Collapse Investigation of a Concrete Garage with Precast Formwork

The expansion of an entertainment complex in the eastern US was under construction in 2003 when a portion of a multi-story parking garage collapsed during an 8th level concrete floor pour. This investigation was initiated to determine the cause of progressive collapse. Garage construction generally consisted of stay-in-place, precast, prestressed concrete formwork and cast-in-place composite floor system connected with mild steel reinforcing to cast-in-place concrete columns and shear walls. The collapse of an approximate 50 by 180 ft. area of the garage occurred during placement of cast-in-place floor system concrete at an upper level of the parking garage. A progressive failure of multiple levels of the stay-in-place precast formwork and cast-in-place composite floor system occurred, but nearly all perimeter columns and shear walls in the area remained standing. The authors were retained to investigate and evaluate the cause of the failure and study the progressive collapse. The investigation included site inspections of the garage collapse site and examination of garage structural components at an off-site location where debris was retained following demolition efforts. Visual inspection of the debris, review of eyewitness accounts, examination of construction documents, and structural analyses were used to evaluate the cause of the progressive collapse. Hand calculations and a three dimensional non-linear fracture mechanics model were also used to assess structural behavior of the composite reinforced concrete garage system. Strength of as-designed and influence of as-constructed connection conditions between the precast formwork floor system and cast-in-place columns and shear walls were evaluated. Results of the investigation were used to successfully assess the cause of the failure and make recommendations on how to avoid such accidents in the future.