Context 161 - September 2019

C O N T E X T 1 6 1 : S E P T E M B E R 2 0 1 9 47 of historical interest, they did not provide any further information for the structural assessment of the roof. As is common in buildings of this age, no calculations could be found. It is known that Hartley tested his floor structure prototypes to destruction and it is reasonable to assume that he did the same for his roof structure. The 1980s appraisal comprised a physical inspection, dimensional checks and the prepara- tion of calculations. The on-site checks showed very little structural damage to the roof, but the trusses were poorly tied to the walls and columns. The internal truss bearing is bolted to the spine beam but this has no physical connection with the isolated column below. The upper column bears on to the lower column using a socket and spigot so is free to rotate. One column was found to have fallen on to the floor below, yet the roof above was able to span 11.6 m for many years. The external wall plate had no connection to the brickwork and the truss had a knife-edge sup- port on to the plate. The structure is therefore free to move. In some cases the external wall was leaning as much as 150 mm in a 3 m height due to the lack of lateral restraint at this connection. The construction is essentially a back-to-back, simply supported span structure, so a single truss could be checked as a three-pin arch. The design was based on the existing dead weight plus a small allowance for the new insulation and turn- coated stainless-steel sheeting. Imposed loads were limited to snow and wind loading only. The checks showed that both the compressive and tensile stresses significantly exceeded the recommended historic allowable stress limits of 4 tons/sq in and 5 tons/sq in respectively. The new fabric allowance increased the total load by 25 per cent, yet the calculations suggested that the existing roof was inadequate for dead load alone. A full-scale load test was commissioned to prove the structure using the guidelines given in Appendix A in BS449: Part 2: 1969, ‘The use of structural steel in buildings. This standard gives advice for stiffness and strength-acceptance tests. It recommends the magnitude of load to be applied and the time for which it must be held to prove the structure. It is important in testing historic structures that the load is applied in increments and the performance of the element is carefully monitored for signs of distress. A secondary scaffold support system was built alongside the chosen truss(es) so that in the event that the test failed the adjacent structure was not compro- mised. Dial gauges were fixed to this framework. Kentledge load was applied incrementally using concrete paving flags supported on cradles hung from the top chord. The results of these tests allowed Curtins to judge that the roof could support the new loads without strengthening. A contract was let to Tarmac Construction in 1982 for two warehouses to be made watertight and for the repair of the structure. The lean on the top floor walls to one section of the Maritime Museum was an issue. Should they be dismantled and rebuilt? They were retained. A design was developed that incorporated an internal reinforced concrete eaves beam supported on new brickwork piers. Fortunately, this structure is hidden behind panelling in the museum’s kitchens. Following the Tall Ships Race in 1984, work on the Tate Gallery commenced. The structural issues were different in this building in that the roof spans parallel to the riverside external wall. A design was developed that made the roof structure act as a plate diaphragm to span the length of each warehouse room.The wrought-iron plates were strengthened at the top and bottom edges so that they act as a deep beam to transfer the wind loading on the external walls back to replacement structure at the three gutter lines. Wall plates have hidden fixings to the masonry so the structure is now well tied together. To most visitors the roof appears to be all original. Much of Jesse Hartley’s work was innovative, since he had a clear understanding of how struc- ture transfers load. What he did not know was that he had developed what a modern engineer would call a stressed-skin roof.The conservation work has merely enhanced his clever idea. A truss bearing on to a spine beam Anthony Clarke, a conservation accredited engineer, is a former technical director of Curtins.