Page 152 - Marine Structural Design

128 PART I Structural Design Principles

be defined by the analyst prior to post-processing the results. The user will generally have to
select the load factors prior to the definition of load combinations and ensure the inclusion of
the material factors.
When a fine mesh is modeled for a local detailed analysis, the loads and boundary conditions
may be taken from a more simplified analysis, which may either include the load factors or
may be supplied unfactored. Therefore, it is recommended that all basic loads be tabulated
along with the appropriate factors for the limit states considered. In this table, it should be
clearly stated whether the load factors are included in the basic loads.

7.2 Ultimate Limit State Design

The codes generally require that the ultimate limit state of the structure complies with two
conditions: ULS-A reflecting extreme permanent loads with regular environmental conditions,
and ULS-B reflecting large permanent loads with extreme environmental conditions.
The structural analysis may be carried out as linear elastic, simplified rigid-plastic, or elastic-
plastic.
7.2.1 Ductility and Brittle Fracture Avoidance
Ductile failure modes will allow the structure to redistribute the forces in accordance to the
structural model. However, regardless of the analysis method used, the model will not be able
to fully represent the redistribution of forces. The redistribution of forces in the structure will
avoid brittle fracture modes or at least verify their excess capacity in relation to the ductile
failure modes.
Brittle fracture should be considered in the following areas:
Unstable fracture caused by a combination of brittle material, high local stresses, and weld
defects
Details where ultimate capacity is reached with only limited plastic deformation thus
making the global behavior brittle
Unstable fracture may occur under unfavorable combinations of geometry, fi-acture toughness,
welding defects, and stress levels, with the risk of such failures being the greatest in steels of
high thickness (Le. > 4Omm) undergoing a deformation.
In general, the steel structure will meet requirements for adequate ductility when:
Material toughness requirements are met
Combinations of high local stresses and undetected weld defects are avoided
Details are designed to develop plastic deformation
Components do not exhibit a sudden drop in capacity when deformations continue beyond
0
the maximum capacity
Local and global buckling interactions are avoided
The maximum allowable defect size can be calculated based on the total stress or strains and
the design hcture toughness using a fracture mechanics approach. It should be shown that
both the maximum undetected defect following fabrication, and the maximum crack size
following fatigue loading over the design life of the structures will be less that the maximum
allowable defect size.