Arching action in beams and slabs: What is the principal effect of arching (developing compressive arch action with horizontal thrust) on the bending moment distribution along a member?

Introduction / Context:
When structural systems can develop horizontal thrust (e.g., due to restrained supports or compressive membrane action), they exhibit arching. This phenomenon alters the internal force distribution compared with pure flexure.

Given Data / Assumptions:

• Member capable of sustaining compression and developing thrust (e.g., masonry lintel, slab with edge restraint).
• Loads predominantly vertical; supports provide some horizontal restraint.
• Elastic behavior with modified load path due to arching.

Concept / Approach:
Arching shifts part of the load-carrying mechanism from bending to axial compression. The thrust line within the depth reduces flexural demands; if the line remains within the kern, tensile stresses reduce and bending moments lower overall.

Step-by-Step Solution:
In pure bending: internal couple M resists load effects.With arching: horizontal reactions develop; a compressive thrust T carries part of the load via a funicular path.Result: required bending moments M_arch are smaller than M_pure for the same external loads due to the axial component assisting load transfer.

Verification / Alternative check:
Comparisons of simply supported beams vs. two-pinned arches under UDL show lower midspan moments for arches due to thrust. Lintel arching over openings in masonry similarly reduces sagging moments.

Why Other Options Are Wrong:
Increase/no effect: contradict the fundamental mechanism of load sharing by axial compression and thrust line action."All of the above"/"Only changes shear": incorrect generalizations.

Common Pitfalls:
Confusing camber (geometric curvature) with arching action; ignoring the need for horizontal restraint to develop thrust.

Final Answer:
To reduce the bending moment throughout.