Ultimate Strength of Steel I-Beams at web openings is investigated. The study includes secondary bending moments, which is called the Vierendeel Effect, in the plastic design of I-beams at unreinforced, concentric and eccentric rectangular web openings. At the opening sections, four-plastic hinge collapse mechanism is assumed in the ultimate strength calculations. Although the analysis is carried out for rectangular openings, the expressions obtained here can easily be applied on other geometric shaped openings with some simple modifications. The numerical results obtained by the proposed methods are compared with available experimental ones. Expressions obtained in this study give satisfactory results.