Pile Design and Construction Practice, Fifth edition

414 Piling for marine structures
where F is the wind force in pounds, V is the sustained wind velocity in m.p.h. at the elevation
of the portion of the structure under consideration, C D is a drag coefficient, and A is the
projected area of the object in square feet (including an allowance for ice accretion).
The values of the drag coefficient for use with equations 8.10 and 8.14 are as listed in
Section 8.1.3 and shielding coefficients (8.14) can be applied for closely spaced members.
Wind velocities can be corrected for height by means of the equation:
(8.15)
where H 2 and H 1 are the two elevations concerned. It should be noted that wind velocities
based on short-duration gusts may be overconservative when considering wind forces on
large ships.
8.1.6 Forces on piles from floating ice
Forces on piles caused by floating ice have characteristics somewhat similar to those from
berthing ships, the principal difference being the length of time over which the ice forces are
sustained. Ice floes are driven by currents and wind drag on the surface of the floe. Typically
a floe consists of a consolidated layer, which may be up to 3 m thick in sub-arctic waters,
underlain by a mass of ‘rubble’ in the form of loose blocks, and wholly or partly covered
by loose debris and snow. When designing a structure to resist ice forces it is necessary to
determine the dominant action, i.e. whether it is the pressure of the wind and current driven
floe against the structure, or the resistance offered by the structure in splitting the advancing
consolidated layer. In an extensive review of the subject Croasdale (8.14) stated that only
on relatively small bodies of water will the wind-induced forces govern the design load.
Wind forces can be calculated from equation 8.10. Croasdale advises omitting the factor
0.5 when using this equation and gives values for CD as 0.0022 for rough ice cover,
0.00335 � CD � 0.00439 for unridged ice, and 0.005 for ridged Arctic sea ice. In equation 8.10
the values for CD are appropriate to m/sec units of the wind velocity at the 10 m level.
Croasdale gives a typical force on a 4 m diameter cylindrical pier as 10 MN caused by an
ice sheet 4.15 � 4.15 km in area, driven by a wind velocity of 15 m/sec.
On striking a vertical pile which is restrained from significant yielding, the consolidated
ice layer is crushed at the point of impact. With further movement of the floe radial cracks
are propagated in the ice sheet followed by buckling. The buckling dissipates the energy of
the moving mass which is brought to rest locally against the pile. The surrounding cracked ice
sheet and the underlying loose rubble are diverted to flow past the pile and in doing so they
generate frictional forces on the contact surfaces. The force is likely to be at a maximum at
the time of initial cracking of the ice sheet followed by lesser peaks due to jamming of the
packed ice and adfreezing of the ice on to the structure (Section 9.4).
Croasdale gives the basic equation for the ice force on a narrow rigid structure as
where
V2 � V1� H2 H1� 1
7
F � p/tb (8.16)
p � effective ice stress
t � ice thickness
b � width of pier