Fire Safety Design

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Fire safety design plays a very important role in the design of buildings. The publication examines the concept and role of fire resistance. It gives guidance when structural elements need to exhibit fire resistance, together with what and why fire resistance may be required. Design methods are reviewed showing how fire resistance may be achieved. A building fire, if unchecked, may cause considerable damage to the building and its contents. In extreme cases the building may collapse and be destroyed. The key to a buildings performance in a fire is the ability of the structural components or elements to resist the effects of a fire.

Structural steel is the first choice of architects and engineers for the framework of single and multi-storey buildings. Steel construction offers many advantages such as fast erection, wide clear spans, light foundations and the cost can be compared very favorably with other methods of construction. However steel structures are often still treated with a particular and unjustified distrust with regard to fire safety. Although steel is not a combustible material, experiments have proved it loses about half of its strength and stiffness at about 600C. ( Kirby,1986) Once the structure is in fire, severe damage can be made, which may be lead to a collapse of the building. If this happens, the evacuation of occupants and entry of firemen are seriously affected, resulting in grate loss of life ( Robinson, 1981) Steel structures can be designed to withstand a level of fire resistance and much research has been carried out to quantify the behavior of fires in buildings and other structures

Current fire design code is questioned by engineers in recent years for its inaccurate representation of real buildings in fire. Generally, BS5950 Part 8 and Euro Code 3 are being used to determine the fire resistance on different types of members. The structures in real fire demonstrate is different with predicted in design code. This is mainly because the code, where the interaction of an integrated structure is ignored, is based on isolated members heated under ISO 834 standard fire. In real building structure elements from part of a continuous assembly, and building fires often remain localized, with the fire-affected region of the structure receiving significant restraint from the cooler areas surrounding it.

In this essay, comparison will be made between current design code and real structural behavior. Typical steel members such as beam and column will be designed by using Euro code 3 and whole structure is modeled by using STAAD PRO under ambient temperature. For real structural behavior under fire condition, Vulcan is used for the analysis. Vulcan is based on the finite element analyze which is developed in recent years by the University of Sheffield.

  • Frame Design

The system proposed is a seven multi-storey structure (ground + 6 floors) and it is an office building. Each storey is 4m height clear spacing and there are 6 bays and 3 bays on plan. The total covered gross floor area is 10206 m2. Whole frame are modeled and designed by using STAAD PRO and it's according to Euro code 3 foremost.

The composite slab with profile depth is chosen which transfer loads on the secondary beams, from secondary beams load transfers to primary beams and then to column. For slab profile depth is 75mm and total slab depth is 180mm.With the span of 9m X 4.5m and with the live load of 4.5KN/m2 and dead load of 4.5 KN/m2 it is found suitable.

The load distribution has been adopted in such a way that the slab transfers its load on secondary beam. From secondary beam load has been transferred into secondary beams and then to the columns and from columns to foundations. It has been decided to make the joints between columns and beams are pinned in this moments does not transfer to the columns only load transfer to the column. The system has been designed elastically so that creation of plastic hinges is avoided it means that sections of the structures are considerably heavier in order to avoid any elastic failure.

After the decision of the plan the modeling has been made on STAAD PRO 2006. Main frame is modeled comprising of secondary beams, primary beams and columns. Composite slab is not modeled but its load and given live load has been applied to secondary beams as a uniformly distributed load. Similarly wind load has been applied on windward elevation only. It is applied on all nodes presents in windward elevation. It has been applied in horizontal direction pointing towards the building. Wind load at each node is 0.6KN/m2 X area (which is sum of half the distance from both side nodes in X direction to sum of half the distance from both nodes in Y direction)

Similarly notional horizontal load has been calculated which is 0.5% the total dead load of the floor applied with proper distribution on the nodes in this building presents in windward elevation only. Notional horizontal load is not applied on non windward side. Joints are assumed to be pin between column and primary beam, similarly pin joint between primary and secondary beams but the joint between column and foundation is assumed as moment resisting or fixed joint. Bracing system is provided in order to provide stiffness and avoid sway occur.

Figure 2-1: 3D model of the structure in STAAD PRO

Figure 2-2: Plan view of the structure

Figure 2-3: Front view of the structure

- 2 sprinklers are provided every 162m2(18m X 9m)

- Compartment such as fire door is provided every 225m2 in order to prevent the flames to be transmitted to other parts of the building.

-There are 2 fire fighting shaft for each floor. According to the fire regulation, 1 fire fighting shaft should be provided less than 900m2 with sprinkler system.

  • Literature review

    • Fire Safety Objectives

    Fire safety in buildings is concerned with achieving a number of fundamental objectives:

    • reduce risk of ignition
    • remove occupants
    • early fire brigade action
    • limit fire spread
    • limit fire severity
    • reduce the financial loss to the property
    • limit causes of death in order of priority

    How can the objectives be achieved:

    1) Prevent ignition (applies to life and property protection)

    a) Choice of material

    -structure, fitting and furniture, to be non-flammable to reduce the risk of ignition, fire spread and heat

    b) Building management and maintenance

    -provision of fire extinguishers, correct use of fire extinguishers

    2) Facilitate escape (applies to life protection only)

    a) Means of escape

    -the ability of rapid escape rapidly from burning building is well recognized as the most effective means of minimizing casualties.

    b) Education and training

    -staff training is important for public premises, where most casualties in non- domestic buildings occur but so too are clearly signed exit routes and smoke control.

    3) Prevent fire development and spread (applies to life and property protection)

    a) Active protection

    -sprinkler systems and firefighting shafts. These measures not only help to extinguish fires and limit fire spread but they also reduce smoke.

    b) Detection of smoke and heat

    -fire alarms provide early warning to building occupants and maximize escape time.

    c) Boundary wall conditions

    -separation distance to reduce the risk of ignition from fires in neighboring property.

    d) Compartmentation

    -Division of building interiors by fire and smoke retaining barriers is well recognized as a means of limiting the consequences of fire.

    e) Venting

    -releasing smoke and heat to the atmosphere is preferable to retaining them inside the building where they can endanger the occupants and hinder fire brigade action.

    4) Prevent structural collapse (applies mainly to property protection)

    a) Passive protection

    -normally only applied to steel and timber frameworks but sometimes to concrete structures.

    b) Structure design

    -significant levels of fire resistance can be achieved in steel famed structure without passive protection. Designer can influence fire resistance by his choice of member stresses, connections, interaction between members and other elements of construction and location of member inside or outside the structure.

    c) Fire engineering

    -quantitative methods of assessing the temperatures that will be generated in natural fires

    One of the more durable frameworks for fire safety assessment is the Fire safety Concepts Tree developed by the National Fire Protection Association (NFPA).

    Figure 3.1-1: Fire Safety Concepts Tree (http://fpemag.com/)

    Figure 3.1-2: Manage Exposed Branch of the Fire Safety Concepts Tree

    3.2 The requirements for fire safety

    According to the Fire Regulations, it is concerned with means of escape from buildings, the training of staff in fire safety, fire spread within and between buildings, and access for the fire services to fight fires. Since the regulations are made in the interests of public health and safety, they do not attempt to achieve non-combustible buildings.

    Fire resistance is used to characterize the performance of elements of a structure on fire. The fire resistance of an element is the length of time the element can continue to perform its functions. These functions may be the ability to not collapse, limit the spread of fire and support other elements. Approved Document B 2000 interprets the requirements of the building regulations and states that the stability criterion will be satisfied if the load bearing elements of the structure of the building are capable of withstanding the effects of fire for an appropriate period without loss of stability.

    For different types of building, the requirements are different. It can be classified into two main sections which are residential and non-residential. There are seven groups of buildings which are residential (dwellings), residential (institutional), office, shop and commercial, assembly and recreation, industrial and storage. For consistency, it is necessary to have a standard way of measuring these proportionately. These rules can be summarized as height, area, cubic capacity, numbers of storey and the height of top storey.

    3.2.1 Period of fire resistance

    The appropriate periods of fire resistance of fire resistance changes for different buildings, depending on their height and the type of occupancy. The height measured for determining the fire resistance of a building, is from the ground to the floor of the top storey. The height of the top storey is not included in the measurement. The definition is given in Figure 3.2-1 and the minimum period of fire resistance for elements of structure is summarized in table 3.2-1.

    Figure 3.2-1: measurement of building height to determine of required fire resistance Table3.2-1: Minimum periods (minutes) of fire resistance for elements of structure

    3.2.2 Protection of escape routes

    A building must be designed and constructed so that in the case of a fire there are means of escape capable of being used and effectively at all .Suitably located routes must be provided. Not all escape routes need to be protected by fire-resisting construction. Escape routes must have adequate lighting. Corridors serving areas where escape is only possible by a single protected stairway must have a minimum fire resistance of 30minutes. Beside that, the design must ensure exits are suitably signed and the ingress of smoke to the escape routes is limited. There are 3 mains criteria to design safe means of escape which are occupant capacity, travel distance and width.

    Table 3.2-2 : Minimum Escape Distances (meters)

    Table 3.2-3: Escape routes & exits Table 3.2-4: Escape stair width

    3.2.3 Compartmentation

    Compartment division must not allow flames to be transmitted to other parts of the building. This includes the roof. It is important therefore to consider the integrity of the roof slab as it becomes severely deflected under the action of fire. This deflection of the roof also affects the compartment walls. The design should consider this severe deflection of the compartment walls to ensure containment of lateral spread of fire. This can be done by designing the walls to take the transferred load from the slab, allowing for the expected movement at the slab wall junction without breaking the compartment boundary or transferring the load to the walls. Fire door is considered as one important element of the compartmentation. All fire doors should be fitted with an automatic self-closing device unless they are to cupboards or service ducts which are normally keep locked shut. The period of fire resistance of door is depended on the position of the door.

    Figure 3.2-2: Example of compartmentation

    Table 3.2-5: Minimum required area for compartmentation

    3.3 Methods of Fire Resistance

    3.3.1 Active method

    Active fire protection systems is a general term used to describe systems designed to control or extinguish fires, or to alert people to their presence. The system that makes the greatest contribution to structural fire safety and property protection is an automatic water sprinkler system. The use of active protection system has been promoted by insurers, who recognize the value of such systems in controlling fire growth and, therefore, in reducing the extent of fire damage in buildings.

    3.3.1.1 Sprinklers

    Sprinklers act as active fire protection systems. Sprinkles are designed to suppress automatically small fire on, or shortly after, ignition or to contain fires until the arrival of the fire service.

    Sprinkler systems can play an important role in reducing financial loss. More recently, it has been recognized that sprinkler systems, particularly those incorporating fast response sprinkler heads, can make significant contribution to improving life safety in a wide variety of building types. Research has shown that over 90% of fires are suppressed by four sprinkler heads or less.

    A reduction of 30 minutes in the required fire resistance may be applied to most types of non-domestic occupancies less than 30 meters in height when an approved sprinkler system is installed. All buildings over 30 meters in height are now required to have sprinklers.

    Most sprinklers in Europe work on the exploding bulb principle. The water nozzle of the sprinkler is sealed by a glass bulb containing a volatile liquid. This liquid expands when it is heated by fire and breaks the bulb, activating the sprinkler head and releasing water. The most common activation temperature is 680c. (Figure 3.3-1 and figure 3.3-2)

    Figure 3.3-1: Typical sprinkler Figure3.3-2: sprinkler head exploding

    There are few requirements to improve the reliability of the sprinkler system. The bypass system is used around the main valve sets to avoid the entire system being turned off during maintenance. To make it more safety, the limiting of the number of sprinklers heads in any zone to a maximum of 200. Beside that, water supply and the positioning of valves is being monitored to ensure that they are in the correct operational position. It should be had the management procedures to control maintenance of the sprinkler system.

    3.3.1.2 Firefighting shafts

    It is important that fire service personnel are able to gain access to buildings in order to reach the seat of the fire and to carry out effective search and rescue. In low-rise buildings without deep basements, this may be achieved by using the normal means of escape facilities within the building and by ensuring that firefighting appliances can get sufficiently close to facilitate ladder access to upper storey. In high buildings or building with deep basements, there will be a need for additional facilities contained within a protected firefighting shaft.

    Firefighting shafts should be located so that every part of each storey is within 60m of the entrance to a firefighting lobby measured along a route which is suitable for laying fire noses. From table 3.3-1, we can obtain the number of firefighting shafts to be provided in a designed building.

    Table 3.3-1: Provision of firefighting shafts in buildings

    3.3.2 Passive Fire Protection Material

    Passive fire protection materials insulate steel structures from the effects of the high temperatures that may be generated in fire. They can be divided into 2 types, non-reactive, of which the most common types are boards and sprays and reactive, of which intumescent coating are the best example.

    3.3.2.1 Board Protection

    Board system (figure 3.3-3) is the most popular type of fire protection in the UK. Insulation reduces the heating rate of a steel member so that its limiting temperature is not exceeded during the required fire resistance period. Base materials include ceramic fibers, calcium silicate, rock fiber, gypsum and vermiculite. Fire protection insulation can be applied by fixing boards to almost any type of steel member. Most products can achieve up to 4 hour rating. The protection board thickness necessary depends on the section factor (Hp/A) of the member and the fire rating required. Boards may be retained by a variety of methods such as noggins for I-section steel and a combination of steel pins or nails, special spiral 'screws' and sometimes a bonding agent. Lightweight galvanized mild steel internal framing members are used with the plaster board and calcium silicate board encasement systems. Longer fire resistance periods often require the use of multiple layers of boards.

    Advantages;

    • Boxed appearance suitable for visible members and suitable for further decoration
    • Clean dry fixing and may not have significant effects on other trades
    • Boards are factory manufactured thus thickness can be guaranteed
    • Often applied to non-primed steelwork

    Disadvantages:

    • May be more expensive and slower to fix than sprays
    • Fitting around complex details may be difficult

    Figure3.3-3: Board protection systems Figure3.3-4: board applied to a column

    3.3.2.2 Sprayed Protection

    Probably the most common combination of protection material is to protect beams with sprays (figure3.3-5 and 3.3-6) and columns with boards. This product also can achieve up to 4 hours rating. Cement or gypsum based materials containing mineral fiber, expanded vermiculite expanded perlite and other lightweight aggregates or fillers, are generally the least expensive forms of fire protection. Mineral fiber based materials are delivered dry to the spray head, where they are mixed with water and compressed air. Vermiculite or perlite sprays are usually premixed with water before being pumped to the spray head. Steel surfaces should normally be degreased, with loose mill scale and rust removed. Any primer applied to the steel should be compatible with the protection material. The coatings appear textured and are often susceptible to mechanical damage. Some may require additional surface protection to prevent air erosion, when used, for example, behind plenum ceilings, or to meet cleanliness requirements. When mechanical retention is required, steel chicken wire, weld mesh or expanded metal lath (EML) is mechanically attached to the steel surface by capacity discharge pins. EML is normally used to provide boxed protection to I-sections. Guidance on the use of sprayed cementations or gypsum based coatings, including thickness measurement.

    Advantages:

    • Spray protection can usually be applied for less than the cost of the cheapest board.
    • Rapid application
    • Easy to cover complex details
    • Often applied to non-primed steelwork
    • Some products may be suitable for external use

    Disadvantages:

    • appearance may be inadequate for visible members
    • overspray may need masking or shielding
    • primer, if used, must be compatible

    Figure3.3-5: Spray protection system figure3.3-6: Sprayed applied to beam

    3.3.2.3Flexible/Blanket Systems

    Flexible fire protection systems (Figure 3.3-7) have been developed as a response to the need for a cheap alternative to sprays but without the adverse effects on the construction program often associated with wet application. The blanket system is comparable with cheap boards. The fixing application is dry and may not have significant effects on other trades. For the appearance, it's unlikely to be used where the steel is visible.

    Figure3.3-7: Pin-fixed blanket applied to truss

    3.3.2.4 Intumescent coatings

    Intusmenscent coating system (Figure 3.3-8 and 3.3-9) are classified as either thin film which are the most commonly used of the intumescent systems or thick film. The majority of intumescent coating applications have been traditionally performed on-site, with off-side application restricted to special situation only.

    Intusmescent coatings are paint like substances which are inert at low temperatures but which provide insulation by swelling to provide a charred layer of low conductivity material at temperatures of approximately 200-2500C.

    There are 2 types of intumescents coatings which are thin film and thick film coatings. Thin film intumescent coatings are most commonly used in industrial and commercial buildings because they are easy to apply and provide a surface finish which can be aesthetically pleasing. It can either be solvent or water borne. The fire resistance period of the thin film intumescent up to 120 minutes. Thick film intmescents tend to be used where more onerous conditions exist, for example, in the petrochemical industry. Thick film intmescent coatings typically have a dry film thickness of up to 45mm for 180 minutes fire resistance, or 30mm for 120 minutes fire resistance.

    Intusmescent coatings are easy post protection fixings to steelwork. They are very attractive on out looking and decorative finishes are possible. In addition, it's easy to cover complex details. But it's more expensive if compare with the application of spray protection and it require blast cleaned surface and compatible primer before apply on the building.

    Figure3.3-8: Automated application Figure3.3-9: Manual Application

    3.3.2.5 Off-site fire protection

    Off-site fire protection, usually using solvent-based thin film intumescent coatings, it is increasingly used in UK. There are 2 applications of the coatings which are manually or automatically. The process offer number of specific advantages:

    • quicker construction time
    • removing the major trade off the construction path
    • simplified installation of services
    • ensuring high standards of finish, quality and reliability
    • eliminating site access and weather problems
    • removing the need to segregate or quarantine areas for fire protection application

    3.3.3 Inherent fire resistant

    In some condition, some structures can exhibit substantial level of fire resistances without applied protection if the members are not fully exposed to fire. The fire resistance of this method can be achieved up to 60 minutes. Generally, this method is applied on the structures which are special, low risk, circumstances that would logically justify its omission. Sometime, it maybe be more reliable than applied fire protection system structures, because it can be damaged or removed during the life of a building.

    3.3.3.1 Unprotected beams

    • Shelf angle floor beams:

    Shelf angle floor beams (figure3.3-10) use angles welded or bolted to the web the support a precast floor slab. The top part of the beam is protected from the fire while the bottom part is exposed by fire. The level of insulation provided increases as the angle is moved down the web. For lightly-loaded beams, it is possible to achieve 60 minutes fire resistance without any additional fire protection.

    • Slim Floor Beams:

    There are 2 types of slim floor option which are SLIMFLOR and slimdek system using an asymmetric beam (ASB) section. A SLIMFLOR beam is comprised with a welded plate between the column section and the bottom flange of the beam. Normally the plate is 15mm thick and floor slab may be constructed using precast concrete slabs or a deep deck composite slab. By using an asymmetric beam, it's also used deep decking but removes the support plate. The bottom flange is wider than the top flange and is used to support the floor slab. The beam is effectively built into the floor and this provide up to 60 minutes fire resistance. For I or H section, generally the web is thicker than the flanges and it can sustain the weight of the building when the bottom flanges loses much of its strength. For hollow section, normally these beams are formed by a welded plate and the application is designed to form edge beams. Hollow section can achieve up to 120 minutes fire resistance without fire protection by filling with concrete and using some reinforcement.

    Figure 3.3-10: Shelf Angle Floor Beam

    Figure 3.3-11: Slim Floor Beam with deep decking

    Figure 3.3-12: Slimdek system using an ASB section

    Figure 3.3-13: RHS Slimflor Edge Beam

    3.3.3.2 Unprotected columns

    1. Block-unfilled columns (Figure 3.3-14)

    This kind of solution is a very lost cost method. Concrete is placed between the flange of the universal column. It can increate the fire resistance to 30 minutes. Longer fire resistance period are possible by protecting only the exposed flanges. Normally the exposed flanges are decorated by paint or other.

    2. Web-unfilled columns (Figure 3.3-15)

    These are same as block-unfilled columns which are filling unreinforced concrete between the flanges of a universal column. But the concrete is only filled between web stiffeners and held in place with nominal shear connectors but it does not extend to the connection area. Therefore, the connection area should be protected by the floor slab or fire protected when it's on fire. Normally it can achieve up to 60 minutes fire resistance. For the block-unfilled and web-unfilled columns, the load will be transferred to the concrete when the steel can not sustain the load because getting weakened by fire.

    • Partially encased column(Figure 3.3-16)

    This is a very high cost solution by filling up with reinforcement concrete between the flanges. But it can achieve up higher fire resistance that is 120 minutes.

    • Concrete filled structural hollow sections

    There are 2 types of solution which are same as above. One with filling with non-reinforced concrete and one with reinforcement concrete the resistance is given by 60 minutes and 120 minutes. In this case, the hollow section is exposed therefore it is being used for external columns and when making a feature of a column.

    Figure 3.3-14: Block-unfilled column

    Figure 3.3-15: Web-unfilled column Figure3.3-16: Partially encased column

    3.4 Fire Safety Engineering

    Fire safety engineering is the application of state of the art scientific and design principles to achieve defined target objectives by providing value engineered protection of life, property and the environment from the fire, while at the same time being commensurate with the client's overall project strategy. Sometimes it's hard to satisfy and achieve the requirement of the building regulations for their design and construction. It is also being influenced while buildings are becoming more innovative. Therefore, authorities have made a lot of research and increase knowledge of how real structures behave in fire to acknowledge that improvements in fire safety may be possible by logical approaches.

    The fire safety engineering offers fire precautions consultancy, fire engineering design and risk assessment. Their background includes fire research, building design, project management and fire brigade activity. Their experience includes application of fire safety design to a wide range of building types including dwellings, offices, schools, theatres, train stations, multi-use premises, industrial premises, hotels, hospitals, leisure complexes, car parks, sports stadium and more.

    According to the approved document B to the Building Regulations, it states that: a fire safety engineering approach that takes into account the total fire safety package can provide an alternative approach to fire safety. It may be the only viable way to achieve a satisfactory standard of fire safety in some large and complex buildings.

    Therefore, fire engineering enables the results of research and development to be applied to practical problems without having to go through the lengthy process of being formally recognized in an Approved Document.

    There are 3 important stage processes which fire engineering should take attention with that are predicating the heating rate and maximum temperature of the atmosphere inside the compartment, predicating the temperature of the steel member, predicating the stability of the structure. The stability of the members depends not only on the temperature it reaches during the fire but also on the applied load and the effects of any composite action, restraint and continuity from the remainder of the structure.

    As an example, fire engineering plays very important role in design of sports stadium. Generally, sports stands can no longer be described as simple bare steel, concrete and block work structures for the sole purpose of watching sport because clients are seeking alternative means of attracting revenue on capital outlay. Therefore fire engineering can overcome those problems which can not be found in the approval documents.

    There are many examples and modification of fire engineering factors that can be cited to justify an appropriate change in the design while maintaining acceptable standards of safety. Typical achievements include extended travel distances or occupancies coupled with smoke control to enhance means of escape, reduction in fire resistance and maximizing the benefits of sprinklers, extended compartment volumes and reductions in the number of firefighting shafts.

    3.5 Steel Property

    3.5.1 Steel Mechanical Property

    Steel suffer a progressive loss of strength and stiffness as their temperature increases. For steel the change can be seen in EC3/4 stress-strain curves (Figure 3.5-1) at temperatures as low as 3000C. Although melting does not happen until about 15000C, only 23% of the ambient-temperature strength remains at 7000C. At 8000C this has reduced to 11% and at 9000C to 6 %.( EC3 Part1.2, 3.2, Table3.1, Fig.3.1)

    Figure 3.5-1: Stress-strain curve at elevated temperatures for s275

    Figure 3.5-2: EC3 Strength reduction for structural steel (SS) and

    Cold-worked reinforcement (Rft) at high temperatures

    The results from figure 3.5-2 are based on an extensive series of tests, which have been modeled by equations representing an initial linear elastic portion, changing tangentially to a part-ellipse whose gradient is zero at 2% strain. When curves such as these are presented in normalized fashion, with stresses shown as a proportion of ambient-temperature yield strength, the curves at the same temperatures for S235, S275 and S355 steels are extremely close to one another. It is therefore possible to use a single set of strength reduction factor for all three grades, at given temperatures and strain levels. In Eurocodes 3 and 4 strengths corresponding to 2% strain are used in the fire engineering design for all types of structural members.

    3.5.2 Steel Mechanical Property

    3.5.2.1 Thermal expansion of steel and concrete

    In most simple fire engineering calculation the thermal expansion of material is neglected, but for steel members which support a concrete slab on the upper flange the differential thermal expansion caused by shielding of the top flange, and the heat-sink function of the concrete slab, cause a thermal bowing towards the fire in the lower range of temperatures. When more advanced calculation models are used, it is also necessary to recognize that thermal expansion of the structural elements in the fire compartment is resisted by the cool structure outside this zone, and that this causes behavior which is considerably different from that experienced by similar members in unrestrained furnace tests. It is therefore necessary at least to appreciate the way in which the thermal expansion coefficients of steel and concrete vary with respect to one another and with temperature.

    Figure 3.5-3: Variation of Eurocode 3/4 thermal expansion coefficients

    of steel and concrete with temperature

    3.5.2.2 Other relevant thermal properties of steel

    Two additional thermal properties of steel affect its heating rate in fire. Thermal conductivity is the coefficient which dictates the rate at which heat arriving at the steel surface is conducted through the metal. A simplified version of the change of conductivity with temperature, defined in EC3, is shown in Fig 3.5-4. For use with simple design calculations the constant conservative value of 45W/m0K is allowed.

    The specific heat of steel is the amount of heat which is necessary to raise the steel temperature by 10C. This varies to some extent with temperature across most of the range, as is shown in Fig3-5-5, but its value undergoes a very dramatic change in the range 700-8000C. The apparent sharp rise to an infinite value at about 7350C is actually an indication of the latent heat input needed to allow the crystal-structure phase change to take place.

    Figure 3.5-4: Eurocode 3 representations of the variation of thermal

    Conductivity of steel with temperature.

    Figure 3.5-5: Variation of the specific heat of steel with temperature.

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