Assessment And Repair Of Fire Damaged Structures Engineering Essay

Assessment And Repair Of Fire Damaged Structures Engineering EssayThis chapter explains how a mental synthesis is assisted and bear oned after(prenominal) the aftermath of a bite. Often the initial response when looking over a fire- prostituted social system is one of despair and horror at the extent of damage. This situation is shown by the amount of non- geomorphological debris lying around together with the unpleasant smell of m any combustion products. In most cuttings, the damage is not as severe as is at premier(prenominal) thought, even though immediate decisions must be taken on the short-term safety device of the structure and whether any temporary propping is unavoidable or, indeed, whether some demolition work is necessary. This judgment will often need to be taken in truth cursorily after the fire and will generally be found on a visual survey and expert judgement. It should be pointed out that the assessment of fire damaged structures is very much a black art in that it relies heavily on experience..4.1 Visual InspectionThe aim of the visual inspection is to modulateStructural stability of the structure andThe extent and hardness of the fire.4.1.2 Structural StabilityIf possible, the original drawings for the structure should be consulted at this stage these allow assessment of how the structure transmits the applied warheading and enables the principal load necessitateing members to be identified, as tumesce as those providing geomorphological stability. The inspection needs to check any excessive deformation, excursion or cracking in the main load-carrying members and integrity at the linkups amidst the main members (1). It is vital to check for structural stability if excessive bowing of structural elements such as masonry cladding or internal compartment walls, which would be observed in the inspection stage of a structure. Anywhere the fire has only moved(p) part of the structure, it is crucial that the inspection also ex tend to any part of the structure not damaged indirectly by the fire it is possible that a signifi female genitalst redistribution of forces can occur into the un change part of the structure. For a example in the Broadgate fire scenario when the structure behaved during the fire in a totally contrasting manner to the way it was designed, in that forces were redistributed out-of-door from the fire by columns acting in tension to transmit forces to the relatively cool upper stories of the structure(2).4.1.3 love of fire severityThe first method of obtaining a rough estimate of the fire severity is by the use of the fire brigade records in equipment casualty of the number of vehicles called out, the length of time taken to fight the fire, the length of time between the fire being noted and the arrival of the brigade, the operation of any mechanical fire detection or fire fighting equipment and the degree of effort required to fight the fire.The second base method is to estimate the temperature reached in the fire by studying the debris caused by the fire and in that respectfore it is essential that no debris is removed until such a study is carried out in order to maintain evidence. Provided the reals generating the debris can be identified, the knowledge may be used to stop an indication of temperature reached, since most materials have known specific melt or softening temperatures. Table.1 gives typical melting temperatures of different materials that could be found in a fire according to the Building ResearchEstablishment (BRE).material BehaviourApproximate temperature (C)Softening or collapse of polystyrene120Melting of polystyrene250Alu arcminuteium softens400Aluminium melts650Softening of glass700-800Melting point of plaque800-1000Melting point of sliver950Melting point of copper1100Melting point of count iron1100-1200Table.1 melting point data (Source Parker and Nurse (1956) BRE)It is very important that c are is taken in consideration when u sing data as the temperatures varies over the height of a fire compartment at that placefore the original seat of a particular artefact is important. This method of assessment only gives anIndication that particular temperatures were reached but not the duration of characterisation to that temperature.The third method that is uncommitted to give an estimate in terms of both the standard furnace streak duration or a known fire, is to measure the charring depth on any sizeable fleck of timber known to have been exposed to the fire from the start of the fire. The charring depth can be related back to the standard furnace exposure since timber of known, or established, density can be assumed to char at a constant rate between 30 and 90 min standard exposure. The position of the timber specimen in the compartment should also be noted.A fourth method is to calculate the fire severity from estimates of the compartment size, the fire load density and the area of openings (ventilation factor). In practice, no one of the above methods is completely reliable and therefore a combination of methods must be used to give a reasonable answer.The visual inspection, once carried out, will have identified those areas which must be either immediately demolished (where the damage is beyond that heart-to-heart of being repaired) or those areas which may be capable of being repaired if sufficient competency can be attained (1). The inspection will also rank where there is no, or only very apparent damage. If repair of a structure is considered feasible, then a much more detailed investigation is required to ascertain the extent and severity of any damage and the residual medium of the structure. To do this, it is first necessary to clear all debris from the structure and to clean as much smoke damage as possible to allow an unhindered examination of all surfaces.4.2 Damage AssessmentIn order to carry out any assessment of damage on a brace structure a number of stages needs to be carried out. The first stage involves a complete fully detailed survey of the structure. The second stage ascertains the residual strength of both the individual members and of the complete structure.4.2.1 Structural surveyFor all structures, the first stage is to carry out, where appropriate, a full line and level survey. This is required to assess the residual deformations and deflections in the structure. The measured deflections should be compared with those for which the structure was designed. Care should be taken to note the effect of any horizontal movements due to thermal actions during the fire. Such effects of horizontal movement are often apparent away from the seat of the fire (Malhotra, 1978 Beitel and Iwankiw, 2005).In steel structures, since most structural steels regain more strength on cooling, there will be a slight loss in strength. However, the resultant deformations are likely to indicate the state of the structure. In this case, it is important to assess the integrity of the connections it is possible that bolts could have failed within the connection or could have become excessively deformed. Where the floors comprise of profile sheet steel decking and in situ concrete, examination should be made for any separation between the decking and the beams. This separation can unsounded occur even if thorough deck stud welding was used. Another potential point of failure is the shear stay between the decking and the in situ concrete. Fig 4.1 shows concrete separated away from the metal deck floor. Even with substantial damage of the types mentioned above, the structure may still be intact as demonstrated after the fire tests on the steel frame structures at Cardington (Bailey, 2004a)(6).Fig 4.1 Measurement of the gap of concrete gap after the fire (http//www.google.ie/images)Whilst carrying out the visual survey, attention should be given to the need for carrying tests on the structural materials to ascertain their residual stren gths. The testing methods used may either be non-destructive or involve the taking of samples from damaged portions on the structure, together with control specimens from undamaged areas.4.3 TestingThere are two approaches that may be used to assess residual steel strengths for steel. The first is to remove test coupons or samples and subject those specimens to a standard tensile test.Fig.4.1 shows test results for a piece of S350GD+Z structural steel. Great Care should taken in removing test specimens in that the damaged structure is not further weakened, and that again any necessary propping should be used.Fig 4.2 Tensile test results for structural steel S350GD+Z, the test pieces taken before and after high temperature compression tests, where the material reached temperatures up to 950C, (Y. C. Wang P6)The second is to use non-destructive tests of which the most suitable is a clumsiness indentation test usually measuring the Brinell hardness. There is a direct, sensibly linear, relationship between the Brinell hardness number (BHN) and tensile strength as shown in fig.4.2. It is important that care is taken in using this test since a number of results are call for before the strength estimates are statistically reliable.Fig4.2 Relationship between steel strengths and Brinell hardness number (BHN) (Kirby, Lapwood and Thompson, 1986, p 370).4.3.1 Residual strengthFor Grade 43A (S275) steel there is no residual strength loss based on the 0,2% proof s0tress when the steel is heated to temperatures up to 600-C but a 30% reduction at a temperature of 1000-C(5). The innovation in residual strength between these temperatures is sensibly linear. The pattern for Grade 50D (S355 J2) steel is similar leave off that the strength loss at 1000-C is only just about 15%. It should be noted that in all the tests, except for the American steel at 800-C, the measured tensile strengths exceeded the minimum guaranteed yield strength.Data on such steels are presented in Fig . 4.4 (Holmes et al., 1982), where it is seen that the yield strength for reinforcing steel shows an increase above ambient strength at temperatures below about 550-C, but a decrease at temperatures above 550-C. Pre-stressing steels show no change in strength below 300-C, but a substantial drop after this point such that at 800-C only around 50% of strength remains Wrought iron appears to show a marginal strength increase at temperatures up to 900-C and thus appears able to perform well in a fire provided however, that excessive deformations do not occur. Cast iron will also perform reasonably well un little undue man-sized bending moments are applied to the member during the fire. The good fire performance in real structures is in part due to the very low stresses to which cast iron members were subjected in design. One problem that can occur is that brittle failure is possible if cast iron is quenched by cold piddle from firemens hoses whilst still red-hot, or if additional load s are induced during the fire (7).Fig.4.4 Variation of residual strengths of reinforcing and pre-stressing steels with temperature (Holmes et al., 1982).4.4 Methods of repairAs far as steelwork is concerned, any repair will be in the form of partial replacement where the original structure has deformed beyond the point at which it can be reused. Where the steelwork is still intact, it is almost certain that the fire protection system used will need partial or total replacement. Any intumescent paint systems will sure need renewing.4.5 Demolition of fire damaged structuresClearly, the same safety hazards that exist for structures being demolished for reasons other than fire damage exist for those so damaged except that problems of stability are exacerbated for fire-damaged structures as the structure itself is naturally weaker, often to such an extent that little physical effort may be involve for demolition.4.6 Re-use of steel after a fireAn often quoted general rule for fire touc hed hot rolled structural steels is that if the steel is straight and there are no obvious distortions then the steel is probably still fit for use. At 600C the yield strength of steel is equal to about 40% of its room temperature value it follows therefore that any steel still remaining straight after the fire and which had been carrying an appreciable load was probably not heated beyond 600C, will not have undergone any metallurgical changes and will probably be fit for re-use. However, where the load in the fire was less than the full design load, and also with high strength steels, this cannot always be held to be true. In such cases it is recommended that hardness tests are carried out on the affected steel. In practice it is recommended that, in all instances, some hardness tests should be carried out. For grade S275 steel, if the ultimate tensile strength resulting from the tests are within the range specify inthe table 2 below, then the steel is reusable.Table.2Ultimatetens ilestrengths (sourcehttp//www.corusconstruction.com/en/design_guidance/structural_design)For grade S355 steel additional tensile test coupons should be taken from fire affected high strength steel members when hardness tests show thatThere is more than 10% difference in hardness compared to non-fire affected steelwork, orHardness test results indicate that the strength is within 10% of the specified minimum.Where deflections are visible, general guidelines on the maximum permissible levels of deflection to ensure satisfactory performance are difficult to specify. The amount of deflection or distortion must be checked so that its effect under load can be metric to ensure that permissible stresses are not exceeded and the functioning of the building is not impaired. Therefore every building should be considered as a separate case and the structural engineer involved in the reinstatement exercise must decide what level is acceptable to satisfy the relevant Codes.4.7 ConclusionIt can b e cogitate that the assessment of steel structures after a fire is crucial in order to judge the structural stability of the structure and seen can the building can be reused after the fire. poise structure can behave different that they have been designed for and this can have a effect on the structural stability of the building, for example the broadgate fire behaved in a different manner than it designed for. It is essential that testing of steel is carried out after the fire in order to see if the steel is capable of been reused.It can be conclude that for Grade 43A steel there is no residual strength loss based on the 0,2% proof stress when the steel is heated to temperatures up to 600-C but a 30% reduction at a temperature of 1000-C. The variation in residual strength and temperatures has a linear relationship as they are directly proportional to each other.1 Steel Construction Industry assembly (SCIF), 1991. Structural Fire Engineering Investigation of Broadgate Phase 8 Fir e, Steel Construction Institute, UK.2 Fire Safety Engineering Design of Structures Second Edition magic A. Purkiss BSc(Eng), PhD3 Outinen,J.Mkelinen,P.,2004.Mechanical properties of structural steel at elevated temperatures and after cooling Fire and Materials, 28 (2-4), pp. 237-251.4 Kirby, B.R., Lapwood, D.G. Thomson, G., 1986. The Reinstatement of Fire Damaged Steel and Iron Framed Structures, British Steel Corporation (now Corus), London, p. 465 Wang Y.C., Wald F., Trk A., Hajpl M., 2008. Fire damaged structures, in Technical sheets Urban habitat constructions under catastrophic events, shanghai Prask technika, Czech Technical University in Prague.6 Bailey, C.G. (2004b) Structural Fire Engineering Design Materials Behaviour- Steel, Digest 487 Part 2, BRE.7 Holmes, M., Anchor, R.D., Cooke, G.M.E., and Crook, R.N. (1982) The effects of elevated temperatures on the strength properties of reinforcing and prestressing steels. Structural Engineer, 60B, 7-138 Barnfield,J.R. and Por ter, A.M. (1984) historic buildings and fire fire performance of cast-iron structural elements. Structural Engineer, 62A, 373-80.4.0 Assessment and repair of fire-damaged structures4.1 Visual Inspection4.1.2 Structural Stability4.1.3 Estimation of fire severity4.2 Damage Assessment4.2.1 Structural survey4.3 Testing4.3.1 Residual strength4.4 Methods of repair4.5 Demolition of Fire-Damaged Structures