The Greenland Library project is within Brasenose College Old Cloisters, a 17th Century Grade I listed building. Our work included transforming the original college cloister into a new multi-use room and extension to the library at first floor, improving circulation in the adjoining spaces and refurbishing and extending the basement archive.
The library was originally constructed at first floor in 1658 over the cloister which was a covered walkway, open to the elements, and served as a burial ground. In 1807, the cloister was infilled with a timber floor built over the original external flag stones with circular columns added in the 20th Century to support the existing oak floor beams, which in turn provide additional support to the library above.
We encountered many structural challenges, all of which were emphasised by working in a historically sensitive location with restricted access. An archaeological investigation uncovered over 40 graves beneath the cloisters floor, and so we used this information to determine the exact working spaces available to insert a new floor structure over. The graves were recorded and blessed so they could remain in place.
The original flag stones were recorded, numbered, lifted, repaired and re-laid onto the new suspended concrete floor. The proposal included an environmentally controlled void underneath the new floor for air distribution, with air drawn through trenches from the adjacent refurbished Deer Quad. However, the location of the graves and existing floor levels restricted the structural and service zones. Close collaboration between the design team led to the use of a steel grillage spanning the full width of the floor with a lightweight composite deck to provide clear service zones beneath the floor while providing support to the re-laid stones. To avoid any visual changes to the shallow vaulted ceiling profile, the retrofitted columns supporting the library oak beams were replaced with cradle beams and inserted between the original oak floor joists spanning the full width and raised into the ceiling.
Improved access between the adjoining rooms was a key part to the brief and was achieved by creating openings through the walls which were up to 1m thick. Some of the walls had 17th Century painting directly applied to the plasterwork and so preservation of this type of decoration was important. The team readjusted the sequence of works in such a way as to not disturb the plasterwork while forming the new openings.
A new small freestanding cantilever stone staircase has been inserted to connect the floors of the library. It winds around a central stone column suspended on an in-situ localised slab, supported on steel beams with mini piles located around the grave locations to prevent disturbance. The stone staircase required precise dimensions to fit between the existing oak support beams and was achieved by CNC cutting the stone treads. Each tread and column section was carved from one piece of stone and installed on top of the tread and column section below. A single Macalloy steel bar passes down the centre of the stone column and is tensioned to apply a compressive force to the column. This holds the treads in place and limits deflections, in particular when the staircase is asymmetrically loaded. Lateral restraint is provided by the library floors, with the vertical load taken at the bottom of the staircase.
The rebated joint between treads required careful detailing along its length and into the central stone drum to prevent any sharp corners where stress concentrations might build up.