Structural Fire Safety through Improved Building Codes and Standards: Recommendations from the Technical Investigation of the WTC Collapse

Transcription

1 Structural Fire Safety through Improved Building Codes and Standards: Recommendations from the Technical Investigation of the WTC Collapse William Grosshandler Building and Fire Research Laboratory Workshop on Structures and Fire: Research Needs June 11, 2007 East Lansing, Michigan

2 Topics to be covered I. Overview of NIST technical investigation into the collapse of the WTC towers on 9/11 II. III. IV. NIST recommendations Implementation of recommendations for structural fire safety Research agenda

3 I. Overview of NIST technical investigation into the collapse of the WTC towers on 9/11 Relevant Technical Objectives: Determine why and how the WTC towers collapsed following the initial impact of the aircraft. Determine procedures and practices used in the design, construction, operation, and maintenance of the WTC buildings. Identify specific areas in current national building and fire model codes, standards, and practices that warrant revision.

4 Four-step Reconstruction Process 1. Aircraft impact simulation 2. Reconstruction of the fires 3. Heating of the structural steel 1 NODAL SOLUTION STEP=7 SUB =1 TIME=.007 USUM (AVG) RSYS=0 DMX = SMN =3.298 SMX = MX MAR :38:28 MN 4. NSimulation of the structural response Z Y X WTC-2 Severe Case Temperature Analysis

5 WTC Investigation Reports WTC Towers Investigation Report Building and Fire Codes Baseline Performance & Aircraft Impact Steel Analysis Active Fire Protection Thermal Reconstruction WTC Towers: Structural Collapse Occupant Behavior & Egress Emergency Response Final report (NCSTAR 1), 8 project reports, and 34 supporting technical reports were released in PDF copies available on line at or on CD by request.

6 II. NIST Recommendations 30 Recommendations in 8 Groups: Prescribe neither specific technologies nor threshold levels. Encourage competition among different systems that can meet performance requirements. Responsibility for establishment of threshold levels belongs in standards and codes development process. International Code Council (ICC) National Fire Protection Association (NFPA) ASTM ASCE ASME AIA UL ACI

7 Group 1. Increased Structural Integrity R1. Tools and guidelines are needed to reliably predict potential for complex failures in structural systems subjected to multiple hazards. R2. Nationally accepted performance standards should be developed for wind tunnel testing and estimating wind loads based on wind tunnel testing data and directional wind speed data. R3. Criteria should be developed for limiting how much tall buildings sway under lateral load design conditions (e.g., winds and earthquakes).

8 Group 2. Enhanced Fire Endurance of Structures R4. Evaluate technical basis for determining structural fire rating requirements considering full evacuation of occupants, timely access by emergency responders, and time required for burnout without local collapse. R5. Improve technical basis for fire resistance testing of components and assemblies and develop guidance for extrapolating results to prototypical building systems. R6. Develop in-service performance criteria for fireproofing to ensure that materials conform to conditions in tests used to establish rating. R7. Adopt structural frame approach (members connected to columns carry the higher fire resistance rating of the columns).

9 Group 3. New Methods for Fire Resistant Design of Structures R8. Require that uncontrolled building fires result in burnout without collapse. R9. Develop performance-based methods to enable design and retrofit of structures to resist real fire conditions, and tools to evaluate fire performance of the structure as a whole system. R10. Develop and evaluate new fire resistive coating technologies with significantly enhanced performance and durability to provide protection following major events. R11. Evaluate suitability of advanced structural steel, reinforced and prestressed concrete, and other high-performance material systems for use under conditions expected in building fires.

10 Group 4: Improved Active Fire Protection R12 - R15. Active fire protection systems (i.e., sprinklers, standpipes, hoses, fire alarms, and smoke management systems) should be enhanced through improvements to: design performance reliability, and redundancy.

11 Group 5: Improved Building Evacuation R16 - R20. Building evacuation should be improved to include: system designs that facilitate safe and rapid egress, methods for ensuring clear and timely emergency communications to occupants, better occupant preparedness for evacuation during emergencies, and incorporation of appropriate egress technologies.

12 Group 6: Improved Emergency Response Technologies and Procedures R21 R24. Technologies and procedures for emergency response should be improved to enable better access to buildings, response operations, emergency communications, and command and control in largescale emergencies.

13 Group 7. Improved Procedures and Practices R25. Entities not subject to building safety code requirements of any governmental jurisdiction should provide safety equivalent to building code requirements of appropriate governmental jurisdiction. R26. Jurisdictions should adopt and aggressively enforce building codes to ensure that egress and sprinkler requirements are met by existing buildings. R27. Building codes should require owners to retain relevant building design/operation documents with information easily accessible to responders during emergencies. R28. The role of the Design Professional in Responsible Charge should be clarified.

14 Group 8: Education and Training R29- R30. The skills of building and fire safety professionals should be upgraded through a national education and training effort for fire protection engineers, structural engineers, and architects.

15 III. Implementation of Recommendations for Structural Fire Safety What are the technical barriers? Why are they hard to overcome? How are they being dealt with today? physical testing computational approaches Windsor Tower Madrid 2005 Empire State Building 1945

16 Technical Barriers Structural engineers design buildings to handle static loads due to gravity and specified building contents. In addition, buildings must handle loads caused by severe natural occurrences (high winds, snow levels and earthquakes), with the loading probability established through historical data. How does one set the severity level for manmade fires (accidental or intentional)?

17 Technical Barriers Setting severity level of fires for building design is complicated by three factors: historical database on structural fires is sparsely populated and/or unreliable; extrapolation of historical data to future events is highly uncertain due to changes in human activities; almost all fires have potential of being severe, given the right set of circumstances not in control of the building designer.

18 Technical Barriers The problem can be stated in two parts: There are no agreed upon protocols for establishing the maximum fire load (analogous to wind and earthquake loads) that a building should be designed to resist. There is no systematic framework nor solid enough scientific foundation upon which to predict, with an established certainty, the maximum fire load that a building could withstand.

19 Why are the barriers hard to overcome? Public policy unique to each jurisdiction sets the ground rules and dictates the applicable building and fire codes. Fire safety design is one of many complex design considerations, and relies not only on reasonable fire growth, smoke spread, and thermal-mechanical analysis of the building, but also on a reasonable prediction of the response of the occupants and fire service to the fire. The heat transfer/mechanical response is coupled, the material properties are variable, the computational domain is huge, and time scales span multiple orders of magnitude.

20 How is the problem solved today? Engineers/building code officials rely upon furnace tests of individual structural elements to rate one design against another, based upon amount of time element can survive furnace environment before failing: ASTM E 119, ISO 834, NFPA 251, UL Courtesy of Underwriters Laboratories 1000 Temperature, o C Hydrocarbon fire Time, minutes ASTM E-119 Wall Furnace

21 How is the problem solved today? Different failure criteria for different materials (steel, concrete, wood); different elements (floors, beams, columns, walls); load-bearing or non-load-bearing elements; and elements mechanically restrained or free to expand. Load criterion: unable to maintain max design load Breach criterion: hot gases penetrate to far side of wall/floor/ceiling and ignite cotton target Temperature criteria: T unexposed surface > 160 o C to 205 o C T exposed structure > 450 o C to 725 o C

22 How is the problem solved today? Shortcomings of Standard Test Method Size of test article limited to size of furnace (~ 5 m x 5 m x 2 m). Thermal environment of furnace does not mimic a real fire. Load conditions do not adequately mimic field use. Edge support conditions do not adequately mimic field use. No universally accepted definition of failure-to-meet-load criteria. Test yields no fundamental information about performance of specimen, nor guidance on how to improve performance. Facility designs are not standardized, and are expensive to operate. Same specimen could receive different ratings if tested in two different facilities. No way to quantify uncertainty or safety factor.

23 How is the problem solved today? Standard test methods continue to be used because massive database using standard fire resistance test method has been established and is in continual use; historical record suggests that test methods are conservative, since number of losses due to collapse of commercial buildings in fires is small; alternative methods have not been developed yet that are acceptable to major parties who have widely divergent interests.

24 Structural materials testing Required Testing Fire resistant materials testing Heat release rate testing of building contents Fire exposure testing of unloaded structural elements Fire resistance testing of loaded structural systems

25 Fire resistance testing of loaded structural systems Field application: 21 m Testing capability: 5 m to10 m Test specimen length, edge conditions Time to exceed 205 o C on top surface 5 m, restrained 157 minutes post-test, local buckling of truss members Maintain load? Yes 10 m, restrained 111 minutes Yes 21 m, restrained???

26 Generalized Computational Approaches Thermal/Mechanical Materials Testing and Data Very Fine (~0.02 cm) Very Fine (~2 cm) Material Constitutive Modeling Mesh Refinement Model Size Component Analyses Subassembly Analyses Coarse (~2-25 cm) Coarse (~30 m) Global Analyses 1 NODAL SOLUTION STEP=15 SUB =18 TIME=6000 UZ (AVG) RSYS=0 DMX = SMN = SMX =4.341 MN OCT :46:42 MX Observables Z X Y WTC1 FL98 - Maximum Damage Case Temperature at 6000 sec 4.341

27 IV. Research Agenda Recommendation: Procedures used in fire resistance design should be enhanced by requiring that uncontrolled fires result in burnout without local or global collapse. This requires: improving technical basis for construction classifications and fire resistance ratings and testing methods; using structural frame approach to fire resistance ratings; developing in-service performance requirements and conformance criteria for spray-applied fire resistive materials; developing and evaluating new fire resistive coating materials and technologies; evaluating fire performance of conventional and high-performance structural materials.

28 Research Agenda (cont.) Measure properties of construction materials Develop new experimental methods and protocols for high temperature measurements of thermal/mechanical properties of construction materials, up to the point of failure. These include: normal/high strength concrete normal/ fire-resistant steel steel/concrete composite aluminum fiber-reinforced composite timber gypsum partitions glazing fire stops intumescent coatings structural fireproofing Standardize measurement methods and use them to accumulate consistent, reliable high temperature (> 500 o C) database on thermal/mechanical properties of construction materials: k(t) c p (T) H α T (T) σ(ε,t,dε/dt)

29 Research Agenda (cont.) Real-scale Structural Fire Endurance Facility for exposing floor and wall composite assemblies to controlled fires under measured loads, to failure; for measuring behavior of fireproofing as installed and when degraded by time, temperature, and stress; and response of structural connections, welds, bolts, rivets and adhesives when exposed to severe fire conditions and loads, including during cool-down period; more efficient non-linear structural algorithms to include creep, concrete cracking, spalling, and fireproofing damage; and which accommodate a wide range of length scales; and verified sub-grid models to better resolve heat transfer from fire environment to structural elements, and for failure of structural connections and interfaces at elevated temperatures.

30 Ultimate scientific goal of structural/fire safety research: Validated models that resolve heat transfer from fire environment to structural elements, and predict failure of structural connections and interfaces at elevated temperatures, up to the point of system collapse; with sufficient accuracy to discriminate performance among alternative designs, construction materials, and active fire protection systems.

31 Acknowledgements NIST associates: Shyam Sunder, Richard Gann, Kevin McGrattan, Anthony Hamins, Fahim Sadek, John Gross, Kuldeep Prasad, Jason Averill, James Lawson, Frank Gayle,. Contractors and Consultants: Harold Nelson, Hughes Associates, Inc., ARA, SGH,.

32 QUESTIONS?

33 Why are the barriers hard to overcome? Phenomena to consider in thermal analysis: fire growth and spread, imposing heat fluxes on boundaries of the structure; heat transfer through building lining materials and structural insulation; evolving temperature distributions within structural elements; and thermal degradation (including phase changes and chemical reactions) and breaching of building partitions.

34 Why are the barriers hard to overcome? Phenomena to consider in mechanical analysis: thermally-induced strain of load bearing elements (including thermal expansion, and elastic, plastic, and creep strains); redistribution of loads as the structure deforms; and mechanical failure of individual components leading to local or global collapse.

35 Why are the barriers hard to overcome? Reference Structural Database Fire Dynamics Analysis 10-3 s Reference Model Conversion Damage to fire barriers Ventilation Fuel Distribution Gas Temperature Histories 1 s Initial Conditions Analysis (e.g., Earthquake, Blast) Fireproofing Damage Thermal Analysis 10-6 s Structural Damage Structure Temperature Histories Structural Model Conversion Reference Model Conversion Structural Analysis 10-3 s Time and length scales vary up to 6 orders-of-magnitude Structure Response/ Failure