1轴向拉压的应变能 F2l 2EA 轴力沿变化: F(x)dx 2EA (15.5) ∑ F 对桁架结构 72E1A 2扭转应变能 T TI 2G1, (15.6) d x 2Gl1.轴向拉压的应变能 l l EA F l V F l N N 2 2 1 2 = = 轴力沿x变化: = l N EA F x dx V 2 ( ) 2 对桁架结构: = i i i Ni i E A F l V 2 2 (15.5) 2.扭转应变能 T T l GIP T l V T 2 2 1 2 = = dx GI T x V l P = 2 ( ) 2 (15.6)