Pile Design and Construction Practice, Fifth edition

402 Piling for marine structures
If the rear of the deck is abutting a retaining wall such as a sheet pile wall, virtually the
whole of the horizontal forces on the deck will be transmitted to the wall.
Where medium to large vessels are accommodated the berthing impact is not absorbed
directly by a pile or by a deck structure supported by piles. Means are provided to cushion
the blow, thus reducing the risk of damaging the ship, and limiting the horizontal movement
of the jetty. It is also more economical during design to provide cushioning devices than to
absorb forces directly on the structure. It must be noted that whereas independent berthing
dolphins can be allowed to deflect over a considerable distance (and large deflections are the
most efficient means of absorbing kinetic energy), the deck slab of a cargo jetty cannot be
permitted to move to an extent which would cause instability in travelling cranes, stacked
containers or mechanical elevators. This limitation restricts the allowable movement of such
cargo jetties to a very small distance.
Where energy absorbing fenders are provided, the work equation 8.4 is modified. Taking the
simplified case shown in Figure 8.4 of a fender pile backed by a cushion block transmitting
the impact to a bent of piles transverse to the berthing line, the work equation becomes
kinetic energy of moving ship absorbed by system in Figure 8.4
� 1
2 � H ��� 1
2 � H(� 1 �� 2)
(8.6)
where H is the impact force of the first blow on the fender, � is the distance moved in
bringing the ship to rest after the first impact, � 1 is the distance moved by the compression
of the cushion block, and � 2 is the distance moved by the pile bent.
In a practical design case a limit is placed on � 2 by the operating conditions on the jetty.
Then if the cushion block is to be fully compressed by the ship moving at the maximum
design approach velocity, � 1 is known and � is the sum of � 1 and � 2. Hence, knowing
the kinetic energy of the moving ship, the impact force H can be calculated. This force
is the sum of the force in the cushion block and the shearing force at the head of the pile.
The bending moment induced in the fender pile by the action of force H over distance � is
compared with the moment of resistance of the selected pile, and the energy absorbing
capacity of the cushion block is checked to ensure that the force required for full compression
is not exceeded by the force H. The condition shown in Figure 8.4, of a single fender pile
transmitting the full force of a moving ship to a single pile bent, does not occur in practice.
In a cargo jetty the fender piles are spaced at equal distances along the berthing face and the
impact is absorbed by a number of piles, depending on the closeness of their spacing, and
Cushion block
fully compressed
∆ 2
∆ 1
∆ =∆ 1 + ∆ 2
Point of
first impact
Cushion block
uncompressed
Fender pile
Figure 8.4 Energy absorption of fender pile cushioned at head.
H