3.3. Structural Analysis The following is a summary assessment of preliminary structural design options: • Structural arrangements of the antenna primary structure leads to selection based on light weight of a rectangular grid beam arrangement, which is 15% to 30% lighter than either a triangular or radial arrangement. • Primary structural member evaluation considering columns of 5, 25 and 100 meter lengths concludes that primary structure caps should be built up as triangular girders. 18 meter long girders made up of 3 meter bays has been selected for cap members. • A determination was made that inertially induced deflections are insignificant, but thermal gradients must be kept low, in the order 2° to 4°C between the upper and lower caps of the primary structure. • Composites offer attractive features in terms of weight savings and thermal properties. • Other factors for future study are the integration of power lines and structure. 3.3.3.1 Antenna Structural Design Arrangements Three structural arrangements were considered as candidates for the baseline design (see Fig. 3. 3-31). • Rectangular Grid Beams - Primary beams at right angles to each other • Triangular Grid Beams - Primary beams arranged in such a manner as to produce geodetic structure • Radial spoke beams - Primary beams emanating from a central core and extending to the periphery’ as spokes in a bicycle wheel. The inertial loads applied to the antenna are relatively low, therefore the total structural weight is a direct function of total beam length. Assuming equal lengths of unsupported beams in each case, the total beam length for the three arrangements were generated. The ratio of their respective total lengths are shown in Fig. 3.3-32. The respective total lengths are approximately 15,760 meters for the rectangular grid, 23, 300 meters for the triangular and 21,330 meters for the radial. Figure 3.3-32 was generated to demonstrate weight relationships using an L/D (length of member/dia. of member) of 20 to 100. In each case aluminum with a thickness of 0. 02 in. (0. 05 cm) and a height between caps of 40 meters was assumed.
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