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Results from an investigation to determine the cause of the collapse of the L' Ambiance Plaza building on April 23, 1987 are presented. The building was being constructed using the lift-slab method; collapse occurred during construction. The investigation included on-site inspections immediately following the collapse, review of eyewitness accounts of the collapse, review of project documentation, laboratory and field tests and analyses of the structure. Several potential failure mechanisms were investigated. The most probable cause of the collapse was determined to be loss of support at a lifting jack in the west tower during placement of an upper level package of three floor slabs. The loss of support was likely due to excessive deformation of the lifting angle in a shearhead followed by a lifting nut slipping off the lifting angle of the shearhead. The postulated failure mechanism was duplicated in laboratory experiments. The local failure propagated as loads were redistributed. The remaining jack rods along column line E supporting the package of floor slabs slipped off the lifting angles and the slabs failed in flexure and shear. These slabs fell causing the lower level slabs to fail.

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