The upper part of the European Extremely Large Telescope (E-ELT) altitude structure is one of the most critical areas of the telescope's structure. This part hosts sensitive optical elements of the telescope. Its structural performance has a major impact on the whole system. The most critical requirements are low optical path obscuration, high static and dynamic performance (high specific modulus), high mechanical safety (high specific strength), low wind cross section and low weight.Composite materials are ideally suited to meet these requirements. This study is carried out in order to quantify the relative advantage of composite material over mild steel, in terms of performance and costs. The mechanical behavior of the steel structure can be easily improved with a structure manufactured with composite materials. This structure is significantly lighter than the steel one and reduces relative displacements between primary and secondary mirror. Consequently, optical performance is improved, assembly process is simplified and transport cost is reduced.The support structure studied takes part of the E-ELT Altitude Structure, and works as the interface between the substructure supporting the primary mirror and the sub-structure supporting the secondary one. It is around 30m height, and a high effective stiffness is desired to minimize the relative movements between the mirrors. To achieve that, trusses legs internally braced with structural ropes are used. Figure 1 shows the configuration selected for this structure. This configuration has been selected after a deep optimization study based on stiffness criterion:

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