The headquarters of the Household Cavalry is the Horse Guards building — built 1750-9, Grade I listed — in central London. It was designed by architect William Kent and built by John Vardy after Kent's death. A museum has been created in the building that tells the history of the Household Cavalry, allowing public access for the first time. Our conservation and structural engineers worked on the detailed restoration and conservation of the historic structure, providing measured drawings, site investigations and structural engineering for the building's adaptation.
Of particular interest is the restoration works in the north pavilion gallery. Here, the ground floor is brick groin vaulted, to a height of 5m. The vaults, internal walls and chimneys of the space above are supported by a grid of six cruciform-plan stone columns plus the external walls. The upper level floor is a suspended timber structure over the brick vaulting below.
When the floorboards were lifted, the suspended floor structure was revealed to be Kent's original design, largely in tact with some insensitive modern replacement parts. The timber joints are very interesting, so in close consultation with English Heritage, they were surveyed and recorded in detail.
The pavilion had suffered 150-250mm differential settlement in the southeast corner and the north wall bulged outwards 60mm at first floor level. In the 1920s, the northwest corner and the north wall had been underpinned, and a concrete joist filler slab inserted into the suspended floor structure. The slab had successfully restrained the vaults from spreading since then.
Opportunities for investigating the suspended floor fully were limited at design stage, so we developed a scheme that would allow flexibility of response during construction. Sloping flat steel sheets were used to restrain various points of the upper vaults. The sheets were tied back to two horizontal trusses installed at upper floor level — the whole acting as a stiff diaphragm.
With the strengthening work in place, the concrete joist slab was sawn out in sections. Potential movement in the building for this operation was difficult to predict, so an extensive detailed monitoring regime was established before work commenced. No movement was observed during the works or after completion.