hy

THE KANSAS CITY BRIDGE. 97

perimeter at 155 feet, 1,483,250 pounds, or a little less than 750 tons. The
form of the caisson was such that the starlings braced themselves, and the only
pressure which had to be carried by interior braces was that on the opposite
long walls, the total strain on the braces being equal to the pressure on 58 feet,
the length of one of these sides, or 483,300 pounds. This would have been
carried by twenty-five braces, each eight inches square, with a strain scarcely
exceeding 300 pounds on the square inch; but to avoid all possibility of acci-
dent, nearly double this strength of bracing was used. No other caisson was
pumped out to nearly this depth; the round tub used at Pier No. 2, from its
circular form, withstood the strains upon it without the aid of interior bracing.

SAND PRESSURE AND FRICTION ON SIDES OF CAISSONS.

The pressure of the sand was considered the same as the thrust of a bank
of earth, the particles of which have no mutual cohesion and computed by the
formula :—

uh?

2
pa tan 2 *
zg 2

(a.)
in which P denotes the total pressure on each horizontal foot ; w, the weight of
a cubic foot of the earth or other material ; A, the height of the bank in feet,
and a, the angle which the natural slope of the material makes with a vertical
line, being the complement of the angle of repose and determined by the
relation :—

Cot. a = coefficient of friction of material on itself.

The application of this formula becomes somewhat complicated when the
earth or sand is submerged. The action of the water is threefold: Ist, it gives
buoyancy to the mass, thereby diminishing the weight ; 2d, by acting as a lubri-
cator on the surfaces in contact, it reduces the friction and increases the value
of a ; 3d, the pressure due to its weight is added to the thrust of the bank, The
two first of these are simple and easily provided for by making the proper
changes in the values of w and a; the latter is of a more complicated
nature, dependent largely on the character of the material. If the bank

 

* This formula is taken from Claudel, Aide Memoire, etc. Tieme Edition, p. 1252. Itis also foundin a
slightly modified form in Rankine’s Civil Engineering, 4th Edition p. 322 (11.)
13