Failures of the cranes’ structural parts unavoidably lead to serious damages or total collapses; these accidents are often followed by very high financial losses and possibly serious injuries or crane-related fatalities. The objective of this research was to identify the causes that led to the failure of the hammerhead tower crane (xl425C) counterjib. The crane is used for assembly works at the hydropower dam. The counterjib collapse resulted from a gusset plate failure and caused such significant damage of the whole crane structure that the crane was dismantled and removed from operation. The study of the accident includes: (1) Identification of the stress-state, where a FEM model is developed to provide a useful tool for studying stress analysis; (2) Laboratory investigations are conducted in order to define the chemical composition and mechanical properties of the material, the tensile properties, hardness, impact toughness, as well as the metallographic analyses. The analysis of the obtained results showed that the principal reasons behind the gusset plate failure originated from design and fabrication faults. The working stress was higher than the allowable one. Also, impact toughness was too low and the fabrication of welds was incorrect.