19.已知图示桁架各杆的拉压刚度为EA,试求各杆轴力。 A人 解:1X1+41 FN, ENL(FN)2 /3-V 2/|-31 3456 0 11/3 0 l/√3 0 F/√3 H/3/31 EA EA 解得F3=K1 F FM= F= F= FNs= F 20.已知桁架各杆的长度均为a,各杆的拉压刚度为EA,试求各杆轴力 解:1X1+41=0 静定基各杆轴力 FML= FM,= FM= FM= FNs= FNe=0 FN7= FN8= FN N9=F F Fm= F 单位力下各杆轴力 FNI= FN=FN3= EN4=Fr FN7= FN8= F FNI= FN =-1131 19. 已知图示桁架各杆的拉压刚度为 EA，试求各杆轴力。 解：  11X1 + Δ1F = 0 i FNi F Ni i l i i i F F N l N i F i l 2 ( N ) 1 F 3 −1 3 l − Fl 3 l 3 2 F 3 −1 3 l − Fl 3 l 3 3 0 1 l 3 0 l 3 4 0 1 l 3 0 l 3 5 0 1 l 3 0 l 3 6 F 3 −1 3 l − Fl 3 l 3 EA Fl l Δ EA = − + 11 = 1F , 1 3  解得 N3 1 N1 N2 N4 N5 N6 1 3 F F F F F F F X = = = = = + = = 20. 已知桁架各杆的长度均为 a，各杆的拉压刚度为 EA,试求各杆轴力。 解：  11X1 + Δ1F = 0 静定基各杆轴力 FN1 = FN2 = FN3 = FN4 = FN5 = FN6 = 0 FN7 = FN8 = FN9 = FN10 = FN11 = FN12 = F 单位力下各杆轴力 F N1 = F N2 = F N3 = F N4 = F N5 = F N6 =1 F N7 = F N8 = F N9 = F N10 = F N11 = F N12 = −1 F 30 30 30 30 F 6 4 5 1 3 2 F F 6 F 4 5 3 1 2 F X1 6 4 5 1 1 2 F 4 F F F F F 3 5 8 11 2 1 7 6 12 9 10 F 4 F F F F F 3 5 8 11 2 7 6 12 9 10 X1 4 3 5 8 11 2 7 6 12 9 10 1