Park Hill, a huge concrete-framed modernist social-housing scheme, was completed in 1961 when Sheffield had near full employment and young architects - in this case Ivor Smith and Jack Lynn - were developing new ways to satisfy the need for affordable flats for rent. Since then the national housing scene has been transformed, a change embodied in the fate of Park Hill, stripped back to its frame and recast for, largely, private ownership.Keith Collie's photographs capture the cliff-like grandeur and formal beauty of this massive structure in ruins and the epic scale of the renovation. David Levitt provides the background to the current renovation project by developer Urban Splashm and Jeremy Till's essay puts the Park Hill story into the wider context of architecture and the welfare state.