4.2. Forces, loads and stress

Loads and forces.

What is a force? What is the relationship between structures and forces?

A force is anything capable of deforming a body (static effect) or altering its state of motion or rest (dynamic effect). The forces acting on a given structure are called loads and can be of two types:

Fixed or permanent loads: they are those that do not vary over time, therefore they are constant loads. Some examples are the weight of a bridge’s own structure or that of the elements that are always on it, such as the asphalt or the railing.

Variable loads: these are those that change over time, for example, the weight of people inside a building, the wind that pushes a bridge or the traffic that is on it.

Structural design

is a methodical investigation to obtain the balance between the lowest economic cost and the best structural performance to carry the predicted load safely. With the application of structural design we obtain and calculate required size, grade, reinforcement etc. of structural members to withstand the internal forces calculated from the structural analysis.

Structual analysis

To perform an accurate analysis a structural engineer must determine information such as structural loads, geometry, support conditions, and material properties. The results of such an analysis typically include support reactions, stresses and displacements. This information is then compared to criteria that indicate the conditions of failure.

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Types of stress or efforts.

What effects do you think the forces within the Bodies produce? From our index finger, if we twist it or squeeze our palms with each other, we experience a tension that grows to our palms one against increasing the force we exert.

We can also notice that the type of feeling is different in each case, because we have applied the forces differently. These forces can cause various kinds of deformations in objects. The reaction of bodies to these forces is called Stress. The opposite is at rest.

A structure, therefore, has to be able to support its own weight, and the external forces to which it is subjected such as wind, waves, etc. Therefore, each element of a structure has to withstand different types of forces without deforming or breaking. The most important types of stress they support are:

Traction/Tensile: If on the ends of a body two opposing forces act that tend to stretch it, the body undergoes traction. It is the kind of effort that suspenders and tensioners endure.
Compression: If two opposing forces act on the ends of a body that tend to compress it, the body undergoes compression. It’s the kind of effort that supports the pillars and foundations.
Bending: If forces act on a body that tend to bend it, the body suffers bending. It’s the kind of strain that girders and trusses endure.
Torsion: If forces act on a body that tend to twist it, the body undergoes torsion. It’s the kind of effort that a key can withstand by turning on a lock.
Cut or shear: If on a body forces that tend to cut or tear it, the body suffers cut. It is the type of effort that suffers the pool trampoline area attached to the tower or the area of union between a beam and a pillar.

Clear ideas

Stresses are internal forces that appear in structures as a result of the action of charges. The distribution of these forces along a section is called tension.

Traction occurs when you intend to stretch the body.
Compression appears when trying to crush or compress.
Bending takes place when we try to bend the body.
Torsion appears when attempting to twist the body.
Shear or shear stress occurs when forces attempt to cut the body.

structural failure

The condition when a structure fails to withstand the stresses imposed upon it. The failure manifests itself by bending, becoming permanently deformed, or breaking.

Causes:

The loss of sustaining capacity or stability.
Rupture of an essential component of the structure.
A marked increase in strain without an increase in load.
A deformation which increases much more rapidly than the increase in imposed load.

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