Photo: Dr. Jean-Paul Rodrigue, 2008.
The purpose of a container overhead gantry crane is to position containers along a linear stacking yard. Although the yard’s length can, in theory, be unlimited, the width of the stacking depends on the width of the crane, but six containers in width is a common configuration. Several gantry cranes can operate along a stack depending on the length of the terminal yard. Containers are usually stacked up to three in height, which enables to minimize rehandlings. A common stacking problem is the repositioning of containers within the stack, particularly when it concerns cargo bound inland. Trucks are coming somewhat randomly to pick an individual container, up until demurrage expires (4 to 5 days). This implies the need to continuously take containers out of the stack as customers come to pick up their containers. If the required container is at the bottom of the stack, those above it have to be removed, which involves non-revenue-generating moves.
In the above photo, there are two rubber-tired overhead cranes (RTG) servicing a stack with a capacity of six containers in width by three in height. The space on the left under the crane is used to load or unload a container from a chassis. The overhead crane has 40 tons in design capacity, which comfortably exceeds the maximum of 34 tons containers may have. It was manufactured by the Chinese company ZPMC, which is the world’s largest.