Presentation on theme: "Workshop at Indian Institute of Science 9-13 August, 2010 BangaloreIndia Fire Safety Engineering & Structures in Fire Organisers: CS Manohar and Ananth."— Presentation transcript:
Workshop at Indian Institute of Science 9-13 August, 2010 BangaloreIndia Fire Safety Engineering & Structures in Fire Organisers: CS Manohar and Ananth Ramaswamy Indian Institute of Science Speakers: Jose Torero, Asif Usmani and Martin Gillie The University of Edinburgh Funding and Sponsorship: Material Behaviour in Fire
Effect of fire on construction materials Chemical decomposition, charring Physical changes in density, softening, melting, spalling Mechanical strength, stiffness, creep, thermal expansion Thermal thermal conductivity, specific heat
Important material properties Properties of materials of interest in terms of structural behaviour in fire Thermal conductivity Specific heat Density Strength Stiffness Creep Thermal Mechanical
Basics Structural steels and concrete reinforcing bars are hot rolled low-carbon ferrite-pearlite steels Pre-stressing tendons are cold drawn high-carbon pearlitic steels with elongated grain structure in cold work directions Properties at room temperature Density ( = 7850 Kg/m 3 Modulus of Elasticity ( = 210 GPa Coefficient of thermal expansion ( = 11.5x10 -6 K -1 Thermal conductivity (k) = 46-65 Wm -1 K -1 Specific heat (c p ) = 450 J Kg -1 K -1
Steel Behaviour at ambient temperature – a review Ductile Ductile Same in compression and tension Same in compression and tension Clear yield point Clear yield point Plastic range with lots of hardening Plastic range with lots of hardening
Steel- Mechanical Properties at high temperature Loss of clear yield point / reduced yield point Loss of clear yield point / reduced yield point Reduction in UTS (above around 400C) Reduction in UTS (above around 400C) Increased ductility Increased ductility
Obtaining stress-strain at ambient temperature Single variable Single variable Easy to perform Easy to perform P P Strain = ΔL/L Stress= P/A
Obtaining stress-strain-temperature data Two variables (temp + strain) Two variables (temp + strain) Either isothermal test Either isothermal test Or an-isothermal test Or an-isothermal test Different results! Different results! –minor in steel –significant in concrete Creep Creep T P P Strain = ΔL/L Stress= P/A T+
Numerical Representation Eurocode (no plastic hardening) Eurocode (no plastic hardening) –Linear range, E function of temperature –Polynomial non-linear –Plastic plateau –Three parameters - E(θ), f y (θ), f p (θ) Eurocode (plastic hardening) Eurocode (plastic hardening) –As non-hardening model –Then increased capacity –Four parameters - E(θ), f y (θ), f p (θ), f u (θ) Osgood-Ramberg Osgood-Ramberg –Non-linear elastic –Can be made temperature dependent
Thermal Expansion of Steel Some variation with temperature Some variation with temperature Often taken as 1.2-1.4x10 -5 ˚ C -1 between Often taken as 1.2-1.4x10 -5 ˚ C -1 between 0-700 C Phase change at 700C results in shrinkage of steel at this point Phase change at 700C results in shrinkage of steel at this point
Steel: Conclusions Highly conductive, heats up quickly and loses strength/stiffness very quickly in fire if left unprotected Cold-drawn pre-stressing steels lose strength quicker than hot-rolled mild steel Slender parts of steel sections under- compression are susceptible to local buckling
Steel: Conclusions Steel expands considerably (> 1mm/m/100 o C) leading to further susceptibility to buckling when restrained L/L=14x10 -6 (T-20) =>(EC3-1995) Plastic deformations in restrained steel structural members lead to large tensile stresses upon cooling Significant creep over 400-500 o C
Basics Hydrated paste forms only 24-43 % of volume, therefore properties vary greatly with aggregate Divided into two major groups: Dense or normal-weight concrete (2150-2450 Kg/m 3 ) Lightweight concrete (1350-1850 Kg/m 3 ). Also siliceous and calcareous aggregates according to aggregate composition
Basics Properties at room temperature (practical values) E=5-35 GPa; f cu = 20-50 MPa (NWC), 10-30 MPa (LWC) k = 1.6 Wm -1 K -1 (NWC-S), 1.3 Wm -1 K -1 (NWC-C), 0.8 Wm -1 K -1 (LWC) c p = 1000 JKg -1 K -1 (NWC) 840 JKg -1 K -1 (LWC) = 18x10 -6 K -1 (NWC-S), 12x10 -6 K -1 (NWC-C), 8x10 -6 K -1 (LWC) - But note LITS
Concrete: Conclusions Concrete loses strength and stiffness when heated beyond 400 o C, its fire behaviour however differs considerably depending upon aggregate used (light- weight aggregates give best performance) When used as a composite with steel, concrete provides fire protection to steel
Concrete: Conclusions Highly insulating (heat propagation is ~50 times slower than steel) Heat propagation is further delayed at ~100 o C due to the latent heat needed to evaporate large quantities of moisture in the micro-pores
Basics Used mainly in residential construction Properties at room temperature (of clear dry wood) oven-dry) = 300-700 Kgm -3 (Douglas firs ~ 450, southern pines ~ 550) True density of the solid forming the walls of wood cells ~ 1500 E=5-15 GPa; f cu = 13-70 MPa (along grain) ~ proportional to k ~ 0.5 W/m o K (across grain) c p = 1500 JKg -1 K -1 = 3-4.5x10 -6 K -1 (along grain), 21-40x10 -6 K -1 (across grain)
Wood: Conclusions Insulating material, but is also inflammable Low thermal conductivity is assisted by charring Rate of char formation varies with wood species and depends upon bulk density and moisture content Strength and stiffness both reduce with heating, compressive strength reduces at faster rate than tensile strength
Wood: Conclusions Design strength of timber members is calculated by omitting the charred portion of sections Thermal expansion is low
MASONRY Properties at room temperature oven-dry) = 1660-2270 Kgm -3 (based on raw materials, moulding and firing techniques) True density ~ 2600-2800kgm -3 E=10-20 Gpa; f cu = 9-110 MPa (~50 MPa average) k ~ 1.0 W/m o K (increasing to ~1.5 at 700 o C) c p ~ 700 J/Kg o K (increasing to ~1200 at 600 o C) = 5.5x10 -6 / o K
MASONRY (BRICKS) Highly insulating and inherently stable in fire Often instability is caused by: movement of connected structure Integrity failure can occur at very high fire loads (bricks can resist temperatures up to a 1000 o C but melt at 1400 o C) Masonry can spall Thermal properties are not of great interest due to secondary and mainly non-load bearing role of brick masonry in modern construction
GYPSUM Well known as plaster of Paris and used as interior linings and in partition walls Low thermal diffusivity and large quantities of chemically bonded water give it good insulating and absorbing properties making it a cheap fire retardant material Practically all strength is lost once the water of crystallisation has evaporated Fibreglass reinforcement improves the strength in fire
GLASS Main use is in glazing for doors and windows In this role, it has little resistance in fire and cracks very quickly because of temperature gradient across the surfaces and because of thermal expansion Double glazing does not help fire behaviour very much Wire reinforcement can provide better performance In general, glazing cannot be relied upon to remain intact in fire
PLASTICS All plastics are inflammable and add fuel to fire Some treatments reduce combustibility but nothing removes it A large amount of plastics can lead to very high rates of heat release in fire leading to very high temperatures
What is Load Induced Thermal Strain? How can we model it? Angus Law, Martin Gillie, Pankaj
Introduction What is Load Induced Thermal Strain (LITS) What is Load Induced Thermal Strain (LITS) –Description –Terminology Why is it important? Why is it important? How can we model it? How can we model it? –Existing models –New model
What is LITS? Most materials…. Most materials…. –Get bigger when they are heated. –Even under load 20ºC250ºC500ºC
What is LITS? Concrete is a bit different…. Concrete is a bit different…. –With load… –and with even more load… 20ºC250ºC500ºC
What is LITS More precisely…(I haven’t just made it up) More precisely…(I haven’t just made it up) Schneider et. al (1988)Khoury et. al (2002)
What is LITS? – Terminology Plastic Elastic Time dependent Lots of different terms… Lots of different terms… –Elastic changes –Shrinkage –Transitional thermal creep –Transient strain –Transient creep –Creep
Why is it important? Larger strains during heating Larger strains during heating Locked in plastic strains… Locked in plastic strains… OriginalWithout LITS With LITS
How can we model it? Elastic –Elastic changes –Shrinkage –Transitional thermal creep –Transient strain –Transient creep
–Shrinkage –Transitional thermal creep –Transient strain –Transient creep How can we model it? Elastic changes (LITS) PlasticElastic
How can we model it? (LITS) Plastic ElasticNon-LITS plastic –Elastic changes –Shrinkage –Transitional thermal creep –Transient strain –Transient creep
How can we model it? Non-LITS plastic (LITS) PlasticElastic –Elastic changes –Shrinkage –Transitional thermal creep –Transient strain –Transient creep ++
How can we model it? Build up strains to make full curve Build up strains to make full curve –Apparent and or Actual modulus?
How can we model it? Column test…. Column test….
How can we model it? Problem solved? Problem solved? What about 2 or 3D cases? What about 2 or 3D cases? Plastic strain occurs in direction orthogonal to yield surface… The trial stress (location relative to the yield surface) is dependent on the elastic modulus… What does this mean?!
How can we model it? The same constitutive curve will give us different strains… The same constitutive curve will give us different strains…
How can we model it? This can be remedied with a new, two step, approach… This can be remedied with a new, two step, approach… Step 1)
How can we model it? This can be remedied with a two step approach… This can be remedied with a two step approach… Step 2)
How can we model it? Does this work? Does this work? Yes!
Conclusions…. LITS is an important factor when modelling concrete; LITS is an important factor when modelling concrete; Representation of LITS needs a more thoughtful approach than simple inclusion in the constitutive curve; Representation of LITS needs a more thoughtful approach than simple inclusion in the constitutive curve; A new method for modelling LITS equations has been successfully modelled. A new method for modelling LITS equations has been successfully modelled.
Further work…. Implementation in more complex material model; Implementation in more complex material model; Full scale validation from experimental results. Full scale validation from experimental results.