Ultimate limit state based design versus allowable working stress based design for box girder crane structures

It is now well recognized that limit states are a much better basis than allowable working stresses for economical, yet safe design of structures. The objective of the present paper is to apply the ultimate limit states as the structural design criteria of a box girder crane. For this purpose, a reference box girder crane structure which was originally designed and constructed based on the allowable working stress criteria and still in operation is selected. The structure is then redesigned applying the ultimate limit state criteria, where other types of limit states such as serviceability limit states and fatigue limit states are also considered. Nonlinear finite element method is applied to analyze the progressive collapse behaviour of the structure until and after the ultimate limit state is reached. While the ultimate strength or maximum load-carrying capacity of the structure has never been realized as far as the allowable working stress based design method is applied, this study starts with identifying it by the nonlinear finite element method. The structural weight was then attempted to minimize by changing structural member dimensions while the ultimate strength remains the same. It is concluded that the ultimate limit state based design method provides more economical, yet safe structures, compared with the allowable working stress based design method. Considerations for the serviceability limit states are addressed in the paper, while those for the fatigue limit states are presented in a separate article.