AISC Specifications

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 CollapsesGROUP 1. Increased Structural Integrity – Recommendations 1, 2 & 3 (out of 30)

2011-11-18:  NIST WTC Recommendations 4-7 > Structural Fire EnduranceGROUP 2.  Enhanced Fire Endurance of Structures – Recommendations 4, 5, 6 & 7



  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 !!!



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.