C O N T E X T 1 2 4 : M A Y 2 0 1 2 23 of movement sensors have been attached to it. These include some within floors supporting the decorative plaster ceilings. Purcell Miller Tritton (conservation architects) and Buro Happold (structural engineers) were appointed to act on behalf of Crossrail and the House of St Barnabas to assess the risks of damage to the historic fabric resulting from structural movement. Our conclusion is that the structure of the house (comprised of bricks in soft mortar) is relatively flexible. Any cracks caused by differential settlement are likely to develop at points of weakness such as through lines of window or door openings. However, the decorative plasterwork is not, for the most part, directly fixed to the brick walls, but spaced off the walls with battens and timber laths. Thus any crack in the walls will not necessarily translate directly into the plaster, as the stresses will be modified and spread through the secondary fixings. Similarly, the ceiling plaster is supported on laths fixed to ceiling joists that span between secondary joists,which in turn are fixed to secondary then primary beams before they reach their bearing on to the supporting brick walls. We checked the key of the haired plaster where we could from above. It appeared largely to be intact and in good condition. We carried out a finger test on the decorative ceilings and found no loose areas. We are therefore as satisfied as we can be that no decorative plaster is under threat of catastrophic failure should there be any differential movement. However, we have taken the precaution of designing and making an emergency kit which can be used to prop ceilings at short notice if concerns arise. Most at risk of failure in the event of even a small displacement in the structure is the fine stone cantilever stair. Cantilever stairs depend for their stability first on the secure bedding of the base tread (in this case on to the brick vault), and then by the resistance of each subsequent tread to over-turning by its embedment into the perimeter wall. Some years back, the author had to specify the repair of a staircase at Somerset House.The deflection of the supporting landing had led to the failure of two treads,which had become true cantilevers and had failed, cracking near the wall.Under these conditions they could have simply dropped out. In order to prevent similar failure at the House of St Barnabas, some inherent weaknesses in the treads were pinned, and a precautionary temporary steel frame was inserted beneath the stair,whose treads are cushionedwith a soft foam. This will prevent treads from falling if they do crack, allowing repairs to take place in situ. Whereas most of the decorative work is of haired plaster on laths, the north wall of the entrance hall has plaster applied directly on to the brickwork. Here there are two door openings, one above the other. Should there be differential movement in the wall, a crack is most likely to develop over or to one side of these openings.This could endanger the adherence of the plaster over the doors, which as a precautionary measure was screwed back to the brick structure to prevent de-bonding. Externally, the north elevation brickwork shows signs of the delamination of the facing bricks from the core of the wall. This potential weakness has been secured by introducing resin-bonded ties through the joints. By contrast with the 18th century house, the Chapel of St Barnabas has a different structural form. It has five apses constructed in thin-jointed sandstone ashlar. The ashlar extends to the inside too, though there does not seem to be a bond between the interior and exterior.The bedding joints are of lime-based mortar, but the thin joints make the masonry inflexible, and consequently any cracks are likely to extend through the stones. No mitigationmeasures are feasible, so careful monitoringwill have to suffice. However, internally there are decorative marble shafts, some of which are likely to be load-bearing. Several of these have diagonal natural fissures which could translate into lines of failure if eccentric pressure is applied to them. Our response has been to repair one detached shaft, which had already spalled, and some other shafts have been wrapped with shrink-wrap to hold the shafts together in the event of failure. The Catholic Church of St Patrick stands on the east side of Soho Square, constructed in 1893 of loadbearing brickwork with a stucco interior, to the designs of the Leeds architects Kelly and Birchall.The bricks are mortared with a hard cementitious mix which will cause the walls to act monolithically. Any differential ground movement is likely to be resisted at first, and cracks might develop some time afterwards.The building is unlikely to accommodate the movement in the way that we expect the House of St Barnabas to. The windows are a mixture of leaded lights within iron frames set into the brickwork and cast iron multi-paned windows. We believe that the leaded lights are likely to accommodate any movement. Even if the glass cracks, it is likely to be held in place by the lead cames and so is unlikely to represent a public risk. However, the cast iron windows could suddenly crack and cause glass to splinter, so we have recommended the temporary filming of the glass to hold it in place as a precaution. The expertise developed in the analysis of these very different buildings will soon be put to the test in Purcell Miller Tritton’s next study for Crossrail, which will consider the effect of underground works on individual buildings adjacent to Paddington Station. Nigel Sunter is a partner at Purcell Miller Tritton. The chapel on Manette Street.The bedding joints are of lime-based mortar, but the thin joints make the masonry inflexible, and consequently any cracks are likely to extend through the stones.
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