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Assessment And Repair Of Fire Damaged Structures Engineering Essay

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This chapter explains how a structure is assisted and repaired after the aftermath of a fire. Often the initial response when looking over a fire-damaged structure is one of despair and horror at the extent of damage. This situation is shown by the amount of non-structural debris lying around together with the unpleasant smell of many combustion products. In most cases, the damage is not as severe as is at first thought, even though immediate decisions must be taken on the short-term safety of the structure and whether any temporary propping is necessary or, indeed, whether some demolition work is necessary. This judgment will often need to be taken very quickly after the fire and will generally be based 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 Inspection

The aim of the visual inspection is to determine:

Structural stability of the structure and

The extent and severity of the fire.

4.1.2 Structural Stability

If possible, the original drawings for the structure should be consulted at this stage these allow assessment of how the structure transmits the applied loading and enables the principal load carrying members to be identified, as well as those providing structural stability. The inspection needs to check any excessive deformation, deflection or cracking in the main load-carrying members and integrity at the connections between 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 affected part of the structure, it is crucial that the inspection also extend to any part of the structure not damaged indirectly by the fire; it is possible that a significant redistribution of forces can occur into the unaffected part of the structure. For a example in the Broadgate fire scenario when the structure behaved during the fire in a totally different manner to the way it was designed, in that forces were redistributed away from the fire by columns acting in tension to transmit forces to the relatively cool upper stories of the structure(2).

4.1.3 Estimation of fire severity

The first method of obtaining a rough estimate of the fire severity is by the use of the fire brigade records in terms 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 automatic fire detection or fire fighting equipment and the degree of effort required to fight the fire.

The second method is to estimate the temperature reached in the fire by studying the debris caused by the fire and therefore it is essential that no debris is removed until such a study is carried out in order to maintain evidence. Provided the materials generating the debris can be identified, the knowledge may be used to give an indication of temperature reached, since most materials have known specific melting or softening temperatures. Table.1 gives typical melting temperatures of different materials that could be found in a fire according to the Building Research

Establishment (BRE).

material Behaviour

Approximate temperature (ËšC)

Softening or collapse of polystyrene

120

Melting of polystyrene

250

Aluminium softens

400

Aluminium melts

650

Softening of glass

700-800

Melting point of brass

800-1000

Melting point of sliver

950

Melting point of copper

1100

Melting point of cast iron

1100-1200

Table.1: melting point data (Source: Parker and Nurse (1956) BRE)

It is very important that care is taken in consideration when using data as the temperatures varies over the height of a fire compartment; therefore the original position of a particular artefact is important. This method of assessment only gives an

Indication that particular temperatures were reached but not the duration of exposure to that temperature.

The third method that is available to give an estimate in terms of either the standard furnace test duration or a known fire, is to measure the charring depth on any sizeable piece 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 capable of being repaired) or those areas which may be capable of being repaired if sufficient strength can be attained (1). The inspection will also identify 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 strength 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 Assessment

In order to carry out any assessment of damage on a steel 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 survey

For 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 still occur even if thorough deck stud welding was used. Another potential point of failure is the shear bond 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 strengths. 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 Testing

There 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 950°C, (Y. C. Wang P6)

The second is to use non-destructive tests of which the most suitable is a hardness 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 needed 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 strength

For Grade 43A (S275) steel there is no residual strength loss based on the 0,2% proof s0

tress when the steel is heated to temperatures up to 600â-¦C but a 30% reduction at a temperature of 1000â-¦C(5). The variation in residual strength between these temperatures is sensibly linear. The pattern for Grade 50D (S355 J2) steel is similar except that the strength loss at 1000â-¦C is only about 15%. It should be noted that in all the tests, except for the American steel at 800â-¦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 550â-¦C, but a decrease at temperatures above 550â-¦C. Pre-stressing steels show no change in strength below 300â-¦C, but a substantial drop after this point such that at 800â-¦C only around 50% of strength remains Wrought iron appears to show a marginal strength increase at temperatures up to 900â-¦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 unless undue large 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 water from firemen's hoses whilst still red-hot, or if additional loads 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 repair

As 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 certainly need renewing.

4.5 Demolition of fire damaged structures

Clearly, 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 needed for demolition.

4.6 Re-use of steel after a fire

An often quoted general rule for fire affected 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 600°C 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 600°C, 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 specified in the table 2 below, then the steel is reusable.

Table.2Ultimatetensilestrengths (source:http://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 that:

There is more than 10% difference in hardness compared to non-fire affected steelwork, or

Hardness 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 calculated 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 Conclusion

It can be concluded 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. Steel 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 600â-¦C but a 30% reduction at a temperature of 1000â-¦C. The variation in residual strength and temperatures has a linear relationship as they are directly proportional to each other.

[1] Steel Construction Industry Forum (SCIF), 1991. Structural Fire Engineering: Investigation of Broadgate Phase 8 Fire, Steel Construction Institute, UK.

[2] Fire Safety Engineering Design of Structures Second Edition John A. Purkiss BSc(Eng), PhD

[3] Outinen,J.Mäkeläinen,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. 46

[5] Wang Y.C., Wald F., Török A., Hajpál M., 2008. Fire damaged structures, in Technical sheets - Urban habitat constructions under catastrophic events, Print Pražská 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, 7-13

[8] Barnfield,J.R. and Porter, 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 structures

4.1 Visual Inspection

4.1.2 Structural Stability

4.1.3 Estimation of fire severity

4.2 Damage Assessment

4.2.1 Structural survey

4.3 Testing

4.3.1 Residual strength

4.4 Methods of repair

4.5 Demolition of Fire-Damaged Structures


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