28 July 2005
It is often cost-effective to orient pile caps (and hence pile bents) transverse to
the length of the structure. This orientation provides improved lateral stiffness for
berthing and mooring forces. When this orientation is used, longitudinal pile caps
are not needed unless crane trackage support or longitudinal seismic resistance
is to be provided. For marginal wharves where lateral loads from mooring and
berthing loads are transferred to the land, a longitudinal orientation of the pile cap
may be considered if feasible for construction. Moments and shears on pile caps
from live loads should take into account the elastic shortening of the piles and the
effect of soil deformation at and near pile tips. For computation of forces from
high concentrated loads, the cap behaves as a beam on elastic foundation, and
distributes the concentrated load to a number of piles adjacent to the load. While
hand calculations are acceptable, a stiffness analysis using a computer is
Pile Bent Framing.
A pile-supported framing system is the most popular form for substructure design
for open piers and wharves. Several framing concepts for open piers and
wharves and marginal wharves are illustrated in Figure 4-2. Many variations and
All Plumb Pile System.
The lateral loads are resisted by "frame action," whereby the piles and the cap
form a moment frame and resist the lateral load primarily by the flexural stiffness
of the piles. However, for narrow structures, lateral deflection may be high for
even small lateral loads. Also, sidesway is not prevented, which increases the
effective length of the pile as a column. If piles vary in unsupported length, the
shorter piles will attract a large portion of the lateral load. Because the piles are
more efficient for axial loads and less so for bending moments, this framing
usually is restricted to shallow waters and light lateral loads. However, for wide
structures with a large number of piles, the total stiffness of the system may
justify a reduced effective length. A more in-depth stability analysis is needed to
validate a reduced effective length. Large diameter steel pipe and
precast/prestressed concrete cylinder piles can provide improved lateral stiffness
and are attractive for use in areas of high seismic activity.
Plumb/Batter Pile Systems.
In this type of framing, all the vertical loads are primarily handled by the plumb
piles, and lateral loads are resisted primarily by the batter piles. The behavior of
the system is one of "truss action." This system is more cost-effective as the
lateral loads are resisted primarily by the axial stiffness of the batter piles.
However, very high forces are transmitted to the caps, which will have to be