Fire Protection - Assignment Example

FV2002 Fire Protection Assignment 1. a). The first equation dMf = 0.092Av√H (1) dt was developed by Kawagoe in (1958) to determine the burning rate of a fire in a room with a single opening. Av refers to the area of the single opening in m2 while the height of the opening is Hv in m. From this equation, the duration of the burning rate determined dividing mass of the combustible wood against the heat load. The second equation is used to calculate the heat release rate Q (MW). The first one is used to determine the burning rate. The second equation is Q = dMf/dt ∆Hc (2) Is used to determine the rate of heat release rate where dMf/dt is a constant which means the rate of heat release is remains constant throughout the burning period of the fire. This is so because the change in caloric value of the fuel is calculated for the whole burning period making the heat release rate is the same. However, the burning rate as calculated using equation (1) is not constant because since the mass of the burning wood changes so is the area and the height of the opening as the burning progresses making the burning rate to change in the process. The difference between the two is the change is caloric value of the fuel which when multiplied by the burning rate gives the heat release rate. b). Solution The specific heat of air: Cp = 1.03 kJ/ (kg.K) The flux of mass is dMf /dt = 0.09 (QpW2)⅓ H = 0.09(Q/1.5W2)⅓H The average temperature after flashover: T = Ta + Qp/( CpdMf/dt) = Ta + (Q/1.5)/0.09W⅔ HCp = 20 + (1053/1.5)⅔ 0.09 X 2⅔ X 1 X 1.03 T = 556.77 (OC) According to the medium t-square fire model, temperature development of the scenario can be calculated as follows: i. At zero seconds (0s) the before the fire starts, the temperature is the ambient temperature T0 = Ta = 20 OC The equation for getting the temperature is: T = Ta + Qp/( CpdMf/dt) The Qp can be calculated from the t-square model Q = at2 ii. At 50 seconds, Q = 0.0117 x 502 = 29.25W T50 = 20 + (29.25/1.5) ⅔ = 69.00 OC 0.09 X 2⅔ X 1 X 1.03 iii. At 100 seconds, Q = 0.0117 x 1002 = 117.00W T100 = 20 + (117/1.5) ⅔ = 144.06 OC 0.09 X 2⅔ X 1 X 1.03 iv. At 150 seconds, Q = 0.0117 x 1502 = 263.25W T150 = 20 + (263.25/1.5) ⅔ = 233.02 OC 0.09 X 2⅔ X 1 X 1.03 v. At 200 seconds, Q = 0.0117 x 2002 = 468.00W T200 = 20 + (468/1.5) ⅔ = 332.61 OC 0.09 X 2⅔ X 1 X 1.03 vi. At 250 seconds, Q = 0.0117 x 2502 = 731.25W T250 = 20 + (731.25/1.5) ⅔ = 440.00 OC 0.09 X 2⅔ X 1 X 1.03 vii. At 300 seconds T300 = 556.77 (OC) A table of developing temperature and time: Time (s) Temperature (oc) 0 20 50 69 100 144.06 150 233.02 200 332.61 250 440 300 556.77 The graph of temperature against time is shown below: 2. a). The spread of fire in and around the building will be by Convention of the air resulting from natural ventilation openings like the air conditioning ducts, the windows and the doors if they are open at the time the fire starts. Conduction of fire through the concrete and the steel walls may cause diffusion of heat from one side of the wall to the other and to the surrounding buildings. Radiation: Solar radiation from the shiny uPVC window frames and the heat radiating from the concrete and steel materials on the structure may assist in fire spread. b). Convection: This will occur in form of air movement resulting from the ventilation openings in the building such as the wooden doors, air conditioning ducts connecting each room and the glazed windows will cause the flames to rise so that they overhang the uPVC combustible plastic material framing the windows. The smoke driven by the central air conditioning ducts will also rise in the same manner adding to the convective heat transfer effect. The fire itself might also drive the free convectional currents which might circulate the air and smoke greatly increasing the convected heat transfer within the building. Radiation: A great deal of solar radiation shining through the shiny uPVC framed windows and the doors will add to the heating effect and assists in the spreading of the fire. The radiation will also dry any fuel available controlling the spread of fire. Smoke and heat trapped in the rooms will heat the floors and walls re-radiating the thermal energy to the burning and the unburnt wooden doors and the uPVC frames. Conduction: The fire might also be spread through conduction through the walls made of concrete and steel. Fire inside the rooms will provide a lot of heat input to the walls which diffuses the heat through them to the other side in a process of conduction. Steel is good heat conductor and the concrete walls can conduct heat to a certain extent. The conducted heat will reach the uPVC window frames and the wooden doors which easily catch fire and the spread of fire continues. c. Use of Fourier’s Law Q = -kΔT Where q is the local heat flux (W/2), ΔT is thetemperature gradient (K/m) K is the thermal conductivity (W/(m.K)). On integration on the materials surface: Q = -kʃs ΔTd, where q is the amount or heat transferred per unit minute. Integrating betwentwo endpoints at constant temperature Q = -kAΔT/Δx Where A is the surface area, ΔT is the temperature difference between ends, Δx is the distance between the ends. Solution ΔT = TA – TB = (585 + 273)K – (250+273)K = 235K Q = -kA ΔT/ Δx Q/A = -k ΔT/ Δx -1250W/m2 = - 0.76W/(m.K) × 235K/ Δx = -178.6W/m/ Δx -1250W/m2 = -178.6W/m/ Δx Therefore Δx = 178.6W/m/1250W/m2 = 0.143m 0.143m is the minimum thickness of the separated wall. 3. Building criteria for a) External wall construction When constructing a building, care must be taken to protect it against fire attacks and it if takes place, hot to reduce or minimize the rate at which the fire spreads. The construction of the external walls is very important because these walls greatly assist in reduction of fire spread by: Preventing the spread of fire from low storey to upper storey Confining the fire within the building until it burns itself out Inhibiting the spread of fire across the relevant boundaries of buildings. Because of the above listed importance of external walls in minimizing fire spread, there are criteria to be followed when they are being constructed. In addition to this, there are designs that have been approved to be adopted when constructing external walls for this particular purpose ((NFPA 221, 2006), pp 8-9)). They should be designed to resist fire for a log time as possible. For the design loads, the external walls must be able to resist a minimum of 5 lb/ m2 and they should be free standing modular external walls engineered and designed to meet the fire resistance needs. The walls should extend through the roof and end at a distance above the roof determined by a code. They should then terminate at the top of the roof with a sheet metal cap for protection against the fire elements (NFPA 221, 2006), pp 12-14). The materials for building the external walls should be made of concrete, reinforced concrete or concrete blocks of masonry unit. The fire barrier external walls should be constructed using partitions of the gypsum board. If there are any penetrations to be made through the external walls of the building such as cables, pipes and air columns should also be protected to reduce fire spread through the wall using the firestop assembly materials (Buchaman, 2005, p 56). However, the penetration of the external walls should not be done in such a way that its structure is weakened during a fire attack and collapses. Openings in the external walls like windows and doors should be fire window assemblies or fire door assemblies rated before being use. b) Roof coverings The roof coverings play similar roles in minimizing fire spread as the external walls. The only difference is that external walls prevent the spread of fire through the process of radiation to the adjacent buildings while the roof coverings do so by preventing the transmission of fire form one building to another. The designed criteria for the building of roof coverings as laid out in British Standard 5588 are: The roof should comprise of an insulated single layer membrane system insulated on a metal deck. The roof pitch must be more than the least level recommended by the manufacturer of the roof system covering. Soffits, trims, perimeter eaves cladding must be fixed by the manufacturer on steel purlins and supported on steel. The floor areas, stairs and lifts to the roof should be finely finished in a mastic asphalt or a single layer of the roofing membrane insulated on board of the metal deck. Rainwater pipes and gutters should be cloaked generally from the external altitudes. Exposed rainwater pipes should be finished naturally with aluminum and fixed in vertical blocks from the gutter to the gully (BS 5588-10, 2006, p 7). 4. a) The three components which comprise standard fire test are; Fire Furnace Test Specimen Measurement Equipment The fire furnace can be horizontal or vertical with different openings and capacity that accommodate any kind pre-prepared test specimen. The horizontal furnace allows for full acquisition of information on full specimens or smaller specimens which are consistent with the laid out standard for the measuring equipment. It comprises of ceilings, floors, roofs, access doors, penetrations and safe security boxes. The Vertical furnace allows loading of a full specimen and the capacity to cycle allowing for acquisition of data for testing. The results are affected by the walls, doors, windows, partitions and the expansion joints. The specimen goes through the furnace as the fire burns and after being heated for certain period of time, the temperature on the backside of the wall of the furnace is measured. b). The fire resistance of a building structure is standard fire test refers to the ability of the building structure to withstand collapse in actual fire. Fire resistance depends on several factors as load intensity, member type (beam, column, wall), dimensions of the structure and the boundary end conditions of the building, incident heat flux from the fire on the wall structure, type of construction material (concrete, steel, wood) and the effect temperature rise has within the structure depending the different properties of the structure. Fire resistance the test specimen can be assessed against three criteria to terminate the test. These are Insulation: The average temperature on surface than is not exposed reaches approximately 140oC. Integrity: Cracks or openings occurring a separating element such that ignition occurs on the insulated side of the structure. Load-bearing capacity: The element being tested loses load-bearing capacity when the element is no longer able to carry the applied loading. c). The three drawbacks of the standard fire test approach are: Expense: It is expensive and time-consuming to carry out the standard fire test as compared to the analytical approach and the numerical method. Fire scenario limitations: The temperature in the furnace may not represent the real exposure to the fire elements and therefore the development of temperature in the test furnace is not equaled to the fire history in practice. Specimen limitations: The type and size of specimen in fire test are common such as the maximum height, width and mass. In addition, the standard fire test tests only the failure mode of a single structural element and does not apply to the failure of complex structure. 5 a). The strength of structural steel used to construct a building reduces with increase in temperature during the fire due to several properties of steel as listed below: The temperature increase affects the thermal expansion of the steel. The thermal expansion increases linearly to a point where the steel reaches the elastic limit at about 700oC and there is seen a sudden shrinkage in the steel with any further increase in temperature and the structure collapses. The elasticity of steel is also affected by the increase in temperature. With increasing temperatures, the steel elasticity is lost resulting in decrease of the strength and stiffness of the steel. at about 400oC, steel loses its stress-strain ability and starts to curve or bend and in the process the structure collapses. b). The fire protection measures of the steel structure include: 1. Insulation of the steel element with spray material or board type protection. 2. Shielding the steel elements with concrete or liquid forms of a heat sink. 3. Filling the hollow sections of the steel elements with concrete materials or liquids to form a heat sink. c). Question 5 (c) Structure strength reduction 0.60 times of the original stregth 100r = -10.0 + 0.064T Where r is the stregnth reduction and T temperature T (C) ISO 834 specifies the temperature time curve as T = Ti + 345log10 (1+8t) Where, Ti is the intial temperature; t – temperature; t(min) time Getting T 100(0.60) = - 10.0 + 0.064T 70 = 0.064T T = 70/0.064 = 1093.75 0.064Ti = 100(0.9) + 10 Ti = 100/0.064 = 1562. 5 Time 1093.75 = 1562.5 + 345log10 (1 + 8t) -468.75 = 345log10 (1 + 8t) Log10 (1 + 8t) = -468.75/345 = -1.4 (1 + 8t) = 10(-1.4) 1 + 8t = 0.04 8t = 0.96 t = 0.12 References BS 5588-10, (2006). Fire precautions in the design, construction and use of buildings — Part 10: Code of practice for shopping complexes , Her majesty’s Stationary Office Buchaman, A.H., (2005). Structural Design for Fire Safety, John Wiley & Sons, 2005 CISBE Guide, (1997). Fire Engineering, The Chattered Institution ofn Building Services, London. NFPA 221, (2006). Standard for High Challenge Fire Walls, Fire Walls, and Fire Barrier Walls, section 4.6 Read More