Seismic performance evaluation of port container cranes allowed to uplift
Kosbab, Benjamin David
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The seismic behavior of port container cranes has been largely ignored-by owners, operators, engineers, and code officials alike. This is despite their importance to daily port operations, where historical evidence suggests that port operational downtime following a seismic event can have a crippling effect on the affected local, regional, and national economies. Because the replacement time in the event of crane collapse can be a year or more, crane collapse has the potential to be the "critical path" for post-disaster port recovery. Since the 1960's, crane designers allowed and encouraged an uplift response from container cranes, assuming that this uplift would provide a "safety valve" for seismic loading; i.e. the structural response at the onset of uplift was assumed to be the maximum structural response. However, cranes have grown much larger and more stable such that the port industry is now beginning to question the seismic performance of their modern jumbo container cranes. This research takes a step back, and reconsiders the effect that uplift response has on the seismic demand of portal-frame structures such as container cranes. A theoretical estimation is derived which accounts for the uplift behavior, and finds that the "safety valve" design assumption can be unconservative. The resulting portal uplift theory is verified with complex finite element models and experimental shake-table testing of a scaled example container crane. Using the verified models, fragility curves and downtime estimates are developed which characterize the risk of crane damage and operational downtime for three representative container cranes subjected to a range of earthquakes. This research demonstrates that container cranes designed using previous and current standards can significantly contribute to port seismic vulnerability. Lastly, performance-based design recommendations are provided which encourage the comparison of demand and capacity in terms of the critical portal deformation, using the derived portal uplift theory to estimate seismic deformation demand.