Guyed masts with tubular shaft and one level of guys are taken into consideration. New equations are derived for global analysis. They are geometrically nonlinear and coupled. Conservation of mass of the members and overall stability of the mast with respect to overturning are the principles. The shaft is subject to wind action and second order forces due to lateral sway of the mast. Sag of guys is evaluated by cubic polynomials which satisfy conditions at the ends of guys. The forces introduced from the guys to the shaft are nonconservative because their directions change during deformation of the system. A scalar effective moment is defined for local instability of the tubular shaft because of interacting bending moment, axial force and shear force. Resistance of the cylindrical shell is assessed for the joint probability of elastic and plastic unstable behaviour. Resistance of the shaft is checked at the critical cross-section where the effective moment reaches its maximum. Deflection of the shaft at the critical point and components of internal forces amplify the effective moment.
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