## Illustrate the theoretical principles behind the design and analysis of a multi$-$span continuous beam in civil engineering. Discuss the key factors that influence the bending moments and shear forces in each span, considering both uniform and point loads. Explain how these factors vary with different support conditions, such as fixed, pinned, and roller supports, and describe how the distribution of internal forces changes along the length of the beam. Additionally, delve into the methods used to calculate deflections and rotations in such beams, incorporating the concepts of stiffness and moment of inertia. Describe the role of the moment distribution method and the importance of understanding the influence lines for continuous beams in determining the maximum moments and shear forces under various loading conditions. Finally, analyze the implications of these theoretical concepts on the material selection and safety considerations in the design of continuous beams, and how these principles are integrated into modern structural analysis software tools.