Grillage is common types of structures in marine and land-based structural system. The plastic collapse modes of those structures are dependent on the loads, which are lateral, in plane or combination of those directions as well as boundaries. In this paper, a plastic analysis method is applied to grillages or grids under a single concentrated load to find the worst load point. The worst load point would be either at the central intersection or on the point of between intersections. In the present paper, general formulae for plastic collapse for the square grillages with simply-supported boundaries are derived.
The rational design procedure requires a sound and scientific background in the following key areas;
Accurate load prediction.
Definition of safety margins and design criteria.
Response analysis methods.
Synthesis and optimization.
In a certain structural system such as grillages in ships and ocean structural system, it is possible to predict the more accurate limiting condition by applying plastic analysis method (Baker, 1951–52; Boufounos, 1975; Kim, 1983; Kim & Park, 1995). In grillage design, the central intersection point load may be used as a worst loading condition especially with larger number of beams in each direction as long as local collapse is prevented. But, a point load may often move around on the grid system. In such case, the worst load point would not necessarily be at the central point. In this case, the worst load point is located between intersections To find the plastic collapse equations, upper bound theorem or mechanism method is applied to the square grillages with simply supported boundaries.
The principle of virtual work is so simple but surprising techniques of calculation can result from its use. If a body in equilibrium is given a set of small displacement.
