Left: Photo6: Charredtimber truss,Doughton, Gloucs. Right: Photo7: Trussafter charringhas beenshot-blastedaway. scaffold and areas weakened by the fire supported. There is also the obvious added benefit that a protected building allows the remedial works to proceed uninterrupted by the weather. Normally the protection takes the standard form of scaffolding with a roof and side sheeting (photo 5), but where safe independent access is possible, and the surviving roof structure is adequate, a quick and economic method is to sheet and batten (photo 4) although this method can create problems in high winds. Wherever feasible it is preferable to have the scaffolding and temporary roof independent of the building to prevent any interference with the restoration work but in certain situations this may not be possible. Difficulties such as the location or the size of the building, the topography of the surrounding ground, or the cost linked to the insurance cover, may necessi_tateinternal supports to carry the temporary roof or horizontal ties through the building to brace the scaffold. Horizontal ties can be particularly disruptive as they impede movement through the building and can dictate the sequence of the remedial works, especially as they often cannot be removed until the later stages of remedial work. The next stage for the engineer is to evaluate in detail the structural adequacy of the surviving structure. It is likely that the fire will have uncovered defects that have remained concealed for many years and the judgement of the engineer becomes crucial in determining what CONTEXT 38 safely existed before the fire and whether or not the defect needs to be rectified. The fire may invalidate the argument that the defect has stood the test of time and it is also possible that the remedial works may airer the load paths through the construction. Material damaged by the fire, such as cast-iron and timber, will need a detailed assessment. There is a general belief that cast-iron always performs badly in a fire but from the evidence available in this country and the USA, and from the writer's own experience, the reputation is not justified. It must not therefore always be assumed that cast-iron which survives a fire cannot be retained, and the material can be tested to enable an assessment of its structural adequacy. The fire resisting properties of timber, however, are more widely appreciated, but the charring that provides the resistance also hides any structural defects that may exist. Shot blasting with a soft sand can effectively remove the charring without damaging the sound timber beneath. Photograph 6 shows a charred timber truss while photograph 7 shows the member after the charring has been blasted away. When this has been done the reduced section can be measure and the effect of any defects on the adequacy of the member assessed. Stonework that appears undamaged by a fire may suffer later from surface spalling when the effects of increased moisture, freezing temperatures, and micro cracking from the heat of the fire begin to take their roll. Immediately after the fire the role of the engineer is crucial in deciding how much of an historic building can be saved, but as the remedial proposals evolve the engineer's contribution becomes one of saving the surviving structure with the minimum of intervention and disturbance of the remaining fabric. Utilising the structure that remains is always preferred but there may be occasions where additional structural members have to be introduced, or a new floor or roof replacing those that have been lost may not be able to repeat the structural layout, materials, or sizes that existed before the fire. These problems may be further exacerbated by the building being under insured with the resulting pressure to select the cheapest construction; for example demolishing a weak wall and then rebuilding is sometimes considered more economic than strengthening the existing with specialist techniques. Insufficient insurance puts pressure on the reinstatement team and it is important to ensure that this factor does not adversely affect conservation decisions. From the moment the fire is extinguished to the completion of the restoration of the structure the structural engineer has an important contribution to make, and although finally most of the engineer's work will be concealed the survival of the historic fabric is the visible statement of what has been achieved. Keith Westonis Senior Structural Engineer with the ConservationEngineeringBranchat EnglishHeritage. 11
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