Wednesday23 August 2017

concrete structures

MJP Architects’ Kendrew Quadrange at St John’s College, Oxford

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MJP Architects’ building uses precast concrete to particular effect in its two spiral staircases

One of the spiral staircases.

Nobel Prize-winning molecular biologist John Kendrew was also a former president and benefactor of St John’s College, Oxford University. His generosity and involvement with the college is why his name has been chosen for the latest and most substantial building to be built on the college grounds.

Kendrew Quadrangle, which is open to the south, is centred on a large and old beech tree. It is of similar proportions to the Canterbury Quad of 1636, one of the most famous in Oxford.

The quadrangle is the third project MJP Architects has carried out for the college, following an extension to the senior common room in 2005, and the larger Garden Quadrangle, finished in 1993.

Kendrew is a five-storey building (including basement) on land to the north of the college and bounded by Blackhall Road to the east and St Giles to the west, with the main entrance being off St Giles. The new building will provide accommodation for students and junior research fellows, as well as a café, library and archival facilities in the east wing and some teaching rooms in the west wing.

In-situ concrete has been used for the building’s main frame, selected for its durability, and its acoustic and aesthetic qualities.

Staircase during construction showing one of the bridges that connects to each floor.
Staircase during construction showing one of the bridges that connects to each floor.

Of particular interest are the two precast concrete spiral staircases, positioned in the east and west corners of the north-facing wing. Both flights of stairs are suspended from a central core and stop at first-floor level, creating a physical separation between the accommodation spaces above and the communal use of the ground floor.

Precast Staircases

The drama of the two precast spiral staircases is the way they float above the ground. Stopping the staircases at first-floor level and resolving how to hang each stair was a new challenge for structural engineer Price & Myers.

“As all the faces of the concrete are exposed to view, we had to find a way of making sure there were no visible structural connections between the precast elements,” explains project engineer Balazs Bicsak. “It’s all been disguised in a clever way that is more difficult to do but looks good in the end.”

The spiral staircase is made up of several precast concrete components all working together: the central core (the only element that extends from the third floor down to the piled foundations below basement level); the stair flights; the cylindrical outer shell; the cruciform at the top; and the three bridges that connect to each floor.

Belgium-based subcontractor Decomo produced a white precast concrete with white dolomite aggregate and white cement and coloured pigmented outer shell wall panels.

The precast stairs.
The precast stairs.

The 1.27m-diameter central core comprises two half circle walls, 250mm thick, with a 300mm-wide vertical slot between them. The central core walls are connected to each other by grouted dowel joints.

The outer shell’s circular external wall panels are 150mm thick and incorporate gaps on either side, drawing in daylight and providing views into the courtyard, as well as providing a connection for the bridges that tie the stairs back to the building.

The welded steel-frame bridges with concrete decks not only provide access at each of the three upper floors but also play a structural part in restraining the stair core. The bridge steelwork is cast into the precast concrete landings at each floor. To hold the outer shell together, vertical steel hanging rods cast into the wall panels at the factory (two rods per panel) are bolted together on site.


The stair flights and outer shell are hung from a welded steel cruciform structure which transfers the loads back to the core. The steel hanging rods that project out from the top of the outer shell are bolted to the cruciform arms.

The welded steel cruciform at the top of the spiral staircase is clad in precast concrete, and transfers the load of the outer shell back to the concrete core.
The welded steel cruciform at the top of the spiral staircase is clad in precast concrete, and transfers the load of the outer shell back to the concrete core.

After the external walls of the outer shell received their limestone cladding, a careful sequence of working was required to post-tension the hanging rods and transfer the weight of the structure off the temporary supports.

Project Team

Client St John’s College, Oxford University
Main contractor Kingerlee
Concrete subcontractor Decomo
Stone contractor Commercial Stone
Steelwork Bar West
Scaffolding K Services


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