Explanation:
In truss analysis, the assumption is made that the loading and the resulting forces are acting only in two dimensions, which means they are confined to a single plane. This assumption allows for simplification of the calculations involved in analyzing and designing truss structures.

Reasoning:
Trusses are structural systems composed of straight members connected at their ends by joints. These members are subjected to various forces, such as axial forces (tension or compression) and sometimes shear forces. To analyze and design truss structures, engineers use the method of joints or the method of sections.

Method of Joints:
The method of joints involves analyzing the equilibrium of forces at each joint of the truss. By considering the forces acting at each joint, engineers can determine the internal forces (axial forces) in the truss members.

Method of Sections:
The method of sections involves analyzing a section of the truss to determine the internal forces in the members that are cut by the section. This method is often used when the forces in specific members need to be determined.

Two-Dimensional Assumption:
The assumption that the loading and the resulting forces are acting only in two dimensions simplifies the analysis of trusses. By assuming that the loading and forces are confined to a single plane, engineers can neglect any forces that act out of this plane. This assumption is valid for many practical applications, as trusses are often designed to carry loads in a specific plane, such as the vertical plane in bridge structures.

Advantages of Two-Dimensional Analysis:
1. Simplifies calculations: By considering only two dimensions, the analysis of forces and the determination of member forces become simpler and more manageable.
2. Reduces complexity: Truss structures can become quite complex, especially in real-world applications. By assuming two-dimensional loading, engineers can simplify the analysis and design process.
3. Saves computational resources: Two-dimensional analysis requires fewer computational resources compared to three-dimensional analysis, making it more efficient and faster.

In conclusion, the statement that the calculations involved in trusses are in 2D is true. This assumption simplifies the analysis and design process for truss structures, allowing engineers to determine the internal forces in the members more easily and efficiently.