, particularly in changing environments where the future is unpredictable andsurprise is likely.
This exposes the strong need for Sustainable Development Governance to embrace resilience thinking. It is not only about being trans-disciplinary and avoiding partial and one-viewpoint solutions; what is needed to solve today’s problems – and especially those linked to sustainable development – is a new approach that considers humans as a part of Earth’s ecosystems, and one in which policies can more effectively cope with, adapt to, and shape change.
In this scenario, the concept and key characteristics of so-called adaptive governance seem to be a practical means for societies to deal with the complex issues that socio-ecological systems are confronted with. Therefore, adaptive governance is best understood as an approach that unites those environmental and natural resource management approaches that share some or all of the following principles: polycentric and multi-layered institutions, participation and collaboration, self-organization and networks, and learning and innovation. Additionally, four interactive crucial aspects for adaptive governance are suggested: (1) to build knowledge and understanding of resource and ecosystem dynamics; (2) to feed ecological knowledge into adaptive management practices; (3) to support flexible institutions and multilevel governance systems; and,(4) to deal with external disturbances, uncertainty, and surprise. Therefore, nine values toward a resilient world are also suggested: diversity, ecological variability, modularity, acknowledging slow variables, tight feedbacks, social capital, innovation, overlap in governance, and ecosystem services.
Finally, three examples analyse practical instances in terms of resilience: (1) the approach taken by the so-called climate change adaptation discourse; (2) the Kristianstad Water Vattenrike, a wetland in southern Sweden that showed problems with loss of wet meadows, decline of water quality, and a disappearing wildlife habitat; and 3) the Goulburn-Broken Catchment from the State of Victoria (Australia). Some lessons can be drawn from these three cases. From the first case, governance structures have direct implications for the level of flexibility in responding to future change as well as variation in local contexts. Sensitivity to feedbacks relates both to the timing as well as where these feedbacks occur. Therefore, learning is more likely if feedbacks occur soon relative to action, and if those most affected by feedbacks are those responsible for the action. Additionally, the way in which a problem is conceptually framed determines the way in which responses are identified and evaluated and therefore influences the range of response characteristics. Second, the example from Sweden revealed that (a) the imposition of a set of rules to protect an ecosystem from the outside will not ensure the natural qualities of a region will be preserved over time. One size never fits all, and an understanding of local history and culture needs to be integrated into the management if local values are to be looked after; (b) for an organization to meaningfully deal with complexity at many scales, it needs to include representatives from each of these levels in the social network; (c) several organizations need to be prepared to contribute to a shared vision and build consensus and leadership – crucial components in adaptability and transformability. Third, the Goulburn-Broken story demonstrates the critical importance of understanding the underlying variables that drive a socio-ecological system, knowing where thresholds lie along these variables, and knowing how much disturbance it will take to push the system across these thresholds.
1.World Trade Center Building 7 was a 47 Storey Office Building located at the north -eastern tip of the WTC Complex in Lower Manhattan, New York City. It had been built on top of an existing Consolidated Edison of New York electric power substation, on land owned by the Port Authority of New York and New Jersey.
On Tuesday, 11 September 2001 … WTC Building 7 was on fire for almost seven hours … from the time of the collapse of WTC Tower 1 – North Tower, just before 10.30 hrs (local time), until 17.21 hrs … when WTC 7 failed completely, collapsing progressively as a result of ‘real’ fires – as distinct from ‘standard test’ fires – on many floors.
There were only two certainties on that fateful day (9-11) … the Fire-Induced Progressive Collapse of WTC Building 7 could no longer be ignored by the International Fire Science and Engineering Community … and the ‘reality’, which Modern Fire Engineering must now confront, was significantly altered. Secondly, it is NEVER acceptable to a general population for buildings to collapse !
Later in 2008, the Mumbai ‘Hive’ Attacks would add a sinister new ingredient to the standard threat profile for buildings, their occupants, and emergency services.
However, long before 9-11 and Mumbai, the growing complexity of modern communities and their rapidly evolving architectural forms had left the Fire Engineer far behind, unable to respond to the new fire safety challenges posed.
2.The second of the NIST Publications being referenced in this New Series of Posts is as follows …
NIST (National Institute of Standards and Technology). August 2008. Federal Building and Fire Safety Investigation of the World Trade Center Disaster: Final Report on the Collapse of World Trade Center Building 7. NIST NCSTAR 1A. Gaithersburg, MD, USA.
This 2008 NIST Report contains, in Chapter 5, a list of 13 Recommendations for Action (A-M), grouped together under the same 8 Subject Headings used in the 2005 NIST Report (NCSTAR 1) …
i) Increased structural integrity … Recommendation A ;
ii) Enhanced fire endurance of structures … Recommendations B, C, D & E ;
iii) New methods for fire resisting design of structures … Recommendations F & G ;
iv) Improved active fire protection … Recommendation H ;
v) Improved building evacuation … Long before its collapse, all occupants/users had evacuated WTC 7 … No Recommendation ;
vi) Improved emergency response … Recommendation I ;
vii) Improved procedures and practices … Recommendations J & K ; and
viii) Education and training … Recommendations L & M.
NIST has clearly stated that “the urgency of these Recommendations is substantially reinforced by their pertinence to the collapse of a tall building that was based on a structural system design that is in widespread use”.
3.The Colour Coding of Texts which I am using in this new series of posts … where NIST has presented new texts relating to WTC Building 7, these are shown in blue … where NIST has chosen to reinforce earlier texts from the 2005 Report on the WTC Towers 1 & 2 Collapses, these are shown in black. The important new paragraphs describing the critical relevance of WTC Building 7 are shown in red.
Please pay particular attention to these Red Paragraphs. Having carefully digested their contents … then if, by any chance, you happen to encounter somebody who still insists that the NIST 9-11 WTC Recommendations have no relevance to the design, construction, management and operation of ALL Buildings … that person is either living in Alice’s Wonderland … or he/she has never bothered to read the NIST Recommendations in the first place !!
4.While it is still essential to distinguish clearly between the two closely related structural concepts below … I would like to take this opportunity to bring to your attention a necessary and important modification … more, a refinement … to the definition of Fire-Induced Progressive Collapse …
The failure of a building’s structural system (i) remote from the scene of an isolated overloading action; and (ii) to an extent which is not in reasonable proportion to that action.
Fire-Induced Progressive Collapse
The sequential growth and intensification of structural distortion and displacement, beyond fire engineering design parameters, and the eventual failure of elements of construction in a building – during a fire and the ‘cooling phase’ afterwards – which, if unchecked, will result in disproportionate damage, and may lead to total building collapse.
This modification/refinement recognizes the following … that Fire-Induced Progressive Collapse may commence long before any breach occurs in a Fire Compartment Boundary … that, as a result of rampant commercial pressures in our societies, the tendency is for Compartment Volumes to become far too large to be any longer effective … and in the case of a Sustainable Building, for example, where natural patterns of air movement in buildings are used for either heating or cooling purposes, there may be no Compartments at all !
Restricting the application of one or both of these structural concepts, in law, to Multi-Storey Buildings, i.e. in many jurisdictions, those buildings having 5 or more storeys … is a purely arbitrary cut-off point.
CIB W14’s Research Working Group IV: ‘Structural Reliability & Fire-Induced Progressive Collapse’ would argue, rationally, that both of these concepts are fundamental to all structural fire engineering design.
5.Structural Fire Engineering is concerned with those aspects of fire engineering which relate to structural design for fire, and the complex architectural interaction between a building’s structure and fabric, i.e. non-structure, under conditions of fire and its immediate aftermath.
As Chair of CIB W14’s Research Working Group IV … I will shortly be making a Workshop Presentation in Europe, the aim of which will be to set the scene for the launch of an International CIB W14 Research WG IV Reflection Document; the specific objective of the Presentation, however, will be to accurately describe the phenomenon that is Fire-Induced Progressive Collapse … and to outline a necessary new design approach which will fulfil future requirements, legal and otherwise, concerning adequate resistance to this phenomenon.
It will be shown that the new design approach is fully compatible with the Recommendations contained in the 2005 and 2008 NIST Reports on the 9-11 World Trade Center Buildings 1, 2 & 7 Collapses – NCSTAR 1 & NCSTAR 1A.
In its final report on the collapse of the World Trade Center Towers (NIST NCSTAR 1), NIST made 30 Recommendations for improving the safety of buildings, occupants, and emergency responders. These encompass increased structural integrity, enhanced fire endurance of structures, new methods for fire resisting design of structures, improved active fire protection, improved building evacuation, improved emergency response, improved procedures and practices, and education and training.
WTC 7 was unlike the WTC Towers in many respects. It was a more typical tall building in the design of its structural system. It was not struck by an airplane. The fires in WTC 7 were quite different from those in the Towers. Since WTC 7 was not doused with thousands of litres of jet fuel, large areas of any floor were not ignited simultaneously. Instead, the fires in WTC 7 were similar to those that have occurred previously in several tall buildings where the sprinklers did not function or were not present. These other buildings did not succumb to their fires and collapse, because they were of structural designs that differed from that of WTC 7.
The Investigation Team has compiled a list of key factors that enabled ordinary fires to result in an extraordinary outcome. In so doing, the Team recognized that there were additional aspects to be included in the content of some of the earlier 30 Recommendations.
Based on the findings of this Investigation, NIST has identified 1 New Recommendation and has reiterated 12 Recommendations from the Investigation of the WTC Towers.
The urgency of the Prior Recommendations is substantially reinforced by their pertinence to the collapse of a tall building that is based on a structural system design that is in widespread use. A few of the Prior Recommendations have been modified to reflect the findings of this Investigation.
The partial or total collapse of a building due to fires is an infrequent event. This is particularly true for buildings with a reliably operating active fire protection system, such as an automatic fire sprinkler system. A properly designed and operating automatic sprinkler system will contain fires while they are small and, in most instances, prevent them from growing and spreading to threaten structural integrity.
The intent of current practice, based on prescriptive standards and codes, is to achieve life safety, not collapse prevention. However, the key premise of NIST’s Recommendations is that buildings should not collapse in infrequent (worst-case) fires that may occur when active fire protection systems are rendered ineffective, e.g. when sprinklers do not exist, are not functional, or are overwhelmed by the fire.
Fire scenarios for structural design based on single compartment or single floor fires are not appropriate representations of infrequent fire events. Such events have occurred in several tall buildings resulting in unexpected substantial losses. Instead, historical data suggests that infrequent fires which should be considered in structural design have characteristics that include: ordinary combustibles and combustible load levels, local fire origin on any given floor, no widespread use of accelerants, consecutive fire spread from combustible to combustible, fire-induced window breakage providing ventilation for continued fire spread and accelerated fire growth, concurrent fires on multiple floors, and active fire protection systems rendered ineffective. The fires in WTC 7 had all of these characteristics.
NIST believes the Recommendations are realistic, appropriate, and achievable within a reasonable period of time. NIST strongly urges that immediate and serious consideration be given to these Recommendations by the building and fire safety communities in order to achieve appropriate improvements in the way buildings are designed, constructed, maintained, and used – with the goal of making buildings safer in future emergencies.
A complete listing of all 13 Recommendations (Recommendations A through L) based on this Investigation follows. Under a few of the Recommendations, the pertinent lesson from the reconstruction of the WTC 7 Collapse is reflected in the form of a modification. For the 12 Reiterated Recommendations, the pertinent codes, standards, and organizations were listed in Table 9-1, and Tables 9-2a through 9-2c of NIST NCSTAR 1 and are not repeated here. For the 1 New Recommendation, B, this information is provided in the text.
5.1.1 GROUP 1. Increased Structural Integrity
The standards for estimating the load effects of potential hazards (e.g. progressive collapse, wind) and the design of structural systems to mitigate the effects of those hazards should be improved to enhance structural integrity.
NIST WTC 7 Recommendation A (NCSTAR 1 Recommendation 1).
NIST recommends that: (1) progressive collapse be prevented in buildings through the development and nationwide adoption of consensus standards and code provisions, along with the tools and guidelines needed for their use in practice; and (2) a standard methodology be developed – supported by analytical design tools and practical design guidance – to reliably predict the potential for complex failures in structural systems subjected to multiple hazards.
Relevance to WTC 7: Had WTC 7 been expressly designed for prevention of fire-induced progressive collapse, it would have been sufficiently robust to withstand local failure due to the fires without suffering total collapse.
5.1.2 GROUP 2. Enhanced Fire Endurance of Structures
The procedures and practices used to ensure the fire endurance of structures should be enhanced by improving the technical basis for construction classifications and fire resistance ratings, improving the technical basis for standard fire resistance testing methods, use of the ‘structural frame’ approach to fire resistance ratings, and developing in-service performance requirements and conformance criteria for sprayed fire resisting materials.
NIST WTC 7 Recommendation B (New)
NIST recommends that buildings be explicitly evaluated to ensure the adequate performance of the structural system under worst-case design fires with any active fire protection system rendered ineffective. Of particular concern are the effects of thermal expansion in buildings with one or more of the following features: (1) long-span floor systems* which experience significant thermal expansion and sagging effects; (2) connection designs (especially shear connections) that cannot accommodate thermal effects; (3) floor framing that induces asymmetric thermally-induced (i.e. net lateral) forces on girders; (4) shear studs that could fail due to differential thermal expansion in composite floor systems; and (5) lack of shear studs on girders. Careful consideration should also be given to the possibility of other design features that may adversely affect the performance of the structural system under fire conditions.
[* F-6 Typical floor span lengths in tall office buildings are in the range of 12-15 metres; this range is considered to represent long-span systems. Thermal effects (e.g. thermal expansion) that may be significant in long-span buildings may also be present in buildings with shorter span lengths, depending on the design of the structural system.]
Building owners, operators, and designers are strongly urged to act upon this Recommendation. Engineers should be able to design cost-effective fixes to address any areas of concern that are identified by these evaluations. Several existing, emerging, or even anticipated capabilities could have helped prevent the collapse of WTC 7. The degree to which these capabilities improve performance remains to be evaluated. Possible options for developing cost-effective fixes include:
More robust connections and framing systems to better resist the effects of thermal expansion on the structural system ;
Structural systems expressly designed to prevent progressive collapse. The current model building codes do not require that buildings be designed to resist progressive collapse ;
Better thermal insulation (i.e. reduced conductivity and/or increased thickness) to limit heating of structural steel and to minimize both thermal expansion and weakening effects. Currently, insulation is used to protect steel strength, but it could also be used to maintain a lower temperature in the steel framing to limit thermal expansion ;
Improved compartmentation in tenant areas to limit spread of fires ;
Thermally resisting window assemblies which limit breakage, reduce air supply, and retard fire growth.
Industry should partner with the research community to fill critical gaps in knowledge about how structures perform in real fires, particularly considering: the effects of fire on the entire structural system; the interactions between sub-systems, elements, and connections; and scaling of fire test results to full-scale structures, especially for structures with long-span floor systems.
Affected Standards: ASCE 7, ASCE/SFPE 29, AISC Specifications, and ACI 318. Development of performance objectives, design criteria, evaluation methods, design guidance, and computational tools should begin promptly, leading to new standards.
Model Building Codes: The new standard should be adopted in model building codes (IBC, NFPA 5000) by mandatory reference to, or incorporation of, the latest edition of the standard.
Relevance to WTC 7: The effects of restraint of free thermal expansion on the steel framing systems, especially for the long spans on the east side of WTC 7, were not considered in the structural design and led to the initiation of the building collapse.
NIST WTC 7 Recommendation C (NCSTAR 1 Recommendation 4).
NIST recommends evaluating, and where needed improving, the technical basis for determining appropriate construction classifications and fire rating requirements (especially for tall buildings) – and making related code changes now, as much as possible – by explicitly considering factors including:*
[ * F-7 The construction classification and fire rating requirements should be risk-consistent with respect to the design-basis hazards and the consequences of those hazards. The fire rating requirements, which were originally developed based on experience with buildings less than 20 storeys in height, have generally decreased over the past 80 years since historical fire data for buildings suggest considerable conservatism in those requirements. For tall buildings, the likely consequences of a given threat to an occupant on the upper floors are more severe than the consequences to an occupant on the first floor or the lower floors. For example, with non-functioning elevators, both of the time requirements are much greater for full building evacuation from upper floors and emergency responder access to those floors. The current height and areas tables in building codes do not provide the technical basis for the progressively increasing risk to an occupant on the upper floors of tall buildings that are much greater than 20 storeys in height.]
timely access by emergency responders and full evacuation of occupants, or the time required for burnout without partial collapse ;
the extent to which redundancy in active fire protection systems (sprinklers and standpipe, fire alarm, and smoke management) should be credited for occupant life safety ;*
[ * F-8 Occupant life safety, prevention of fire spread, and structural integrity are considered separate safety objectives.]
the need for redundancy in fire protection systems that are critical to structural integrity ;*
[ * F-9 The passive fire protection system (including the application of fire protection insulation, compartmentation, and fire stopping) and the active sprinkler system each provide redundancy for maintaining structural integrity in a building fire, should one of the systems fail to perform its intended function.]
the ability of the structure and local floor systems to withstand a maximum credible fire scenario* without collapse, recognizing that sprinklers could be compromised, not operational, or non-existent ;
[ * F-10 A maximum credible fire scenario includes conditions that are severe, but reasonable to anticipate, conditions related to building construction, occupancy, fire loads, ignition sources, compartment geometry, fire control methods, etc., as well as adverse, but reasonable to anticipate operating conditions.]
compartmentation requirements (e.g. 1,200 sq.m*) to protect the structure, including fire rated doorsets and automatic enclosures, and limiting air supply (e.g. thermally resisting window assemblies) to retard fire spread in buildings with large, open floor plans ;
[ * F-11 Or a more appropriate limit, which represents a reasonable area for active fire fighting operations.]
the effect of spaces containing unusually large fuel concentrations for the expected occupancy of the building ; and
the extent to which fire control systems, including suppression by automatic or manual means, should be credited as part of the prevention of fire spread.
Relevance to WTC 7: The floor systems in WTC 7 failed at lower temperatures because thermal effects within the structural system, especially thermal expansion, were not considered in setting the fire rating requirements in the construction classification, which are determined using the ASTM E 119 or equivalent testing standard.
NIST WTC 7 Recommendation D (NCSTAR 1 Recommendation 5).
NIST recommends that the technical basis for the century-old standard for fire resistance testing of components, assemblies and systems be improved through a national effort. Necessary guidance also should be developed for extrapolating the results of tested assemblies to prototypical building systems. A key step in fulfilling this Recommendation is to establish a capability for studying and testing components, assemblies, and systems under realistic fire and load conditions.
Of particular concern is that the Standard Fire Resistance Test does not adequately capture important thermally-induced interactions between structural sub-systems, elements, and connections that are critical to structural integrity. System-level interactions, especially due to thermal expansion, are not considered in the standard test method since columns, girders, and floor sub-assemblies are tested separately. Also, the performance of connections under both gravity and thermal effects is not considered. The United States currently does not have the capability for studying and testing these important fire-induced phenomena critical to structural safety.
Relevance to WTC 7: The floor systems failed in WTC 7 at shorter fire exposure times than the specified fire rating (two hours) and at lower temperatures because thermal effects within the structural system, especially thermal expansion, were not considered in setting the endpoint criteria when using the ASTM E 110 or equivalent testing standard. The structural breakdowns that led to the initiating event, and the eventual collapse of WTC 7, occurred at temperatures that were hundreds of degrees below the criteria that determine structural fire resistance ratings.
NIST WTC 7 Recommendation E (NCSTAR 1 Recommendation 7).
NIST recommends the adoption and use of the ‘structural frame’ approach to fire resistance ratings.This approach requires all members that comprise the primary structural frame (such as columns, girders, beams, trusses, and spandrels) be fire protected to the higher fire resistance rating required for the columns. The definition of the primary structural frame should be expanded to include bracing members that are essential to the vertical stability of the primary structural frame under gravity loading (e.g. girders, diagonal bracing, composite floor systems that provide lateral bracing to the girders) whether or not the bracing members carry gravity loads. Some of these bracing members may not have direct connections to the columns, but provide stability to those members directly connected to the columns. This Recommendation modifies the definition of the primary structural frame adopted in the 2007 supplement to the International Building Code (IBC). The IBC considers members of floor or roof construction that are not connected to the columns not to be part of the primary structural frame. This Recommendation ensures consistency in the fire protection provided to all of the structural elements that contribute to overall structural stability. State and local jurisdictions should adopt and enforce this requirement.
Relevance to WTC 7: Thermally-induced breakdown of the floor system in WTC 7 was a determining step in causing failure initiation and progressive collapse. Therefore, the floor system should be considered as an integral part of the primary structural frame.