The behaviour of heated structures is strongly governed by thermal induced deformation and degradation of material properties. This book presents an augmentation of OpenSees to enable thermomechanical analysis of structures exposed to fire. OpenSees is an open source software framework developed at UC Berekeley and has been extended for analysis of structures in fire. The extension enable the 3D modeling of heated composite steel-framed structures. The performance of the proposed model has been comprehensively verified and validated against experimental and numerical results. The behavior of steel framed structures with composite slabs under fire conditions is investigated and recommendations for practical design are proposed.