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The building under survey was incinerated during a major fire after a gas explosion in the basement. Our evaluation began immediately after the end of the Fire Department investigation on the causes of the incident.
It is well known that fire damage extends beyond simple combustion. Table 1 (below) attempts to summarise the various mechanisms by which damage is caused.
In considering structural repair strategies in general, the basic choice to be made is that between demolition/renewal, and some form of repair. The choice of repair method is then a secondary decision. Schneider proposes a classification for the severity of damage, summarised in Table 2.
There is a wealth of information available on the repair of fire-damaged concrete structures. Reinforced concrete is a composite material and it is necessary to consider the effect of the fire on the concrete and the steel components individually, and on their composite action.
Steel and Cast Iron
Structural steel is less well served in the literature. The major problem is gross distortion caused by a combination of reduced yield strength, reduced Young's modulus and increased stresses above a critical temperature. This is addressed in a British Steel report, which notes a good recovery of properties on cooling.
The same report notes that cast iron performs well in fire. The main problem is that thermal shock from extinguishment water may cause cracking which has particular implications for the re-use of cast iron beams.
There is little information on masonry repair. Schneider notes that if there is no visible gross damage the strength of the bricks may be taken as similar to their original values, due to the high temperatures bricks undergo during manufacture, and that mortar behaves in a similar way to concrete. This is, of course, only true for clay brickwork. Calcium silicate bricks have more in common with concrete and it is appropriate to deal with calcium silicate brickwork, and concrete blockwork in a similar way to unreinforced concrete.
There is little or no information readily available on how timber performs in a fire, except for data on charring rates, which is available from several sources. The most authoritative is BS 5268 . Schneider places the decision between repair and renewal in damage Class 4 on structural grounds but acknowledges that Ã¦sthetic requirements may be decisive even down to Class 1 damage.
There is little published information on the reinstatement of non-structural components. Therefore, specifiers must work with 'ordinary' maintenance methods for the components in question, or rely on their own extrapolation of those methods.
It is likely that non-structural works are seen as less interesting, and less important than structural works, although survey respondents agreed strongly that smoke and water caused more damage than the fire itself, and such damage is often non-structural in nature.
Aim of repair and general requirements
The aim of the repair of a building that has suffered damage from fire is to be restored in its initial situation. The evaluation of the downgrading of the concrete construction and the method and the material of repair should be selected according to the requirements of official models and codes of construction of reinforced concrete. The carrying capacity, the functionalism and resistibility in the erosion and the fire should be compatible with the requirements of the buildingâ€™s codes. The codes, nevertheless, do not forecast re-establishment, and consequently the requirements are modified so that they correspond in each particular case. These adaptations should correspond to the principals of design of Reinforced concrete and they are based on the mechanical attributes of materials that are used for repair or reinforcement.
The following functional requirements should be considered for any repair:
The repair must protect the armament from the erosion during the expected life of operation of the structure
The material of repair should have the same resistance in time with the repaired material
The repair it is supposed to restore the required carrying capacity and to ensure acceptable deformity
The repair it is supposed to ensure the resistibility in fire of the construction, as it is required.
Restrictions in the repair of concrete structures
If the damage is so serious that the existing bars of armament are completely revealed after the abstraction of the destroyed concrete and the sizes and the intervals of armament do not obey the recommended limits, then the existing armament should be ignored in the re-designing of elements. In extreme cases, the replacement of the entire element with conventional concrete can be necessary.
The quality of repair that can be achieved into practice depends almost entirely from the level of constructional details and work done. In case of repair of concrete, the repair should not be considered that it ideally collaborates with the initial concrete. Methods exist (eg Î³-rays, ultrasounds) with which, the material of repair can be examined for the existence of imperfections (voids etc). The cost however of the application of these methods, in the entire structure is very high, while if they are applied, a fact that should be taken into consideration in the planning is the degree of precision that can be obtained.
The abstraction of destroyed materials
The work follows the model of demolition.
The units that should be removed entirely can be demolished with any of the usual methods, under the term that excessive disturbance is not transmitted in the remainder parts of manufacture.
Attention should be given in the effect the release of trapped tensions at the replacement of units in adjacent units. Armament of units that is repaired is removed with attention, so that it is caused as much as possible smaller damage in the armament that remains. At certain degree, the method of abstraction and the discrimination between healthy and harmed concrete are connected. Where possible, the regions that are demolished it should not be near the regions that are repaired and consequently it should passes a sufficient period of time between the repair of units and demolition, when the demolition concerns:
any work, if the demolition it is likely to harm or to influence the repair of a unit
any panel or plate that is supported or is connected with a repairing unit
Usually, the period that is interfered between the repair of the unit and the demolition it should be sufficient in order to ensure that the repair has developed the 50% of its characteristic resistance.
Line of work
The line of work should be programmed taking into consideration two ruling parameters:
resistance: no part of the structure can be overcharged beyond his carrying capacity
stability: any permanent or provisional work should never causes instability in any part of the structure
The line of work that is followed in new constructions, that is to say the work begins by the foundations and it is continued to upper levels, it is not necessarily effective in the case of repairs, even if this is the simplest method. In big and extensively destroyed buildings it can be possible and desirable to gain time by working simultaneously in more than one level, if this is allowed, under the term that the two previously reported parameters are ensured.
The choice of method
The engineer it is supposed to decide for the method of repair and to receive the necessary approvals before the detail planning. In first stage it is essential to appreciate the repair that will satisfy her criteria paragraph 6.1.
The engineer will be supposed then to examine:
the viability of repairs
The cost should be examined from the engineering with the usual processes calculation. The viability of the method of repair will depend from subjects as access, the place and the scale of repair. The engineer should estimate the suitability of every of the available methods of repair.
Methods of repair
The repair of a building harmed from any cause is, in almost every case, possible and has a specific cost. Depending on the classification of damage, the nature of repair can be selected. In the case where the repair from fire of the destroyed building is examined, the gravity of the damages will be not uniform but it will vary.
The re-establishment can oscillate from repairs of decorative character to important interventions in the structural system. Certain methods of repair which are applied are:
cleaning and colorations
repair of architectural elements
replacement of elements from steel
addition of armament
aid of structural system with addition of young persons of holder elements
repair with resin
repair with polymer materials
In all repairs, the preparation and the implementation of work are very important. Because there is a big variety of available technique, in a given case more than one solution of repair will exist. In a frame of general repair directives, specific methods of repair should not be given.
There are a lot of different methods for the repair of destroyed constructions by fire. First of all the criteria of repair should be decided, that is to say if the repair concerns in the re-establishment of
the resistance in fire
The selected method or methods of repair, often depends from the extent of the damage, the cost, the possibility of access and the available time. The methods of repair have been divided in the following categories:
Repairs with resins
Reconstruction of harmed building
This method of repair should be applied where there has been caused extensive damage, or where it is necessary the initial cross-sections of the elements to be restored and perhaps it is suitable when the possibility of application of other methods is difficult. The reconstruction requires the following steps:
Demolition: The demolition of heavy harmed structural elements should be performed according with the models and the codes that are in effect. The demolition of harmed structural elements should be drawn in order to be ensured that the requirements of planning are filled for both the temporary and the final new structure
Methods of manufacture: The materials and the work of manufacture should follow the codes in effect
Connections: Supposing that all destroyed portions of concrete is removed in the phase of demolition what is required is the usual preparation of manufacture of connection. The armament can be covered etc. mechanically, depending on the operation and the place of structural element in the manufacture. The welding of concrete categories of steel can cause loss of resistance and the calculation of such losses can be taken into account considering that it has the attributes of cast-iron.
The repairing resins are varied mixes of epoxy, polyester and acrylic mortar. The resins are often used for the repairs of lightly shelled regions and although they behave satisfactorily in regular conditions, available data do not exist for the behavior in conditions of fire, neither data from proportional experiments.
By the data that exist, included certain reports, is indicated that resin can be soften in relatively low temperatures (80 Â°C). Accordingly, it is likely that certain repairs with resin do not provide the sufficient fire safety in the armament and possibly they fail to maintain the structural sufficiency. Consequently it is recommended that the repairing resins are only used when:
Exist data that can show that the particular manufacture has sufficient fire safety and maintains her structural attributes in conditions of fire, or
The material is sufficiently protected against fire from other materials and maintains his structural attributes in expected temperatures of fire in the depth of cross-section, or
The loss of form of material will not cause not acceptable reduction of cross-section
For covering, various materials can be used as cardboard, metal coverings and boards. These elements are used for the re-establishment of appearance, fire safety and, potentially, resistibility. Where the structural re-establishment is required, materials of casing are used in combination with the concrete.