THICKNESS OF VOUSSOIRS, (RING STONES).

253. The thickness of the voussoir, or arch stone, depends upon the form and size of the arch, the nature of the masonry, and the character of the stone. No authority gives more reliable results than Gauthey, who, for stone of average quality, with hammer dressed beds, laid in cement, gives the following proportions between the span and depth of key:—

For spans under six feet the depth should be thirteen inches.

From six to fifty feet, 13 inches plus 1
48 of the span.

From fifty to one hundred feet, 1
24 of the span.

For over one hundred feet, 1
24 of 100 plus 1
48 of the remainder.

Thus for a span of one hundred and ninety-six feet we have

100 × 12
24 + 96 × 12
48,

or, 50 + 24 equal in all to seventy-four inches, or six feet and two inches; whence the following table:—

Span of arch in feet.Thickness of voussoir in inches.
613 +0 = 13
813 +2 = 15
1013 +3 = 16
1213 +3 = 16
1513 +4 = 17
1813 +5 = 18
2013 +6 = 19
2513 +7 = 20
3013 +8 = 21
3513 +9 = 22
4013 +10 = 23
4513 +11 = 24
5013 +13 = 26
60 = 30inches.