Abstract: To study the effect of side column failure on collapse resistance of steel frames, two full-scale beam-column substructures were built and tested. The resistance mechanism, internal force change and failure mode of the specimen were studied by pushdown loading tests. For the side span of steel frames, the side column failure forms an ultra-long cantilever beam, which is prone to overall buckling due to compression on the lower flange. Lateral supports can change the failure mode into local buckling of the beam bottom and increase the bearing capacity. The impact of end plate thickness and beam length on the resistance mechanism were investigated by the finite element analysis, which shows that increasing end plate thickness and shortening beam length can enhance anti-instability performance. Considering that the constraint of semi-rigid joints on cantilever beams is weaker than that of rigid joints, the steel beam's calculated length needs adjustment in overall buckling bearing capacity calculations. A bearing capacity calculation model were thusly adopted to calculate the bearing capacity under different failure modes. By comparing the experimental results with finite element analysis ones, the accuracy of the calculation method is verified.