The problem of optimal structural design is considered. The goal of de
sign is to minimize an integral functional which characterized the qua
lity of construction. Classes of optimal microstructures of composites
made of given materials are described and analysed. The proposed meth
od deals with the averaged description of minimizing sequences of mate
rials layout which physically are the composites with a special micros
tructure. In contrast with the known approach of the G-closure descrip
tion, the suggested procedure does not require the description of the
full variety of effective properties of an arbitrary composite, but in
stead selects the class of structures which are the only candidates to
optimal design. The approach is illustrated by the structural design
problems of conducting and elastic composites.