Forces

Forces can be as:

  • Compressive
  • Tensile
  • Shear

All loads acting on structures are a combination of these forces.

In addition, a force can also create a turning affect around a point, which can be of two types, as:

  • Bending Moment (or simply “Moment” because it is very frequent)
  • Torsional Moment (os simply “Torsion”, because it is far less frequent)
Types of forces in statics

In Figure 1, the fundamental ways of forces act on bodies and some very basic concepts are shown. In these figures members are shown as beams but they could be anything, such as column or a plate or a foundation, or a deck, or any structural element.

In Fig.1a, you see a structural member in compression.

Fig.1b shows a member in tension.

Fig.1c shows the concept of shear stress, which occurs when two different surfaces try to slide on each other. If they can not move against each other, then shear stress is created there. It is shown as if moving just to animate it in readers mind.

Fig.1d shows a uniform (also called distributed) load. The uniform load in this figure is linear, with 1 dimension, such as acting linearly on a beam, but it could also be 2 dimensional, acting on a surface, such as the weight of snow over a roof area.

Fig.1e shows the concept of “Bending Moment”. It is the turning effect of force, which is equal to Moment=Force x Distance, which tries to bend a structural member. Here this moment equals the magnitude of one of the force, times the distance between the forces. Here two forces equal but opposite in direction are acting at a certain distance from each other. This creates a turning effect on the beam, which means there is moment. The second figure is equal to the first figure, because the equal forces in opposite direction cancels out and all remains is the turning effect. As you can see we drew the moment on the side of the beam, but it really does not matter where the moment is drawn. It is just the turning effect, so it can be drawn anywhere (it is not a linear force, which can create additional turning effect, with respect to a point for instance). You may wonder, what would happen if these two forces were not equal in magnitude? Then the second shape would include the moment, plus, a resultant force equal to the difference of forces, which would act in the line of action of the larger force (the smaller force would be totally consumed). So if it was floating in space, this beam would rotate forever, given there is no opposing friction or reaction.

Just to make sure, something is clear… Shear stress is sometimes harder to understand as a beginner. If it is not so clear from the figure 1c above, also see below figure. Shear stress is very important to understand, and we will mention it so many times in this series later.

Torsional force

Fig. 1h shows the concept of Torsional Moment or simply Torsion. Again Moment = Force x Distance but here the moment occurs on a different axis. While the bending moment tries to bend the beam, which is very, very common in structural analysis and occurs almost always, torsional moment tries to twist the beam as you can see in the figure, and it happens only in special circumstances (at least for our engineering analysis purposes as it is very negligible most of the time).

Figure 1 i below shows a very common occurence, a simply supported beam in bending. There is nothing new here than what we discussed above. We will discuss what simply supported means later.

Simply supported beam in bending

In the next post, we will introduce the term “Equilibrium Concept, Action/ Reaction”

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