Internal Forces

Definition

Internal forces are the forces that develop within a structure as a response to external loads, supporting the equilibrium of the structure. These forces are crucial for understanding how structures behave under loads and help determine the distribution of stresses throughout members.

5 Must Know Facts For Your Next Test

1. Internal forces can be calculated using methods like free-body diagrams and equilibrium equations, which help in determining the forces acting on individual members of a structure.
2. In statically determinate structures, internal forces can be directly determined from external loads and support reactions without needing additional information.
3. The method of sections allows for analyzing specific parts of a structure to find internal forces by 'cutting' through members and applying equilibrium principles.
4. Influence lines are used to determine how internal forces vary as loads move across a structure, helping in understanding critical points of stress and design considerations.
5. For indeterminate structures, internal forces require compatibility conditions in addition to equilibrium conditions, as there are more unknowns than equations available.

Review Questions

• How can internal forces be determined in statically determinate structures, and what role do free-body diagrams play in this process?
  • In statically determinate structures, internal forces can be determined by applying equilibrium equations to free-body diagrams of individual members. The free-body diagram visually represents all external loads and reactions acting on a member. By solving these equations for each member, one can find the internal forces such as axial forces, shear forces, and bending moments that ensure the member remains in equilibrium.
• Describe how the method of sections aids in finding internal forces within a complex structure and the implications for design.
  • The method of sections involves cutting through a structure to isolate a portion and applying equilibrium equations to solve for internal forces. This technique is especially useful for complex structures where determining forces directly from reactions can be challenging. By focusing on specific sections, engineers can identify critical internal forces, allowing for more informed design decisions regarding materials and safety factors.
• Evaluate the differences in analyzing internal forces between statically determinate and indeterminate structures, emphasizing the implications for structural stability.
  • The analysis of internal forces in statically determinate structures relies solely on external loads and support reactions since there are enough equilibrium equations available. In contrast, indeterminate structures require additional compatibility conditions due to the presence of redundant supports or constraints. This added complexity means that understanding how members interact under load is crucial for ensuring structural stability. Failure to properly analyze these internal forces can lead to unexpected stress concentrations and potential failure, highlighting the importance of thorough analysis in design.

Related terms

Shear Force: A type of internal force that acts parallel to the cross-section of a structural member, causing it to shear or slide apart.

Bending Moment: An internal force that causes a member to bend, resulting from external loads applied to the member, which creates a moment about a point along its length.

Axial Force: An internal force that acts along the length of a member, either in tension or compression, depending on the direction of the applied load.