27.槽形截面梁的受力及尺寸如图6.84所示。已知: _(2)=1.729times (10)^8m(m)^4 ,_(1)=-|||-183mm ,_(2)=317mm ,材料为铸铁,其许用拉应力 [ (a)_(i)] =40MPa ,许用压应力 [ 0,] =-|||-80MPa ,试求该梁的许用荷载[F]。-|||-F 日-|||-C-|||-14 2F B z-|||-1.4m 1.6m-|||-yi-|||-图6.84