Material and Structural Response through Fire Experiments A Way Forward

Transcription

1 Material and Structural Response through Fire Experiments A Way Forward Venkatesh Kodur Michigan State University 1

2 Outline Background Fire Resistance Assessment Elemental / System Tests Test Methods Drawbacks Data State-of-the-art Way Forward Material Property Tests Test Methods Drawbacks Data State-of-the-art Way Forward Summary 2

3 Fire Hazard & Structural Fire Safety Fire can be Primary event natural origin (lightning) Accidental, Arsonist or Terrorist Secondary event Occur after earthquake, blast, explosion, or impact Accidental or terrorist related Fire represents most severe condition Buildings, transit systems, Tunnels Fire safety - design requirements Minimize loss of life and property Fire resistance to structural elements - objectives Safe evacuation of occupants & fire personnel Minimize property damage Control spread of fire 3

4 Fire Resistance Measure of the ability of a building element to resist a fire Usually expressed in time as the duration during which a building element exhibits resistance with respect to: Structural integrity Stability Temp transmission during a fire-resistance test Fire resistance is a property of building elements & function of: Fire severity material, geometry and support conditions of elements restraint from the surrounding structure applied loads at the time of the fire Materials do not possess fire resistance 4

5 Fire Resistance FRR for building elements are specified in codes In time units -range from 1/2 hour to 2, 3 or 4 h Time-temp. curves followed in the tests are the standard curves for each country Although do-able, fire-resistance tests are not intended to simulate real fires Purpose of fire-resistance tests is to allow for a standard method of comparison between fire performance of building elements 5

6 Fire Resistance Assessment Rating/Listings Standard fire resistance tests Simple Calculation Methods Validation through experiments Basic material properties Advanced Calculation Methods Validation through experiments High-temperature material properties. Prescriptive approach Performance based approach Material properties 6

7 Fire Resistance Tests - Current Approach Standard fire tests Assemblies addressed: Columns, beams Floor-ceilings, roof-ceilings Specifications in standards Fire scenarios Types of furnace Instrumentation Specimen dimensions Test procedure Failure criterion 7

8 Fire Resistance Tests - Background Denver - first record of test NYC Bldg. Dept - series of comparative tests - loaded, full scale Columbia Fire Testing Station ASTM P Committee 1908 first test standard for floors 4 hour exposure, 3 criteria wall test standard 2 hour partition test first comprehensive std. with different ratings ¾, 1, 1½, 2, 3, 4 hr duration added temperature criterion on unexposed side 8

9 Fire Resistance Tests - Standards Standards for fire-resistance testing North American ASTM E119 NFPA 251 UL 263 ULC-S101 (2004) Canada International ISO 834 used by many countries Most European standards similar to ISO 834» BS 476 Parts (BSI, 1987) 9

10 Process of Conducting Tests Sponsor develops specifications for assembly Test lab contracted by sponsor Sponsor hires contractors to construct assembly Test lab witnesses construction Test conducted by lab & data collected Proprietary report issued If pass, assembly is listed. If fail, not reported Experiments mainly by researchers Limited 10

11 Fire Resistance Tests - Equipment A typical fire-test furnace consists of a large steel box lined with fire bricks or ceramic fibre,& has: a set of burners fuelled by gas or by fuel oil an exhaust chimney several thermocouples for measuring gas temp. small observation windows Data monitored during fire tests Temperatures, deformations and loads inside the furnace on the unexposed face of test specimens, & on rebars Temp. are measured at other locations within test specimen or in the furnace (Optional) floor/ceiling sample column sample Furnace wall sample 11

12 Fire Resistance Tests - Equipment Size of furnaces specified by most testing standards: Wall furnaces m x 3.0 m Floor furnace - 16 m 2 with span at least 3.7m (ASTM E119) & 3x 4 m (ISO 834) Special furnaces are available in some labs for testing individual columns or beams Test specimens should reflect actual Furnace for testing walls 12 construction

13 Column Furnace - Loaded 13

14 Std. Time-Temperature Exposure Standard time temp. curve - Heat exposure across the surface Natural / Propane gas burners, controlled by thermocouple readings in the furnace Typically more severe exposure at: - Center of the specimen compared with side - Top of wall assembly Temperature (C ) ASTM E119 fire Hydrocarbon fire Time (min) 14

15 Applied Loads Load applied not always Max. allowable design load (according to structural codes) for the assembly (lighter loads permitted as exceptions) - Limitations Applied with water barrels or hydraulic jacks or sand bags Wall assemblies - load varies over length because of deflection Restraint - varies during the test Ambiguity in the load calculations Loading arrangement for testing of floors 15

16 Column Furnace - Unloaded 16

17 Restraint Real structures have restraint from adjoining members in frame Flexural continuity & axial restraint can have a significant effect on fire resistance fire test has restraint from a rigid frame restraining forces vary during the test over the depth of the specimen increases fire resistance, often substantially ASTM E119 requires that restrained: floors be reasonably restrained in the furnace beams are to be tested to simulate the restraint in the construction represented 17

18 Failure (Performance) Criteria Three main failure criteria: Insulation (Barrier) Fire spread (Temp.) Stability (Strength) - Collapse Integrity Fire spread (Flame) Very often not tested to failure 18

19 Failure (Performance) Criteria Insulation/Integrity criteria - To test the ability of a barrier to contain fire spread Temp. rise on unexposed side: 250 F (140 C) average 325 F (180 C) single point (barrier) Prevent passage of flame or hot gases (barrier) Hose stream - prevent passage of water (wall) Stability - Sustain load (load-bearing elements) Temperature of structural components 1,000 F Av., 1,200 F single point (steel columns) 800 F pre-stressing steel, 1,100 F reinforcing steel Unexposed Side of Wall Assembly 19

20 Drawbacks Test Methods Fire scenario - Standard fire Limitations on specimen size Scaling effects Furnace size Furnace parameters Pressure Temperature Emissivity Instrumentation Failure scenario Fire test furnace (Intermediate Scale) 20

21 Drawbacks Furnace Parameters Heat flux from fire exposure Actual heat flux to assembly dependent on the material exposed to the fire combustible assemblies: less heat energy from burners needed to maintain temp. in the furnace Actual heat flux a function of: furnace volume furnace surface area thermal properties of furnace boundaries fuel gas properties 21

22 Drawbacks Furnace Parameters Furnace Pressure ASTM E119 test does not specify any pressure requirements Floors, columns - typically neg. press. airflow Furnace Height (m) Pressure Difference (Pa) ISO 834 specifies: a + ve pressure of 10 Pa under a horizontal test specimen (floors) for vert. test specimens (walls), the pressure gradient must be inear, with 10 Pa at the top & at least 2/3 of the specimen subjected to + ve pressure 22

23 Drawbacks Fire Scenario Standard fire Unrealistic Temperature rise Cooling phase Initial growth Emissivity Natural fire Realistic 23

24 Drawbacks Test Specimens Covers only elements Beams, columns, slabs Does not include interactions (Frames) Does not cover connections Does not account for systems Support conditions Restraint / Unrestraint New materials RH (during test day) 24

25 Drawbacks - Loads Application Sand bags, water cans Load level Service, dead Live Ambiguity in load calculations Eccentricity, lateral loads Often unloaded Not maintaining the same load level Spillage can occur 25

26 Drawbacks - Instrumentation Relative humidity Data monitoring Strains at high temperature C/S temperatures Deflections Spalling progression Cracking and pore pressure Bond 26

27 Drawbacks Failure criterion Strength / Stability not always considered Temperature in steel Hose Stream test Loss of cover to steel Spalling Performance-based design Strength Deflection Rate of deflection 27

28 Fire Resistance Tests - Drawbacks (Summary) Major Drawbacks Prescriptive Restrictive scenarios Expensive ($40k/column) Time consuming (1 year) Not realistic Minimum data for validation Limited application 28

29 State-of-the-Art: Test Data RC / PC Elements Columns - Large data set Beams Scarce Slabs Limited Walls Limited Connections Limited Generic No detailed measurements Researchers / industry Mostly standard tests 29

30 State-of-the-Art: Test data Steel / Composite Elements Columns - Large data set for limited types Beams Limited Beam-Slab assembly Limited Wall / Floor systems Large data set. Connections Limited Proprietary / industry Specific insulation system 30

31 State-of-the-Art: Test data Wood / Timber elements Columns - Limited data Beams Limited Wall / Floor systems Large data set. Connections Limited Proprietary / industry Specific insulation system Industry Groups 31

32 State-of-the-Art: Test data Other materials - FRP FULL - SCALE FIRE TESTS Phase 1: Fall 2002 Restrained Against Rotation (both ends) Columns Limited Steel End -Plate FRP-Wrapped Reinforced Concrete Column (instrumented w/ thermocouples and strain gauges) NRC Column Furnace Beams Limited CAN/ULC S -101 Standard Fire Wall / Floor systems Large data set. A A Test Setup for Column Fire Tests Proprietary / industry Specific FRP system Load Applied from Below Phase 2: Winter 2003 A A Applied Load FRP-Strengthened Reinforced Concrete Beam (Instrumented with thermocouples & strain gauges) CAN/ULC S -101 Standard Fire FRP sheet and various fire protection schemes to be tested Section A-A Test Setup for Beam - Slab Fire Tests 32

33 State-of-the-Art System Approach Frame tests Very limited Cardington Tests Full/large scale systems steel, concrete, wood buildings frames, connections Realistic scenarios fire exposure loads, connections restraint, size 1/2 bay frame AISC, Europe 33

34 Way Forward - General Fire resistance experiments Absolute necessity Validation, comparison Dependent on local materials Dependent on local construction Improved test methods To overcome current drawbacks Scaled fire resistance tests System approach Need data from experts 34

35 Way Forward Standards Revision Specimen preparation Conditions RH Instrumentation C/S thermocouples, strain gauges Failure criterion Continue till failure Data collection Load Calculations Applications Implementation Simple steps 35

36 Way Forward Fire Scenarios Current Standard fire Different fire scenarios 1500 Falling branch (Temperature-time curve) Furnace parameters Furnace pressure Furnace temperature Temperature (C ) ASTM E119 fire Hydrocarbon fire Fire I Fire II (shielded) Fuel type, Emissivity Lining, Unlined Time (min) Implementation Moderate effort 36

37 Way Forward Test Furnace Development of small scale and intermediate scale test equipment Scaling calibration System approach Portal frames Connections Non-furnace tests Realistic fire and loading scenarios Implementation Significant effort 37

38 Way Forward Systems Approach FR Assessment Conventional approach single element - columns, beams conservative - ideal conditions fire, loading, connections, restraint Improved approach overall structural behavior - frame real fires, loading Realistic failure criterion Realistic fire & load scenarios Implementation Large effort 38

39 Way Forward Overall Summary Revision/Development of Standards To address current drawbacks Collecting/Reporting of all useful data Development of test equipment Development of high-temperature sensors Development of US / International collaboration Development of relevant resources 39

40 Fire Resistance Assessment Material Characterization Fire performance Depends on properties of materials Vary with temperature Simple calculation methods Validation through experimentation Elemental tests Basic properties Advanced calculation methods - Validation through experimentation - Elemental/system tests - Detailed material properties - Constitutive Models 40

41 Material Characterization General Critical for developing models Variability affects fire resistance Usually evaluated through tests Small scale specimens (coupons) Lower heating rates Steady state conditions Significant variations due to material composition 41

42 Material Characteristics General Elev. Temp Thermal - conductivity, sp. heat, mass loss Mechanical stress-strain, elastic mod. Deformation - expansion, creep, shrinkage Material-specific properties Concrete bond, pore pressure, moisture migration Steel bond Wood charring Insulation stickability (adhesion, cohesion), impact resistance Properties - function of temperature 42

43 Thermal Properties Test methods and standards Thermal conductivity Hot Wire Method (ASTM C1113 Test Method) Guarded Hot Plate Method (ASTM C 177 Test Method) ASTM E1530 Guarded Heat Flow Meter Method TPS (Hot Disk Technology) Specific Heat Differential scanning calorimetry (DSC-based methods) TPS (Hot Disk Technology) Thermal diffusivity TPS (Hot Disk Technology) Laser Flash method Mass and density changes Thermogravimetry (TG-based) methods 43

44 Mechanical Properties Test methods and Standards Universal hydraulic compressive machine with built-in furnace Stress-strain curves Compressive strength Modulus of elasticity Tensile strength Bond properties Creep Heating rate Steady state Standards - RILEM Schematics for HT Strength tests 44

45 Drawbacks Summary No standard test methods Different guidelines needed for new materials Lack of specifics on Heating rate Load Residual strength Steady/transient state Temperature, load range Reporting RH, heating rate Strength (test day) Mix design (aggregate type) Burning g of resin g at 500 o C,, 12 diameter G12.7mm GFRP l sample 45

46 Drawbacks Summary (Continued) Not easy availability of test equipment Small size of specimens Hard to instrument Cost-effective Lack of instrumentation To measure bond To measure strains Need of significant resources Time consuming Large number of parameters GFRP bar at 450 o C Developing of Constitutive Models 46

47 Material Properties: State-of-the-Art Concrete Thermal Properties NSC: Limited data HSC: Limited data Mechanical Properties NSC: Large data set HSC: Large data set Deformation Properties NSC: Large data set HSC: Limited data set Special Properties HSC Fire-induced Spalling: Limited data HSC, FRC Permeability: Not available Constitutive Relations Limited Not fully validated Variability Stress (psi) C 260 C 538 C 760 C Strain 47

48 Material Properties: State-of-the-Art Variations in test data for heat capacity of NSC 48

49 Material Properties: State-of-the-Art Steel Thermal Properties Reinforcing: Limited data Prestressing: Limited data Structural: Limited data Mechanical Properties Reinforcing: Large data set Prestressing: Large data set Structural: Large data set Deformation Properties Reinforcing: Limited data Prestressing: Limited data Structural: Limited data Special Properties Debonding: Scarce data Creep under fire: Limited data Constitutive Relations Available Not well validated Less variability 49

50 Mechanical Properties: Constitutive Model Stress (MPa) C 200 C 400 C 500 C 600 C 800 C Stress (MPa) C 200 C 400 C 500 C 600 C 800 C Strain Concrete Strain Strain Reinforcing Steel 50

51 Material Properties: State-of-the-Art Variation of compressive strength with temperature for NSC. 51

52 Material Properties: State-of-the-Art Variation of compressive strength with temperature for HSC. 52

53 Material Properties: State-of-the-Art Wood Thermal Properties Scarce data Mechanical Properties Scarce data Deformation Properties Scarce data Special Properties Charring: Scarce data Constitutive Relations Very limited Variations with species Not well validated 53

54 Material Properties State-of-the-Art Protective Materials Thermal Properties Gypsum board: Scarce data Spray applied: Limited data Deformation Properties Gypsum board: Limited data Spray applied: Limited data Special Properties Gypsum board-cracking: Scarce data Spray applied Debonding, Stickability: Limited data Constitutive Relations Not available Significant variations 54

55 Material Properties: State-of-the-Art Thermal conductivity of insulation as a function of temperature 55

56 Material Properties: State-of-the-Art Specific heat of Type X Gypsum board core as a function of temperature 56

57 Way Forward - Standards Development of standards urgent need Test methods Test procedure Provisions in Standards Sample preparation Test conditions load, moisture content Test equipment Test procedures Instrumental TC, Strain gauges Observations/Monitoring Data Collection Failure criteria Post-test evaluation (observation) Not applicable to new materials Pin 950mm 950mm 200mm Kiln Specimen Figure 5: Schematics of test rig Loading jack Displacement transducer 57

58 Way Forward Reporting Guidelines Test conditions, heating rate Data collection, every minute Specimen details, RH Observations Spalling, Charring, Insulation fall-off Post-test assessment Implementation Simple steps 58

59 Way Forward Data collection / Instrumentation Properties Strength Specimen conditions -RH Instrumentation (HPM) TC (multiple locations) Strain gauges Deflections Spalling, Debonding Implementation Moderate effort 59

60 Way Forward More Experiments Research studies Multiple tests High level of instrumentation Repeatability Varying parameters Implementation Significant effort Large resources 60

61 Summary Fire resistance assessment continues to be based on standard fire tests There is an urgent need to develop test procedures, guidelines and standards for undertaking rational fire experiments There is lack of test data for validating computer models 61

62 Summary Material properties at elevated temperature are critical for PB fire design and there is large variation in available data With moderate effort (& resources) some of the drawbacks in current fire test standards can easily be overcome 62

63 Thank You Questions 63