General internal actions in an arch Under arbitrary loading and realistic boundary conditions, what internal actions can an arch rib develop at a section?

Introduction / Context:
Unlike a cable, which ideally carries pure tension, a rigid arch rib can carry a combination of internal actions. Understanding these actions is vital for sizing the rib and checking serviceability and ultimate limit states.

Given Data / Assumptions:

• Rigid arch rib (steel, concrete, or masonry) with finite depth.
• Arbitrary loading (dead plus live plus wind, etc.).
• Hinged or fixed boundary conditions typical of arch structures.

Concept / Approach:
Equilibrium of any free body cut from the arch requires three internal resultants in plane: normal thrust (axial force), shear force, and bending moment. Special geometries (e.g., a parabolic three-hinged arch under full UDL) may reduce bending moment to near zero, but the general case admits all three.

Step-by-Step Solution:
Isolate a segment of the arch and apply ΣFx = 0, ΣFy = 0, ΣM = 0.Solutions require internal axial (thrust), shear, and moment to satisfy equilibrium.Hence, the complete set of actions is present in general.

Verification / Alternative check:
Design codes for arches require checks for axial compression, shear, and bending (including interaction diagrams), confirming the presence of all actions.

Why Other Options Are Wrong:
Pairs of actions listed in other options are incomplete subsets; they omit at least one necessary component for general equilibrium.

Common Pitfalls:
Assuming an arch always follows a funicular line; in practice, load changes and construction tolerances introduce bending.

Final Answer:
thrust, shear force and bending moment