What are the Loads Acting on Marine Concrete Structure?

Pressure and submergence

At first let us discuss about floating concrete structure; the structure that are considered as floating, the dead weight of it is approximately compensated by the buoyancy. When a structure is submerged in sea water it is subjected to continuous hydrostatic pressure. These hydrostatic pressure definitely exerts loads on structure additionally it force water to penetrate into the concrete through Through any weak point, if any. Microcracking in concrete is very common; though durability is not concern of this post, the weight of concrete will be increased with this penetration leading an increase in density by 2% upon absorption, which may increase the net buoyant weight up to 40 to 50 percent.

Impact loads from debris and ice

Impact of ship with crane result failure in Sendai Port (2011 Japan Tsunami, Chock et al., 2013)

Moving debris and ice may produce impact loads directly on concrete structures. These materials may accumulate on structure and increase effective area, thus exert additional load. When ice is accumulated as solid sheet, it will fail by crushing. In this case ice breakers should be installed to pile up ices until it fails in tension. 

Debris is that mass that can float in flow path with an inundation depth Which cannot withstand water flow. When debris are carried with tsunami wave, they may be trees, cars, wooden poles used for utility supply, complete or fraction of timber framed houses. Sometimes some debris that don’t float may transported by wave when they are very strong like pieces of concrete and boulders. In normal case, debris are bottles, waste of ships, cans, fishing equipment, automobile tires, nets and abandoned fishing line.

Debris and ice cakes often become wedge within adjacent piles which exerts torsion and may lead to break of piles. An example may be included here, broke down of better piles adjacent to piles where large mass of ice were accumulated; during spring the mass was thawed and at low tide it slide down to exert load to that piles. Break water may be also effected by formation of ice cakes resulting wedging units apart from each other and during high tide they are lifted up.

It seems that the loading of debris and ice is same on different structural type like concrete, steel and timber, but the consequences of it are significantly different, as it depends on physical and structural characteristics in many cases. A concrete pile may be more susceptible to failure under impact and its resultant torsion whereas reinforced concrete sea walls or caissons often have more resistance as they possess local rigidity, mass and to some extent have ductility at low temperature.

When water is allowed to enter inside concrete structure and get frozen it will exerts a bursting pressure. If a pile or caisson have hollow-core, must have means to impede freezing like providing Styrofoam or wood log to float side and/or provision to connect with unfrozen water beneath surface like vents. If such precaution is not taken serious bursting and subsequent failure may occur.

Load exerted by current

Movement of the sea water produces different types of loads on the concrete structure which are required to evaluate overall stability of marine structure.

Nature of current

Speed of current in ocean usually run from 0.5 nautical miles per hour (knot) which attain maximum value of about 2 knots. In restricted channel or narrow inlets, even higher velocity may occur. In contrast to conventional belief, in some location it is often found that current in ocean may be almost same at bottom as that in surface. In case of estuaries where salt water meet with fresh water, salt water flooding at the bottom is observed while the surface zone is still ebbing.

Prolonged winds in same course can generate strong currents in open sea. In some region the current may rotate during alternate folding and ebbing.

Current force

The load exerted by current can be computed by following formula

Where
P = Total load exerted by current in lb
A = Area exposed perpendicular to the current, SFT
V = Velocity of current, FPS
g = 32.2 ft/sec²
w = unit weight of water 62.4 lb/ft³
k = shape constant (0.75 for circular and 1.33 for rectangular shape)

An alternative calculation of current forces can be used based on naval architecture approach where forces are expressed as summation of two components, Surface friction (P_f) and eddy action (P_e):

Where 
Θ=angle at which the flow is inclined with face of srucutre 
C=constant can be taken as 0.5 
S=area of immersed surface, Sft 
A=area exposed perpendicular to the current, Sft

Current loading found from equation (2) is 50% more than that derived from equation (1). Observations on actual structure showed that the actual forces are nearer to force found from equation (2).

Uplift and downward pressure

We know current produces pressure on the concrete structure. It often results an eddy action when they flow over the top surface which will exert force having tendency to lift the structure that remain on the bottom. On the contrary to that, when a structure hangs just above the bottom, there a pulling down tendency of current resulting to increase in the effective weight of the structure.

Dynamic and cyclic loading

On a submerged structure, waves exerts cyclic and dynamic loading. The waves indeed may break on the surface with enormous energy. It has been measured that the stress induced by local impact may be as large as 3 Tons/Sq. ft. For a complete submerged structure, during the wave cycle a dynamic uplift forces may occurred; It is fortunate that these forces are not usually coupled with maximum lateral forces.

Differential pressure

When differential pressure exists on the either side of any bottom supported marine structure, tunneling underneath may occur which is followed by serious erosion in the bottom sediments. This can reach up to composite structure like back pressure, break water may disintegrate them into the individual components and left out them with an explosive force generated by hydraulic ramming.

This phenomenon of conversion of wave force into kinetic energy can be used deliberately to dissipate and imbibe wave energy. This is done by providing holes in vertical walls of caissons of quay wall, offshore storage facility or breakwater to absorb fraction of the wave energy.

Standing waves:

When non-breaking waves are reflected from a marine structure may take form of standing waves just in front of structure. Under the crest of standing wave erosion may occur. If the term standing waves is new to you, we can include standing wave is often called stationary wave of which peak amplitude is constant in respect of time at any point in the space. The interesting thing is that there have oscillation in the wave but peak amplitude profile remain stationary in space.

Ship impact load

Ship impact load was ignored in design of marine structure in the recent past. The damages due to such impact are mitigated by providing suitable fendering systems. The transportation of goods, machinery and fuels are increasing day by day and the size of vessels are also increasing to meet such requirements. Such large vessels can produce enormous impact which cannot be mitigated by fendering system alone and critical marine structures seek special consideration to minimize damage by absorbing energy. This is done by increasing mass of marine facilities, well-arrangement of reinforcements, prestressing and designing structure to act as diaphragm.