A Cylindrical Tank Is Assembled by Welding Steel Sections in a Heli-

A cylindrical tank is assembled by welding steel sections in a heli- cal pattern with angle $\alpha=50^{\circ}$ . Tank diameter is 1.6 m, thickness is
20 mm, and internal pressure is 2.75 MPa. Modulus E = 210 GPa and
Poisson's ratio $v=0.28$ . The normal stress acting perpendicular to the
Weld is approximately: (A) $39 \mathrm{MPa}$
(B) $48 \mathrm{MPa}$
(C) $78 \mathrm{MPa}$
(D) $84 \mathrm{MPa}$