Semi-rigid frames are frames for which the connections joining the beams and columns are neither fully rigid nor perfectly pinned. In reality, all steel frames are semi-rigid in nature as all connections exhibit a certain degree of flexibility under loads. The use of the rigid m:: pinned connection model in frame analysis serves the purpose of simplifying the analysis and design processes but by no means represents the actual behavior of the structure. For cases in which the connections exhibit negligible deformation under service loads, the rigid frame idealization can be used without incurring noticeable errors. Similarly, for cases in which the connections possess negligible moment capacity, the use of the pinned connection model is often justifiable. However, for cases in which the rigidity of the connection falls between the two extreme cases, a more realistic analysis method which explicitly takes into account the effect of connection flexibility should be used.