Pile Design and Construction Practice, Fifth edition

Example 8.3

A cross-section of an approach trestle giving roadway access to a cargo jetty is shown in
Figure 8.20. The trestle is sited at right-angles to the direction of maximum current velocity
and travel of storm waves. The distribution of current velocity with depth is shown on the
cross-section. The deck slab and other components of the superstructure impose a total
horizontal wind force of 25 kN on each pile bent. Storm waves have a maximum height from
crest to trough of 3 m. Determine the distribution of current and wave forces on the pile bent
and calculate the bending moments on the piles produced by these forces.
The maximum horizontal force on the piles will be due to the combined current and wave
action at HWST(6.0 m). At this stage of the tide the storm wave crest will be at 7.5 m.
The underside of the transom beam is at 8.0 m and therefore the wind force on the exposed
length of pile from 7.5 to 8.0 m will be relatively small and can be neglected. It is con-
venient to divide the length of the pile into 2 m elements. Allowance is made for barnacle
growth on the piles. Thus,

giving 9.7y 0.26, y 0.26^10

3

9.7

27 mm

Piling for marine structures 431

Figure 8.20

0 0.5 1.0 1.5 2.0

Current velocity in m/s

Wind formce-25 kN

HWST 5 + 6.0

Wave trough 5 + 4.5

0.00

610 mm steel

tubular pile

(12.7 mm wall

thickness)

Distribution

of current velocity Stiff boulder

clay Cross-sectionof pile bent

Distribution of

current and wave force

• 5.5
  
  
  • 3.5
    
    
    • 1.5
    
    
    
    
  
  
  
  

+ 0.5

Combined current

and wave force

+ 6.5

+ 4.5

+ 2.5

11.91 kN/m^2

9.93 kN/m^2

8.37 kN/m^2

7.14 kN/m^2

6.71 kN/m^2

6.14 kN/m^2

6.14 kN/m^2

D 5 0.800 m

D 5 0.68 m

• 1.5

+3

+ 8.0

+ 8.5 Deck slab Transom beam

Sea bed

• 6.5

D 5 0.610 m

Wave crest 5 +7.5