109 OF ENGINEERING F0HMULÆ.
Appboximate Rules fob the Thickness of
Arches and Abutments. (Hurst.)
D = Depth or thickness of crown in feet.
H = Height of abutment to springing in feet.
R — Radius of arch at crown in feet.
T = Thickness of abutment in feet.
W = Weight of 1 foot in length of half arch in
cwts.
n — Constant.
D = n kJ R
Series of arches:—
Block Stone n = ’35
Brick .. »i = 45
Hubble .. n = 5
Single arches:—
Block Stone n — ’3
Brick .. n = ‘4
Rubble .. n — ’45
Wv W
.11/ " H ‘
A/ iR» + fRl +
T =
This formula gives the thickness T of abutment
without wing-walls or counterforts just sufficient
to balance the thrust of the half arch, the depth
at the crown being equal to • 4 kJ K, and the
material in the arch and abutment being the same.
A considerable margin of safety must be allowed on the
dimensions thus given.
RULE SOMETIMES USED FOR RAILWAY BRIDGES.
For spans between 25 and 70 feet—Rise = _
5
Thickness of arch = —
18
S S
Thickness of abutments t to -
5 4
... S 8
Thickness of pier to -
Batter (if any') 1 inch to the foot.