Design of Piers

Pier - dock which projects into the water; can be used for docking on both sides

Design Considerations

Type of Design

- A pier may be designed as a rigid structure in which the lateral forces are taken by batter piles or by rigid frame action; some movement may still take place due to lateral deformation and bending but is usually ignored in absorbing the impact of the ship

- Some installations - FLEXIBLE to absorb the docking impact (e.g. wood pile clusters because they can absorb energy of impact through the large movement which they are capable of undergoing without permanent distortion - for barges and small vessels only)

- For large vessels, design the pier with structural steel framing and steel piles to provide an adequate resisting force: See Fig. 5.2 and 5.71 for examples.

Loads to be used in Design

-Lateral and vertical loads for which the dock is to be designed:

1. Lateral Loads - from the mooring lines that pull the ship into or along the dockor hold it against the force of the wind or current. (a)

10 psf ≤ Maximum wind force = A * 1.3W ≤ 20 psf (10-20 range taken from wind velocities 55 to 75 mph respectively based on the wind pressure formula p = 0.00256v², multiplied by the shape factor 1.3 where p=pressure in psf and v = velocity in mph)

where: A = exposed area in sq.ft. of the broadside of the ship in a light condition

W = wind pressure in psf (1.3 shape factor; a combined factor taking into consideration the reduction due to height and the increase in suction on the leeward side of the ship.

***

cause of suction on leward side: (for buildings)

As wind blows against a vertical surface of a home such as a wall or steeply pitched roof, it exerts a positive force or “pressure” against that surface. As the wind flows over or around the home, it exerts a negative force or “suction” on the walls or roof planes parallel to or away from the direction of the wind. The combination of these pressure and suction forces can result in the following damaging effects to structures: Uplift; Sliding ; Overturning ; Racking

source: Federal Emergency Management Agency

***

! For 2-sides berthing : the total wind force acting on he pier shall be increased by 50% to allow for wind against the seconf ship

Maximum of 20 psf was ideally established by the author because a ship would not remain alongside the dock in a light condition, in a storm approaching hurricane intensity. The ship would either put to sea or take on ballast to reduce its exposed area to the wind.

!"Ballast is seawater taken in by a ship to keep it stable as it unloads cargo. The ships take in the water at one port and releases it at another" http://www.jamaicaobserver.com/news/html/20041004T230000-0500_67114_OBS_BALLAST_WATER_FROM_CARGO_SHIPS_A_BIG_THREAT.asp

(b) Wind against the pier structure and a warehouse or transit shed on a pier may be a more severe condition than wind on the ship as the sruface area may be larger and the wind intensity is greater. Wind pressures for this case shall be determined for the max velocity in the area and applicable shape factor ranging from 1.3 to 1.6 e.g. in a hurricane area, wind velocity = 125 mph, and total wind pressure = 64 psf

(c)Force of the current (F) = (w/(2g)) * v² : for salt water F = v²

where F = in psf

w = weight per cubic foot of water

v = velocity of current in fps

g = 32.2 ft/s²

The velocity of current is usually 1 and 4 fps which will result in a pressure of 1 to 16 psf

This force F will be applied to the area of the ship below the water line when FULLY LOADED. since the ship

is generally berthed parallel to the current, this is seldom a controliing factor.

2. Docking Impact - caused by the ship ship striking the dock when berthing

Desing assumptions:

(a) Maximumm impact when ship is FULLY LOADED (displacement tonnage) striking the dock

(b) at an angle 10 degrees with the face of the dock

(c) velocity normal to the dock = 0.25 to 0.5 fps

A few installations habe been designed for 1 fps but this is considered excessive, except for small ships, as this corresponds to velocity of approach = 3.5 knots at an angle 10 degrees and could damage the ship

Fender systems - designed to absorb docking impact; resulting force to be resisted by the dock will depend upon the type and construction of the fender and deflection of the dock (if designed as flexible structure)

3. Earthquake Forces - consider if pier is within an area of seismograph disturbance. Horizontal force may vary between 0.025 and 0.1 (verify if seismicity map) of the acceleration of gravity g times the mass, applied at its center of gravity which can be expressed as 0.025 to 0.10 of the weight (g * mass = weight) respectively

Weight to be used = total dead load plus one half of the live load

Unless the dock is of massive or greavity type, the effect on the design will usually be small as the allowable stresses when combined with dead and live load stresses may be increased by 33 1/3 percent.

If batter piles are used to carry the horizontal earthquake force: this must be checked to see that they will carry the horizontal earthquake force without increasing the allo loaing by more than 33 1/3 %, otherwise addtl piles will be needed. (applies to both transverse and longitudinal directions of the pier.

4. Vertical Loads

(a) dead load - dead weight of the structure

(b) live load - usually consists of a uniform load plus wheel loads from trucks, railroad cars or locomotives, cargo-hndling cranes, and equipments; range (250 to 1000 psf of deck area, 250 can be used for oil docksor similar structures where materials are handled by pipelines; usually piers are designed from 600 to 800 psf. used 1000 psf for piers handling heavy metals such as copper ingots and containers)

The uniform live load will control the design of the piles and pile caps, whereas the concentrated wheel loads, including impact will usually control the design of the deck slabs and beams

A reduction of 33 1/3 % is sometimes made in the uniform live load in figuring the pile loads and in designing the pile caps or girders based on the assumption that the entire deck area of adjoining bays will not be fully loaded at one time

Reference: The above are taken from this book unless otherwised referenced to other sources.

DeFQuinn, A. Design and Construction of Ports and Marine Structures. 1972. McGrawHill. p293-298