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Brian Morton relates some experiences with
fire protection and talks about problems he found
rebuilding a mill gutted by fire.


SUPPORTING
COLUMNS
This article relates some of my own experiences with reference to fire protection and the fire effects on structures whilst dealing with the practical problems associated with a fire at Little Braxted Mill in Essex.
In considering the strength and stability of an historic structure the conservation engineer is always thinking of ways of extending the life of the building by carry ing out the minimum amount of work whilst not impairing the visual factual historic value of the structure. Fire provisions can be an important part of this.
A few years ago we were involved in the complete restoration of two timber- framed buildings at the end of a terrace on High Street, Gravesend. These houses which had been converted into shops on the ground floor retained much of their original external weatherboard finish and internal panelling; the frames were in poor condition, the building racking down the hill necessitating extensive propping to the end of the terrace. Elemental parts of the timber frame needed to be repaired but our solution to the racking problem was to board the external face of the timber frame with marine quality ply directly beneath the weatherboarding which was taken off and replaced. There was some difficulty in adapting this method to the party wall with the adjoining slightly lower property, thus we internally lined the wall with ply then placed plasterboard onto this ply. Above the roof of the adjoining building the timber frame was weather boarded; the assumption was that some of the building would be left exposed on this gable. Towards the end of the six months snagging period the adjoining property burnt down, to our surprise, with very little effect to our buildings. There was a certain amount of smoke penetration, but without any doubt at all the internal ply facing to the party wall saved our structure.
In two places the fire had attacked the ply very seriously and the face had charred; there were only two areas where the ply had almost burnt through, and generally the plasterboard on the internal face was not affected in any serious way.
For many years the Practice has adopted the use of good quality ply sheeting (minimum 5-ply, if possible 7) to strengthen floors by underlaying timber boarding with it and also using it for strengthening walls. Using this treatment for floors where the joists are exposed and an appropriate number of screws is inserted linking the joists and the ply, fire resistance can be increased sufficiently to overcome the charring effect on the joists which the Code of Practice requires be taken into account.
When taking account of the period of fire resistance of a structural member and considering the charring effect there is generally not a great problem with hardwoods. This is because although one is required to allow for deduction in section due to charring, the Code also allows increased stresses during the period of the fire.
Generally the fire protection of the timber structures of historic buildings has not caused the practice many problems because of course the principal members are generally much larger in section than required by the Building Regulations. If there is a change of use to a building that requires higher floor loads there can be considerable difficulty. Very often we have found that where floor loads are to be increased stresses are satisfactory while deflection calculations indicate that the size of the member should be substantially increased. We have successfully argued that, when the governing criterion is fire, deflection is not important anyway because the requirement of the Code relates to limiting deflection to avoid damage to finishes on the soffit. Of course, this is not applicable in the case of a fire.
Another practical aspect that has to be taken into account was our experience at Little Braxted Mill, and also the effects of the fire at Hampton Court. Where timber beams have substantial shakes the fire penetrates those shakes as well as charring the outside of the timber causing a substantial reduction in the size of the timber member. The solution to this is to fill the shake with some compatible material.
Dealing with a fire that has

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