Shearing and Bending in Ship Structures: External and Internal Forces Explained

Shearing and Bending in Ship: Numerous forces and moments can be applied to any type of floating vessel. External loads from the water medium that are operating on the vessel can be broadly classified into two categories: internal and external.

Additionally, various types of external loads can be classified into different categories. These include buoyancy loads, which are primarily a result of force vectors generated by hydrostatic pressure acting directly on the hull structure. There are also forces from hydrodynamics, the movement and interaction of the vessel, wind forces, weather loads, and other time-varying loads like operational loads on board and other effects. Internal loading, or simply a vessel’s weight distribution, originates from the vessel itself.

As we know from the fundamental Archimedes’ principle, the balancing buoyancy forces or an upward response push support the entire weight of the vessel acting on the water in order to maintain a condition of physical balance such that the vessel remains afloat.

When discussing the hull girder in its entirety, the issue can be conceptualized as a single body in an equilibrium state where the vessel’s total weight acting through its center of gravity (CG) is balanced or supported by the opposing buoyancy force acting through its center of buoyancy, keeping the vessel afloat.

The center of gravity and the center of buoyancy for a symmetric body in perfectly still water are colinear. This is only the most basic instance of the floating box that we were introduced to when learning about Archimedes’ principle, which states that a body is in hydrostatic equilibrium. The box-shaped vessel floating in stillwater is depicted in the following basic free-body diagram.

Though fundamentally the same, the concept of floating takes on a new and advanced level for more practical real-world challenges, such as a seagoing vessel.

Shearing and Bending

Shearing and Bending
(Credit: MarineGyaan)

Let’s review what a shear force is now. As demonstrated, shear force is a tangential component of force that acts coplanarly, parallel to the plane of the object’s face. Shear strain is the term for when an object deforms in relation to the line of reference.

According to solid mechanics, any body that is subjected to tangential or lateral forces acting in the opposite direction will shear.

Now, have a look at the accompanying figure, which serves as the most basic explanation of this idea. A force couple (F1 and F2) operating on the two faces in opposition to one another is applied to the body. The body has a tendency to deform relative to its two faces because of a counteracting force pair. Additionally, as a result of this force pair, a moment is also observed, and as a result, the body tends to bend, or is exposed to a bending moment, M.

F1 and F2
(Credit: Marine Insight)
F1 and F2
(Credit: Marine Insight)

Check out the following figure now. At both ends, there is a basic beam that is supported. Imagine a downward force emanating from one of its ends, let’s say A, at an arbitrary distance, let’s say x. What takes place? The beam naturally bends in the direction of the supplied force.

As a result, have a look at the free body diagram from end A to point P, which represents the point at which the load is acting. In addition to acting tangentially coplanar to the beam’s cross-section at point P, this force, F, is also a shear force acting perpendicular to the beam overall. It is a normal force acting globally. At support point A, a reaction occurs as a result of this applied force, acting in the opposite direction.

As shown, this force pair produces a moment in a clockwise direction by effectively shearing the beam section between points A and P.

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Similarly, the bending moment is in the opposite or counter-clockwise direction, meaning if we look at the free-body force diagram in the other part of the beam, from P to the other end-support point, let’s say B.

Because of the applied load P, the beam as a whole experiences a net bending moment that results in a flexural tendency in the downward direction.

The fundamental relationship between the bending moment and the flexural or bending behavior of beams under load is established. In mechanical terms, shear stresses operating coplanar to the material cross sections per unit area is a more acceptable term to employ when discussing the flexural nature of bodies subjected to varying degrees of loading.

Hence, shear forces are tangential and coplanar forces acting along the body’s face; the body is said to be under a shearing effect when two shear forces acting in opposition to one another are present. When a body is sheared, shear forces occur on it, causing the body to bend and producing the bending moment.

Force multiplied by a linear measure (distance), as we have always known from first principles, equals just moment. Therefore, in the scenario above, the bending moment that the beam experiences at point P is equal to the applied force F times the distance x.

Because it is a function of distance, the bending moment is therefore zero at point A, where x = 0, and climbs linearly to the bending moment’s reached value, F * x, at point A, as illustrated. From A to P, the shear force is always equal to F, and from P to B, it is equal to -F. The diagrams for shear force and bending moment are shown as follows:

Now examine the same beam with a loading of, let’s say, R, distributed uniformly. N/m. The following describes the characteristics of the bending moment and shear force diagrams.

The bending moment is, mathematically speaking, the integration of shear forces. On the other hand, shear forces result from the differential of bending moment. The differential of shear force at a given place produces the loading or force distribution at that particular point. Shear force, in turn, can be thought of as the sum of all the acting loads over the region of study.

Within the field of statical solid mechanics, the bending or flexural nature of beams subjected to loads under various situations is a particularly important domain.

There are many different types of scenarios with numerous combinations of loads (combined, distributed, uniformly varying, random, time-varying, and so on) and support types (fixed, free, partially fixed, cantilever, roller, hanging, etc.), even though the simply supported beam with a point load acting is the simplest of all beam problems. These are not to be explored here as they are all fundamental to the general theory of beams in solid mechanics.

Shearing and Bending in Ships

Any given time, a ship can be thought of as a simply sustained beam. Let’s now combine the instances of the simple beam subjected to uniformly distributed loading and Archimedes’ simple block floating in water.

The buoyancy forces of a floating body are spread throughout in accordance with the body’s shape, its weight distribution, and, of course, the water medium it floats on.

Similar to all other forces, the buoyancy forces distributed throughout the body ultimately result in the net buoyancy force that acts through the center of buoyancy, as seen in Figure 1. This is a reaction to the body’s weight, which is also spread along its span, and the net action that results through the CG.

As demonstrated, in the ideal scenario of a homogenously constructed simple block issue in perfectly still water, the buoyancy and weight forces are spread uniformly throughout. As a result, the weight or load component operating downward and the buoyancy component acting upward are two counteracting force vectors at any given position along the vessel’s hull.

The weight distribution is precisely uniform in the homogeneously constructed block. Additionally, the buoyancy distribution that is enforced has a completely uniform distribution because of the water’s perfectly calm circumstances and its shape.

The resulting load curve, assuming perfect external conditions, is just a uniformly distributed loading where the shear force at any given place is equal to the difference between the buoyancy force and the weight acting at that point.

Therefore, in essence, the shear force and bending moment are the same as when a simply supported beam with an evenly distributed load is considered above.

We now know that, in the absence of any external factors, a body’s buoyancy must match its entire weight in order for it to float in water. Apart from the weight-buoyancy equivalency, under the ideal scenario of the aforementioned block dilemma, the weight and buoyancy should always be the same. As a result, the weight at any given time is W/l if the body’s total weight is W and its length is, let’s say, l.

From here on, the buoyancy at the same location should ideally equal -W/l, where -W is the weight-equivalent net buoyancy force. What takes place? The net load is zero throughout the body, which is to be expected! The shear and bending curves remain 0 and coincide with the reference axes, indicating that there is no shearing or bending!

Nothing is perfect in the actual world, though. Because of the nearly ideal circumstances and the body’s homogeneity, shearing and bending do occur even for a block suspended in a water tank, albeit at very low levels.

In the real world, vessels navigating the water are extremely intricate instances of flotation. In the end, any vessel that wants to stay afloat must have net buoyancy, which is equal to the total physical weight it has imposed. This value is commonly expressed as displacement, which is essentially the volume of water the vessel has displaced spatially. The interaction of loads and forces is highly complex and random.

Now, going back to the beginning of this article, when we had briefly discussed the various types of loads acting on a vessel, it is crucial to understand that the following forces are always at play at any given place on the hull:

  • The vessel’s weight at that point acts directly on the water
  • Buoyancy loads act on the hull with both the Stillwater and the wave component, mostly as outer hull pressure force vectors.
  • Hydrodynamic loads in any sea state
  • Random loads occur due to vessel motions and dynamics, the interaction of waves with the structure, and related effects.
  • Wind forces and other kinds of miscellaneous loads
  • Any other time-varying physical effects the vessel may be subjected to.

But practically speaking, we consider the self-weight distribution of a vessel to be fixed for two main scenarios when evaluating the strength of common vessels, such as cargo: the departure or full-load condition (also called the deep seagoing case) and the arrival or light-load condition (also called the light seagoing case).

The weight distribution pattern can be maintained consistently for the majority of other vessels, whose deadweight tons are presumed to be mainly fixed every time.

buoyancy

The most crucial factors are the buoyancy loads and the ship’s construction weight. The wave component is quite dynamic, but the Stillwater component is constant for a given draft. Examine the following figure, which shows a hull’s longitudinal section. The wave profile makes it clear that point P experiences more buoyancy forces than point Q.

Therefore, the buoyancy load vector values at any two random sites on the vessel can differ significantly from one another, even though the total buoyancy of the entire ship is equal to the displacement for equilibrium.

In technical terms, the system is acting like a simply supported beam when you isolate it for the purpose of evaluating the loads, but it is actually acting under the combined effect of extremely complex force actions, as previously mentioned.

These loads can have any size at any given time and can behave in any unpredictable manner. Therefore, for the system in question, the solution of all the forces mentioned at various points in time can be linked to the net load at any given location within the hull. This aggregate value may also be positive or negative, depending on the local intensities of each of the aforementioned elements.

Now think about the following scenarios:

  • When a high wave crest provides a high buoyancy vector upwards at a particular point in the hull, for instance, and the weight at that point is less than the buoyancy force, the effect of all the other force factors above is negligible in comparison, and the resultant loading is most likely positive and acting upwards.
  • Now assume a random place in the aft where the buoyancy force vector is exceeded by a very large load because of the weights of the machinery, the super-structure, and heavy above-hull outfitting loads such as boat davits. As a result, the resulting force has a negative sign and is downhill!
  • Now think of a more complex scenario. Because the sea is often calm, waves are not wildly unpredictable. A ship now sails over a sizable undersea ridge. Do you recall squatting? Go here to learn about squatting. Every time the draft rises above the ridge at any randomly selected place on the hull, there is an immediate increase in draft that results in a net downward force (negative) at that precise location. Every point on the hull feels this effect as soon as they pass the ridge’s tip.
  • Likewise, at a different point on the hull, let’s say in the bow area, the weight of the ship nearly equals the buoyancy force. But there’s bow slamming, where the bow hits the water’s surface every now and again. What takes place? The net load is working downward with a negative value when the vessel makes contact with the water’s surface, and upward with a positive sensation when it is rising!
  • A frame or station is a handy tool to define a specific spot on the hull while evaluating loads on ships. This idea, however, can be applied to any randomly selected spot within the vessel’s hull. Assume, for simplicity, that we select station A and find a specific random loading value, F1, acting upwards, derived from the sum of the aforementioned effects.

We now select a different station B, say B, which is located h distance from A, and there is a net load acting downwards at station B, say F2. The block between A and B, over the span h, is effectively stated to be under shearing because of the dissimilarity! Furthermore, shearing inevitably results in a bending moment.

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Therefore, it is possible to determine the vessel’s shearing behavior at any given time by integrating all of the load points along the length of the hull; this is also known as the shear force diagram technically. Similarly, the vessel’s bending behavior—technically referred to as the bending moment diagram—is produced by integrating the shearing forces along the vessel’s length.

All vessels must undergo shearing and bending as part of the longitudinal strength evaluation process. Shearing and subsequent bending are more problematic in longer vessels than in smaller ones since the flexural character of any beam is directly related to its span or length.

It is beyond human capacity to estimate the precise shearing and bending characteristics of a seagoing vessel at any given moment. Nonetheless, we consistently adhere to the strength design principle, meaning that the vessels are built and engineered to provide maximum strength against worst-case loads. Since shearing directly results in bending moment, all seagoing vessels are built to withstand the greatest amount of bending moment that can be achieved, taking into account the projected amount of bending moments during their service life.

Practically speaking, a maximum bending moment is calculated taking into account the circumstance in which the length of the vessel is equal to one appropriate wavelength. This is the worst-case scenario that provides us with a clear image of the maximum amounts of bending moments the vessel can encounter, even if it might not be feasible given the unpredictability of the sea surface.

This maximum bending moment taken into consideration is merely an estimate, but it’s reasonable enough to offer an indication of the loads and moments the vessel can withstand during worst-case scenarios. As with anything in the design and engineering domain, no value is 100% correct or exact.

Two situations occur when we assume that the wavelength or length of one wave is equivalent to the length of the vessel:

  • The wave’s troughs, or lowest parts, are located close to the ends, and the crest, or peak, is located at around midship, as indicated.
  • The trough is located close to the middle, with two subsequent crests near the ends, as indicated.
    Hogging and Sagging
  • In the former scenario, the endpoints have a low or negative aggregate buoyancy effect because of the low wave pressure from the troughs, whereas the center body has a substantial and considerable component of positive buoyancy force. The hull girder therefore acts as though it is going up toward the middle and down near the ends due to the nature of these forces.

Hogging is the term for the structure’s characteristic pattern, which makes it lean upward toward the center. The hogging moment is the greatest bending moment that the structure can undergo. The bottom shell or fiber is in a compressed state in the hogging situation, while the main deck is in a tensioned state.

In the latter scenario, the reverse occurs as a result of strong upward forces from the wave crests close to the ends. Sagging is an effect when the hull girder exhibits a curvature that goes down near the middle; this sagging moment is the maximum bending moment that occurs on the structure. The bottom shell is in a condition of tension in the sagging case, while the main deck is in a state of compression.

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As shown, waves that come from the side, like in a beam sea or an oblique situation, do not seem to make a big difference in hogging or sagging when longitudinal strength testing is done, since they tend to cause hull deflections in a lateral sense.

All vessel structural designs are evaluated for longitudinal structural strength as part of fundamental design, research, and analysis to look for shear forces and bending moments for conceivable worst-case scenarios.

Then, these are compared to the maximum values allowed by law for shear forces and bending moments. This is done in line with state laws and classification guidelines. The design is deemed safe if the computed values fall within the specified bounds; otherwise, they do not.

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