Final answer:
The reaction at built-in support A is a combination of forces and moments, including axial forces, shear forces, bending moments, and possibly torsional forces. This is due to the fixed nature of the support, resisting movements in all directions and rotations about all axes.
Step-by-step explanation:
When determining the reactions at a built-in support, you must consider all possible interactions between the support and the structure it's supporting. These interactions typically include axial forces, shear forces, bending moments, and sometimes torsional forces. A built-in support is fixed, meaning it can resist movements in all directions and rotations about all axes. Thus, it is capable of exerting a combination of forces and moments on the structure.
To analyze these reactions, one would need to construct a free-body diagram of the structure, identifying all external forces and moments. Equilibrium conditions require that the sum of horizontal forces, the sum of vertical forces, and the sum of moments (torques) about any point must each be zero. From there, one can solve the balance equations to find the reaction forces and moments at the support.
Therefore, the reaction at built-in support A would be a combination of forces, typically axial forces, shear forces, bending moments, and possibly torsional forces, depending on the loads applied to the structure.
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