Structural Fire Engineering Engineering

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Integration of Structural Engineering into Fire Engineering Design LEUNG Siu-man Chief Structural Engineer Buildings Department

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Building (Construction) Regulation 90 – Fire Resisting ConstructionEvery building shall be designed and constructed so as to – Inhibit the spread of fire within the building and to nearby buildings by dividing the building into compartments; Provide adequate resistance to the spread of fire and smoke by the separation of different uses in a building by compartment walls and floors and by the separation of the building from any adjoining building or site; Maintain the stability of the building in case of fire; and Provide adequate resistance to the spread of fire over the roof of one building to another having regard to the position of the building. In other words, every building shall be designed and constructed so as to provide adequate resistance to spread of fire and smoke and to maintain its stability.

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High casualty fire incidents in HKGarley Building (1996) : 40 fatalities Mei Foo Sun Chuen (1997) : 9 fatalities Practice Note for AP & RSE 204 issued in 1998 : to provide guidance on fire engineering approach to meet the fire safety objective and performance requirements of B(C)R90

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Fire Engineering Design Active Fire Services Installation Passive Fire Design Fire Safety Management System Total Fire Safety in Buildings

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Smoke: A deadly hazard to life in a fireFire hazard chart

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Some measures in Fire Engineering Design to minimize the hazard caused by smoke

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Three dominant criteria to ensure that the fire resistant compartment is maintained so as to allow sufficient time for safe evacuation and rescue operation – The “Fire Resistance Period” conceptInsulation (I) to prevent developing excessive temperature on the unexposed surface of the building element; structural Integrity (E) to maintain the separating function in preventing spread of flame and smoke; ability for Load-Bearing (R) structural element to support the load under fire.

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Structural Fire Engineering generally comprises the consideration of three aspects : Modeling of possible fire scenarios Calculation of heat transfer to the structure Assessment of the structural response at elevated temperatures

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Example of a restrained partition compartment wallThe 3m high concrete compartment partition wall, which is constructed of d=100mm thick and restrained both at the top by a fire protected steel beam and the bottom by the concrete floor slab, is subjected to a natural fire condition with the fire exposed wall surface at a temperature T0 higher than the fire unexposed wall surface.

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Assuming there is negligible flexible movement at the top and bottom restraints, the restrained bow (or lateral thermal displacement) yR [1] 2 H T0 gap  H   For T0 = 900 C ;  = 9.0 x 10-6 /C; E = 20,000 N/mm2 H = 3000mm ; the gap at the top of compartment wall = 10mm yR = 132mm The restraint stress R E ( T0 gap ) H R = 95 MPa  (i.e. which is much greater than the crushing strength of partition wall of 20 MPa) [1] O’Connor D J, Structural Engineering design for fire safety in buildings. The Structural Engineer/ Volume 73/ No. 4, 21 February 1995

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Sensitivity study of parameter on integrity of compartment wall under fire condition(N.B. The basic case in bold : Gap = 10mm, L = 3m and T0 = 900 oC) From the sensitivity analysis, the following findings are observed: - Sufficient gap size would significantly reduce the restraint stress in the compartment wall The increase in compartment wall height would increase the restraint stress mildly whereas the increase in bow deflection is much greater The change in temperature would significantly change the restraint stress in the compartment wall

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Example of a platform supported by steel hanger rods

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According to BS 5950 Part 8, the limiting temperature of steel members in tension is as follow:Table 1

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The forces taken by each steel rod at fire limit state calculated by using a finite element program SAFE, the structural capacities of steel rods at ambient temperature provided by the manufactures, the calculated load ratio of the steel rods, and the respective limiting temperatures derived by interpolation of Table 1 are as follow:

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Deficiency The platform is supported by hanger rods which can only take tensile loads. Excessive relative elongation of individual rods heated up under fire may render these affected rods losing their supporting action (i.e. no longer in tension). This will cause a load redistribution and increase the load on the other remaining hanger rods jeopardizing their original fire limit state designed for.

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Common Problems FRP ratings are mainly evaluated for individual building elements only Structural aspect concentrated on strength and relied on FRP ratings as guideline on the use of respective element and material types Large deformation and plastic strains allowed in fire limit state design, but may hinder the overall fire performance of the compartment

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Thermal analysis relating to structural response of building elements ignored or not properly considered Structural Fire Engineering Engineering Gap

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Factors to consider for Fire Engineer & Structural Engineer(a) The selected fire cases should be critically reviewed to ascertain if they generally cover the worst scenario. (b) The derivation of the fire load should be conversant with the local condition rather than simply refer to overseas fire load data. Some unprotected steel members might be subjected to a temperature close to the limiting temperature in case of fire. However, the structural design of the steel structure should take into account the stress reduction factor of steel at elevated temperature. The derived surface temperature of the material subjected to elevated temperature has to account for the heat effects of thermal conductivity of materials, which may further raise the surface temperature. The behaviour of natural fire set up of an open frame structure in the laboratory may be different from the real fire on the spot. In particular, the prevailing wind and other actual building layout, which may accelerate and increase the temperature of the structural material, have to be taken into consideration. Calculation of fire resistance ratings, based on full compartment burn-out. Development of suitable acceptance criteria for the design and use of timber, steel, concrete and glazing and their material limitation.

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ConclusionImportance of other design parameters affecting the structural design in performance base approach to be observed Design criteria should be incorporated in the coming Code of Practice (Fire Engineering Design)

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Thank You - Q & A

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Last Updated: 8th March 2018

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