Modes of failure of arches — identify comprehensive causes Which of the following can cause failure in a masonry or concrete arch under service or overload conditions?

Introduction / Context:
Arches carry loads primarily through compression along curved paths. Stability depends on the thrust line remaining within the arch’s middle third. Deviations due to settlement, excessive loading, or poor geometry can trigger various failure modes. Understanding these helps in assessment and retrofitting of historic and modern arch structures.

Given Data / Assumptions:

• Arch may be masonry or plain concrete with distinct voussoirs or a monolithic ring.
• Supports (abutments or piers) may move or rotate slightly.
• Loads include self-weight and superimposed actions.

Concept / Approach:
Uneven abutment settlement shifts the thrust line, introducing tension or excessive compression in zones, potentially causing cracks or hinge formation. Sliding of voussoirs occurs when shear along joints exceeds friction or when inadequate interlock exists. Crushing happens if compressive stresses exceed material capacity, often near the crown or haunches where thrust concentrates. Any of these can precipitate partial or progressive failure of the arch ring or spandrel.

Step-by-Step Solution:

Verification / Alternative check:
Limit analysis using the three-hinge mechanism criterion illustrates how settlements and loads can create hinge formation, corroborating these failure causes.

Why Other Options Are Wrong:

• Choosing only one mode ignores other common and interacting failure mechanisms.
• none of these: incorrect because all listed causes are recognized in arch engineering.

Common Pitfalls:
Assuming arch action guarantees safety without checking support movements; neglecting shear keys or dowels in voussoir joints.

Final Answer:
all the above