This research aims to theoretically derive a closed-form evaluation formula forthe coupled local buckling strength of I-shaped steel members under uniformbending. Power trigonometric functions are used to devise a particular displace-ment function. Because the function is fast-converging compared to the Fourierfunction, it is possible for the buckling deflection to be described using only twoterms of the trigonometric function. Subsequently, a closed-form evaluationequation is proposed based on the displacement function. An eigenvalue analysisbased on the finite element method is then conducted to verify the reliabilityand effectiveness of the proposed equa