C O N T E X T 1 0 1 : S E P T E M B E R 2 0 0 7 9 SUPPORTING COLUMNS Repairing timber-frame buildings Elements of a poor timber structure can be left in place as long as there are adjacent timbers to redistribute the loads, writes engineer Brian Morton. Over the past six months I have been working on two timber-frame buildings, both proceeding in parallel. The emphasis has been on retaining as much of the individual elements of these listed buildings as possible. The obvious solution when working on buildings that are in a very poor condition, with sole plates and eaves wall plates in a poor state, is simply to take out those elements, replacing them exactly as existing. However, in my case, the two buildings are both listed and of some significance. The conservation officers concerned emphasised that saving the existing structure should be the first consideration. When looking at this situation, an understanding of the way structure works and how loads are distributed is vital, particularly in terms of load distribution from one structural member to another when one fails. The principal posts of a timber-framed building generally support principal beams at floor level, and trusses at eaves level. Alternatively, the principal posts can simply carry up to the underside of the eaves wall plate, supporting rafter couples without there being any principal trusses. If a post has failed, with not enough timber left in it to support the dead and imposed load, there is a tendency simply to take it out and repair it, or splice in a new section of post. In fact, in many instances when a vertical supporting member in a timber frame building deteriorates, the load distributes itself on to the timber stud posts. If it is due to a failure at high level, the load carries down through the building so that by the time it reaches ground level the load is probably more or less where it would have been if the existing post had still been strong. It is important not to change the current distribution of load at ground level, to avoid possible future foundation settlement. I remember some years ago being involved in a discussion about a timber-framed building in a conservation area, which the owner wanted to demolish. My advice was that there was little chance of that happening because, as seen from the inside, it was a substantial timber-framed structure and certainly worthy of retention. I was telephoned some months later by the owner. He had opened up the render facing the structure, which had been my rather tentative suggestion, to establish the condition of the timber frame. I went back to look at it. I found that the principal posts (I believe there were ten in all) had simply lost their bottoms.The loads had redistributed themselves around the studwork on to the existing sole plate and girding beam at first-floor level. There were no signs of imminent structural failure of the building, indeed very little cracking. In this case the hard render on the outside face of the building was simply a diaphragm holding it up. Loads do distribute themselves quite adequately around the frame. One can afford to leave in place elements of a poor structure as long as there are adjacent timbers to redistribute the loads. On one of the projects I saw a problem that often occurs. The eaves wall plate was in a very poor state, and the tenons from the rafters into the eaves wall plate had been lost, so the rafters did not transmit the horizontal load from the rafters into the wall plate any more. In addition to this, the room had a coved ceiling which we did not want to destroy. To add to the difficulties, the wall had bowed out in its length, perhaps by 150mm. Fortunately, the sliding of the roof meant that we had some spare space to get a new facing to the existing wall plate. Once the additional timber had been connected, we had a problem dealing with the fixings of the rafters down to that plate.We decided to place a piece of oak running along the length of the new piece of timber, flush with the outside face, making this timber 60mm2. We then cut away a section of the rafter at its foot, to accommodate this new section of timber which would satisfactorily transmit the horizontal The junction of the wall plate with the adjoining roof
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