In 1907, the first span of the cantilever reached over the water, and it became clear to McLure that certain steel chords were beginning to bend. He sent a letter to Cooper, who was stationed in New York, and later met with Cooper to discuss causes of the bending. Cooper later sent a telegraph to the Phoenix Bridge Company to “add no more load to [the] bridge till after due consideration of facts”, but McLure resumed construction. At the end of the work day on August 29, 1907, the bridge collapsed killing 75 of the 86 workers on the bridge.
A Royal Commission was assembled to investigate the cause of the collapse, and the majority of the blame was on Cooper and Hoare. Cooper was at fault for not knowing what was happening at the worksite, and for miscalculations. The report found that Hoare should not have been appointed as the chief engineer because he lacked technical competence to control the work. Communication and organization were also blamed for this collapse.
The bridge was rebuilt, and on September 11, 1916, the bridge collapsed again killing 13 men. From an investigation by the St. Lawrence Bridge Company, the company assigned to build the bridge, the collapse was due to a material failure in one of the four bearing castings that supported the central span.
These collapses were the driving force in regulating the engineering profession. The Iron Ring worn by most engineers today is rumoured to be made of the steel that collapsed, to remind engineers of their importance and their duty. Some lessons learned by this collapse include: paying engineers well for their work, defining clear duties for individuals, discussing design plans in groups, communicating effectively, making sure the work is monitored regularly, and only assigning tasks to capable individuals.