Brick Arches.

Progress, Volume VI, Issue 1, 1 November 1910, Page 454

Brick Arches.

Tt is usual in so-called "straight," "flat," or "camber" gauged arches (the terms are applied indifferently) t.> cut and rub the intrados or soffit ends of the bricks to a slight curve to counteract the appearance of sagging, or bending down that all horizontal lines over voids appear to have. This is called, for convenience, an optical illusion, but is in reality merely due to the association of ideas. Persons familiar with building are so accustomed to see upturned curves or arches over voids, and have come to associate this in the mind with . strength, that the absence of it appears to convey weakness. A perfectly straight beam always appears to say so to me, but I have asked various people not accustomed to them, and they have all replied "No"; so i

take it that it is not an optical illusion, but, as I say, merely the association of an idea we are familiar with. It is not advisable to camber the extrados or top part of the arch, because the bed joints of the course immediately over it would appear crippled, if it were so. A true arch should, of course, have its edges parallel curves, otherwise increased stresses result. A S a "g e « straight arch is merely an ornamental 'feature in a building, and should not carry any great weight, that is,, anything greater than can be resisted by the cement joints, because such an arch is really a lintel or brick beam and has no constructive strength apart from the adhesive properties of the joints; therefore there is, or should be, no settlemeut to provide against.