2005 NIST WTC RECOMMENDATIONS
NIST WTC Recommendations 12-15 > Improved Active Protection
Previous Posts in This Series …
2011-10-25: NIST’s Recommendations on the 9-11 WTC Building Collapses … GROUP 1. Increased Structural Integrity – Recommendations 1, 2 & 3 (out of 30)
2011-11-18: NIST WTC Recommendations 4-7 > Structural Fire Endurance … GROUP 2. Enhanced Fire Endurance of Structures – Recommendations 4, 5, 6 & 7
2011-11-24: NIST WTC Recommendations 8-11 > New Design of Structures … GROUP 3. New Methods for Fire Resisting Design of Structures – Recommendations 8, 9, 10 & 11
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2011-11-25: SOME PRELIMINARY COMMENTS …
1. Reliability has always been an issue with Active Fire Protection Systems … but, it is neither acknowledged, nor fully understood, that … Reliability Is Equally An Issue With Passive Fire Protection Measures !
Furthermore, the following should always be taken into account when considering the Safety Factors to be applied in calculating the level of satisfactory fire safety and protection which is provided in a specific project … one of the design objectives in Ethical Fire Engineering.
For example, if Category C below is indicative of the design and construction quality on a particular building site … just think of the Priory Hall Apartment Development in Dublin (!) … the Safety Factors to be applied in the design should be high … and with regard to actual construction, it should be expected that the Reliability of both Active Fire Protection Systems and Passive Fire Protection Measures will be initially low … with Life Cycle Reliability being entirely non-existent.
Quality of Fire Engineering Design & Related Construction
Category A
(a) Design of the works is exercised by an independent, appropriately qualified and experienced architect/engineer/fire engineer, with design competence relating to fire safety and protection in buildings … and, most importantly, that he/she interacts directly with the Project Design Professional in Responsible Charge ;
(b) Installation/fitting of related construction products/systems is exercised by appropriately qualified and experienced personnel, with construction competence relating to fire safety and protection in buildings ;
(c) Supervision of the works is exercised by appropriately qualified and experienced personnel from the principal construction organization ;
(d) Regular inspections, by appropriately qualified and experienced personnel familiar with the design, and independent of the construction organization(s), are carried out to verify that the works are being executed in accordance with the fire engineering design.
Category B
(a) Design of the works is exercised by an independent, appropriately qualified and experienced architect/engineer/fire engineer ;
(b) Installation/fitting of fire-related construction products/systems is exercised by appropriately qualified and experienced personnel ;
(c) Supervision of the works is exercised by appropriately qualified and experienced personnel from the principal construction organization.
Category C
This level of design and construction execution is assumed when the requirements for Category A or Category B are not met.
2. With regard to Recommendations 12 & 13 below … in an earlier post in this series, and elsewhere, I have defined Disproportionate Damage … and differentiated that structural concept from the related concept of Fire-Induced Progressive Collapse.
A significant number of countries include a requirement on Resistance to Disproportionate Damage in their national building codes. Often, it is only necessary to consider this requirement in the case of buildings having 5 Storeys, or more … a completely arbitrary height threshold. I would consider that adequately tying together the horizontal and vertical structural elements of a building … any building … is a fundamental principle of good structural engineering !!
Putting it simply … for the purpose of showing compliance with this structural requirement … it is necessary to demonstrate that a building will remain structurally stable if a portion of the building’s structure is removed … always remembering that every building comprises both structure and fabric, i.e. non-structure.
In reality this may happen, and quite often does happen, when, for example, a large truck runs into the side of a building, which can happen anywhere … or there is a gas explosion in some part of the building, which happened in Dublin’s Raglan House back in 1987, and many times in other countries … or a plane hits a high-rise building, which happened to Milan’s iconic Pirelli Tower in 2002, and to New York’s Empire State Building way back in 1945 … etc., etc. Raglan House collapsed … the Pirelli Tower and the Empire State Building did not.
[ The World Trade Center Towers were originally designed to absorb the impact of a large plane and to remain structurally stable afterwards ... in ambient conditions. However, what was not considered in the ambient structural design was 'fire', i.e. the fuel tanks were empty and no fire in the building would be initiated as a result of the mechanical damage caused by the plane impact ... which, on 11 September 2001, proved to be a ridiculous basis for any structural design ! This is why 9-11 should be regarded, at its core, as being a very serious 'real' fire incident.]
What I am leading up to is this … the concept of removing a portion of a building, and it remaining structurally stable afterwards … should now – logically and rationally – also be incorporated into the fire engineering design of Active Fire Protection Systems. In other words, if a portion of a building is removed, will any particular Active Fire Protection System continue to operate effectively in the rest of the building ? This has implications for the location and adequate protection of critical system components in a building … and for the necessary redundancy, zoning and back-up alternative routeing which must be designed into the system from the beginning !
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2005 NIST WTC RECOMMENDATIONS
GROUP 4. Improved Active Fire Protection
Active fire protection systems (i.e. sprinklers, standpipes/hoses, fire alarms, and smoke management systems) should be enhanced through improvements to the design, performance, reliability, and redundancy of such systems.
NIST WTC Recommendation 12.
NIST recommends that the performance and possibly the redundancy of active fire protection systems (sprinklers, standpipes/hoses, fire alarms, and smoke management systems) in buildings be enhanced to accommodate the greater risks associated with increasing building height and population, increased use of open spaces, high-risk building activities, fire department response limits, transient fuel loads, and higher threat profile. The performance attributes should deal realistically with the system design basis, reliability of automatic/manual operations, redundancy, and reduction of vulnerabilities due to single point failures. Affected Standards: NFPA 13, NFPA 14, NFPA 20, NFPA 72, NFPA 90A, NFPA 92A, NFPA 92B, and NFPA 101. Model Building Codes: The performance standards should be adopted in model building codes by mandatory reference to, or incorporation of, the latest edition of the standard.
NIST WTC Recommendation 13.
NIST recommends that fire alarm and communications systems in buildings be developed to provide continuous, reliable, and accurate information on the status of life safety conditions at a level of detail sufficient to manage the evacuation process in building fire emergencies; all communication and control paths in buildings need to be designed and installed to have the same resistance to failure and increased survivability above that specified in present standards. This should include means to maintain communications with evacuating occupants that can both reassure them and redirect them if conditions change. Pre-installed fire warden telephone systems can serve a useful purpose and may be installed in buildings and, if so, they should be made available for use by emergency responders. All communication and control paths in buildings need to be designed and installed to have the same resistance to failure and increased survivability above that specified in present standards. Affected Standards: NFPA 1, NFPA 72, and NFPA 101. Model Building and Fire Codes: The performance standards should be adopted in model building and fire codes by mandatory reference to, or incorporation of, the latest edition of the standard.
NIST WTC Recommendation 14.
NIST recommends that control panels at fire/emergency command stations in buildings be adapted to accept and interpret a larger quantity of more reliable information from the active fire protection systems that provide tactical decision aids to fire ground commanders, including water flow rates from pressure and flow measurement devices, and that standards for their performance be developed. Affected Standards: NFPA 1, NFPA 72, and NFPA 101. Model Building and Fire Codes: The performance standards should be adopted in model building and fire codes by mandatory reference to, or incorporation of, the latest edition of the standard.
NIST WTC Recommendation 15.
NIST recommends that systems be developed and implemented for: (1) real time off-site secure transmission of valuable information from fire alarm and other monitored building systems for use by emergency responders, at any location, to enhance situational awareness and response decisions, and maintain safe and efficient operation;* and (2) preservation of that information either off-site, or in a black box that will survive a fire or other building failure, for purposes of subsequent investigations and analysis. Standards for the performance of such systems should be developed, and their use should be required. Affected Standards: NFPA 1, NFPA 72, and NFPA 101. Model Building and Fire Codes: The performance standards should be adopted in model building and fire codes by mandatory reference to, or incorporation of, the latest edition of the standard.
[ * F-35 The alarm systems in the WTC towers were only capable of determining and displaying: (a) areas that had at some time reached alarm point conditions; and (b) areas that had not. The quality and reliability of information available to emergency responders at the Fire Command Station was not sufficient to understand the fire conditions. The only information transmitted outside the buildings was the fact that the buildings had gone into alarm. Further, the fire alarm system in WTC Building 7, which was transmitted to a monitoring service, was on 'test mode' during the morning of 11 September 2001, because routine maintenance was being performed. Under test mode conditions: (1) the system is typically disabled for the entire building, not just for the area where work is being performed; and (2) alarm signals typically do not show up on an operator console.]
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NIST WTC Recommendations 8-11 > New Design of Structures
Previous Posts in This Series …
2011-10-25: NIST’s Recommendations on the 9-11 WTC Building Collapses … GROUP 1. Increased Structural Integrity – Recommendations 1, 2 & 3 (out of 30)
2011-11-18: NIST WTC Recommendations 4-7 > Structural Fire Endurance … GROUP 2. Enhanced Fire Endurance of Structures – Recommendations 4, 5, 6 & 7
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2011-11-24: SOME PRELIMINARY COMMENTS …
1. The first of two NIST Publications being referenced in this Series of Posts is as follows …
NIST (National Institute of Standards and Technology). September 2005. Federal Building and Fire Safety Investigation of the World Trade Center Disaster: Final Report on the Collapse of the World Trade Center Towers. NIST NCSTAR 1. Gaithersburg, MD, USA.
The 2005 NIST Report concludes, in Chapter 9, with a list of 30 Recommendations for Action, grouped together under the following 8 Subject Headings …
i) Increased structural integrity ;
ii) Enhanced fire endurance of structures ;
iii) New methods for fire resisting design of structures ;
iv) Enhanced active fire protection ;
v) Improved building evacuation ;
vi) Improved emergency response ;
vii) Improved procedures and practices ; and
viii) Education and training.
NIST has clearly stated that “the numerical ordering (of the Recommendations) does not reflect any priority”.
From my point of view, the 2005 NIST Report is especially noteworthy for the emphasis placed on:
(a) The 3 R’s … Reality – Reliability – Redundancy ;
(b) Evacuation Way Finding … should be ‘intuitive and obvious’ … a major challenge for building designers, since buildings are still typically designed for ‘access’ only. In order to find the evacuation routes in a building, it is usually necessary to have a compass, a map, a magnifying glass, a torch … and a prayer book !!! More about this in later posts …
2. However, following on from NIST’s emphasis on Reality … and just between you, me and the World Wide Web … there is a lot of misunderstanding in the International Fire Science and Engineering Community about what exactly is the Realistic End Condition. But, here it goes …
Realistic End Condition: A ‘real’ fire in a ‘real’ building, which is used by ‘real’ people with varying abilities in relation to self-protection, independent evacuation to a ‘place of safety’, and participation in the Fire Defence Plan for the building.
It is strange, therefore … and quite unacceptable … to have to point out that the Realistic End Condition IS NOT … a test fire or an experimental fire in a laboratory … or a design fire in a computer model, even IF it is properly validated !
3. With regard to Recommendation 8 below … NIST’s contention that “Current methods for determining the fire resistance of structural assemblies do not explicitly specify a performance objective” is not strictly the case.
If we examine Technical Guidance Document B (Ireland) and Approved Document B (England & Wales) once again, as examples close to home … Part B: ‘Fire Safety’ in both jurisdictions should be read in conjunction with its associated Part A: ‘Structure’, which contains a requirement on Disproportionate Damage.
In everyday practice, however, this never happens. Instead, people dealing with Part B in both jurisdictions enter a sort of bubble … a twilight zone … and, if there is anything to do with structural performance in fire, they immediately refer to the Appendices at the back of both Guidance Documents (ignoring Part A altogether) … where we find a ‘single element’ approach to design, no consideration of connections, etc., etc., etc.
And … this fundamental error is further reinforced in Ireland because, under the national system of Fire Safety Certification for buildings, it is only Part B which is relevant.
At European Level, I would make the same point … under EU Regulation 305/2011 on Construction Products … Basic Requirement for Construction Works 2: ‘Safety in Case of Fire’ must be read in conjunction with Basic Requirement 1: ‘Mechanical Resistance & Stability’ … where we will again find a direct reference to Disproportionate Damage … and an indirect, but explicit, reference to Serviceability Limit States under normal conditions of use … including fire !
A major gap … the missing link at international level … is the failure, still, to elaborate and flesh out the structural concept of Fire-Induced Progressive Collapse. More about this in later posts …
4. With regard to Recommendation 10 below … and amplifying my earlier comments concerning Recommendation 6 … the manufacturers of all Lightweight Structural Fire Protection Systems … not just the Sprayed Systems … have a lot to answer for.
Major question marks concerning Life Cycle Durability, and Resistance to Mechanical Damage at any stage in a building’s life cycle, hang over all of these systems.
Fire testing, alone, does not show that a Lightweight Structural Fire Protection System is ‘fit for its intended use’ ! And manufacturers well know this !!!
And as for the Installation of Lightweight Structural Fire Protection Systems on site … it’s a hornets’ nest that nobody wants to touch !
Vested interests … vested interests … vested interests !!!
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2005 NIST WTC RECOMMENDATIONS
GROUP 3. New Methods for Fire Resisting Design of Structures
The procedures and practices used in the fire resisting design of structures should be enhanced by requiring an objective that uncontrolled fires result in burnout without partial or global (total) collapse. Performance-based methods are an alternative to prescriptive design methods. This effort should include the development and evaluation of new fire resisting coating materials and technologies, and evaluation of the fire performance of conventional and high-performance structural materials.
NIST WTC Recommendation 8.
NIST recommends that the fire resistance of structures be enhanced by requiring a performance objective that uncontrolled building fires result in burnout without partial or global (total) collapse. Such a provision should recognize that sprinklers could be compromised, non-operational, or non-existent. Current methods for determining the fire resistance of structural assemblies do not explicitly specify a performance objective. The rating resulting from current test methods indicates that the assembly (component or sub-system) continued to support its superimposed load (simulating a maximum load condition) during the test exposure without collapse. Model Building Codes: This Recommendation should be included in the national model building codes as an objective, and adopted as an integral pert of the fire resistance design for structures. The issue of non-operational sprinklers could be addressed using the existing concept of Design Scenario 8 of NFPA 5000, where such compromise is assumed and the result is required to be acceptable to the Authority Having Jurisdiction (AHJ). Affected Standards: ASCE-7, AISC Specifications, ACI 318, and ASCE/SFPE 29.
NIST WTC Recommendation 9.
NIST recommends the development of: (1) performance-based standards and code provisions, as an alternative to current prescriptive design methods, to enable the design and retrofit of structures to resist real building fire conditions, including their ability to achieve the performance objective of burnout without structural or local fire collapse; and (2) the tools, guidelines, and test methods necessary to evaluate the fire performance of the structure as a whole system. Standards development organizations, including the American Institute of Steel Construction, have already begun developing performance-based provisions to consider the effects of fire in structural design.
This performance-based capability should include the development of, but not be limited to:
a. Standard methodology, supported by performance criteria, analytical design tools, and practical design guidance; related building standards and codes for fire resistance design and retrofit of structures, working through the consensus process for nationwide adoption; comprehensive design rules and guidelines; methodology for evaluating thermo-structural performance of structures; and computational models and analysis procedures for use in routine design practice.
b. Standard methodology for specifying multi-compartment, multi-floor fire scenarios for use in the design and analysis of structures to resist fires, accounting for building-specific conditions such as geometry, compartmentation, fuel load (e.g. building contents and any flammable fuels such as oil and gas), fire spread, and ventilation; and methodology for rating the fire resistance of structural systems and barriers under realistic design-basis fire scenarios.
c. Publicly available computational software to predict the effects of fires in buildings – developed, validated, and maintained through a national effort – for use in the design of fire protection systems and the analysis of building response to fires. Improvements should include the fire behaviour and contribution of real combustibles; the performance of openings, including door openings and window breakage, that controls the amount of oxygen available to support the growth and spread of fires and whether the fire is fuel-controlled or ventilation-controlled; the floor-to-floor flame spread; the temperature rise in both insulated and un-insulated structural members and fire barriers; and the structural response of components, sub-systems, and the total building system due to the fire.
d. Temperature-dependent thermal and mechanical property data for conventional and innovative construction materials.
e. New test methods, together with associated conformance assessment criteria, to support the performance-based methods for fire resistance design and retrofit of structures. The performance objective of burnout without collapse will require the development of standard fire exposures that differ from those currently used.
Affected National and International Standards: ASCE-7, AISC Specifications, ACI 318, and ASCE/SFPE 29 for fire resistance design and retrofit of structures; NFPA, SFPE, ASCE, and ISO TC92 SC4 for building-specific multi-compartment, multi-floor design basis fire scenarios; and ASTM, NFPA, UL, and ISO for new test methods. Model Building Codes: The performance standards should be adopted as an alternative method in model building codes by mandatory reference to, or incorporation of, the latest edition of the standard.
NIST WTC Recommendation 10.
NIST recommends the development and evaluation of new fire resisting coating materials, systems, and technologies with significantly enhanced performance and durability to provide protection following major events. This could include, for example, technologies with improved adhesion, double-layered materials, intumescent coatings, and more energy absorbing SFRM’s.* Consideration should be given to pre-treatment of structural steel members with some type of mill-applied fire protection to minimize the uncertainties associated with field application and in-use damage. If such an approach were feasible, only connections and any fire protection damaged during construction and fit-out would need to be field-treated. Affected Standards: Technical barriers, if any, to the introduction of new structural fire resisting materials, systems and technologies should be identified and eliminated in the AIA MasterSpec, AWCI Standard 12 and ASTM standards for field inspection, conformance criteria, and test methods. Model Building Codes: Technical barriers, if any, to the introduction of new structural fire resisting materials, systems, and technologies should be eliminated from the model building codes.
[ * F-34 Other possibilities include encapsulation of SFRM by highly elastic energy absorbing membranes or commodity grade carbon fibre or other wraps. The membrane would remain intact under shock, vibration, and impact but may be compromised in a fire, yet allowing the SFRM to perform its thermal insulation function. The carbon wrap would remain intact under shock, vibration, and impact, and possibly under fire conditions as well.]
NIST WTC Recommendation 11.
NIST recommends that the performance and suitability of advanced structural steels, reinforced and pre-stressed concrete, and other high-performance material systems be evaluated for use under conditions expected in building fires. This evaluation should consider both presently available and new types of steels, concrete, and high-performance materials to establish the properties (e.g. yield and ultimate strength, modulus, creep behaviour, and failure) that are important for fire resistance, establish needed test protocols and acceptance criteria for such materials and systems, compare the performance of newer systems to conventional systems, and the cost-effectiveness of alternative approaches. Technical and standards barriers to the introduction and use of such advanced steels, concrete, and other high-performance material systems should be identified and eliminated, or at least minimized, if they are found to exist. Affected Standards: AISC Specifications and ACI 318. Technical barriers, if any, to the introduction of these advanced systems should be eliminated in ASTM E 119, NFPA 251, UL 263, ISO 834. Model Building Codes: Technical barriers, if any, to the introduction of these advanced systems should be eliminated from the model building codes.
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NIST WTC Recommendations 4-7 > Structural Fire Endurance
First Post in This Series …
2011-10-25: NIST’s Recommendations on the 9-11 WTC Building Collapses … GROUP 1. Increased Structural Integrity – Recommendations 1, 2 & 3 (out of 30)
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2011-11-18: SOME PRELIMINARY COMMENTS …
1. Before launching into the next Group of NIST WTC Recommendations, it would be useful to distinguish between the following technical terms … which have been adapted from ISO/TR 10158: ‘Principles and Rationale Underlying Calculation Methods in Relation to Fire Resistance of Structural Elements’ …
Real Fire: A fire which develops in a building and which is influenced by such factors as the type of building and its occupancy; the combustible content (fire load); the ventilation, geometry and thermal properties of the fire compartment, or building space (should no fire compartmentation exist); the fire suppression systems in the building and the actions of the fire services.
Real Fires are complex phenomena. Consequently, in structural fire engineering, idealized versions of ‘real fires’ are employed.
Experimental Fire: A full or reduced scale fire with specified and controlled characteristics.
Design Fire: A fire with specified exposure data intended for use in connection with structural fire engineering calculations.
A Design Fire may either be representative of the thermal exposure described by the standard time-temperature-pressure relationship in an International/European/National Standard, or some non-standard exposure intended to simulate particular fire exposure conditions.
However, in SDI Technical Guidance Note 95/102: ‘Proper Evidence of a Fire Test Result within the European Economic Area (EEA)’, issued on 22 May 1995, I included the following caution …
#1.7 A Fire Test in a Fire Test Laboratory, involving exposure of a test specimen or prototype to ‘test fire’ conditions, gives only a limited indication of: (a) the likely performance of a particular product, material or component when exposed to ‘real fire’ conditions; and (b) the suitability of a product, material or component for a particular end use.
2. In conventional fire engineering, much confusion arises because of a failure to properly distinguish between these two concepts …
Fire Resistance
The inherent capability of a building assembly, or an ‘element of construction’, to resist the passage of heat, smoke and flame for a specified time during a fire.
Structural Reliability
The ability of a structural system to fulfil its design purpose, for a specified time, under the actual environmental conditions encountered in a building.
[ In structural fire engineering, the concern must be that the structure will fulfil its purpose, both during the fire - and for a minimum period afterwards, during the 'cooling phase'.]
3. Therefore, with regard to Recommendation 6 … it is more correct and precise to refer to ‘Steel Fire Protection Systems’, rather than to ‘steel fire resisting materials’ ! AND … the same questions must be asked about All Lightweight Steel Fire Protection Systems … not just the sprayed systems.
Lightweight Fire Protection Systems are also used to protect concrete in buildings and tunnels.
4. These 2005 NIST Recommendations will later be confirmed, and further reinforced, by the 2008 NIST Recommendations. Bringing Recommendation 7, below, closer to home … it is interesting to note that a very necessary discussion on the technical adequacy of the approach taken to structural performance in fire … in both Technical Guidance Document B (Ireland) and Approved Document B (England & Wales) … has yet not even commenced !
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2005 NIST WTC RECOMMENDATIONS
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 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-23 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. It is not clear how the current height and areas tables in building codes consider 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-24 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-25 The passive fire protection system (including 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-26 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-27 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.
Adoption of this Recommendation will allow building codes to distinguish the risks associated with different building heights, fuel concentrations, and fire protection systems. Research is needed to develop the data and evaluate alternative proposals for construction classifications and fire ratings. Model Building Codes: A comprehensive review of current construction classifications and fire rating requirements and the establishment of a uniform set of revised thresholds with a firm technical basis that considers the factors identified above should be undertaken.*
[ * F-28 The National Fire Protection Association (NFPA) 5000 model code and the International Building Code (IBC) both recognize the risks associated with different building heights and accepted changes in 2001 and 2004, respectively. Both model codes now require that buildings 126 metres and higher have a minimum 4 hour structural fire resistance rating. The previous requirement was 2 hours. The change provides increased fire resistance for the structural system leading to enhanced tenability of the structure and gives firefighters additional protection while fighting a fire. While NIST supports these changes as an interim step, NIST believes that it is essential to complete a comprehensive review that will establish a firm technical basis for construction classifications and fire rating requirements.]
NIST WTC 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.
This effort should address the technical issues listed below: *
[ * F-29 The technical issues were identified from the series of four fire resistance tests of the WTC Floor system, and the review and analysis of relevant documents that were conducted as part of this Investigation.]
a. Criteria and test methods for determining:
- structural limit states, including failure, and means for measurement ;
- effect of scale of test assembly versus prototype application, especially for long-span structures that significantly exceed the size of test furnaces ;
- effect of restraining thermal expansion (end-restraint conditions) on test results, especially for long-span structures that have greater flexibility ;
- fire resistance of structural connections, especially the fire protection required for a loaded connection to achieve a specified rating ; *
[ * F-30 There is a lack of test data on the fire resistance ratings of loaded connections. The fire resistance of structural connections is not rated in current practice. Also, standards and codes do not provide guidance on fire protection requirements for structural connections when the connected members have different fire resistance ratings.]
- effect of the combination of loading and exposure (time-temperature profile) required to adequately represent expected conditions ;
- the repeatability and reproducibility of test results (typically, results from a single test are used to determine the rating for a component or assembly) ; and
- realistic ratings for structural assemblies made with materials that have improved elevated temperature properties (strength, modulus, creep behaviour).
b. Improved procedures and guidance to analyze and evaluate existing data from fire resistance tests of building components and assemblies for use in qualifying an untested building element.
c. Relationships between prescriptive ratings and performance of the assembly in real fires.
Affected National and International Standards: * ASTM E 119, NFPA 251, UL 263, and ISO 834. Model Building Codes: The standards should be adopted in model building codes by mandatory reference to, or incorporation of, the latest edition of the standard.
[ * F-31 While the NIST Recommendations are focused mainly on U.S. national standards, each U.S. standard has counterpart international standards. In a recent report (ISO/TMB AGS N 46), the International Organization for Standardization (ISO), through its Advisory Group for Security (AGS), has recommended that since many of the ISO standards for the design of buildings date back to the 1980's, they should be reviewed and updated to make use of the studies done by NIST on the World Trade Center disaster, the applicability of new technology for rescue from high buildings, natural disasters, etc. ISO's Technical Advisory Group 8 co-ordinates standards work for buildings.]
NIST WTC Recommendation 6.
NIST recommends the development of criteria, test methods, and standards: (1) for the in-service performance of Sprayed Fire Resisting Materials (SFRM, also commonly referred to as fire protection insulation) used to protect structural components; and (2) to ensure that these materials, as installed, conform to conditions in tests used to establish the fire resistance rating of components, assemblies, and systems.
This should include:
- Improved criteria and testing methodologies for the performance and durability of SFRM (e.g. adhesion, cohesion, abrasion, and impact resistance) under in-service exposure conditions (e.g. temperature, humidity, vibration, impact, with/without primer paint on steel*) for use in acceptance and quality control. The current test method to measure the bond strength, for example, does not distinguish the cohesive strength from the tensile and shear adhesive strengths. Nor does it consider the effect of primer paint on the steel surface. Test requirements that explicitly consider the effects of abrasion, vibration, shock, and impact under normal service conditions are limited or do not exist. Also, the effects of elevated temperatures on thermal properties and bond strength are not considered in evaluating the performance and durability of SFRM.
[ * F-32 NIST tests show that the adhesive strength of SFRM on steel coated with primer paint was a third to half of the adhesive strength on steel that had not been coated with primer paint. The SFRM products used in the WTC towers were applied to steel components coated with primer paint.]
- Inspection procedures, including measurement techniques and practical conformance criteria, for SFRM in both the building codes and fire codes for use after installation, renovation, or modification of all mechanical and electrical systems and by fire inspectors over the life of the building. Existing standards of practice (AIA MasterSpec and AWCI Standard 12), often required by codes for some buildings need to be broadly applied to both new and existing buildings. These standards may require improvements to address the issues identified in this Recommendation.
- Criteria for determining the effective uniform SFRM thickness – thermally equivalent to the variable thickness of the product as it is actually applied – that can be used to ensure that the product in the field conforms to the near uniform thickness conditions in the tests used to establish the fire resistance rating of the component, assembly, or system. Such criteria are needed to ensure that the SFRM, as installed, will provide the intended performance.
- Methods for predicting the effectiveness of SFRM insulation as a function of its properties, the application characteristics, and the duration and intensity of the fire.
- Methods for predicting service life performance of SFRM under in-service conditions.
Affected Standards: AIA MasterSpec and AWCI Standard 12 for field inspection and conformance criteria; ASTM standards for SFRM performance criteria and test methods. Model Building Codes: The standards should be adopted in model building codes by mandatory reference to, or incorporation of, the latest edition of the standard. (See Recommendation 10 for more on this issue.)
NIST WTC Recommendation 7.
NIST recommends the adoption and use of the ‘structural frame’ approach to fire resistance ratings. This approach requires that structural members – such as girders, beams, trusses, and spandrels having direct connection to the columns, and bracing members designed to carry gravity loads – be fire protected to the same fire resistance rating as columns. This approach is currently required by the International Building Code (IBC), one of the model codes, and is in the process of adoption by NFPA 5000, the other model code. This requirement 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.
[ * F-33 Had this requirement been adopted by the 1968 New York City building code, the WTC floor system, including its connections, would have had the 3 hour rating required for the columns since the floors braced the columns.]
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NIST’s Recommendations on the 9-11 WTC Building Collapses
2011-10-25: Since shortly after my visit to Lower Manhattan in mid-October 2001 … we have maintained an Archive Page on ‘Structural Fire Engineering, World Trade Center Incident (9-11) & Fire Serviceability Limit States‘ … at SDI’s Corporate WebSite. And I have referenced here … many, many times … the Recommendations contained in the 2005 & 2008 Final Reports of the U.S. National Institute of Standards & Technology (NIST) on the 9-11 World Trade Center Building 1, 2 & 7 Collapses.
In this post (and a series of future posts) … I find it most necessary that the 2005 & 2008 NIST Recommendations now be presented for everyone to read. Yes, some of Recommendations apply specifically to Tall and Very Tall Buildings … and Building Designers in India, China, Brazil, Russia & South Africa (BRICS), the Arab Gulf Region, Europe and North America, etc., should be fully aware of their contents.
BUT … I am also strongly convinced … precisely because I am an Architect, a Fire Engineer and a Technical Controller … that most of the NIST Recommendations apply to ALL Buildings … so catastrophic was the failure exposed on that fateful day (11 September 2001) … in all of our common design and construction practices … and our operation, maintenance and emergency response procedures !
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PRELIMINARY COMMENTS
1. Extract from Paragraph #9.2, Chapter 9, NIST Final Report on the Collapse of the World Trade Center Towers – Report Reference NIST NCSTAR 1 (2005) …
- NIST believes that these Recommendations are both realistic and achievable within a reasonable period of time, and that their implementation would make buildings safer for occupants and emergency responders in future emergencies.
- NIST strongly urges that immediate and serious consideration be given to these Recommendations by the building and fire safety communities – especially designers, owners, developers, codes and standards development organizations, regulators, fire safety professionals, and emergency responders.
- NIST also strongly urges building owners and public officials to: (i) evaluate the safety implications of these Recommendations for their existing inventory of buildings; and (ii) take the steps necessary to mitigate any unwarranted risks without waiting for changes to occur in codes, standards, and practices.
2. At the time of writing … it is important to point out that although they are related Structural Concepts … and there is still, to this day, a lot of confusion about these concepts in the USA … it is important to clearly distinguish between …
Disproportionate Damage
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 distortion, displacement and 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.
3. Recommendation 2, below, would certainly need to be understood and implemented within today’s additional design constraints of Sustainable Climate Change Adaptation and Resilience to Severe Weather Events. Therefore … Design Wind Speeds must be increased, accordingly, for ALL Buildings.
4. As such a high level of performance is expected … indeed demanded … of a Sustainable Building … Sustainable Fire Engineering must be ‘reliability-based’. In other words, it must have a rational, empirical and scientifically robust basis … unlike conventional fire engineering, which is yet aimlessly wandering around in pre-historic caves !
5. Finally … there is no use trying to hide the fact that progress on implementing the NIST Recommendations, within the USA, has been lamentably slow. Outside that jurisdiction, the response has ranged from mild interest, to complete apathy, and even to vehement antipathy. The implications arising from implementation are much too hard to digest … for long established fire safety professionals and researchers who are unswervingly committed to the flawed and out-of-date practices and procedures of conventional fire engineering and, especially, for vested interests !
However … is it either in society’s interest, or in the interests of our clients/client organizations … that, to give you a simple example which is relevant close to home, British Standard 9999 (published on 31 October 2008): ‘Code of Practice for Fire Safety in the Design, Management and Use of Buildings’ takes absolutely no account of any of the NIST Recommendations ? As far as the British Standards Institution is concerned … 9-11 never happened … which I think is an inexcusable and unforgivable technical oversight !
For this reason, the General Public in ALL of our societies and Clients/Client Organizations in ALL countries should also be fully aware of the contents of these Recommendations …
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Colour photograph showing the two World Trade Center Towers immediately after the impact of the second plane. At a fundamental level, this was a very serious 'real' fire incident ... which was extensively, and very thoroughly, investigated by the U.S. National Institute of Standards & Technology (NIST) ... and resulted in the important 2005 & 2008 NIST Recommendations. Click to enlarge.
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2005 NIST WTC RECOMMENDATIONS
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 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.
a. Progressive collapse* should be prevented in buildings.
[ * F-19 Progressive collapse (or disproportionate damage) occurs when an initial local failure spreads from structural element to structural element resulting in the collapse of an entire structure or a disproportionately large part of it.]
The primary structural systems should provide alternate paths for carrying loads in case certain components fail (e.g. transfer girders or columns). This is especially important in buildings where structural components (e.g. columns, girders) support unusually large floor areas.*
[ * F-20 While the WTC towers eventually collapsed, they had the capacity to redistribute loads from impact and fire damaged structural components and sub-systems to undamaged components and sub-systems. However, the core columns in the WTC towers lacked sufficient redundant (alternative) paths for carrying gravity loads.]
Progressive collapse is addressed only in a very limited way in practice and by codes and standards. For example, the initiating event in design to prevent progressive collapse may be removal of one or two columns at the bottom of the structure. Initiating events at multiple locations within the structure, or involving other key components and sub-systems, should be analyzed commensurate with the risks considered in the design. The effectiveness of mitigation approaches involving new system and sub-system design concepts should be evaluated with conventional approaches based on indirect design (continuity, strength and ductility of connections), direct design (local hardening), and redundant (alternate) load paths. The capability to prevent progressive collapse due to abnormal loads should include: (i) comprehensive design rules and practice guides; (ii) evaluation criteria, methodology, and tools for assessing the vulnerability of structures to progressive collapse; (iii) performance-based criteria for abnormal loads and load combinations; (iv) analytical tools to predict potential collapse mechanisms; and (v) computer models and analysis procedures for use in routine design practice. The federal government should co-ordinate the existing programmes that address this need: those in the Department of Defence; the General Services Administration; the Defence Threat Reduction Agency; and NIST. Affected Standards: ASCE-7, AISC Specifications, and ACI 318. These standards and other relevant committees should draw on expertise from ASCE/SFPE 29 for issues concerning progressive collapse under fire conditions. Model Building Codes: The consensus standards should be adopted in model building codes (i.e. the International Building Code and NFPA 5000) by mandatory reference to, or incorporation of, the latest edition of the standard. State and local jurisdictions should adopt and enforce the improved model building codes and national standards based on all 30 WTC Recommendations (2005). The codes and standards may vary from the WTC Recommendations, but satisfy their intent.
b. A robust, integrated predictive capability should be developed, validated, and maintained to routinely assess the vulnerability of whole structures to the effects of credible hazards. This capability to evaluate the performance and reserve capacity of structures does not exist and is a significant cause for concern. This capability would also assist in investigations of building failure – as demonstrated by the analyses of the WTC building collapses carried out in this Investigation. The failure analysis capability should include all possible complex failure phenomena that may occur under multiple hazards (e.g. bomb blasts, fires, impacts, gas explosions, earthquakes, and hurricane winds), experimentally validated models, and robust tools for routine analysis to predict such failures and their consequences. This capability should be developed via a co-ordinated effort involving federal, private sector, and academic research organizations in close partnership with practicing engineers.
NIST WTC Recommendation 2.
NIST recommends that nationally accepted performance standards be developed for: (1) conducting wind tunnel testing of prototype structures based on sound technical methods that result in repeatable and reproducible results among testing laboratories; and (2) estimating wind loads and their effects on tall buildings for use in design, based on wind tunnel testing data and directional wind speed data. Wind loads specified in current prescriptive codes may not be appropriate for the design of very tall buildings since they do not account for building-specific aerodynamic effects. Further, a review of wind load estimates for the WTC towers indicated differences by as much as 40 % from wind tunnel studies conducted in 2002 by two independent commercial laboratories. Major sources of differences in estimation methods currently used in practice occur in the selection of design wind speeds and directionality, the nature of hurricane wind profiles, the estimation of ‘component’ wind effects by integrating wind tunnel data with wind speed and direction information, and the estimation of ‘resultant’ wind effects using load combination methods. Wind loads were a major factor in the design of the WTC tower structures and were relevant to evaluating the baseline capacity of the structures to withstand abnormal events such as major fires or impact damage. Yet, there is lack of consensus on how to evaluate and estimate winds and their load effects on buildings.
a. Nationally accepted standards should be developed and implemented for conducting wind tunnel tests, estimating site-specific wind speed and directionality based on available data, and estimating wind loads associated with specific design probabilities from wind tunnel test results and directional wind speed data.
b. Nationally accepted standards should be developed for estimating wind loads in the design of tall buildings. The development of performance standards for estimating wind loads should consider: (1) appropriate load combinations and load factors, including performance criteria for static and dynamic behaviour, based on both ultimate and serviceability limit states; and (2) validation of wind load provisions in prescriptive design standards for tall buildings, given the universally acknowledged use of wind tunnel testing and associated performance criteria. Limitations to the use of prescriptive wind load provisions should be clearly identified in codes and standards.
The standards development work can begin immediately to address many of the above needs. The results of those efforts should be adopted in practice as soon as they become available. The research that will be required to address the remaining needs also should begin immediately and results should be made available for standards development and use in practice. Affected National Standard: ASCE-7. Model Building Codes: The standard should be adopted in model building codes by mandatory reference to, or incorporation of, the latest edition of the standard.
NIST WTC Recommendation 3.
NIST recommends that an appropriate criterion be developed and implemented to enhance the performance of tall buildings by limiting how much they sway under lateral load design conditions (e.g. winds and earthquakes). The stability and safety of tall buildings depend upon, among other factors, the magnitude of building sway or deflection, which tends to increase with building height. Conventional strength-based methods, such as those used in the design of the WTC towers, do not limit deflections. The deflection limit state criterion, which is proposed here is in addition to the stress limit state and serviceability requirement; it should be adopted either to complement the safety provided by conventional strength-based design or independently as an alternate deflection-based approach to the design of tall buildings for life safety. The recommended deflection limit state criterion is independent of the criterion used to ensure occupant comfort, which is met in current practice by limiting accelerations (e.g. in the 15 to 20 milli-g range). Lateral deflections, which already are limited in the design of tall buildings to control damage in earthquake-prone regions, should also be limited in non-seismic areas.*
[ * F-22 Analysis of baseline performance under the original design wind loads indicated that the WTC towers would need to have been between 50 % and 90 % stiffer to achieve a typical drift ratio used in current practice for non-seismic regions, though not required by building codes. Limiting drift would have required increasing exterior column areas in lower stories and/or significant additional damping.]
Affected National standards: ASCE-7, AISC Specifications, and ACI 318. Model Building Codes: The standard should be adopted in model building codes by mandatory reference to, or incorporation of, the latest edition of the standard.
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