This lecture covers the principles of axial constraints and material deformations in structural engineering. It begins with the resolution of non-triangulated systems, emphasizing the distinction between compression and tension bars. The instructor discusses the geometric variations that can be leveraged in force diagrams, leading to a recap of triangulated versus non-triangulated systems. Key concepts such as stress, strain, and the elastic and plastic deformations of materials are introduced, along with their implications for structural integrity. The lecture also addresses the behavior of materials under different loading conditions, including the effects of scale on structural performance. The instructor illustrates these concepts with practical examples, including the behavior of various materials like wood, concrete, and metals under stress. The session concludes with a summary of critical equations related to rigidity and deformation, reinforcing the importance of understanding material properties in the design of stable structures.