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