Fire Limit State Design (FLS) for concrete structure

Fire Limit State Design (FLS) under Code of Practice for Structural Use of Concrete 2013

1. Definition:
cl. 1.4.1 & 2.2.3.1	Fire Limit State relating to the structural effects of a fire in a building or part of a building
2. Requirement for fire resistance
cl. 2.1.1	Adequate structural resistance for the required fire resistance period.
cl. 2.2.4.7	Durability and fire resistant
cl. 4.1.2	Appropriate degree of fire resistance (a) flame penetration; (b) heat transmission; and (c) collapse.
cl. 4.3	Requirement for fire resistance Cover specified for durability will not be sufficient for fire protection in accordance with Code of Practice for Fire Safety in Buildings; Concrete compressive strength greater than 60 MPa, possible reduction of strength at elevated temperatures and the associated risk of spalling should be investigated, taken into account the relevant factors including moisture content, type of aggregate, permeability of concrete, possible heating rate and the silica fume content.
cl. 4.3.1 for high strength concrete	1. Prevention of spalling in high strength concrete -          content of silica fume not exceed 6% by weight of total cementitious content -          Pfa and ggbs comply with cl 4.2.5.5 for normal strength concrete 2. Method to reduce risk of concrete spalling. At least one of the following method  (a) Method A: A reinforcement mesh with a nominal cover of 15mm. This mesh shall have wires with a diameter ≥ 2mm with a pitch ≤ 50 x 50mm. The nominal cover to the main reinforcement shall be ≥ 40mm; (b) Method B: Include in the concrete mix not less than 1.5 kg/m3 of monofilament propylene fibres. The fibres shall be 6 – 12 mm long and 18 – 32 μm in diameter, and shall have a melting point less than 180°C; (c) Method C: Protective layers for which it is demonstrated by local experience or fire testing that no spalling of concrete occurs under fire exposure; or (d) Method D: A design concrete mix for which it has been demonstrated by local experience or fire testing that no spalling of concrete occurs under fire exposure. 3. For high strength concrete exceeding C80, at least one fire test should be carried out to demonstrate that the main reinforcing bars of a structural member shall not be exposed.
3. Design parameters:
cl. 2.2.3.2, 3.6, Table 3.5 to 3.7	Strength reduction factors Elevated temperature Reduction factors for concrete Reduction factors for reinforcement Reduction factors for tendon  using wires & stands Reduction factors for tendon using bars 20 °C 1.00 1.00 1.00 1.00 100 °C 1.00 1.00 1.00 1.00 200 °C 0.95 1.00 0.82 1.00 300 °C 0.85 1.00 0.64 0.78 400 °C 0.75 0.87 0.44 0.55 500 °C 0.60 0.60 0.21 0.25 600 °C 0.45 0.36 0.09 0.09 700 °C 0.30 0.11 0.08 0.08 800 °C 0.15 0.08 0.06 0.06 900 °C 0.08 0.06 0.05 0.05 1000 °C 0.04 0.04 0.03 0.03 1100 °C 0.01 0.02 0.02 0.02 1200 °C 0.00 0.00 0.00 0.00 Elevated temperature see BS EN 1992-1-2
cl. 2.3.2.7 & Table 2.2	Partial safety factors for loads: Loads γf Dead load 1.00 Imposed loads: a) permanent: 1) those specifically allowed for in design, e.g. plant, machinery and fixed partitions 2) in storage buildings or areas used for storage in other buildings (including libraries and designated filing areas) b) non-permanent: 1) in escape stairs and lobbies 2) all other areas 1.00 1.00 1.00 *0.80 Wind loads 0.33 Note: *The value may be reduced to 0.50 when suitable justification is available
cl. 2.4.3.2 & Table 2.3	Partial safety factors for materials: Material/design consideration γm for ULS γm for FLS Reinforcement (prestressing steel included) 1.15 1.00 Concrete in flexure or axial load 1.50 1.10 Concrete shear strength without shear Reinforcement 1.25 1.10 Bond strength 1.40 1.10 Others (e.g. bearing stress) ≥1.50 ≥1.10 Effects of exceptional loads or localised damage. γm may betaken as 1.3 for concrete in flexure and 1.0 for steel.

Reference:

1. Code of Practice for Structural Use of Concrete 2013

2. Code of Practice for Fire Safety in Buildings

3. BS EN 1992-1-2 Eurocode 2 Design of concrete structures – Part 1-2 General rules:  Structural fire design

4. The Effect of Elevated Temperature on Concrete Material and Structures - A Literature Review by US Nuclear Regulatory Research

                       

Basics of Fire Science

(Draft)

I. Basics of Fire Science

A. Definition:
- Fire: It is a rapid oxidation process accompanied by evolution of heat, light, flames and emission of sound.

B. Ignition:
- Pilot ignition: Fire ignited by an external heating source;
- Spontaneous ignition: Fire is ignited by itself under elevated temperature.

Materials	Pilot Ignition Temperature (°C)	Spontaneous Ignition Temperature (°C)
Cotton	230 – 266	254
Paper	230	230
White pine	228 – 264	260
Polyethylene	341	349
PVC	391	454
Perspex	280 - 300	450 - 462
Polystryrene foam	346	491
Polyurethane	310	416

C. Combustion:
- a series of very rapid chemical reactions. (i.e. fuel, heat, oxygen).
i. Smouldering Combustion - Burning process without flame due to limited supply of oxygen.
ii. Flaming Combustion - Visible manifestation of combustion between gaseous fuel and oxygen.
iii. Heat of Combustion (ΔHc):

- Heat of combustion is the energy released as heat when a material undergoes complete combustion with oxygen under standard conditions.

- Different fuels have different heat of combustion. Usually, fuels with carbon-rich molecules have higher heat of combustion but also require higher ignition energy.
iiii. Combustion Reaction:
- Propane (C₃H₈)

- C₃H₈ + O₂ ―> 3CO₂ + 4H₂O

-The reaction produces 2044kJ/mole of C₃H₈ Or 46.45kJ/g of C₃H_8.

- The energy produced by the combustion reaction will be presented in form of light and heat. (e.g. a flame)

D. Flammability & Flame Structure

E. Flames 

i. Premixed flame - Fuel gas and oxygen are mixed before combustion.

ii. Diffusion flame - Fuel gas and oxygen are separated before combustion (e.g. bunsen burner)

F. Buoyant plume

- The heat produced by the combustion reaction will heat up the surrounding air.

- When the air temperature increase the density is reduced.

- The density different between the hot gas and the surrounding ambient air increases.

- The buoyancy force of the hot air increases and pushes the hot air to higher level. 

- The hot gases created from the fire forms a hot gas column extending to the ceiling of the compartment.

- The upward movement of the hot gas column induces the entrainment of the surrounding ambient air by turbulent mixing and molecular diffusion.

- Due to the air entrainment, the temperature and velocity of the plume decreases along the upward direction. Therefore, a plume shape can be approximated by an inverted cone.

G. Ceiling Jet

- When the hot gas reaches the ceiling, it can not penetrate through the slab. It spreads not penetrate through the slab. It spreads radially under the slab soffit.

-The air entrainment along the horizontal spread of the hot gas is not efficient.

- The speed of the ceiling jet is fast due to the thin layer of the hot smoke under the ceiling.

- The ceiling jet in contact with sprinkers and detectors.

Modelling of Ceiling Jet Temperature:
- Alpert's equation (for temperature)

H. Hot and cold layers
- When the ceiling jet reaches the wall boundaries, it reflects back and accumulates at the upper part of the compartment.
- A thermal interface exists in the compartment to demarcate the upper hot gas layer and the lower air layer. The interface is quite stable.
- It is defined as the level which the largest change in temperature.
- When the thermal interface reaches the door soffit, the hot gas emerges out of the compartment.

I. Neutral Plane
- At the upper part of the door opening, the hot gas is emerging out of the  compartment.
- At the lower part of the door opening, the ambient air entering into the compartment.
- There exist a level at the door opening in which the air velocity is zero. It is defined as Neutral Plane.

J. Modelling Hot Gas Temperature

i. McCaffrey et al. equation

ΔT_g = 480 [Q / (g^1/2 c_pρ_∞ T_∞ A_o H_o ^1/2)] ^2/3 [ h_k A_T / (g^1/2 c_pρ_∞ A_o H_o ^1/2)] ^-1/3

- h_k is the heat transfer coefficient which is time dependent

- If t ≦ t_p, h_k = ( kρc /t)^1/2, otherwise h_k = k /δ

- t_p = (ρc /k) (δ/2) ² is the penetration time

ii. Foote et al.
ΔT_g /T_∞ = 0.63 [ Q/(m_g c_p T_∞]0.72   [h_k A_{T /} (m_g c_p ) ]-0.36

- mg is the mass ventilation rate in (kg/s)

K. Flashover
- The fire plume and the hot gas layer emit radiation to all unburnt combustible materials inside the compartment
- When the radiation is sufficiently high, it will ignite all combustible materials
- All fuels inside the compartment are involved in the fire
- The heat release rate / temperature are rapidly increased

L. Determination of Flashover
- Hot gas temperature at 10mm below ceiling soffit ≧ 600°C
- Radiation at the floor of the compartment ≧ 20kW/m²
- Minimum heat release rate can be estimated by use of the McCaffrey equation by setting the
ΔT_g  = 600- ambient temperature in °C.

- Method of Babrauskas
Q = 750 Ao Ho1/2

- Method of McCaffrey et al.
Q = 610( h_k A_TA_o Ho1/2 )1/2

- Method of Thomas
Q = 7.8 A_T + 378 A_o Ho1/2

M. Growth stages of enclosure fire
- Ignition
-Grown stage (pre-flashover stage)
- Flashover
- Fully developement stage (post-flashover stage)
-Decay stage

N. Typical Fire Growth Curve

O. Design Fire Growth Rate
- t² fire is commonly adopted in fire engineering.
- It owns a parabolic increasing profile
   * Extra fast growth (75s to reach 1MW)
   * Fast growth (150s to 1MW)
   * Medium growth  (300s to 1MW)
   * Slow growth  (600s to 1MW)
- The fire size increases parabolically until it reaches the maximum heat rekease rate.

P. Smoke / Hot Gas Production Rate
Smoke Production Rate is approximated by the air entrainment rate

m smoke = m fuel + m air
m fuel << m air => m smoke ≒ m air

R. Smoke Production Rate

If (z –z fire) / L flame ≧ 1 (smoke height > flame length

mg = 0.071 Qc ^1/3 (z – z fire – L fame + 0.166Qc ^2/5) ^5/3 x [1 +0.026 Qc^2/3 (z – z fire – L flame + 0.166 Qc ^2/5)^-5/3]

If 0 < (z –z fire) / L flame < 1

Mg = 0.0054Qc (z – z fire)

Where

Qc ≈ 0.65 Qtotal

L flame = -1.02 D + 0.235Qc ^2/5

D id the diameter of fire bed

S. Mechanical Smoke Extraction

- Smoke is removed by extraction fan

- The mass extraction rate should be higher than the mass of smoke generation rate

- Volumetric flow rate = mass flow rate / density

- By ideal gas law with constant pressure, we have  ρT = constant.

ρT =ρ₂₇₃T₂₇₃ = 1.2922 x 273 =352.77

T. Static Smoke Vent

- Smoke is drive by its buoyancy to the atmosphere naturally (natural vent)

Mg = C_d,v Av ρo [2 g z_lay (T – To)]^1/2 / [ T/To + (C_d,v Av / C_d,i Ai)² (To / T] ^1/2

where Cd,v and Cd,I are the discharge coefficients of vent outlet and inlet respectively.

U. Zone Modelling

- Zone model  approximate the smoke layer can be defined as one zone; the clear air underneath is another

- Heat and mass are transferred from lower zone to upper zone is treated as the third zone

- Heat release rate should be specified as a function of time (e.g. t² fire)

- Generally responds quick and useful in sample geometry

V. Field models

- Computational fluid dynamics (CFD) techniques

- Computational domain is divided into many small volumes

- A large set of partial differential equations to describe the chemistry of combustion, heat soot production of one volume and its neighboring volumes

- The problem is solved iteratively by numerical approach until the solution converge

W. Meshing (finite element approach)

- Some CFD models provide equation to determine the grid size. Others may require grid sensitivity study.

X. Fire Size and Growth

- Some of the CFD model can solve the fire chemistry and determine the fire size by its own

- However, pre-determining a maximum fire size can cope with the worst scenario but it should be determined reasonably with supports from literature

- T-square fire is usually adopted

Y. Smoke Generation

- With only heat release rate specified, the fire source is only a heating element. The soot yield rate should also be pre-determined since it will affect the visibility

Z. Boundary Condition Setting

- Boundary materials

- Extended regions should be provided at openings

- Patching of fire location

- The fire bed area

e.g. When door is opening, the pressure will reduce. Fire/smoke will be extract suddenly.

AA. Convergence

- Some of the CFD packages can terminate the simulation of the preset convergence criterion

- Some of the CFD packages (e.g. LES model) require user to determine the simulation time. To confirm the convergence, the time averages of two consecutive time frames should be compared

Reference:
1. NIST Engineering Laboratory
2. Fire standards and flammability standards developed by ASTM
3. A New Curve for Temperature - Time Relationship Compartment Fire, Thermal Science 2011.
4. International Journal on Engineering Performance-Based Fire Codes - PolyU
5. International Journal on Architectural Science - PolyU

II. Compartment Fire Behaviour & Fire Fighting 

A. Compartment Fire Behavior Training

B. Combustion Theory
- Heat: conduction, convection & radiation
- Oxygen: does not support combustion at >15%
- Fuel: Liquid, gas & solid (Pyrolysis); Only 25% of gaseous fuel will be burnt and the rest will be accumulated in the atmosphere

a. Pyrolysis
- Decomposition of a substance by heat
- Does not involve catalyst and oxygen
- Pyrolysis can start to be product at about 80°C
- At 150°C - 200°C pyrolysis will occur in wood

b. Fire Gases (Smoke)
- Non flammable gases - mainly CO2 and water vapour
- Flammable gases - due to pyrolysis and incomplete combustion, includes Carbon Monoxide
- Air - entrained in by rising temperature
- Soot - small solid particles of carbon

c.  Limits of Flammibility

Gas	Limits of Flammability (%)	Auto Ignition Temperature (°C)
Acrolein	3 – 31	278
Ammonia	16 - 25	651
1,3 Butadiene	2 – 11.5	429
Carbon Monoxide	12.5 – 74	609
Formaldehyde	7 - 73	430
Hydrogen cyanide	6 – 41	538
Hydrogen sulphide	4.3 - 46	260

Reference:
1. Resources CFB-US 
3. Publication and Code of Practices-  HKFSD

2. Fire safety engineering of structures (Blog)

Fire safety engineering of structures

DRAFT

Introduction

The relevant Building Fire Safety Regulations and Procedures in Hong Kong may be referred to HKU website: http://www.arch.hku.hk/teaching/firesafety.html.

The fire resistance period (FRP) in HK is specified in Section 6 and Table 2 of Code of Practice of Fire Resisting Construction 1996 (FRC).

A number of different approaches to ensure fire safety, which includes:

·        A prescriptive approach (deemed-to-satisfy rules)

·        A performance based approach to address a particular part of the design with the rest of the design following a prescriptive based approach

·        A full performance based approach

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(a) Prescriptive method defines a structural fire design fairly in terms of the materials used, shape and size of structural elements, thickness of fire protection materials and construction details etc.
PNAP 192 & FRC table A to F: Fire resisting material.
PNAP 195 & MOE: Provision of means of escape
CoP FSI: Fire service installation of equipment
PNAP 87 & BO 21(6)(d): Permanent Water Supply to Fire Service Installations
PNAP 182, MOA & B(P) 41, 41A, 41B, 41C & 41D: Means of Access for Firefighting and Rescue in Buildings
PNAP 202: Application of fire safety codes
PNAP 212 & Fire Safety (Commercial Premises) Ordinance: Prescribed commercial premises when floor area exceeds 230m². 
PNAP 302 & Fire Safety (Buildings) Ordinance: For composite building or domestic building

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Concrete

Three methods for design of concrete for fire resistance:

A. Tabulated data

B. Fire test
C. Fire calculation

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A. Tabulated data
Control of cover, finish and minimum dimension of elements:
(i) For BS8110-2, estimation of concrete cover:-
C ave = (A1C1 + A2C2 + ...) / (A1 + A2 +...)
where A1, A2,... = Area of reinforcement; C1,C2,... = Cover / distance from heated faces
(ii) In FRC 1996, consideration of design of fire resisting elements: -
Column/beam - exposure condition, steel ratio and minimum dimension (table E);
Steel column/beam - concrete cover, finish or gypsum board (table D)
Slab/stair - minimum dimension and concrete cover (table C & F)
Wall - Steel ratio & cover (table A) and finish (table B)
(iii) Applied finish for additional protection. i.e. gypum or gypum plaster board (Table B & D of FRC)
(iv)  Additional requirement for high strength concrete of grade greather than 60D according to s4.3 of CoP of Concrete 2004. The Buildings Department may impose the following conditions in approval letter
B. Fire test
(i) A quality assurance proposal is to be submitted with sufficient preliminary test results to confirm that reliable and consistent concrete can be produced. This should include a detailed assessment of the concreting materials, the mix design and the quality control procedures of the batching plant. In this respect the designed mean resistance to crushing shall initially exceed the specified resistance to crushing by a margin of not less than 12 MPa.
(ii) Adequate facilities are to be provided on site, for sampling the fresh concrete, making, curing and storing the test cubes.
(iii) An experienced and competent person is to be provided full time to supervise the whole work so as to ensure compliance with the approved plans.

(iv) All concrete arriving onsite shall come from a concrete supplier registered under the Quality Scheme for the Production and Supply of Concrete (QSPSC).

(v) Sampling of fresh concrete, making, curing, storing and compression testing of concrete test cubes should be carried out in accordance with the methods specified in CS 1:1990 by a laboratory accredited under HOKLAS for the particular test concerned. Both sampling and test results should be reported on a HOKLAS Endorsed Certificate and submitted within 21 days after sampling and/or testing.

(vi) At least one sample of concrete shall be taken from every ready mixed vehicle arriving on site. If concrete is site batched one sample shall be taken from every 10m³ of concrete produced.
(vii) Insitu core testing to the completed structure shall be carried out. Tests for compressive strength of concrete cores should be carried out in accordance with the method specified in CS1: 1990, by a laboratory accredited under HOKLAS for the particular test concerned. Test results should be reported on a HOKLAS Endorsed Certificate and submitted within 21 days after testing.
(viii) Qualified site supervision of the drilling of concrete core samples, by an experienced and competent person, should be provided. The person supervising the drilling of core samples should be indepedent from the contractor.
(ix) Details of any subsequent revision of the mix design are to be submitted.
(x) Consent to commence the work will not be granted until the following documents have been submitted and found satisfactory.
·        the quality assurance proposal and confirmation of the mix proportion to be adopted;
·        proposals for the onsite facilities for sampling of fresh concrete and making, curing and storing the test cubes;
·        the name of the laboratory accredited under HOKLAS, which will carry out the onsite and laboratory sampling and testing work;
·        the name and professional details of the person who is to supervise the works on a full time basis; and

·        proposal for insitu core testing of the finished concrete structure.