I'm a masonry contractor in Truro, Mass. Recently, my crew and I laid a series of stone arches below the wood-framed terrace of a new residence. The deck above was independently supported on structural steel columns that would be concealed within the stonework, so in this case the arches were decorative. Nonetheless, they were traditionally built and totally capable of supporting a structural load. Before trusses, steel beams, and reinforced concrete became commonplace, nothing beat the durability and bearing capacity of a stone masonry arch.
A masonry arch gets its structural quality from wedge-shaped "ring stones" placed in compression around the perimeter. The overhead pressure is redistributed to the ring stones and down to the springer stone at the base of the arch.
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