Determining the internal loading in a beam is an essential step in the design process. It helps engineers understand how the beam will respond to external forces and moments, allowing them to design the beam to withstand these loads without failure. The correct answer to the question is option 'D', which states that the determination of the internal loading is usually done to design the beam. Let's explore this further.

Internal Loading in Beams:
When a beam is subjected to external forces and moments, such as loads or bending moments, it experiences internal loading. This internal loading consists of internal forces (shear forces) and internal moments (bending moments) that are distributed throughout the beam's cross-section. Determining these internal loads is crucial because they directly affect the beam's strength and stability.

Designing the Beam:
Designing a beam involves selecting appropriate dimensions, materials, and cross-sectional shapes that can withstand the anticipated loads and prevent failure. The determination of internal loading is a fundamental step in this process. By analyzing the internal loading, engineers can assess the maximum stresses and deformations experienced by the beam and ensure that these values are within safe limits.

Key Steps in Designing a Beam:
1. Identify the External Loads: Determine the types and magnitudes of the external forces and moments acting on the beam. These can include concentrated loads, distributed loads, and moments.

2. Determine the Support Conditions: Understand how the beam is supported, such as fixed, simply supported, or cantilevered. The support conditions affect the internal loading and beam behavior.

3. Apply Equilibrium Equations: Use the principles of statics to establish equilibrium equations for the beam. This involves balancing the external loads with the internal reactions at the supports.

4. Calculate Internal Shear Forces and Bending Moments: Apply the equilibrium equations to determine the internal loading at various points along the beam's length. This involves calculating the shear forces and bending moments.

5. Evaluate Stresses and Deflections: With the internal loading known, engineers can assess the stresses and deflections in the beam. This helps ensure that the beam can withstand the applied loads without excessive deformation or failure.

6. Select an Appropriate Cross-Section: Based on the calculated internal loading and desired material strength, engineers can choose a suitable cross-sectional shape and dimensions for the beam. This involves considering factors such as beam depth, width, and moment of inertia.

7. Verify the Design: Finally, engineers check the design by analyzing the beam's internal loading and stresses using appropriate structural analysis methods. This ensures that the beam will perform as intended and meet the required safety factors.

In summary, determining the internal loading in a beam is crucial for designing a beam that can withstand external forces and moments without failure. It allows engineers to evaluate the stresses and deflections, select suitable dimensions and materials, and ultimately ensure the beam's integrity and performance.