A dimensional approach to structural optimization is presented. Different from the usual analysis use of the concept, its synthesis use is developed to acquire the basic feature of artificial intelligence technology in that a problem is solved by matching it with a class of problem of which the solution is known. For physically similar structures expressible in terms of structural indices, a prototype can be optimally proportioned directly by using design charts one the optimal proportion of corresponding model is known. For a set of recommended input parameters such as strength and loading statistics, and a target system reliability, for a given offshore location which may be characterized by a design wave describable in terms of wave length and wave steepness, the approach is implemented in the generation of practical design charts for truss models of jacket structures. These charts may effectively facilitate layout selections in an early design situation.