Pile Design and Construction Practice, Fifth edition

the extent to which they are tied together by intercostal beams or by a longitudinal berthing
beam. An approximate rule is to assume that the blow is absorbed over a length of berthing
face equal to twice the width of the jetty. BS6349 recommends a minimum distance between
ships moored along a jetty of 15 m.
The design process is one of trial and adjustment to determine the most economical
combination of vertical fender piles with rubber or spring cushion blocks that will limit the
movement of the protected jetty structure to the desired value. If the impact is delivered at
a point below the head (Figure 8.5) some of the energy is absorbed by the soil, some by
the deflection of the pile considered as a beam fixed at the lower end and with a yielding
prop at the upper end, and some by the yielding at the prop position (i.e. the yielding of the
cushion block).
As alternatives to the system of fender piles, each backed by a cushion block as shown in
Figure 8.4, a group of piles can carry a rubber fender (Figure 8.6a) or a link-suspended
clump fender (Figure 8.6b). For these designs the energy transmitted to the supporting piles
is equal to the kinetic energy of the moving ship, less than the energy expended in
compressing, displacing, and raising the fender from its neutral position.
Forces act in a direction parallel to as well as normal to the berthing line. Assuming
that there are no objects projecting beyond the side of the ship, the force acting parallel to
the berthing line is equal to the coefficient of friction between ship and fender times the
reaction normal to the berthing line. The longitudinal force tends to cause the twisting of
fender piles and of pile bents set transversely to the berthing line. The rotational force on
the pile bents is a maximum when the ship makes contact near the end of the jetty, and it is
desirable to provide piles raking in a longitudinal direction at the two ends of the structure.
The end piles in a jetty head are vulnerable to impact below the water-line from the bulbous
bows of vessels provided with bow-thrust propellers.
Damage to fender piles or their connections to the main structure by longitudinal forces
can be avoided by spiking timber rubbing strips onto the faces of the fenders. These will be
torn off by a severe impact but the pile will remain relatively undamaged.
Rubber fenders are designed to deflect in a longitudinal as well as a transverse direction
and are thus capable of absorbing impact energy from both directions. Suspended fenders
are given a degree of freedom to swing in a longitudinal direction and they fall clear as the
ship sheers off after the first impact. Fenders can also be provided with rollers mounted on
vertical axles to reduce the longitudinal frictional force on the structure.

Piling for marine structures 403

Figure 8.5Impact force below head of raking fender pile.

Cushion block

Point of impact

Fender pile