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Strength Evaluation of Connectors for Intermodal Containers



Hai S. Lew, Fahim Sadek, E D. Anderson


Intermodal containers are used widely in marine cargo handling. In order to facilitate unloading of empty containers from ship to shore, two containers are lifted frequently in tandem. This operation is referred to as vertical tandem lifting. In this lifting process, the lower container is attached to the upper container by means of interbox connectors at the four corners of the container. During the vertical tandem lifting operation, if one or more connectors are disengaged the bottom box may be separated from the upper box, thereby creating an unsafe condition to workers below. This safety concern prompted the Occupational Safety and Health Administration (OSHA) to request the National Institute of Standards and Technology (NIST) to study the suitability of the use of of interbox connectors for the vertical tandem lifting.The NIST study included site visits to port facilities and laboratory tests of interbox connectors. Test specimens included semi-automatic twistlocks and latch locks. The experimental program included testing of twistlocks in tension, twistlock and latch lock assemblies with corner fittings in tension and compression, and shafts of twistlocks for the stress-strain relationship. In addition, measurements were made of the bearing area of the top and bottom cones of twistlocks and the latch on the inner surface of the corner fitting.
NIST Interagency/Internal Report (NISTIR) - 6557
Report Number


container, corner fittings, interbox connectors, latch locks, marine transportation, twistlocks, vertical tandem lifts


Lew, H. , Sadek, F. and Anderson, E. (2000), Strength Evaluation of Connectors for Intermodal Containers, NIST Interagency/Internal Report (NISTIR), National Institute of Standards and Technology, Gaithersburg, MD, [online], (Accessed July 18, 2024)


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Created August 1, 2000, Updated February 19, 2017