Truss Analysis Assumptions Explained

Truss analysis is a method used to determine the forces in the members of a truss structure. Certain assumptions are made to simplify the analysis and make it solvable using basic principles of statics.

Understanding Truss Assumptions

Here are the common assumptions made when analyzing simple trusses:

• Pinned Joints: The joints connecting the members are assumed to be perfect pins, allowing members to rotate freely at the joints. This means members are subjected only to axial forces (tension or compression).
• Joint Loading: All external loads and reactions are assumed to be applied only at the joints. No loads are applied along the length of the members.
• Straight Members: The members connecting the joints are assumed to be perfectly straight between the joints.
• Rigid Members: The members are assumed to be rigid, meaning they do not deform under load. This assumption allows us to analyze the truss geometry as fixed. While members are elastic in reality and do deform, the "rigid" assumption is often used in the initial static equilibrium analysis to determine forces, ignoring the deflections.
• Elastic Material: Although rigidity is assumed for geometry in statics, the material is assumed to be linearly elastic, following Hooke's Law, which is important for calculating member forces.
• Uniform Members: Members are often assumed to have a constant cross-sectional area and homogeneous material properties throughout their length.

Analyzing the Options

Let's examine each option based on the standard assumptions of truss analysis:

1. Members are not rigid: As discussed above, a standard assumption in truss analysis (particularly for determining forces using statics) is that members are rigid (or at least the deformations are small enough not to significantly change the geometry). Therefore, assuming members are *not* rigid is contrary to a common assumption.
2. Forces are acting only at joints: This is a fundamental assumption in truss analysis. All external loads and support reactions are considered to act only at the pin joints.
3. The members of trusses are straight: This is another standard assumption. Truss members are idealized as straight bars connecting the joints.
4. Cross sectional area of members of truss is same through out their length: While not as fundamental as joint loading or pinned joints, it is a common simplifying assumption that truss members have a uniform cross-section and material properties along their length.

Conclusion

Based on the analysis, the statement that is not a standard assumption made in the analysis of trusses is that "Members are not rigid". The assumption is actually that members are rigid, or that deformations are negligible for force calculations.