4-3.2

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

recommended.

4-4

4-4.1

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

combinations of the illustrated concepts are possible.

4-4.1.1

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.

4-4.1.2

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

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