What’s the problem?
Container handling quay cranes have existed for 54 years and the only collapse due to an earthquake occurred in 1995 at Kobe, Japan. Liquefaction caused that collapse. Quay cranes have a history of successfully resisting earthquake damage. However, this is only because cranes were light and one or two legs could lift a few inches off the rails with only minor crane damage. This is no longer the case; large heavy cranes are different. The forces required to lift a heavily loaded leg are so great that the crane cannot resist them without damage, perhaps even collapse, and the crane lateral load on the wharf may be large enough to damage the wharf.
The Japanese were first to recognize this problem. Since the Kobe earthquake, all new quay cranes in Japan are designed to resist major earthquakes. Only recently has the danger of crane collapse been recognized outside of Japan. The Ports of Los Angeles (POLA) and Long Beach (POLB) recognize that seismic forces from cranes can damage wharves. The ports’ recent codes have requirements that limit the crane’s impact on the wharf. However, though the codes limit the effects of the cranes on the wharf, they do not address a crane’s seismic performance.
Protecting the crane and the wharf
To protect the crane and the wharf, the usual practice in Japan is to add base isolation systems to the cranes at the wharf level. The isolation systems include a trigger, a hydraulic damper, and a reset device. The system is complex and expensive. Fortunately, quay cranes have a structural element that can be easily converted to a device that will protect the crane. A friction damper can be inserted in the diagonal to portal tie connection (see Figure 1). The friction damper works by allowing the connection plates to slide as the crane structure deforms in an earthquake. The damper provides a large clamping force on the sliding plates to dissipate significant energy during the earthquake. The phenomenon is similar to braking a moving wheel on a car.
For new cranes, the cost of the friction damper is relatively small. For existing cranes, friction dampers are an economical retrofit. The friction damper needs little maintenance and can be easily reset after an earthquake.
Friction dampers on container cranes
The idea and use of a friction damper is not new. A number of researchers have studied friction dampers. Egor Popov and Carl Grigorian published Energy Dissipation with Slotted Bolted Connection in 1995, reporting their study of bolted friction dampers. Friction dampers have already been used in a number of building structures. Four new container cranes with friction dampers are being delivered to APL terminals at the Port of Los Angeles in the summer of 2012. The cranes will have resettable friction dampers and are the first of their kind for quay cranes. The dampers will protect the cranes and the wharf.
The POLA and POLB wharf design criteria require that the wharf be functional after the ‘Operational Level Earthquake’ (OLE), with a 50 percent probability of exceedance in 50 years, and with no collapse after a ‘Design Earthquake’ (DE), a major earthquake level used for designing structures for life safety. However, most quay cranes were not designed to these criteria. The APL cranes are designed to remain operational after the OLE and to not collapse in the DE. Most likely, the cranes will be operational even after the DE, after the damper is reset and the crane frame is realigned by adjusting the restraint rods.
The POLA and the POLB codes
The codes contain requirements for both new container cranes and modifications to existing cranes to protect the wharves from overload during earthquakes.
The design of four APL terminal cranes presented many challenges. The project schedule was tight. Fabrication needed to start before the design of the wharf was complete. Consequently, many wharf design parameters were unknown during the crane design process. The new wharf was expected to be more flexible than a typical wharf. A reasonably stiff crane was needed to meet operational requirements. Making the crane structure unusually flexible to limit its impact on wharf response was not an option, some type of seismic isolation mechanism was required. Studies of several seismic isolation concepts indicated that friction dampers are the most practical and economical solution.
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