In stone arches built with stratified rock, how should the bedding planes of the voussoirs (arch stones) be oriented relative to the line of thrust? Select the best practice used to ensure stability and durability.

Introduction / Context:
Stratified stones possess natural bedding planes that represent planes of potential weakness. In arches, correct orientation of these planes is crucial for safely transmitting compressive thrust to the supports without promoting delamination or shear along bedding.

Given Data / Assumptions:

• Arch behavior is dominated by compressive thrust along a curved line (line of pressure).
• Stratified stones can split along bedding if loaded unfavorably.
• Voussoirs should seat firmly with compression acting favorably.

Concept / Approach:
Best practice is to set the stones so that bedding planes are radial—i.e., they point toward the arch center—making the compressive thrust act normal (at right angles) to the bedding. This reduces sliding on the planes and enhances interlock and long-term performance under variable loads and weathering.

Step-by-Step Solution:

Verification / Alternative check:
Historic masonry guidelines and conservation practices consistently specify radial bedding orientation to limit delamination, especially in weathered stratified stones such as sandstones and limestones.

Why Other Options Are Wrong:

• Parallel/perpendicular to the arch axis: may put components of load along bedding, increasing slip risk.
• None of these: incorrect because a well-established best practice exists.

Common Pitfalls:
Misreading 'perpendicular to axis' as 'perpendicular to bedding'; confusing geometric references; neglecting to dress bed joints to the correct radial angle during cutting.

Final Answer:
Radial to the curve (beds normal to pressure)