Barrel Vault and Semi-Circular Arch
A barrel vault exerted a continuous thrust along its sides.
Usually the thrusts were dissipated in the heavy mass of the haunching and the supporting walls.
In the semicircular masonry arch the line of pressure does not conform to the shape of the arch and therefore the crown tends to fall while the sides buckle out.
In a semicircular arch where the stones can slide the crown (C) will fall while the sides (B) are pressed out above a secure springing (D) or at the springing itself (A).
Though the pointed Gothic arch better fits the ideal line of pressure, if too acutely pointed the crown tends to rise while the sides fall inwards. (Similar to saddle failure in pointed corbel vaults)
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