Example $20\mathrm{.}1$

Part of a wing section is in the form of the two$-$cell box shown in Fig. $20\mathrm{.}4($a$)$ in which the vertical spars are connected to the wing skin through angle sections all having a crosssectional area of $300m{m}^2.$ Idealize the section into an arrangement of direct stress carrying booms and shear stress only carrying panels suitable for resisting bending moments in a vertical plane. Position the booms at the spar$/$skin junctions. in this question how we know the sigma$1$ sigma$2$ and sigma$3....$etc