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Re: cantilever versus arch



Jack Uretsky wrote:

Of course the presence or absence of glue amounts to much more
than a "nitpick".

1) That's debatable at best. The absence of glue was quite strongly
implied when you chose the word "stack".
http://www.m-w.com/cgi-bin/dictionary?va=stack

2) In any case, that's irrelevant to the questions of whether
half an arch is a cantilever or vice versa, or whether an arch
and a cantilever operate on similar physical principles.

The performance of the "glued cantilever" would be proportional
to the strength of the glue. The performance of a "glued arch"
wouldn't be. Nitpickers should assume the arch is used in the
ordinary way.

There is good physics here, or at least good engineering. It
has been known for thousands of years that ordinary masonry
materials are incomparably stronger in compression than in
tension.

http://www.masonryonline.com/members/history_of_masonry.htm

] When masonry materials are stacked vertically,
] they are very stable; every part is undergoing
] compression. The real problem of construction,
] however, is spanning. Ways must be found to
] connect walls so as to provide a roof. The two
] basic approaches to spanning are post-and-lintel
] construction and arch, vault, and dome
] construction. In post-and-lintel construction,
] lintels, or beams, are laid horizontally across
] the tops of posts, or columns; additional
] horizontals span from beam to beam, forming decks
] that can become roofs or be occupied as floors.
] In arch, vault, and dome construction, the
] spanning element is curved rather than
] straight. In the flat plane of a wall, arches may
] be used in rows, supported by piers or columns to
] form an arcade; for roofs or ceilings, a sequence
] of arches, one behind the other, may be used to
] form a half-cylinder (or barrel) vault; to span
] large centralized spaces, an arch may be rotated
] from its peak to form a hemispherical dome.
]
] Post-and-lintel solutions can be executed in
] various materials, but gravity subjects the
] horizontal members to bending stress, in which
] parts of the member are in compression while
] others are in tension. Wood, steel, and
] reinforced concrete are efficient as beams,
] whereas masonry, because it lacks tensile
] components, requires much greater bulk and weight.
] Vaulting permits spanning without subjecting
] material to tension; thus, it can cover large
] areas with masonry or concrete. Its outward
] thrust, however, must be counteracted by abutment,
] or buttressing.