Tham khảo tài liệu 'bruhn - analysis and design of flight vehicles structures episode 3 part 10', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | CH. 18 DIAGONAL SEMI-TENSION FIELD DESIGN In single uprights the outstanding legs are relieved to a considerable extent by virtue of the act that the compressive stress decreases with distance from the web the allowable stresses for single uprights are therefore somewhat higher than those for double uprights Because the forced crippling is of local nature it is assumed to depend on the peak value ill_ of the upright stress rather than max. on the average value. The upright stress at which final collapse occurs is obtained by the following empirical method - 1 Compute the allowable value of FUrnAY for a perfectly elastic upright material by the formula - For 2024-T3 Aluminum Alloy -Fu 21000ka 3 tu t I5 For double uprl gilts - 61a Fij 26000k2 3 tu t J1 3 For single uprights - 51b For 7Q75-T6 Aluminum Alloy - Fu 26000ka 3 tu t 1 3 For double uprights - 610 . Fu 32500k2 3 tự t x 3 For single uprights - old 2 If Fu exceeds the proportional limit for the upright material use as allowable value the stress corresponding to the compressive strain Fu E. Fu can also be obtained for various materials from Fig. . Natural Crippling Failure The term natural crippling failure is used to denote a crippling failure resulting from a compressive stress uniformly distributed over the cross-section of the upright. By this definition It can occur only in double uprights. To avoid natural crippling failure the peak stress m the upright F y in the upright should be less than the crippling stress of the section for L p 0. It appears that crippling failure does not appear to be a controlling factor in actual designs. General Elastic Instability of Neb and Uprights Test experience so far has not indicated that general elastic instability need be considered In design. Apparently the web system is safe against general elastic Instability If the uprights are designed to fall by column action or by forced crippling at a shear load not much less than the shear strength of the web. CH. 23 Web
