Context 161 - September 2019

46 C O N T E X T 1 6 1 : S E P T E M B E R 2 0 1 9 1850s Liverpool Lime Street Station, for exam- ple; but less common in utility buildings built in 1846. Slate-covered, timber king-post trussed structure was the convention for warehouses. So why did the dock engineer Jesse Hartley choose to use a different form of construction? At that time Liverpool was the second most important dock in the UK. Liverpool’s Old Dock was the first commercial wet dock to be formed in a city centre. Constructed in 1715, it proved to be a great success but by 1800 it was too small. The Liverpool dock trustees wanted to expand the port and chose to build a much larger dock out in the estuary. The 1841 trustee committee meeting minutes show that much discussion was held about the cost of insurance for dock buildings due to the high risk of fire and theft. They instructed their dock engineer Jesse Hartley to prepare a design for fireproof build- ings. Hartley responded with a superstructure made solely from brickwork and iron. The use of a timber roof would have compromised the fireproofing of the buildings, so Hartley chose an iron roof. Figure 1 shows the roof layout of the warehouse that would become the Maritime Museum. In the large room Hartley introduced a spine beam running parallel with the long axis of the building. These beams bear on to isolated cast-iron columns, reducing the roof span to 15.6 m. Arches are evident in the floors, walls and in the substructure of these buildings, so it is no surprise that he chose the same form for the roof. The chosen roof is sheer simplicity and elegance. Slender trusses placed at 5 ft (1.5 m) centres are constructed using a single curved 78 mm x 60 mm T-shaped top chord and an 18 mm diameter inclined eaves chord tie. Nine drop rods support this tie member. At the truss bearing, the web is forged to provide a flat surface which in turn sits on a 100 mm x 15 mm wrought-iron wall plate built on the inner side of the external wall. Above this bearing is a stone parapet concealing a gutter (see figure 2). There are no purlins or any form of plan bracing in this roof. Instead Hartley used flat wrought-iron plates, 10ft x 27 x 3/16 thick (3 m x 0.7 m x 5 mm). These plates are laid out in stretcher bond, overlapping on their longitudinal seams. They are riveted together and also to the trusses. In effect, Hartley produced an eggshell roof structure, having the advantage of being light weight, strong and capable of being deliv- ered to the site in small sections which could be easily erected by manpower alone. Our brief in 1982 was to assess the capacity of the buildings. Mersey Docks and Harbour Board, the then owners, had provided a number of record drawings from their archives. While Figure 3: Roof strengthening to the Tate Gallery Figure 2: Roof truss profile Figure 1: Maritime Museum roof truss layout