COFFERDAMS. 10/
where w is the weight of the whole dam per lineal foot and b the breadth of
the base.
(2) Considered as a loaded cantilever, the outer row of piles will be
subjected to tension and the inner row to compression, or both rows will be
subjected to tension and compression alike, according to whether we treat
the structure as rigid or deformable. Assuming the former condition, if a
be the sectional area of single piling per foot run and/j and /, the resistance
of the material per unit area to tension and compression respectively, then
the linear moment of resistance is
M2=a/,6 or af„b.........................(7)
Assuming the latter condition, the resistance of each row of piles must be
considered disconnectedly, and
M2=| a/(d and ^ afQd, (8)
where d stands for the depth, or thickness, of the piles.
Fig. 66. — Cofferdam at Liverpool.
(3) If a be the sectional area of one of the external struts at a distance, 3,
from the base, and s the horizontal distance apart of the struts, then the
linear moment of resistance due to any number of such struts is
M„=S-—.............................
3 s
This is on the assumption that the struts lie directly in the axis of stress.
Should this not be the case, and the angle of inclination to the horizontal be
Ô, we must write
cos 0
3 s
A similar and additional modification would have to be made if the struts
were also raking on plan.