Seismic Performance of Cranes : A NEES Grand Challenge Project

Reginald DesRoches and Roberto T. Leon

Recent earthquakes have highlighted the vulnerability of cranes and other material handling equipment to damage resulting from even moderate ground motion. At the Port of Kobe during the 1995 Hyogoken Nanbu earthquake, cranes suffered extensive damage due to local buckling of plates and global buckling of the legs, often leading to collapse. Most of these cranes are unique structures from the design, fabrication and functional standpoints, and replacement for a failed structure can take more than a year. Their continued operation following an earthquake is critical to the operation of the port.

Cranes are usually designed as rigid frames with little or no seismic detailing, are fabricated from thin welded shapes, and are by necessity non-redundant structures. They typically have a large mass since they must be designed to carry heavy service loads and resist large environmental loads and berthing loads from ships. This results in large seismic forces being transferred to these structures.

Furthermore, these structures are often subject to large differential displacements being imposed at the base of the crane due to the liquefaction-related ground failures. The primary research issue to be considered here is the development of performance-based seismic design guidelines for crane structures, with special emphasis on the effects of large ground displacements. A second research issue to be addressed in this study is the potential use of protective systems for retrofitting crane structures. These systems would dramatically reduce the horizontal forces to which the crane structures are exposed during the earthquake, thereby protecting the cranes from damage and derailing, as well as minimizing reaction forces transferred to the wharf.

In order to further understand the seismic vulnerability of container cranes, detailed analytical models are being developed in OpenSees, a finite-element platform designed specifically for earthquake response analysis.  Both two and three-dimensional models of typical container cranes are being detailed to illustrate the complex nonlinear response.  Special emphasis has been placed on capturing the important dynamic coupling between crane uplift and elastic/inelastic response, as well as on evaluating the portal frame’s ultimate ductility.  Sensitivity studies using these models are being employed to identify key design criteria for both retrofit of existing cranes as well as recommendations for future cranes.