Application and implementation of approximate explicit models in optimal structural design A strictly modular and flexible optimization package OPTIMA-S is introduced as a tool for shape finding and sizing. This package makes extensive use of standard systems for analysis (e.g. ASKA, LARSTRAN) and modelling (e.g. PATRAN). The optimizer is based on a behaviour model simulating structural properties. Various approximation schemes are available in the form of a library. The inclusion of second order effects is prepared. This system is applied to a series of problems embracing shape finding and resizing, weight and stress reduction. Beyond that also a shape redesign for a three-dimensional body is performed. Finally an adaptive behaviour model for optimization of structures exposing nonlinear behaviour is presented and applied to an introductory example.