In major building projects, concrete wall and floor slabs are often assembled using the in situ, or cast-in-place, method of construction. But a cheaper and faster alternative, that infamously played a major role in a tragic construction accident, takes the 'raising the roof' expression literally.

The Huron Towers in Ann Arbor, Michigan, utilized the lift slab technique, image by Dwight Burdette via Wikimedia Commons

The lift slab method of construction involves casting post-tensioned floor slabs on the ground and then lifting them into place. Successive levels are then cast on top of the previous slab and raised to their respective positions with hydraulic jacks. A parting compound between the slabs prevents bonding. Narrow openings around the columns allow steel collars to be embedded, which are then used for lifting the slab. The technique is ideal in buildings with conventional and repetitive floor plates. 

Construction of L'Ambiance Plaza, image via NIST

The lift slab method was fatefully implemented in the L'Ambiance Plaza development in Bridgeport, Connecticut. What was set to be a 16-storey residential project suddenly collapsed on April 23, 1987, killing 28 construction workers. A settlement was reached through mediation, avoiding long litigation, but also closing active investigations into the disaster. As a result, the exact cause of the collapse remains a mystery, though most observers point the blame towards lift slab construction. A nationwide investigation into the method was launched and the practice was temporarily banned in Connecticut. The collapse effectively sparked the end of lift slab construction in the United States.

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