Walkway Collapse at the Botanical Gardens: A Study in Movement

On 19 December 2008, a large portion of the S-shaped, steel-framed walkway under construction at the Atlanta Botanical Gardens suddenly collapsed during concrete placement operations. The approximately 575 ft long cable-stayed canopy bridge was designed to be supported by four masts along the curved spline segments and two opposing inclined columns at the transitional straight spline segments. At the time of collapse, the walkway structure was supported on temporary shoring tower frames and helical piles. A detailed study of movement and structural behavior was used to investigate the cause of collapse. Construction photos made it possible to compare erected locations of shoring tower frames and grillage members with their post-collapse locations and stacked configurations. Spline impact marks on the ground and shoring members below showed varying degrees of post-impact translation at points along the length of the collapsed walkway structure. Type and directionality of bolt failures at spline beam splices and abutments, and angles of inclination of shoring tower support piles above the ground surface were also considered. The movement displayed in these various elements pointed toward likely locations of collapse initiation among the numerous shoring towers. This study of movement allowed for a focused investigation to establish probable cause of the collapse.